Pathophysiology and pharmacologic treatment of acute spinal cord injury

Pathophysiology and pharmacologic treatment of acute spinal cord injury

The Spine Journal 4 (2004) 451–464 Review Article Pathophysiology and pharmacologic treatment of acute spinal cord injury Brian K. Kwon, MD, FRCSCa,...

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The Spine Journal 4 (2004) 451–464

Review Article

Pathophysiology and pharmacologic treatment of acute spinal cord injury Brian K. Kwon, MD, FRCSCa,b,c,*, Wolfram Tetzlaff, MD, PhDc,d, Jonathan N. Grauer, MDa, John Beiner, MDa, Alexander R. Vaccaro, MDa,e a

Department of Orthopaedic Surgery, Thomas Jefferson University and The Rothman Institute, 719-1015 Chestnut Street, Philadelphia, PA 19107, USA b Combined Neurosurgical and Orthopaedic Spine Program, University of British Columbia, Vancouver General Hospital, 2733 Heather Street, Vancouver, British Columbia, Canada, V5Z 3J5 c International Collaboration on Repair Discoveries, University of British Columbia, 2469-6270 University Boulevard, Vancouver, British Columbia, Canada, V6T 1Z4 d Department of Zoology, University of British Columbia, 6270 University Boulevard, Vancouver, British Columbia, Canada, V6T 1Z4 e Delaware Valley Regional Spinal Cord Injury Center, Thomas Jefferson University Hospital, 132 South 10th St., Philadelphia, PA 19107, USA Received 7 January 2003; accepted 2 July 2003

Abstract

BACKGROUND CONTEXT: The past three decades have witnessed increasing interest in strategies to improve neurologic function after spinal cord injury. As progress is made in our understanding of the pathophysiologic events that occur after acute spinal cord injury, neuroprotective agents are being developed. PURPOSE: Clinicians who treat acute spinal cord injuries should have a basic understanding of the pathophysiologic processes that are initiated after the spinal cord has been injured. A familiarity with the literature on which the current use of methylprednisolone is based is also essential. STUDY DESIGN/SETTING: Literature review. METHODS: Literature review of animal data on pathophysiologic mechanisms, and of both animal and human trials of neuroprotective agents. RESULTS: The mechanical forces imparted to the spinal cord cause primary damage to the neural tissue, but a complex cascade of pathophysiologic processes that imperil adjacent, initially spared tissue to secondary damage rapidly follows this. Attenuating this secondary damage with neuroprotective strategies requires an understanding of these pathophysiologic processes. Many researchers are investigating the role of such processes as ischemia, inflammation, ionic homeostasis and apoptotic cell death in the secondary injury cascade, with hopes of developing specific therapies to diminish their injurious effects. Beyond methylprednisolone, a number of other pharmacologic treatments have been investigated for the acute treatment of spinal cord injury, and even more are on the horizon as potential therapies. CONCLUSIONS: This review summarizes some of the important pathophysiologic processes involved in secondary damage after spinal cord injury and discusses a number of pharmacologic therapies that have either been studied or have future potential for this devastating injury. 쑖 2004 Elsevier Inc. All rights reserved.

Keywords:

Spinal cord injury; Secondary damage; Pathophysiology; Apoptosis; Neuroprotection; Methylprednisolone; GM1 ganglioside

FDA device/drug status: not approved for this indication: methylprednisone, GM1 Ganglioside, Naloxone, Gacyclidine, Nimodipine, COX-2 Inhibitors, Riluzine, Minocycline, FK506 (Tacrolimus), Cyclosporin, Erythropoletan. Nothing of value received from a commercial entity related to this research. * Corresponding author. Department of Orthopaedics, University of British Columbia, D-6 Heather Pavilion, VGH, 2733 Heather St., Vancouver, BC, V5Z 3J5, Canada. Tel.: (604) 875-5857; fax: (604) 875-5858. E-mail address: [email protected] (B.K. Kwon) 1529-9430/04/$ – see front matter doi:10.1016/j.spinee.2003.07.007

쑖 2004 Elsevier Inc. All rights reserved.

Introduction Individuals paralyzed by trauma to the spinal cord are left with one of the most physically disabling and psychologically devastating conditions known to humans. Over 10,000 North Americans, most of them under the age of 30 years, experience such an injury each year [1]. A decade ago, the cost for the medical, surgical and rehabilitative care for spinal cord–injured patients was estimated at over $4 billion

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annually [2], a societal expense that has undoubtedly increased substantially in the new millennium. Although undeniably enormous, this economic impact fails to recognize the incalculable loss experienced by these patients, who are often young, otherwise healthy and in the most productive years of their lives. This has prompted much investigation from the clinical and scientific communities to develop therapeutic strategies for spinal cord–injured patients, in order to enhance neurologic function in what was historically deemed an untreatable condition [3,4]. Substantial insight has subsequently been generated about the pathophysiology of acute spinal cord trauma, giving rise to a number of clinically applicable neuroprotective treatments to maximize the functional integrity of the remaining spinal cord. In this review we summarize these pathophysiologic mechanisms that contribute to the “secondary injury” that follows the initial mechanical trauma to the spinal cord, and discusses current and experimental pharmacologic treatments. As evidenced by the poor neurologic recovery after most spinal cord injuries and the paucity of pharmacologic treatments currently available, it should be obvious to even the most casual observer that our present understanding of these pathophysiologic processes and how to manipulate them is fairly rudimentary. Concepts of primary and secondary damage after spinal cord injury Blunt injuries to the spinal cord occur as the osteoligamentous spinal column fails under a variety of loading conditions, including flexion, extension, axial load, rotation and distraction. These forces impart the primary mechanical insult to the spinal cord, which in its mildest form causes a cord concussion with brief transient neurologic deficits [5,6] and in its most severe form causes complete and permanent paralysis. Whereas the former represents local axonal depolarization and transient dysfunction, the latter represents a primary axonal and neuronal injury followed by a spreading of secondary tissue damage that expands from the injury “epicenter” (Fig. 1). The extent of both the initial tissue disruption and subsequent secondary injury is likely directly related to the energy delivered to the spinal cord at the moment of impact [7]. Our current medical and surgical interventions for the acutely cord-injured patient attempt to minimize this secondary injury and protect the neural elements that initially survived the mechanical injury. The existence of such spared neural tissue has been observed in postmortem studies of patients with “complete” injuries, demonstrating that the spinal cord is rarely anatomically transected after blunt injuries, even in patients whose motor and sensory paralysis is deemed complete [8,9]. It is not currently known how much of the spinal cord needs to remain intact in humans to mediate meaningful distal neurologic function, although minimal residual motor function has been observed in an incompletely paralyzed patient with approximately 7% of the

