Imaging of Spinal Cord Injury: Acute Cervical Spinal Cord Injury, Cervical Spondylotic Myelopathy, and Cord Herniation

Imaging of Spinal Cord Injury: Acute Cervical Spinal Cord Injury, Cervical Spondylotic Myelopathy, and Cord Herniation

Imaging of Spinal Cord Injury: Acute Cervical Spinal Cord Injury, Cervical Spondylotic Myelopathy, and Cord Herniation Kiran Talekar, MD,* Michael Pop...

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Imaging of Spinal Cord Injury: Acute Cervical Spinal Cord Injury, Cervical Spondylotic Myelopathy, and Cord Herniation Kiran Talekar, MD,* Michael Poplawski, MD,† Rahul Hegde, MD,* Mougnyan Cox, MD,† and Adam Flanders, MD* We review the pathophysiology and imaging findings of acute traumatic spinal cord injury (SCI ), cervical spondylotic myelopathy, and briefly review the much less common cord herniation as a unique cause of myelopathy. Acute traumatic SCI is devastating to the patient and the costs to society are staggering. There are currently no “cures” for SCI and the only accepted pharmacologic treatment regimen for traumatic SCI is currently being questioned. Evaluation and prognostication of SCI is a demanding area with significant deficiencies, including lack of biomarkers. Accurate classification of SCI is heavily dependent on a good clinical examination, the results of which can vary substantially based upon the patient's condition or comorbidities and the skills of the examiner. Moreover, the full extent of a patients' neurologic injury may not become apparent for days after injury; by then, therapeutic response may be limited. Although magnetic resonance imaging (MRI) is the best imaging modality for the evaluation of spinal cord parenchyma, conventional MR techniques do not appear to differentiate edema from axonal injury. Recently, it is proposed that in addition to characterizing the anatomic extent of injury, metrics derived from conventional MRI and diffusion tensor imaging, in conjunction with the neurological examination, can serve as a reliable objective biomarker for determination of the extent of neurologic injury and early identification of patients who would benefit from treatment. Cervical spondylosis is a common disorder affecting predominantly the elderly with a potential to narrow the spinal canal and thereby impinge or compress upon the neural elements leading to cervical spondylotic myelopathy and radiculopathy. It is the commonest nontraumatic cause of spinal cord disorder in adults. Imaging plays an important role in grading the severity of spondylosis and detecting cord abnormalities suggesting myelopathy. Semin Ultrasound CT MRI 37:431-447 C 2016 Elsevier Inc. All rights reserved.

Acute Cervical Spinal Cord Injury

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pinal cord injury (SCI) is a devastating, life-altering event. Approximately 12,000 new injuries occur annually in the United States,1 and currently there are approximately 227,080-300,938 individuals living in the United States with the sequelae of SCI including permanent paralysis. Not

*Section of Neuroradiology, Sidney Kimmel Medical College at Thomas Jefferson University, Philadelphia, PA. †Department of Radiology, Sidney Kimmel Medical College at Thomas Jefferson University, Philadelphia, PA. Address reprint requests to Kiran Talekar, MD, Section of Neuroradiology, Sidney Kimmel Medical College at Thomas Jefferson University, Philadelphia, PA 19063. E-mail: [email protected]

http://dx.doi.org/10.1053/j.sult.2016.05.007 0887-2171/& 2016 Elsevier Inc. All rights reserved.

surprisingly, the costs to society of SCI are staggering and in 1998 were estimated at $9.7 billion per year.2 The lifetime direct costs of a high tetraplegic injured at age of 25 years can exceed $3 million.1 Males are disproportionately affected with a 4:1 male-to-female ratio, and most of injuries occur between the age of 16 and 30 years. Mirroring the increasing age of the U.S. general population, the average age at injury has increased from 28.7 years of age in the mid-1970s to 39.5 years since 2005. There are currently no “cures” for SCI and the only accepted pharmacologic treatment regimen for traumatic SCI is high dose methylprednisolone (MP), which has been reported to show efficacy in Phase II randomized trials.3 Subsequently, MP administration for acute SCI has become widespread in the United States. Recently the efficacy of this treatment has been 431

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432 questioned and currently is the subject of ongoing debate.4,5 Much of the debate has centered on whether the magnitude of reported improvement with MP is clinically important. The controversy regarding the utilization of MP highlights the critical need for new treatment strategies. To date, the treatment of acute SCI has been characterized, unfortunately, by the paucity of clinical trials. Although the efficacy and safety of MP remains controversial, there is general agreement that any pharmacologic measure should be employed in the first few hours after injury. Patient selection for a specific therapy can be problematic in the acute period because the classification system used to grade neurologic impairment is completely dependent upon the accuracy of the neurologic examination. The neurologic examination is accurate and reproducible in ideal conditions. However, the results can vary substantially based upon the level of cooperation, communication, and consciousness of the patient, associated patient comorbidities and the skills of the examiner. Moreover, the full extent of a patient's neurologic injury may not become apparent for days after injury. By then, late implementation of a drug based upon a delayed neurologic assessment is less likely to demonstrate a therapeutic response. In that respect, it is proposed that in addition to characterizing the anatomic extent of injury, metrics derived from conventional magnetic resonance imaging (MRI), and diffusion tensor imaging (DTI), in conjunction with the neurological exam, can serve as a reliable objective biomarker for determination of the extent of neurologic injury and early identification of patients who would benefit from treatment.