Fig. 1. Primary and secondary damage after spinal cord injury. The mechanical forces imparted to the spinal cord at the time of injury cause immediate tissue disruption. This “primary” injury rarely transects the spinal cord but rather leaves an intact rim of tissue. This adjacent tissue that survives the primary injury is vulnerable to acute pathophysiologic processes that quickly follow. Neuroprotective interventions aim to minimize the destructive effects of these processes.

normal number of axons below the injury level [9,10]. Animal studies have demonstrated the maintenance of significant neurologic function with sparing of 1.4% to 12% of the total number of axons across the spinal cord injury site [11–13]. It is under the pretense that even small gains in neuroprotection might effect functionally relevant neurologic recovery that it becomes extremely important for these patients to receive expeditious trauma resuscitation and clinical care, supplemented by neuroprotective pharmacologic interventions when appropriate [14].

Acute pathophysiologic processes It is worth noting at the onset that very little of our current understanding of the pathophysiologic processes initiated in the human spinal cord after blunt injury is actually derived from human studies. The overwhelming majority comes from animal models of spinal cord injury, which use a variety of animal species and injury paradigms [15]. This is important to consider, particularly when interpreting studies of neuroprotective interventions that appear promising in the laboratory setting but have failed to demonstrate efficaciousness in human trials [16]. For the most part, the study of secondary damage after spinal cord injury is best performed in blunt injury models, such as weight drop or clip/balloon compression, whereas axonal regeneration is more easily studied after sharp injury (Table 1). Such studies have identified a number of interrelated processes that are thought to contribute to secondary damage after spinal cord injury, including

B.K. Kwon et al. / The Spine Journal 4 (2004) 451–464 Table 1 Animal models for studying secondary injury mechanisms after spinal cord injury ●





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Many animal species have been used in models of spinal cord injury, including primates, dogs, cats and rabbits. Mice and rat models are the most commonly used currently. Models of spinal cord injury in which the cord is bluntly injured are useful for studying acute pathophysiologic processes, but leave axons spared at the periphery and are therefore difficult to use for studies of axonal regeneration. Models in which the cord is completely or partially cut in a sharp manner are useful for studying axonal regeneration, but poorly represent the typical human injury and are therefore less appropriate to use for studies of acute pathophysiology. Blunt spinal cord injury paradigms have commonly used weight drop and clip or balloon compression to produce graded severities of injury. In rodent models, the New York University and Ohio State University spinal cord impactors are widely used and produce consistent injuries by precisely striking the dorsal aspect of the spinal cord.

alterations in microvascular perfusion, free radical generation and lipid peroxidation, necrotic and apoptotic cell death and dysregulation of ionic homeostasis (Fig. 2). Vascular abnormalities Local vascular alterations and ischemia within the spinal cord are thought to be one of the most important aspects of the secondary injury, although the exact mechanisms by which ischemia is produced are not entirely clear. Mechanical disruption of the microvasculature causes petechial hemorrhage and intravascular thrombosis, which in combination with vasospasm of intact vessels and edema at the injury site can lead to profound local hypoperfusion and ischemia [17]. This is primarily a microvascular phenomenon, and the larger-caliber vessels, such as the anterior spinal artery, are normally spared [18]. Human postmortem studies have demonstrated that vascular perfusion is substantially worse in the grey matter than in the white matter, which may be related to the disruption and/or thrombosis of the sulcal

Fig. 2. Acute pathophysiologic processes after spinal cord injury. The initial trauma initiates a number of different processes that contribute to the necrotic and apoptotic death of cells within the spinal cord. These are interrelated processes that often positively feedback on one another to worsen injury.

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arterial network that centrifugally supplies much of the grey matter [19]. The high metabolic requirements of neurons make the grey matter exquisitely sensitive to ischemia injury, which can be compounded by the loss of autoregulatory mechanisms that normally provide tight control of the microvasculature hemodynamics within the spinal cord [20]. The loss of autoregulation, which normally maintains fairly constant local cord hemodynamics during systolic blood pressure fluctuations between approximately 50 and 130 mm Hg, makes the cord vulnerable to systemic arterial pressure [21]. Because these acutely traumatized patients can present with systemic hypotension secondary to hypovolemia from additional injuries and/or neurogenic shock and bradycardia from their loss of sympathetic tone, the prompt trauma resuscitation and aggressive monitoring and maintenance of systolic blood pressure is a critical aspect of their initial care. In this regard, it has been proposed that mean arterial pressure be maintained, if possible, at or above 90 mm Hg after spinal cord injury [22]. Ironically, after this period of ischemia, the spinal cord can undergo a period of reperfusion during which a significant increase in oxygen-derived free radicals may actually further exacerbate the secondary damage [23,24]. Free radicals and lipid peroxidation Free radicals are molecules that possess unpaired electrons that make them highly reactive to lipids, proteins and DNA. Molecular oxygen itself (O2) possesses two such unpaired electrons. The addition of one electron to oxygen produces superoxide, of two electrons produces hydrogen peroxide and of three electrons produces the highly reactive hydroxyl radical. The generation of these free radicals can be catalyzed by free or protein-bound iron. Another highly reactive free radical, peroxynitrite (ONOO⫺), is formed by the interaction of superoxide with nitric oxide. Free radicals can cause a progressive oxidation of fatty acids in cellular membranes (lipid peroxidation), whereby the oxidation process geometrically generates more free radicals that can propagate the reaction across the cell surface [25] (Fig. 3). This oxidative stress can also disable key mitochondrial

Fig. 3. Lipid peroxidation by free radicals. Notice that there is a geometric “chain reaction” progression to the lipid peroxidation process. The free radical OH● generates a lipid radical L● from fatty acids in the lipid membrane. After oxidation of L●, another lipid molecule from the membrane is claimed in an oxidation reaction that generates yet another lipid radical, which can propagate the reaction further. If this process goes unchecked, one can envision how the cell membranes would be progressively disrupted.