Pathophysiology of SCI Similar to acute traumatic brain injury, acute SCI can be divided into primary and secondary injury models. Compared with the brain, the mechanism of acute SCI is less well understood, and most of the research data currently available is derived from animal trials.6 Although transection injuries of the spinal cord do occur in acute trauma, most of acute SCI in humans is caused by blunt trauma, usually in the setting of motor vehicle accidents.7,8 The primary insult to the cord is initiated by transient or fixed loss of integrity of the surrounding bony and ligamentous structures with resultant blunt impact on the spinal cord. In general, the amount of force transmitted to the cord determines the severity of the underlying cord injury.9 The injury may range from transient neurologic deficits because of abnormal axonal firing to dense neurologic deficits due to axonal disruption.7 Aside from preventive measures like lowering the speed limit and enforcing drunk-driving laws, primary SCI is immutable and current interventions are aimed at mitigating secondary spinal injury. Secondary SCI is characterized by subsequent cellular dysfunction, necrosis, and death of initially intact neurons adjacent to the site of primary affect. Several processes are triggered by the injured or dying neurons at the site of primary impact that spread to nearby normal axons and result in propagation of the initial injury. Immediately following acute injury, spinal cord edema occurs, resulting in decreased

perfusion pressure and ischemia related to microvascular perfusion abnormalities.10 The injured axons also release glutamate in large amounts, a potent excitatory neurotransmitter.11,12 Exposure of uninjured neurons to excessive amounts of glutamate is toxic, resulting in influx of calcium and sodium into the cells and initiating a number of deleterious processes including cell death in some cases.11,12 Vulnerable intact neurons close to the site of initial SCI may undergo cell death in the ensuing hours or days via necrosis or apoptosis.13 Apoptosis is controlled cell death that results in minimal inflammation, whereas necrosis is a disorderly process of cell death that causes significant inflammation.10 Severe cord injury tends to result in more extensive necrosis.7 Another important mechanism of secondary SCI is generation and propagation of free radicals. Free radicals are highly reactive molecules that interact with lipids and proteins in cell membranes to cause cellular dysfunction. Free radicals are especially abundant during the reperfusion phase of SCI, and several therapies are specifically targeted at halting the production of free radicals.14 The processes in secondary spinal injury are complex and interdependent. Optimal management of acute SCI in the future will likely be directed at inhibiting multiple sites in the secondary spinal injury cascade in hopes of achieving a synergistic therapeutic effect.

Clinical Assessment of Acute SCI Initial clinical presentation of patients with acute cervical SCI is the main factor determining triage, defining therapeutic options, and predicting prognosis. As such, the initial neurologic assessment should be accurate, consistent, and reproducible in defining the neurological deficits. In addition, an ideal neurologic assessment scale should have prognostic value in determining patient's potential for recovery. Numerous assessment scales have been employed to evaluate SCI patients, and can be divided into 2 general types. The first type focuses on the neurological deficits resulting from SCI and is examination specific. The International Standards for the Neurological Classification of SCI (ISNCSCI) is the most widely used and validated system, having undergone multiple revisions, most recently in 2011.15 The second type of scale focuses on SCI patient's functional skills, including ability to care for oneself, perform personal hygiene, ambulate or transfer. These scales aim to determine patient's ability or inability to functionor live independently. In general, the first type of scale is used to acutely assess patients with SCI, while both scales are used to define the chronically injured patient. Scales for functional outcomes include the Barthel Index, the modified Barthel Index, the Functional Independence Measure (FIM), the Quadriplegic Index of Function, the Spinal Cord Independence Measure (SCIM), the Walking Index for SCI, the SCI Functional Ambulation Inventory, and more recently proposed SCI Computer Adaptive Test.16 The central aspect of the ISNCSCI is classification of SCI patients into American Spinal Injury Association (ASIA) Impairment Scale—the AIS, which is a 5-point ordinal scale that classifies individual's injury from A through E (Table). The neurologic level of injury (NLI) refers to the most caudal

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Table AIS Scale A B

C

D

E

Using ND

Complete No sensory or motor function is preserved in the sacral segments S4-5 Sensory Incomplete Sensory but not motor function is preserved below the neurological level and includes the sacral segments S4-5 (light touch [LT] or pin prick [PP] at S4-5 or deep anal pressure [DAP]) AND no motor function is preserved more than three levels below the motor level on either side of the body Motor Incomplete. Motor function is preserved at the most caudal sacral segments for voluntary anal contraction (VAC) OR the patient meets the criteria for sensory incomplete status (sensory function preserved at the most caudal sacral segments [S4-S5] by LT, PP, or DAP), and has some sparing of motor function more than three levels below the ipsilateral motor level on either side of the body (This includes key or nonkey muscle functions to determine motor incomplete status.) For AIS C—less than half of key muscle functions below the single NLI have a muscle grade Z 3 Motor Incomplete. Motor incomplete status as defined above, with at least half (half or more) of key muscle functions below the single NLI having a muscle grade Z3 Normal If sensation and motor function as tested with the ISNCSCI are graded as normal in all segments, and the patient had prior deficits, then the AIS grade is E. Someone without an initial spinal cord injury does not receive an AIS grade To document the sensory, motor and NLI levels, the ASIA Impairment Scale grade, and the zone of partial preservation (ZPP) when they are unable to be determined based on the examination results

ND, not determined; NLI, Neurologic level of injury; ISNCSCI, International Standards for Neurological Classification of Spinal Cord Injury.

segment of the cord with intact sensation and antigravity muscle function strength, provided that there is normal (intact) sensory and motor function rostrally. Figures 1-4 show AIS (“A” in AIS stands for ASIA) grades A through D, respectively. A required portion of the ISNCSI motor examination is testing of key muscle functions corresponding to 10 paired myotomes (C5-T1 and L2-S1) with each muscle group receiving a score

A

B

of 0-5. These scores are then summed across myotomes and sides of body to generate a single motor score each for the upper and for the lower limbs (maximum score of 50 for each the upper and lower extremities), and provides means of numerically documenting changes in motor function. The ASIA motor scores are routinely used and reported in the literature to describe functional impairment following SCI and

C

D

Figure 1 Spinal cord injury in a 43-year-old female hit by a train. At presentation patient was categorized as AIS grade A. (A) Sagittal CT image demonstrates traumatic anterolisthesis of C6 on C7, with a flexion teardrop fracture at C7, and spinous process fracture of C6. Also present were bilateral jumped facets at C6 or C7 with bilateral comminuted fractures of superior articular C7 facets (not shown). (B) T1-weighted MRI, (C) T2-weighted, and (D) STIR imaging additionally demonstrates significant cord compression at C6 or C7 with cord edema extending from C5 to superior aspect of T1. Heterogeneity of cord T2 signal abnormality with foci of low signal suggest hemorrhagic contusion of the cord (arrow). Note: epidural hemorrhage posterior to C6 vertebral body (arrow head). There is diffuse posterior soft tissue edema extending inferiorly from C1.

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A

C

B

D

Figure 2 Spinal cord injury in a 58-year-old male following a motor vehicle collision. At presentation patient was categorized as AIS grade B. (A) Sagittal CT demonstrates a perched right facet at C5-C6 (arrow). No fracture was seen. (B) T1-weighted MRI, (C) T2-weighted, and (D) short tau inversion recovery (STIR) imaging further demonstrates traumatic anterolisthesis of C5 on C6 with disruption of the disc (arrow head). There is disruption of interspinous ligaments and anterior longitudinal ligament (arrows). Cord edema extends through C5 and C6 level, without evidence of cord hemorrhage.

as predictors of outcome.16 The ASIA standards cannot be accurately employed in SCI patients who cannot be accurately examined because of confounding factors,17 and are not applicable to adolescents and children.18 Despite these limitations, the ASIA standards provide the most validated, consistent, and reliable system for neurological assessment, and is recommended (Level II evidence) as the preferred tool in the care of acute SCI patients.16

A

B

C

Although the AIS does not assess functional ability, several functional recovery measures have been developed as outcome measures for persons living with SCI. Anderson et al19 reported the consensus analysis of a multinational work group in 2008. Overall, they recommend SCIM III as the one best measure of global disability and functional recovery specific for SCI. The SCIM aims to quantify the ability to perform everyday tasks and captures the economic burden of disability, as well as the

D

E

Figure 3 Spinal cord injury in a 47-year-old male who was toppled by a wave. At presentation patient was categorized as AIS grade C. (A) Initial outside hospital T1-weighted MRI, (B) T2-weighted, and (C) STIR imaging demonstrates prevertebral edema from C1 to C4-C5 and mild widening of anterior C3-C4 disc space with focal disc T2 signal abnormality (arrow head). There is spinal cord edema spanning the C3 level. There is interspinous edema extending from C2 to C5. Patient was transferred to our facility because of worsening neurological function and a 32-hour follow-up (D) T2-weighted, and (E) short tau inversion recovery (STIR) MRI demonstrated progression of cord edema with a new focus of T2 hypointensity (arrow) within the cord at C3-C4 indicating progression to hemorrhagic contusion.

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B

Figure 4 Spinal cord injury in a 25-year-old following a hyperextension injury while playing rugby. At presentation patient was categorized as AIS grade D. (A) T1-weighted MRI and (B) T2-weighted imaging demonstrates acute transmembraneous disc herniation with discontinuity of the posterior longitudinal ligament at C3-C4 (arrow) causing significant spinal cord compression, and mild spinal cord edema. Additional disc bulge is seen at C4-C5. No fracture was identified on computed tomography.

effect of their disability on the patient's overall medical condition and comfort. SCIM consists of 3 subscales that cover the related but distinct subsets of self-care (6 items; score range: 0-20), respiration and sphincter management (4 items; score range: 0-40), and mobility (9 items; score range: 0-40). The total score ranges from 0-100. The mobility subset is further subdivided into 2 subscales: room and toilet, and indoors and outdoors. Individual item scores range from 2-9 points. SCIM scores a task higher in patients who accomplish it with less assistance, aids, or medical compromise than other patients.20 With level I evidence, the SCIM III is recommended as the preferred functional outcome assessment tool for care and follow-up of SCI patients.16

Neurologic Recovery in SCI In general, spontaneous neurologic recovery from SCI is inversely related to the severity of the initial neurologic injury. However, there is reported variability in recovery within similar AIS grades. Most of neurological recovery in SCI patients occurs during the first 6-9 months.21-23 Afterward, the rate of improvement rapidly drops off with a plateau being reached 12-18 months postinjury with little additional improvement after that time. Early improvement in neurological status is associated with greater recovery than slow improvement.24 Late recovery following complete SCI, defined as motor recovery greater

than one year after injury, is rare but can occur. Recovery of motor function distal to the zone of injury in patients with complete SCI is relatively rare, and when it does occur it tends to be minimal and nonfunctional. The national SCI database indicates that about 15% of all AIS grade A patients admitted within 1 week of injury convert to incomplete status by 1 year. However, only 2.3% of initially complete patients regain significant motor function below the injury level, that is, to AIS grade D.25 Other studies have reported complete to incomplete conversion rates ranging from 4%-34%.21,22,26-28 Recovery in motor complete, sensory incomplete injuries (AIS grade B) is mixed, with about 50% attaining ambulatory status.29,30 By definition, individuals with an AIS grade B injury have some initial preservation of distal sensation, including the S4-5 dermatomes, but no accompanying motor function. It is known that this type of sensory sparing influences prognosis—those with sacral or lower extremity pin prick sensation—have a better chance of walking than those with only light touch sensation.30-34 While recovery in motor complete injuries is generally poor, recovery from incomplete injuries (AIS grades C and D) is generally good, although this is influenced by the degree of motor deficit and the age of the patient. For AIS grade C patients younger than 50 years of age at the time of injury, the chance of walking exceeds 90%, while those over age 50 have only a 42% chance of walking.35 Up to 95% of individuals with AIS grade D injuries will recover the ability to ambulate.29