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respiratory chain enzymes, alter DNA and DNA-associated proteins and inhibit sodium-potassium ATPase, collectively conspiring to induce the metabolic collapse and subsequent necrotic or apoptotic death of the cell [26]. The process of lipid peroxidation in particular is thought to be extremely important in the acute pathophysiology of spinal cord injury, both during the initial period of hypoperfusion and possibly even more significantly during the period of reperfusion [27]. Products of these oxidative reactions, such as malonyldialdehyde, are observed to rapidly increase after blunt injury in animal models of spinal cord injury [28], whereas such antioxidants as alpha-tocopherol ascorbate decrease as they are presumably consumed [29]. The involvement of free radicals in spinal cord injury is particularly relevant in that the inhibition of lipid peroxidation is thought to be one of the principle mechanisms of action for pharmacologic agents that have been evaluated for spinal cord injury, including methylprednisolone, tirilazad mesylate and GM1 ganglioside. Excitotoxicity and electrolyte imbalances Glutamate release and accumulation occurs rapidly after spinal cord injury in response to ischemia and membrane depolarization, reaching toxic levels as early as 15 minutes after experimental injury [30]. Glutamate is the most prevalent excitatory neurotransmitter in the central nervous system (CNS), acting on both ionotropic and metabotropic receptors. Ionotropic glutamate receptors include the N-methylD-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazolepropionate (AMPA)/kainate receptors through which ions pass (calcium and sodium in particular). Metabotropic glutamate receptors are coupled to G proteins that act as secondary intracellular messengers to mediate a wide spectrum of cellular functions. Excitotoxicity refers to the deleterious cellular effects of excess glutamate stimulation of these receptors [31]. Glutamate receptors, particularly the NMDA receptors, have been studied extensively over the past two decades, because it has been hoped that a better understanding of their role after CNS trauma might lead to pharmacologic interventions. Glutamate activation of NMDA receptors allow extracellular calcium (and sodium) to move down a massive concentration gradient into the cell, where the cytosolic calcium concentrations are normally extremely low and tightly controlled. NMDA receptor activation may also trigger the release of calcium from intracellular stores into the cytoplasmic compartment [32]. Elevated calcium concentrations within the cytosol and mitochondria can trigger a multitude of calcium-dependent processes that can lethally alter cellular metabolism [33,34]. These processes include the activation of lytic enzymes, such as calpains, phospholipase A2 and lipoxygenase; the generation of free radicals and the dysregulation of mitochondrial oxidative phosphorylation leading to the apoptotic death of the cells. Indeed, it would be difficult to overstate the significance of altered calcium homeostasis in CNS injury, because

increased cytosolic and mitochondrial calcium may represent a common final death pathway for numerous insults [35]. For this reason, NMDA receptor blockade has been extensively evaluated as a potential treatment of spinal cord and other CNS injuries and neurodegenerative disorders. Sodium dysregulation has also been implicated in the pathophysiology of spinal cord injury, in particular with regard to damage of the axonal and glial components of the white matter. An intracellular influx of sodium can result from glutamate activation of NMDA, AMPA and kainate receptors (another manifestation of excitotoxicity), as well as from activation of voltage-gated sodium channels and activity of a sodium-calcium exchanger that shuttles calcium out of the cell in exchange for sodium. Whereas action potential propagation normally involves the transient influx of sodium through the activation of voltage-gated sodium channels, the massive depolarization and loss of adenosine triphosphate (ATP)-dependent ability to move the sodium back into the extracellular compartment lead to a toxic accumulation of sodium (and hence water) within the axon. Animal studies in which ion channel blockers have been applied to the spinal cord shortly after injury have highlighted the relevance of dysregulated ionic homeostasis in the pathophysiology of spinal cord injury. Focal microinjections of tetrodotoxin, a potent blocker of voltage-gated sodium channels, into the injured spinal cord was shown to protect against axonal loss and improve locomotor function [36,37]. Similarly, glial cell susceptibility to sodium influx through the glutamate activation of AMPA/kainate receptors was mitigated by focal intraspinal injections of 2,3-dihydro-6nitro-7-sulfamoyl-benzo (f) quinoxaline (NBQX), an AMPA receptor antagonist [38]. The acute, time-dependent nature of these processes is highlighted by the demonstration that tetrodotoxin injections, although effective when performed within 5 minutes of injury, lose their effect when applied 4 hours after spinal cord injury [39]. Necrotic and apoptotic cell death The manner in which cells die during normal development and aging and after injury can take on different morphologic appearances (Table 2). On the one hand, necrotic cell death Table 2 Apoptotic cell death ● ●

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The term “apoptosis” was coined in the 1970s to describe a morphologic appearance of dying cells that was quite different from that of necrotic cells. Apoptotic death is an important component of tissue homeostasis during normal development and aging. A predilection toward undergoing apoptotic death is implicated in certain neurodegenerative disorders, whereas the failure to initiate apoptotic death may cause malignancy. Many extracellular and intracellular stimuli/injuries can trigger apoptosis, including ischemia, oxidative stress and inflammatory cytokines. These stimuli activate complex intracellular pathways that ultimately result in the activation of enzymes called caspases. Targeting caspases and the pathways that activate them represent a potential means of preventing apoptotic cell death.