436 In contrast to recovery below the zone of injury, most people with complete tetraplegia have some local recovery immediately cephalad to the zone of injury (ie, the zone of partial preservation). Most of individuals with complete tetraplegia gain a motor level, although there are differences dependent on the initial level. If the initial motor level is C4, 70% will gain C5 motor function; the corresponding rates for C5 to C6 and C6 to C7 are 75% and 85%, respectively.23 Recovery more than 2 levels below the most caudal level with motor function is rare, being seen in only 1% of cases.22 One factor that accounts for this wide range in recovery is the difficulty performing an accurate neurological assessment during the early phase of injury. Influences that can affect reliability of this clinical examination include those affecting cognition (traumatic brain injury, drug effects, and psychological disorders) as well as communication (ventilator dependency and language barrier).17 There is a potential therefore to initially misclassify patients, contributing to ambiguity in our understanding of the relationship of the initial injury to spontaneous recovery. As new novel therapies for SCI become routinely available, it is imperative that we improve our precision in classifying patients as close as possible to the time of injury to maximize benefits of these therapies. Other objective methodologies are needed for identifying appropriate patient populations for treatment delivery; optimal therapy may need to be delivered in the first few hours or days, yet neurologic examination may require a week's delay before providing an accurate prognosis.17 The variability in recovery for patients with incomplete SCI has hindered the study of promising treatments in the acute injury period for such patients. As a panel sponsored by the International Campaign for Cures of SCI Paralysis (ICCP) reported “trials involving motor incomplete SCI patients, or trials where an accurate assessment of AIS grade cannot be made before the start of the trial, will require large subject numbers or better objective assessment methods.”36 The substantial variability in recovery within similar injury classification suggests that the arbitrary delineation among injury classifications (AIS: A-E) may not be representative of all of the potential functional categories. Potentially functional subclasses may exist that need to be defined through means other than the neurologic examination, such as MRI. With this additional information, the ability to perform rigorous patient selection before initiation of therapy would be greatly enhanced and measuring efficacy of these therapies would be much more feasible.

Conventional MRI of Acute Cervical SCI Currently, MRI provides the only means to directly inspect the damaged spinal cord, therefore it has the potential to complement the assessment provided by the subjective neurologic examination in gauging the degree of injury in SCI. Moreover, MRI evaluation is not operator dependent and the assessment of the MRI features is reproducible among observers. MRI provides excellent definition of intramedullary hemorrhage and edema in animal models. On MRI, cord edema is indicated by intramedullary hyperintensity on T2-weighted sagittal

K. Talekar et al. images, and acute cord hemorrhage (deoxyhemoglobin) is indicated by focal decreased signal on T2-weighted and gradient echo images. The combination of MRI lesion length, cord caliber, and degree of preservation of white matter in MRI cross-section has a significant relationship to functional status in animals and the pathologic findings at autopsy. The MRI appearance of experimentally induced SCI has been used to explain the variability in functional deficit among animals subjected to identical injuries. A significant shortcoming of MRI is its limited capability in demonstrating preserved white matter tracts at the level of injury; this observation becomes significant in estimating preserved functional capacity. With the advent of diffusion techniques and tractography algorithms based upon diffusion parameters, MRI now has the capacity to assess the integrity of spinal white matter. The depiction of parenchymal SCI on MRI not only correlates well with the degree of neurologic deficit, but it also bears significant implications regarding prognosis and potential for neurologic recovery.37-43 Many clinical investigations have reported that the MRI patterns of SCI correlate with the neurologic deficit at presentation. Kulkarni et al initially proposed 3 MRI injury patterns for SCI and correlated these with the 5-part AIS and total motor scores. Intramedullary hemorrhage (Type I pattern of injury) equated with a severe neurologic deficit and a poor prognosis. Cord edema alone (Type II pattern of injury) was found in patients with mild to moderate initial neurologic deficits who subsequently showed neurologic improvement.42 Schaefer et al40 refined the MRI patterns of SCI by including the size of the injured segment. Cord edema that extended for more than the span of one vertebral segment was associated with a more severe initial deficit than smaller areas of edema. Cord hemorrhage was associated with the most severe neurologic abnormalities.40 Flanders et al38 demonstrated that spinal cord hemorrhage in the cervical region was a strong predictive finding for a complete neurologic injury. The location of the hemorrhage corresponded anatomically to the level of neurologic injury. Although the location of spinal cord edema related imprecisely to the neurologic level, the proportion of spinal cord affected by edema was directly related to the severity of initial neurologic injury. Schaefer et al41 correlated the MRI appearance of the spinal cord on admission to the change in total motor index score (MIS) in 57 patients. Patients with hemorrhagic spinal cord lesions showed no statistical improvement in MIS at follow-up. The group of patients with small areas of edema (less than 1 vertebral segment in length) demonstrated the largest improvement in MIS (72% recovery), whereas larger areas of edema showed intermediate recovery of MIS (42%).41 Flanders et al37 assessed the prognostic capabilities of MRI in forecasting motor recovery in 104 cervical SCI patients. Individual manual muscle test scores were compiled for the upper and lower extremities both at the time of admission and 12 months after injury. A motor recovery rate for the upper and lower extremities was also determined. The injured spinal cord segment on MRI was measured using a unique method that quantified spinal cord hemorrhage and edema by length and location relative to known anatomic landmarks. Lesion

Imaging of spinal cord injury length was directly proportional to neurologic impairment at the time of injury (P o 0.001). In addition, spinal cord hemorrhage was associated with the most severe injuries (P o 0.001). Although improvement in motor function after 1 year was observed in all patients, subjects with spinal cord hemorrhage on MRI had lower initial motor scores and had less improvement than those without hemorrhage. Nonhemorrhagic MRI lesions were associated with significantly higher motor recovery rates in the lower and upper extremities and had a higher proportion of useful muscle function. Multiple regression analysis was used to determine the contribution of MRI in predicting the outcomes parameters of motor function independent of the initial clinical evaluation. Initial motor scores, the presence of hemorrhage and the length of edema were independent predictors of final motor score and the proportion of muscles with useful function at one year. The addition of the MRI parameters to the initial clinical information improved the statistical power of the SCI model by 16% for the upper extremities and 34% for the lower extremities.37 Flanders et al44 also compared the MRI parameters of edema and hemorrhage to a standardized measurement of disability (FIM). A total of 4 distinct motor scales from the FIM assessment were determined at the time of admission to rehabilitation and subsequently at discharge from rehabilitation. The individual motor scales included tasks related to self-care, sphincter control, mobility, and locomotion. Patients without spinal cord hemorrhage on MRI had significant improvement in self-care and mobility scores compared to patients with hemorrhage. The upper limit of the lesion (edema) correlated with admission and discharge self-care, admission mobility, and locomotion scores. Edema length correlated negatively with all FIM scores at admission and discharge. Moreover, at the time of admission to rehabilitation, all patients were completely dependent on equipment or caregivers to perform the FIM tasks. At the time of discharge, only patients with nonhemorrhagic MRI lesions improved to a modified dependence category.44 Boldin et al45 did a prospective analysis of 29 SCI patients by comparing an absolute measurement of the size of the injured segment on a postoperative MRI to the initial clinical examination and changes in long-term neurologic status. The authors also found that the presence of intramedullary hemorrhage had a higher association with a complete neurologic deficit and patients with hemorrhages that measured greater than 4 mm in cranial-caudal length showed no clinical improvement at follow-up. Both the length of edema and hemorrhage were shown to be predictive variables for complete injuries. Patients with hemorrhages measuring o4 mm had incomplete injuries upon admission and showed clinical improvement at follow-up. Although their patient cohort was small and the authors were unable to control for time to clinical follow-up or time to imaging, their data suggest that there may be an absolute threshold for lesion size that predicts neurologic recovery.45 Boghosian et al46 correlated the NLI with the anatomic location of the spinal cord lesions on the MRIs of 109 cervical spinal cord injured patients. The authors found a statistically

437 significant correlation between the location of the upper margin of spinal cord edema and hemorrhage as well as the lesion epicenter. The upper boundary of hemorrhage showed a stronger correlation than either edema or lesion epicenter. The lesion length showed no statistical significance with NLI. Lesion epicenter and edema length were the best predictors of NLI. The implication of this work is that some MRI measures may be used as an objective measure of the NLI when determination by clinical examination is either inaccurate or unavailable.46

DTI in SCI Although MRI is the best imaging modality for the evaluation of spinal cord parenchyma, conventional MR techniques do not appear to differentiate edema from axonal injury. They are therefore limited to providing anatomic information about the spinal cord parenchyma. The water content or hemorrhagic content does not necessarily reflect the status of the white matter tracts and consequently the functional status of the spinal cord is not well assessed. As with white matter tracts in the brain, anisotropy in the spinal cord is probably a result of diffusion barriers encountered as water moves in the direction perpendicular to the fibers. Figure 5 shows DTI in a normal individual with axial fractional anisotropy (FA) maps and DTI tractography. These barriers are believed to be cellular membranes and myelin sheaths, which result in a low transverse apparent diffusion coefficient (tADC). As water diffuses longitudinally in the spinal cord, these diffusion barriers are not encountered, and the longitudinal ADC is therefore large in comparison to tADC. Using either diffusion-weighted imaging or DTI techniques, the preferred direction of anisotropic water diffusion in spinal cord white matter tracts has been shown in numerous ex vivo47-53 and in vivo54-61 experimental studies, as well as in vivo human studies,62-70 to be parallel, or longitudinal, to the long axis of the axons. A critical goal of spinal cord imaging research is a noninvasive quantifiable predictor of axon loss. Schwartz et al71 have shown that the natural variation of differing axon morphometric parameters (including axon density, axon spacing, and axon diameter) between normal spinal cord tracts significantly correlates with different directional water diffusion values. Ford et al72 showed that alterations in ADC values were more sensitive than conventional MR techniques in detecting experimental SCI. Following injury, tADC values increased and longitudinal ADC values decreased in both normal and abnormal appearing white matter. These changes resulted in decreased anisotropy. These results imply that there are consequences of SCI that dramatically alter axon structure without changing water content or T2, and therefore would not be detected by conventional MR imaging. Nevo et al51 have shown that measurement of ADC values and anisotropy can be used to quantify SCI and neuroprotection. Changes in ADCs in spinal cord white matter have been correlated with behavioral recovery following cervical lateral funiculus lesion and transplantation of fibroblasts genetically modified to express brainderived neurotrophic factor.48

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A

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Figure 5 Spinal cord DTI. Normal study. (A) Normal color coded axial FA maps. (B) DTI tractography. (Color version of figure is available online.)

In the earliest case report which used diffusion-weighted imaging in acute human SCI it was noted that diffusion values decreased acutely at the site of injury, potentially due to cellular and axonal swelling.73 In another case report of a patient with syringomyelia, DTI was able to identify spared white matter around the periphery of the syrinx, underscoring the potential for visualizing spared white matter following trauma.74 In patients with spondylosis and spinal cord compression, it has been seen that diffusion MRI improves sensitivity to cervical myelopathy, however, there have been conflicting reports of both increased and decreased ADC values, and it may be that the age and clinical severity of a lesion may be important in relating the imaging finding to pathophysiology.75,76 DTI may also be able to determine the degree and directionality of glial scarring in the gray following injury, which may go undetected with conventional MRI.77,78 As some current therapies are focused on decreasing the degree of glial scarring following injury, DTI may provide an important noninvasive outcome measure. Although the majority of published works on the application of DTI in SCI use animal models, there are limited published series that illustrate the utility of DTI in human SCI.78-87 Ellingson et al83,84 reported significant decreases in FA and MD in a group of chronic spinal cord injured patients compared to normal controls. MD was measurably lower throughout the spinal cord in the injured group and FA reduction was indirectly related to clinical severity. In a small clinical series, Shanmuganathan et al85 demonstrated the feasibility of clinical DTI in acute SCI by reporting a consistent change of DTI parameters in 20 SCI patients compared to normal controls using a standard clinical MRI unit. Whole cord ADC values were significantly lower in patients and both the ADC values and FA values were decreased at the site of injury compared with controls. Interestingly, the authors reported a decrease in regional ADC values remote from the site of injury suggesting that the DTI parameters can vary in normal appearing spinal cord on conventional MRI. This supports

the concept that DTI may have greater value in mapping the full extent of injury in conjunction with features from conventional MRI. In a subsequent study, Cheran et al86 found statistically significant differences in mean diffusivity (MD), FA (FA), radial (RD), and longitudinal diffusivity (AD) for hemorrhagic and nonhemorrhagic SCI patients compared to controls. For nonhemorrhagic SCI, the investigators found strong correlations between admission motor scores (total MIS) and average MD, FA, RD, and AD at the injury site. This same relationship did not hold for hemorrhagic SCI. In a small cohort of human cervical SCI patients, Chang found that DTI indices correlated better than conventional anatomic MRI.87 While MRI and DTI will likely never provide the same high level of granularity as that of a good quality neurologic examination, it does offer three distinct advantages over the INSCSCI assessment: (1) speed, (2) objectivity and (3) direct visualization of the end organ (the spinal cord). Pharmacologic companies that are engaged in development of SCI therapeutics not only need a reliable measure of neurologic function (ie, the clinical examination) but they also rely upon MR imaging to demonstrate the intrinsic changes to the spinal cord immediately after injury and temporally after direct administration of a therapy. As such, it may be more suitable to select a patient for a clinical trial primarily on the basis of the MRI and DTI findings and secondarily on the initial neurologic assessment.