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involves the swelling of the cell, disruption of organelles, then membrane lysis and release of the intracellular contents that can incite a local inflammatory reaction. Alternatively, apoptotic cell death, or programmed cell death, involves a cellular shrinkage with intact organelles and fragmentation into apoptotic bodies that are subsequently cleared by phagocytosis without a significant inflammatory response [40] (Fig. 4). Although necrosis and apoptosis are morphologic descriptions of cell death, they mechanistically occur by means of distinct pathways. Necrotic cell death can be thought of as a form of cellular homicide in which a severe insult overwhelms the cell’s homeostatic mechanisms, leading to membrane and organelle damage, the loss of ATP production and the passive swelling and eventual disruption of the cell. Apoptosis on the other hand requires the active participation of the cell itself and can be considered a form of cellular suicide in which specific extrinsic or intrinsic stresses initiate

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a cascade of intracellular pathways. These pathways ultimately lead to the activation of enzymes called caspases, which then target various cytoskeletal and nuclear proteins to affect an orderly dismantling of the cell [41]. Whereas necrosis is characterized by the loss of ATP production and energy failure, the apoptotic process is actually ATP dependent and requires the de novo synthesis of proteins in order to be carried to completion. Although it is convenient to consider apoptosis and necrosis as distinct pathophysiologic and morphologic entities, it is likely that cell death occurs along a spectrum between the two. Both necrotic and apoptotic cell death are known to occur after human spinal cord injury [42]. Both are initiated by many of the same insults, such as ischemia, oxidative stress and excitotoxicity, although the more severe the injury, the more likely the cell will undergo necrosis. From a practical point of view, the distinction between necrotic and apoptotic cell death is

Fig. 4. Differences between necrosis and apoptosis. This schematic illustrates some of the fundamental differences between necrotic (left) and apoptotic (right) cell death. It is important to recognize that apoptosis is an energy-dependent process by which the cell activates enzymes (caspases) to dismantle itself in a somewhat orderly fashion. Targeting such caspases is a potential therapeutic approach to prevent this form of cell death after spinal cord injury.

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relevant because of the increased potential for therapeutic intervention in the latter. It may be impossible to prevent a cell subjected to massive injury at the site of impact from being rapidly overwhelmed and succumbing to a necrotic type of death. However, cells around the epicenter of injuries that are spared this severe initial trauma may experience a sufficient secondary biochemical insult to initiate an apoptotic program of self-destruction effected by the subsequent activation of caspases. Finding ways to intervene in this apoptotic program is currently the subject of intense investigation. Apoptotic cell death has been observed in studies of animal and human spinal cord injuries and, interestingly, can occur for weeks after injury at quite a remote distance from the point of mechanical impact [42–44]. This suggests that not only is there a cellular substrate to protect but that a window of opportunity exists to prevent apoptotic death. Oligodendrocytes in particular appear quite vulnerable to apoptotic cell death and after spinal cord may even begin to express receptors, such as fas and p75, that when stimulated initiate the apoptotic process [45]. The death of these oligodendrocytes can result in the demyelination of otherwise spared axons, thus contributing to the loss of distal neurologic function. The general inhibition of protein synthesis with cycloheximide has been shown to inhibit apoptosis, reduce secondary damage and improve functional outcome after experimental spinal cord injury, demonstrating the requirement for the cell to actively contribute to its own apoptotic demise by synthesizing new proteins [46]. Other strategies to prevent apoptosis involve pharmacologic caspase inhibitors and the application of genes for proteins that inhibit the pathways leading to caspase activation, such as Bcl-2 [47–49]. These strategies have thus far been evaluated in animal models only and, although promising, are in fairly early stages of development. Inflammatory/immunologic response Inflammation is a universal defense and reparative response to tissue injury. Although the spinal cord is no exception, it appears that the inflammatory and immunological response to injury within the CNS is qualitatively and quantitatively different than that which is occurring in other tissues [50]. The inflammatory and immunologic responses involve cellular components, such as neutrophils, macrophages and T cells, and noncellular components, such as cytokines, prostaglandins and complement (Fig. 5). After spinal cord injury, the injury site is rapidly infiltrated by blood-borne neutrophils, which secrete lytic enzymes and cytokines that may further damage local tissue and recruit other inflammatory cells [51]. Blood-borne monocytes/macrophages are recruited, as are locally activated resident microglia, both of which subsequently invade to phagocytose the injured tissue [52]. These and other reactive cells produce cytokines, such as tumor necrosis factor (TNF-α), interleukins and interferons, that mediate the inflammatory response and can contribute to further tissue damage [53,54].

Fig. 5. Inflammatory/immunologic response to spinal cord injury. The inflammatory and immune response to central nervous system injury involves a complex interaction between cellular and noncellular elements, both of which are implicated not only in secondary damage but also in the native reparative response.

Such cytokines can induce the expression of cyclooxygenase (COX) 2 and thus promote the breakdown of arachidonic acid into proinflammatory prostanoids (prostaglandins, prostacyclin and thromboxanes) that mediate vascular permeability/resistance and platelet aggregation/adherance [55,56]. Excess cytoplasmic calcium can activate phospholipases that generate arachidonic acid from phospholipids within the cell membrane [57]. It is by the antagonism of COX activity that nonsteroidal anti-inflammatory agents attain their anti-inflammatory properties. Both COX1 and COX-2 have been shown to increase after blunt spinal cord injury, although more attention has been placed on the highly inducible COX-2 isoform [58,59]. Prostacyclin (PGI2) has vasodilatory properties that promote vascular permeability and edema at sites of inflammation, whereas thromboxane A2 tends to worsen venous thrombosis and ischemia by promoting platelet aggregation and vasoconstriction. The involvement of the cyclooxygenases in the generation of these inflammatory mediators represents a potential target for intervention, because inhibitors of these enzymes are in widespread clinical use (Fig. 6). Among the various cytokines involved in secondary CNS injury, TNF-α is perhaps the most extensively studied. It can be produced by a number of different cell populations, including neutrophils, macrophages and microglia, astrocytes and T cells [60], and has been shown to accumulate quickly at the site of spinal cord injury [61]. Its release soon after injury promotes the further migration of bloodborne activated leukocytes into the spinal cord and can stimulate additional cytokine production. Animal studies have revealed both neurotoxic and (somewhat surprisingly) neuroprotective properties of TNF-α. For example, inhibiting TNF-α after CNS injury with antibodies or other cytokines

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Fig. 6. Arachidonic acid metabolism phospholipases can mobilize arachidonic acid from the cell membrane. Cyclooxygenase metabolism of arachidonic acid produces thromboxane, prostacyclin and prostaglandins, all of which influence the inflammatory process. Prostacyclin (PGI2) has vasodilatory properties that promote vascular permeability and edema at sites of inflammation, while thromboxane A2 tends to worsen venous thrombosis and ischemia by promoting platelet aggregation and vasoconstriction.