Chronic Cervical Spondylotic Myelopathy Cervical spondylosis is a common disorder affecting predominantly the elderly. The term spondylosis encompasses the range of degenerative changes that occur in the structures forming the spine including disc degeneration, disc herniation, degenerative spondylolysthesis, uncovertebral and facet arthropathy and ligamentum flavum infolding. These

Imaging of spinal cord injury degenerative changes have a potential to narrow the spinal canal and thereby impinge or compress upon the neural elements—the spinal cord and the nerve roots—leading to cervical spondylotic myelopathy (CSM) and radiculopathy. Cervical spondylosis is an extremely common disorder with radiographic evidence of degenerative changes in the cervical spine seen in as many as 80% of those over the age of 75 years. The incidence and prevalance of cervical spondylotic myelopathy has been estimated to be 41 and 605 per million in North America, respectively and it is the commonest nontraumatic cause of spinal cord disorder in adults.88 The onset of spondylosis and CSM is insidious with a slow progression leading to difficulty in its clinical detection. This is further compounded by the difficulty in predicting the progress of myelopathy and its irregular correlation with degree of severity of spondylosis. Imaging plays an important role in grading the severity of spondylosis and detecting cord abnormalities suggesting myelopathy.

Pathophysiology of CSM Although the exact pathophysiology for CSM remains uncertain, the central cause for its development is compressive injury to the cord. This results from static factors leading to spinal canal stenosis, dynamic factors which involve structures producing strain on the cord with cervical motion leading to repetitive direct mechanical injury to the cervical cord and possibly a component of spinal cord ischemia. The static factors consist of canal stenosis produced by the various structures that form the spinal canal. Canal compromise is caused by various factors: (1) Anteriorly—disc herniation or bulges, osteophytes. (2) Anterolaterally—uncovertebral joint hypertrophy or osteophytes. (3) Posteriorly—facet hypertrophy, ligamentum flavum infolding (Fig. 6). These factors decrease the CSF reserve around the spinal cord and contribute to mechanical injury. The canal can be developmentally small in caliber, which is an independent risk factor for development of CSM. Edwards

439 and LaRocca89 predicted a canal size o10 mm as a high risk factor for development of myelopathy, those with a canal size of 13-17 mm were less prone to myelopathy but were more prone to symptomatic cervical spondylosis, and those with a canal size 417 mm were asymptomatic. Morishita et al90 proposed on MRI that a congenital sagittal diameter o13 mm was a significant risk factor for development of symptomatic disease. Cervical motion can aggravate spinal cord damage precipitated by static compression. The canal dimension changes with motion and gets narrower in flexion and extension. In flexion, the spinal cord lengthens and occupies the anterior portion of the spinal canal.91 The cord can then get stretched over the disc and osteophytes.92 In extension, buckling of ligamentum flavum causes reduction in subarachnoid space posteriorly and overall greater decrease in the spinal canal dimension compared to flexion.93 Also, this is compounded by the fact that in extension, the spinal cord shortens with a slight increase in the spinal cord thickness further increasing risk of mechanical injury to the cord.94 Apart from extrinsic compression during motion, abnormal motion can also produce shear and strain forces within the cord contributing to cord injury.94 On a cellular level, these factors result in direct injury to neurons and glia and lead to a cascade of events including ischemia, excitotoxicity, and apoptosis.95 This pathophysiology is not unlike what is postulated for traumatic SCI though the time course in CSM is over months to years. Grossly, the central cord, in particular the gray matter of the spinal cord and the anterior horn cells are most severely affected in CSM, especially with severe compression of the cord. Among the white matter tracts, the lateral corticospinal tracts are most prone to injury even with minor compression. The posterior column and more so the anterior column white matter are more resistant to compressive injury.96 In comparison, demyelinating disease preferentialy involve the peripheral white matter rather than the central grey.

Clinical Features

Figure 6 Schematic diagram at the level of a cervical intervertebral disc. Note: the narrowing of the central canal by the disc-osteophyte complex and the ligamentum flavum infolding. The neural foramen on the left is narrowed by osteophytic spurring from the uncovertebral and facet joints.

Cervical spondylosis has an insidious onset and slow progression. It commonly starts of as neck pain, which in itself is a nonspecific symptom and may even be caused by various structures outside the cervical spine. The source for neck pain related to cervical spondylosis can be from the nerves innervating paraspinal muscles, intervertebral discs, and facet joints.97 Narrowing of the spinal canal and neural foramina can lead to impingement and compression on the nerve roots, which can lead to radiculopathy wherein the symptoms would occur in a specific dermatomal distribution in the upper extremity. The symptoms may be sensory, most frequently pain or motor loss corresponding to the involved nerve root. Cervical myelopathy can lead to varied symptoms and signs depending predominantly on the area of affection of the cord. Crandall and Batzdorf described 5 broad categories of cervical spondylotic myelopathy based on the location of insult in the cord: (1) Transverse lesion syndrome, in which all the motor

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“Torg-Pavlov ratio,” or the “canal-to-body ratio.” Measuring canal diameter using Torg-Pavlov ratio may have value in predicting myelopathy. Yue et al101 showed a significant difference in T-P ratio between patients with and without CSM (0.72 vs 0.95 with P o 0.001). However, the ratio has a low positive predictive value due to variability in vertebral size in the population.102 A narrower vertebral size will underestimate severity of canal narrowing by this ratio whereas a wider vertebral body will overestimate narrowing. The utility of this ratio has been disputed by many and a study performed on CT by Blackley et al103 to detect the correlation of this ratio with true canal diameter found this ratio to be a poor predictor of canal stenosis. Although radiographs and particularly CT give a fair idea of the osseous anatomy and severity of canal narrowing, they fail to provide the detailed visualization of the narrowing produced by the soft tissue components of disc and ligaments and are not sensitive in providing accurate information of canal dimensions. MRI with its exquisite soft tissue resolution is the imaging modality of choice for depiction of cervical spondylosis as well as changes in the cervical spinal cord (Fig. 7). A routine MRI protocol for evaluation of cervical degenerative disease would usually consist of imaging in 2 planes— sagittal and axial. The protocol used in our institution consists of sagittal T1, T2, and STIR sequences. Axial T2 sequence at the level of the intervertebral disc allows the ability to further localize the site and cause of canal and foraminal narrowing. Axial T2 is obtained with both spin echo and gradient sequences. Spin echo sequence is better for evaluation of cord

and sensory tracts were involved. (2) Motor system syndrome, in which corticospinal tracts and anterior horn cells were involved leading to spasticity. (3) Central cord syndrome, in which motor and sensory deficits affected the upper extremities more severely than the lower extremities. (4) BrownSéquard syndrome, which consisted of ipsilateral motor deficits with contralateral sensory deficits. (5) Brachialgia and cord syndrome, which consists of radicular pain in the upper extremity along with motor or sensory long-tract signs.98 Though this compartmentalization may not hold always hold true and a mixed pattern of deficit may be produced.

Imaging of Cervical Spondylosis and CSM The primary role of imaging is determination of the static factors leading to spinal canal narrowing. The bony component of the narrowing can be detected by radiographs or more accurately by computed tomography (CT). Radiographs and CT can depict vertebral subluxation, disc space narrowing, which indicates underlying disc degeneration, osteophytes, uncovertebral, and facet arthropathic changes leading to central canal and foraminal narrowing. The sagittal canal dimension measured on a lateral cervical radiograph is not reliable indicator of bony canal dimension and is prone to measurement error due to magnification and positioning issues. To partially offset these issues, Torg et al99 and Pavlov et al100 proposed measurement of the ratio of the sagittal diameter of the cervical canal divided by the corresponding diameter of the vertebral body, also referred to as the

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Figure 7 Cervical spinal stenosis. (A) Plain lateral radiograph of the cervical spine demonstrating osseous changes of cervical spondylosis—multilevel disc space narrowing and osteophytes. (B) Corresponding sagittal T2W MRI of the same patient demonstrating the associated soft tissue components—disc (anterior arrows) and ligaments (posterior arrows) that lead to multilevel spinal canal stenosis.

Imaging of spinal cord injury signal and gradient sequence is better for evaluation of osteophytes. On the anterior aspect of the cervical spinal canal, disc disease and osteophytes are the primary causes for canal narrowing. The normal disc demonstrates central hyperintense signal on T2 because of presence of hydrophilic glycosoaminoglycans. Disc degeneration begins with desiccation of the disc. The disc loses its normal hyperintensity on T2. There can be accompanying degenerative endplate changes in the vertebral bodies. Disc height narrowing and bulging of the annulus occur with more advanced degeneration. A rent in the disc referred to as an annular fissure is often a precursor to disc herniations. Contrary to disc bulge, herniations involve less than one-fourth (901) of the disc circumference. Disc herniations with a base wider than dome are referred to as “protrusions” whereas those with a base narrower than the dome are referred to as “extrusions.” Less commonly, the herniated disc material may lose contact with the parent disc and is referred to “sequestered” disc. In the cervical spine, disc material is often accompanied by marginal osteophytes from the vertebral body and these are referred to as “disc-osteophyte” complexes (Fig. 8). The location of the bulge or herniation and its severity may correlate with neural compression. The centrally located disc herniations can impinge on the ventral cord or the ventral nerve roots. The more peripherally

441 located disc herniations in the foramina or extraforaminal locations can impinge on the exiting nerve roots. Foraminal stenosis, apart from disc herniations is often contributed by osseous narrowing due to osteophytes from the uncovertebral and facet joints. On the posterolateral aspect, the ligamentum flava which run along the lamina of the cervical vertebra can show infolding decreasing the CSF space on the posterolateral aspect of the cord and with greater severity may impinge on the cord. Another component that can produce narrowing from the anterior aspect of the spinal canal is ossification (and thickening) of the posterior longitudinal ligament. Though pathophysiologically a distinct entity, OPLL also produces symptoms due to chronic compression of the cervical spinal cord. Apart from evaluation of the anteroposterior spinal canal diameter on the sagittal sequences, various additional quantitative measurements like the transverse area of the spinal cord, compress ratio, maximal canal compromise, and maximal spinal cord compression can be calculated on MRI.104 Numerous grading systems have been proposed to grade the severity of the canal compromise. Muhle et al105 proposed a numeric grading system consisting of grade 0, normal; grade 1, partial obliteration of the anterior or posterior subarachnoid space; grade 2, complete obliteration of the anterior or posterior subarachnoid space; and grade 3, cervical cord

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Figure 8 MR axial imaging of disc disease. Axial GRE images demonstrates diffuse disc bulge (A) and a focal disc protrusions (C and D) (arrows). Additional presence of uncovertebral hypertrophy (B) resulting in foraminal stenosis (arrow heads), commonly referred to “disc-osteophyte” complex in the cervical spine. GRE, gradient echo.