has been shown to promote functional recovery, suggesting a cytotoxic role for TNF-α [61,62]. Conversely, some in vitro studies have demonstrated TNF-α to be neuroprotective against excitotoxic death [63,64]. Along the same line, transgenic mice lacking TNF-α receptors (making them presumably insensitive to the effects of endogenous TNF-α) have actually demonstrated greater tissue loss and functional deficits than wild-type mice after spinal cord injury, suggesting that TNF-α would have mediated a neuroprotective effect [65]. The conflicting actions of TNF-α described above reflect a growing awareness that the view of inflammation as a detrimental, neurotoxic process best inhibited by antiinflammatory agents, such as corticosteroids, is a gross oversimplification [66]. Inflammation is perhaps more appropriately considered as a “dual-edged sword,” with both neurotoxic and neuroprotective properties after spinal cord injury [67] (Fig. 7). TNF-α, for example, clearly has beneficial and deleterious effects, probably dependent on when after the injury it is being released and on which cellular populations it is acting on. And although many cytokines propagate an inflammatory response, other cytokines such as interleukin (IL)-10 are considered to have potent antiinflammatory properties [68]. IL-10 is produced by many of the same cells as TNF-α, including leukocytes, macrophages, astrocytes and microglia, and its administration has been shown to be neuroprotective after experimental spinal cord injury, possibly by inducing antiapoptotic genes. [61,69]. Also, although phagocytic macrophages that rapidly invade the spinal cord injury site have traditionally been implicated in the further destruction of neural tissue [70,71], it has more recently been suggested that the macrophage

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Fig. 7. Neuroprotective and neurotoxic elements of the inflammatory response. The inflammatory response can be considered a dual-edged sword, with both neurotoxic and neuroprotective properties. Notice that some of the inflammatory elements themselves, such as TNF-α, macrophages and nitric oxide, have both beneficial and detrimental effects—likely related to when they are expressed after spinal cord injury and on which cells they act.

response to CNS injury is in fact an inadequate one and that the poor regenerative response after spinal cord injury as compared with peripheral nerve injury is related in part to the more pronounced macrophage invasion in the latter [50,72,73]. This line of investigation has led to a phase 1 human clinical trial in which activated macrophages have been implanted into the spinal cords of acutely injured patients with complete injuries. These examples provide some insights into the enormous complexity of the inflammatory and immune response to spinal cord injury. There is increasing consensus that the early phases of inflammation are deleterious in nature, whereas later inflammatory events appear to be protective. The development of applicable neuroprotective strategies that target these responses will require a more sophisticated elucidation of the beneficial and detrimental aspects of inflammation and neuroimmunology.

Pharmacologic interventions for acute spinal cord injury Corticosteroids The use of corticosteroids in the setting of acute spinal cord injury began over 30 years ago, rationalized by its well-recognized anti-inflammatory properties that were thought to reduce spinal cord edema [74,75]. A sizeable body of animal literature supports the administration of steroids in experimental spinal cord injury, although it is important

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to realize that animal studies have not universally demonstrated a beneficial effect [76]. The precise mechanisms by which corticosteroids affect neuroprotection are not completely understood but are proposed to include the inhibition of lipid peroxidation and inflammatory cytokines, modulation of the inflammatory/immune cells, improved vascular perfusion and prevention of calcium influx and accumulation [77]. The inhibition of lipid peroxidation has been hypothesized to be the most neuroprotective property of glucocorticoids, and in this regard, methylprednisolone appears to be particularly efficacious compared with other glucocorticoids [78]. The current widespread use of methylprednisolone has stemmed largely from three large-scale prospective randomized double-blinded multicenter clinical trials reported as the National Acute Spinal Cord Injury Studies (NASCIS) I, II and III. (Table 3). NASCIS I evaluated the efficacy of 10

Table 3 Summary of the National Acute Spinal Cord Injury Studies clinical trials of methylprednisolone NASCIS I [79] 330 patients randomized and treated within 48 hours of spinal cord injury Treatment arms: 1. Methylprednisolone: 100-mg bolus, then 25 mg every 6 hours for 10 days 2. Methylprednisolone: 1,000-mg bolus, then 250 mg every 6 hours for 10 days Findings: ● No significant difference in neurologic recovery between the two groups at 6-month follow-up NASCIS II [82, 83, 107] 487 patients randomized and treated within 12 hours of spinal cord injury Treatment arms: 1. Methylprednisolone: 30-mg/kg bolus, then 5.4 mg/kg/hour × 23 hours 2. Naloxone: 5.4-mg/kg bolus, then 4.5 mg/kg/hour × 23 hours 3. Placebo Findings: ● No significant difference in neurologic recovery among the three groups at 6 or 12 months after injury ● In patients receiving methylprednisolone within 8 hours of injury, significant motor and sensory improvement was observed at 6 months [82] and at 12 months after injury [83]. Naloxone was not shown to be effective. ● In patients with incomplete lesions, naloxone was subsequently shown to promote significant neurologic recovery [107]. NASCIS III [84, 85] 499 patients randomized and treated within 8 hours of spinal cord injury Treatment arms: 1. Methylprednisolone: 30-mg/kg bolus, then 5.4 mg/kg/hour × 23 hours 2. Methylprednisolone: 30-mg/kg bolus, then 5.4 mg/kg/hour × 47 hours 3. Tirilazad mesylate: 2.5-mg/kg every 6 hours for 48 hours Findings: ● No significant difference in neurologic recovery among the three groups at 6 or 12 months after injury ● If treatment was initiated 3 to 8 hours after injury, patients receiving methylprednisolone for 48 hours had significant recovery over those who received methylprednisolone for 24 hours; p⫽.01 at 6 months after injury [84]; p⫽.53 at 12 months after injury [85]. Neurologic recovery with tirilazad was equivalent to that observed with 24-hour methylprednisolone. NASCIS⫽National Acute Spinal Cord Injury Studies.

daily doses of either 100 mg or 1,000 mg of methylprednisolone begun within 48 hours of spinal cord injury in 330 patients [79]. No difference in motor or sensory recovery was observed between the two regimens, but a placebo group was not included in this study to enable a comparison between methylprednisolone and the natural history of spinal cord recovery. Animal studies suggested that the 1,000-mg dose was far lower than that required for effective neuroprotection and that an initial dose of 30 to 40 mg/kg followed by intravenous maintenance was more appropriate [80,81]. The subsequent NASCIS II trial therefore used an initial bolus of 30 mg/kg followed by a 23-hour infusion of 5.4 mg/ kg per hour [82,83]. A total of 487 patients were randomized within 12 hours of injury to methylprednisolone, naloxone (an opioid receptor antagonist) or placebo. The authors of NASCIS II reported that methylprednisolone, when administered within 8 hours of injury, resulted in statistically significant motor and sensory recovery in both complete and incomplete spinal cord injuries when evaluated at 1.5, 6 and 12 months after injury. As the first clinical study to demonstrate the efficacy of a pharmacologic agent in the treatment of spinal cord injury, the NASCIS II trial established the widespread use of methylprednisolone, validated the relevance of secondary injury and its potential to be influenced pharmacologically and encouraged the development of other neuroprotective strategies. NASCIS III was then performed to compare different durations of methylprednisolone treatment and also to evaluate the efficacy of tirilazad mesylate [84,85]. Tirilazad mesylate is a member of the 21-aminosteroid family of antioxidant molecules called lazaroids that was developed to prevent lipid peroxidation but without activating glucocorticoid receptors, thereby avoiding some of the complications of steroid use [86–88]. A total of 499 patients within 8 hours of injury received the initial 30-mg/kg bolus of methylprednisolone and then were randomized to 24- or 48-hour methylprednisolone infusions or to tirilazad mesylate over 48 hours. No placebo arm was included in this study because it was considered unethical. The motor and sensory recovery was similar among all study arms when treatment was initiated within 3 hours after injury, suggesting that the 24-hour infusion of methylprednisolone was sufficient in these patients; however, when initiated between 3 and 8 hours, there appeared to be a benefit to extending the methylprednisolone infusion to 48 hours. Although NASCIS II and III have ingrained the administration of methylprednisolone into the standard of clinical practice for acute spinal cord injury across North America, much criticism has recently been directed at the interpretation and conclusions of these studies, leading to its discontinuation in some centers. A number of authors have published in-depth analyses of the NASCIS II and III studies [89–93]. The primary questioning of the validity of these studies relates to the fact that in NASCIS II (on which NASCIS III was subsequently based), the primary outcome analysis of motor and sensory recovery in all randomized

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patients was in fact negative, and only after a post hoc analysis (establishing the 8-hour cutoff) was an arguably small yet statistically significant benefit accrued (Table 4). Similarly, the primary outcome measures of NASCIS III were negative, but with a post hoc analysis delineating a benefit of 48-hour methylprednisolone treatment for those in whom treatment was initiated after 3 hours. It has also been pointed out that despite the widespread use of methylprednisolone, the NASCIS II and III trials did not study pediatric spinal cord injuries, penetrating spinal cord injuries and cauda equina injuries, leaving the applicability of the NASCIS results in these settings unsubstantiated. It should also be recognized that the administration of methylprednisolone was not found to be benign in the NASCIS papers, because wound infection rates, pulmonary embolism, severe pneumonia and sepsis and even death secondary to respiratory complications appeared to be higher with steroid use, in particular with the 48-hour regimen of NASCIS III. Although statistical significance was not achieved in these adverse outcomes, it is unlikely that either study was powered sufficiently to establish such significance. Although current medicolegal implications may deter physicians from deviating from what has become accepted clinical practice for many years, it is hoped that increased awareness of the limitations of NASCIS II and III will allow physicians to carefully weigh the reportedly small clinical benefit of methylprednisolone administration against the potential complications of its use. At the very least, the ongoing controversy regarding the benefits of methylprednisolone has highlighted the compelling need to develop better neuroprotective agents with more convincing efficacy. Gangliosides Gangliosides are sialic acid–containing glycosphingolipids that are highly expressed on the outer surface of cell Table 4 Primary criticisms of the National Acute Spinal Cord Injury Studies trials ●







● ●

The primary outcome measure of NASCIS II was 6- and 12-month motor and sensory recovery in patients randomized and treated within 12 hours of injury. This was negative. A statistically significant effect for methylprednisolone in NASCIS II was achieved only after a post hoc analysis of patients receiving the drug before and after 8 hours. The primary outcome measure of NASCIS III was 6- and 12-month motor and sensory recovery in patients randomized and treated within 8 hours of injury. Similar to NASCIS II, this was also negative. A statistically significant effect for the extended dosage of methylprednisolone in NASCIS III was achieved only after a post hoc analysis of patients receiving the treatment between 3 and 8 hours after injury. Numerous statistical comparisons were made in both NASCIS II and III without adjusting for multiple comparisons. Both NASCIS II and III reported increased incidences of adverse effects with methylprednisolone, such as pulmonary embolism, wound infection, severe pneumonia, sepsis and respiratory-related deaths. Although not statistically significant, neither study was powered to establish such differences. NASCIS⫽National Acute Spinal Cord Injury Studies.