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442 compression or displacement (Fig. 9). Kang et al106 suggested a modification of this grading system replacing absence or presence of cord deformation as criteria for grades 1 and 2 instead of partial or complete obliteration of the CSF spaces which they considered less reliable. Recently, combined task forces of the North American Spine Society, the American Society of Spine Radiology and the American Society of Neuroradiology have suggested a simple technique for grading spinal stenosis that can be graded as mild, moderate, or severe, if the canal is narrowed by less than a third, one-third to twothirds, or greater than two-thirds of the original diameter respectively; this can also be used to grade spinal stenosis.107 Evaluation of the dynamic component of canal narrowing can be performed with imaging in flexion and extension sequences.108 This would better define the dynamic component of compression but has not gained widespread acceptance, partly due to the time consumed for performing additional sequences and a lower yield in the general patient population. However, certain cases with suspected higher grade stenosis should benefit from additional imaging in flexion and extension. Kinematic cine MRI sequences depicting real time motion changes in the spinal canal using fast EPI sequences is also a promising tool in better understanding the dynamic component of the disease. Flexion MRI may also aid diagnosis of Hirayama disease, which classically manifests in adolescent males with progressive weakness that stabilizes after

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several years. It is characterized by asymmetric muscle weakness and atrophy in the C8-T1 distribution. MR imaging findings reported include loss of attachment of the dura to the lamina, asymmetric lower cervical spinal cord atrophy, spinal cord T2 hyperintensity, loss of cervical lordosis in the neutral position, and forward displacement of the dura with flexion MR imaging.109,110 MRI evaluates the cord by identifying the degree of spinal cord deformation and compression and detecting spinal cord signal abnormality. The normal cord has a rounded shape on axial imaging. Loss of CSF reserve around the cord with deformation of the cord and flattening of its surface may indicate early cord compression. With chronicity and severity of cord compression, signal abnormalities develop in the spinal cord. T2 hyperintensity in the cord in early stages may represent reversible edema (Fig. 10). More advanced stages tend to have associated T1 hypointensity. This in combination with cord flattening and volume loss is consistent with myelomalacia, which indicates irreversible injury111 (Fig. 11). CT accurately depicts osseous narrowing of the spinal canal compared to radiographs. It is more sensitive in depiction of ossification of posterior longitudinal ligament compared to MRI (Fig. 12). It can have a complementary role to MRI in certain cases, especially in preoperative planning. CT myelography also plays a role in imaging patients, especially when the patient has a contraindication for getting an MRI study

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Figure 9 Grading of severity of central canal stenosis. (A) Sagittal T2-WI and (B) sagittal STIR images in the same patient. Note: varying levels of central canal stenosis: Grade II at C6-C7 (effacement of CSF without cord deformity—small arrow on A), Grade III at C4-C5 (effaced CSF with cord compression—large arrow on A—and increased cord signal—arrow on B ) and Grade I at other levels. CSF, cerebrospinal fluid.

Imaging of spinal cord injury (Fig. 13). However, intradural injection of contrast agent is an invasive procedure with associated risks and thus should not be considered a first-line test in patients who are able to undergo MRI.

443 significantly lower and ADC significantly higher in cases with cord compression.113

Idiopathic Spinal Cord Herniation DTI in CSM Diffusion properties can be evaluated using quantitative indices such as the apparent diffusion coefficient (ADC), mean diffusivity (MD) and fractional anisotropy (FA). The white matter tracts in the spinal cord have low ADC and high FA values because of tightly packed directional orientation of the fiber bundles. Wen et al112 detected that FA values were significantly lower in the cervical spinal cord of patients with CSM compared to controls. Compared to T2 signal abnormality, FA values were better able to prognosticate the severity of the myelopathy. Kerkovsky et al also detected that FA values were significantly lower and ADC values were significantly higher at site of maximal compression in patients with CSM compared to healthy controls. In addition, among patients with CSM, symptomatic patients demonstrated lower FA values and higher ADC values empowering DTI to prognosticate the severity of CSM. Studies have shown FA values to be

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Idiopathic spinal cord herniation is defined as herniation of the spinal cord through a dural tear which occured without trauma or surgery. It may be asymptomatic, however, prolonged and severe cases can present with symptoms of myelopathy. It is more common in the mid-thoracic spine with the dural defect often noted in the ventral aspect of the spinal canal. The typical imaging feature is of a focally ventrally displaced deformed cord with prominent dorsal CSF space (Fig. 14). This makes a dorsal arachnoid cyst a close imaging differential and a CT myelogram study may be needed in difficult case to differentiate these two entities.114

Summary Mechanical damage to the cord can occur in the setting of trauma, chronic compression or in the rare cases of cord

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Figure 10 Cervical spondylotic myelopathy with cord edema. Sagittal T2 (A) and axial T2 (B) show a large disc protrusion at C5-C6 causing severe spinal canal narrowing, cord compression and cord edema. Note: the edema predominantly affects the central gray (double white arrows). In contrast, in a case of multiple sclerosis, the areas of T2 signal abnormality are almost exclusively seen involving the white matter fiber tracts—as seen in (C) where the black arrows point to foci of demyelination in the posterior and left lateral white matter columns.

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Figure 11 Cervical spondylotic myelopathy with myelomalacia. Sagittal T2 (A) and sagittal T1 (B) MRI of the cervical spine demonstrate hyperintense T2 and slight hypointense T1 signal in the spinal cord at C3-4 disc level (white arrow). Axial T2 GRE (C) and axial T2 spin echo (D) demonstrate a central disc protrusion and the cord signal abnormality (thick double white arrows on C and D, respectively) predominantly involving the central gray matter of the cord.

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Figure 12 Ossification of posterior longitudinal ligament (OPLL). Sagittal and axial CT (A and B) and MRI (C and D) depicting OPLL as a cause of canal narrowing (arrows).

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References

Figure 13 CT cervical myelogram. Multilevel central canal narrowing, most severe at C6-7 (arrow) where the CSF reserve is lost both anteriorly and posteriorly. CSF, cerebrospinal fluid.

herniation. MRI is the modality of choice for evaluation of cord injury. In the acute traumatic setting, areas of hyperintensity on T2-WI probably reflecting edema and particularly hypointensity on GRE images reflecting hemorrhagic changes correlate with clinical findings and worse prognosis. Similarly, abnormal signal in the cord with CSM—usually prominent in the central gray matter—heralds important cord damage.

Figure 14 Idiopathic spinal cord herniation. Focal ventrally displaced cord with prominence of the dorsal subarachoid space. This appearance is typical for cord herniation through a dural defect. When the cord deformity is less marked than this a dorsal arachnoid cyst can be considered as a differential.

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