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membranes within the CNS. The systemic administration of monosialotetrahexosylganglioside (GM1 or Sygen [Fidia Pharmaceutical Corporation, Washington, DC]) has been associated with neuroprotective effects in a variety of models of experimental CNS injury, with the proposed mechanisms of action including the augmentation of neurite outgrowth and plasticity, the inhibition of excitotoxicity and the prevention of apoptosis [94–96]. Promising results from a single-center prospective double-blinded randomized trial of 37 spinal cord–injured patients with 100 mg of intramuscular GM1 daily for 30 days [97] prompted a large-scale multicenter randomized trial, the results of which were published in December of 2001 [98]. This trial randomized 797 patients between 1992 and 1997 to placebo, low-dose GM1 (300-mg loading dose, then 100 mg/day for 56 days), or high-dose GM1 (600-mg loading dose, then 200 mg/day for 56 days). For ethical reasons, all patients received the NASCIS II methylprednisolone protocol, and the GM1 therapy was initiated after its completion. The primary outcome measure of this large study was the proportion of patients who achieved marked recovery at 26 weeks after injury, defined as an improvement of at least two grades in a modified Benzel classification of motor/sensory function over their baseline American Spinal Injury Association (ASIA) score. GM1 treatment was not associated with a significantly higher proportion of patients with marked recovery at 26 weeks as compared with placebo, and similar to the NASCIS studies, the primary efficacy analysis of this well-conducted trial of GM1 was negative. Nevertheless, there did appear to be a more rapid rate of recovery in patients treated with GM1, and many parameters, including motor and sensory scores and bowel and bladder function, showed trends of improvement in GM1 treatment over placebo, particularly in incomplete patients. Although the lack of efficacy in complete patients was indeed disappointing, the study’s findings were somewhat more encouraging for patients with incomplete paraplegia. Because the authors have made the primary data from this study available for other investigators, it is likely that more insights into the potential benefits of this therapy will be forthcoming. Opioid antagonists The nonspecific opioid receptor antagonist naloxone was investigated extensively in the early 1980s after it was found to reverse spinal shock and improve spinal cord blood flow, with associated functional and electrophysiologic improvements in animal models of spinal cord injury [99–103]. As is the case for many pharmacologic strategies, this beneficial response of naloxone, which was thought to be mediated by antagonizing the rise in endogenous opiates that was observed after spinal cord injury, was not reproduced by all authors [104,105]. Nonetheless, naloxone in the form of a 5.4-mg/kg intravenous bolus, then 4-mg/kg infusion for 23 hours, was one of three treatment arms in NASCIS II, although it was later suggested that this represented a subtherapeutic dose [106]. The initial conclusions from this study

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indicated that naloxone did not confer any therapeutic benefit over placebo [82], but in a subsequent reevaluation of this data by two of the authors, it was suggested that naloxone did in fact promote motor and sensory recovery in incompletely injured patients [107]. The opioid and nonopioid receptor effects of these pharmacologic agents in the setting of spinal cord injury have not been clearly delineated [108], and largescale clinical evaluations of naloxone or other antagonists for specific opioid receptors [109,110] have not been since undertaken. Glutamate receptor and ion channel antagonists The recognition that NMDA and non-NMDA (AMPA/ kainate) receptor activation play a role in excitotoxic damage after spinal cord injury in addition to other traumatic and nontraumatic CNS disorders [111] has stimulated a great deal of interest in the development of pharmacological interventions. NMDA receptor antagonists, such as MK801 and gacyclidine (GK11), have demonstrated significant neuroprotective effects after experimental spinal cord injury in animal studies [112,113]. The development of NMDA antagonists as clinical therapies has been hampered by the widespread distribution of glutamate (and its receptors) in neurotransmission throughout the human CNS, making it difficult to avoid significant side effects with systemically administered treatment. Nevertheless, the NMDA receptor antagonist gacyclidine has been evaluated in France in a phase 2 double-blind, randomized study of 280 spinal cord– injured patients. This study apparently failed to show significant improvement in ASIA scores compared with placebo treatment. The focal microinjections of NBQX, an AMPA receptor blocker, into the spinal cord after blunt injury has been shown to reduce white matter glial loss and improve locomotor function [38], but this invasive form of receptor antagonism has not been developed into a pharmacologic application. Calcium channel blockers have also been investigated as a potential means of reducing the pathologic influx of calcium into cells by means of nonglutamate receptors (NMDA receptors are only one of many routes for calcium entry into cells). It would appear, however, that the predominant physiologic effects of calcium channel blockers are mediated through their pharmacologic activity on vascular smooth muscle rather than by altering calcium flux across neuronal and glial cell membranes. Nimodipine, for example, has been shown to enhance spinal cord blood flow and reverse hypoperfusion in experimental spinal cord injury [114,115]. Other animal studies, however, have not demonstrated significant neurologic recovery with nimodipine treatment after spinal cord trauma or ischemia [116–118]. Nimodipine was evaluated in France in a prospective randomized trial of 106 patients who were randomized to one of four arms within 8 hours of injury: methylprednisolone according to the NASCIS II recommendations; nimodipine 0.15 mg/kg per hour for 2 hours, then 0.03 mg/kg per hour for 7 days; both

methylprednisolone and nimodipine; or placebo [119]. This study failed to show any benefit in neurologic outcome at 1 year after injury for any of the three treatment arms, although it likely was underpowered to do so. The use of calcium channel blockers in acute spinal cord injury invokes some concern regarding the potential for systemic hypotension, which in the setting of impaired spinal cord autoregulation could be detrimental. Sodium channel blockade with local microinjections of tetrodotoxin have demonstrated significant neuroprotection of white matter and improved functional outcomes after blunt experimental spinal cord injury [36,37]. Although the clinical translation of these results would involve surgical intervention, the systemic administration of riluzole, another sodium channel blocker, has recently been shown to have similar neuroprotective effects after a clip compression injury to a rodent spinal cord, with sparing of white and gray matter and improved locomotor function [120]. Riluzole has received Food and Drug Administration approval for the treatment of amyotrophic lateral sclerosis, and many human pharmacokinetic and toxicity issues have therefore been addressed. Human studies of its application in spinal cord injury are currently lacking. Cyclooxygenase inhibitors The important role of inflammatory prostaglandins in mediating secondary injury has stimulated interest in the potential application of the widely used cyclooxygenase inhibitors in the setting of spinal cord injury. Ibuprofen and meclofenamate, two commonly used nonsteroidal anti-inflammatory agents, have been shown to maintain spinal cord blood flow after spinal cord injury in cats [121]. Similarly effective in this study was a combination of a thromboxane inhibitor with a prostacyclin analogue. The expression of COX-2 has been observed to increase in the rat spinal cord after contusion injury, and the specific pharmacologic inhibition of the COX-2 isoform was shown to improve functional outcome in moderately severe injuries [59,122]. Although the human application of COX-1 or COX-2 inhibition for acute spinal cord injury has not yet been reported in the literature, their widespread use in other musculoskeletal and rheumatologic conditions at least overcomes many of the safety and pharmacokinetic issues that most other pharmacologic treatments must otherwise contend with (Table 5). Other novel potential pharmacologic interventions Because the options for effective neuroprotection after spinal cord injury are scarce, it is not surprising that many pharmacologic and nonpharmcologic approaches are currently in various stages of development. A number of pharmacologic interventions have yet to proceed past animal models of spinal cord injury and into clinical trials, but their current use in other human applications makes this step at least conceivable in the not so distant future. Cyclooxygenase inhibitors are a good example of this. Other such potential

B.K. Kwon et al. / The Spine Journal 4 (2004) 451–464 Table 5 Summary of human trials of neuroprotective agents (excluding methylprednisolone) GM1 ganglioside Geisler et al. [97] ● A prospective randomized trial of 37 patients, receiving either GM1 or placebo within 72 hours of spinal cord injury ● Significant neurologic recovery was observed with GM1 treatment at 1 year after injury. Geisler et al. [98] ● A prospective randomized trial of 760 patients, receiving either highdose GM1, low-dose GM1 or placebo within 72 hours of injury (and after receiving methylprednisolone according to the the NASCIS II recommendations) ● GM1 did not increase the fraction of patients who achieved marked functional recovery 26 weeks after injury, as compared with placebo. ● A more rapid recovery and a trend toward greater neurologic recovery with regard to motor, sensory and bowel/bladder function was observed with GM1. Opioid antagonists: naloxone (evaluated in NASCIS II) Bracken et al. [82, 83], Bracken and Holford [107] ● Although the initial reports of NASCIS II at 6 months and 1 year did not report a beneficial effect for naloxone, the post hoc analysis reported in 1993 indicated that naloxone promoted neurologic recovery in incomplete spinal cord injuries. Antioxidants: tirilazad mesylate (evaluated in NASCIS III) Bracken et al. [84, 85] ● Patients treated with tirilazad demonstrated neurologic recovery equivalent to those treated with the 24-hour methylprednisolone protocol. ● In keeping with the lack of glucocorticoid effect, severe sepsis and pneumonia appeared to be lower with tirilazad. Calcium channel antagonists: nimodipine Pointillart et al. [119] ● A prospective randomized evaluation of 106 patients receiving either methylprednisolone, nimodipine, both or placebo within 8 hours of injury ● Nimodipine was not associated with significant neurologic recovery. NMDA antagonists: gacyclidine ● A prospective randomized evaluation of 280 patients has been performed, with reportedly no benefit observed with gacyclidine (unpublished). GM1⫽monosialotetrahexosylganglioside; NASCIS⫽National Acute Spinal Cord Injury Studies; NMDA⫽N-methyl-D-asparate.

agents that have generated some recent interest include the tetracycline antibiotic minocycline, the immunosuppressants FK506 and cyclosporin and the hematopoietic agent eryrthopoietin. Minocycline, a tetracycline antibiotic, has been shown to inhibit excitotoxicity [123] and provide neuroprotection in models of Parkinson disease [124], autoimmune encephalomyelitis [125], amyotrophic lateral sclerosis [126] and adult and neonatal brain ischemia [125,127,128], most likely through its inhibition of microglial activation. It is currently under investigation in animal models of contusive spinal cord injury, with preliminary results suggesting a promising reduction in tissue damage and apoptotic death at the injury site, and improved locomotor function [129]. FK506, or tacrolimus, and cyclosporine are immunosuppressants that have demonstrated some beneficial effects in the setting of peripheral nerve injury [130,131]. Cyclosporin acts at the mitochondrial membrane to impede apoptosis and has been shown to promote tissue sparing and inhibit lipid peroxidation in models of brain and spinal

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cord injury [132–135]. FK506 has been shown to be beneficial for promoting axonal regeneration within the CNS and functional recovery after experimental spinal cord injury [136,137]. Erythropoeitan is thought to have anti-inflammatory, antioxidant and antiapoptotic properties [138–141] and in addition to demonstrating neuroprotection in the setting of experimental brain injury [142] has been shown to prevent motor neuron apoptosis and improve neurologic function in a global ischemia model of spinal cord injury in rabbits [143]. Clearly, much work remains to be done to bring these experimental strategies to clinical fruition, but they do represent promising potential interventions. Conclusion The pathophysiologic processes initiated acutely after spinal cord injury are extremely complex, and the extent of our understanding of them is reflected in the limited neuroprotective strategies currently available beyond rapid trauma resuscitation and attentive clinical care. Promising research is, however, being carried out to delineate the aspects of vascular dysregulation, inflammation, lipid peroxidation and apoptotic cell death that may be amenable to pharmacologic intervention. Although few agents such as methylprednisolone and GM1 have been subjected to large-scale human trials, a number of others have demonstrated efficacy in animal models of spinal cord injury and may become appropriate for testing in the human setting in the near future. References [1] Nobunaga AI, Go BK, Karunas RB. Recent demographic and injury trends in people served by the Model Spinal Cord Injury Care Systems. Arch Phys Med Rehabil 1999;80(11):1372–82. [2] Stripling T. The cost of economic consequences of traumatic spinal cord injury. Paraplegia News 1990;8:50–4. [3] Kwon BK, Tetzlaff W. Spinal cord regeneration: from gene to transplants. Spine 2001;26(24 suppl):S13–22. [4] McDonald JW, Sadowsky C. Spinal-cord injury. Lancet 2002; 359(9304):417–25. [5] Zwimpfer TJ, Bernstein M. Spinal cord concussion. J Neurosurg 1990;72(6):894–900. [6] Del Bigio MR, Johnson GE. Clinical presentation of spinal cord concussion. Spine 1989;14(1):37–40. [7] Blight AR, DeCrescito V. Morphometric analysis of experimental spinal cord injury in the cat: the relation of injury intensity to survival of myelinated axons. Neuroscience 1986;19(1):321–41. [8] Bunge RP, Puckett WR, Becerra JL, Marcillo A, Quencer RM. Observations on the pathology of human spinal cord injury. A review and classification of 22 new cases with details from a case of chronic cord compression with extensive focal demyelination. Adv Neurol 1993;59:75–89. [9] Kakulas BA. The applied neuropathology of human spinal cord injury. Spinal Cord 1999;37(2):79–88. [10] Kaelan C, Jacobsen P, Morling P, Kakulas BA. A quantitative study of motoneurons and cortico-spinal fibers related to function in human spinal cord injury (SCI). Paraplegia 1989;27(148):153. [11] Blight AR. Cellular morphology of chronic spinal cord injury in the cat: analysis of myelinated axons by line-sampling. Neuroscience 1983;10(2):521–43.

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