Pathophysiology and Acute Management

Section 1: Primary Injury and the Secondary Cascade

Bottom line: five primary injury mechanisms (compression, contusion, distraction, laceration, shear) with compression the most common mechanism and contusion the most common pathologic finding; true anatomic transection is rare even in clinically complete injuries; secondary injury extends damage over hours to weeks via vascular failure, glutamate excitotoxicity with calcium influx, free-radical lipid peroxidation, neutrophil and macrophage infiltration, oligodendrocyte apoptosis, and glial scar formation.

Primary injury is the immediate mechanical damage that occurs at the moment of impact. It is irreversible and initiates everything downstream. Five principal mechanisms are testable. Compression is the most common mechanism: bone fragments, disc material, or ligamentous structures are displaced into the canal and compress the cord. Compression may be transient (hyperextension pinches the cord between hypertrophied ligamentum flavum and osteophytic ridges) or sustained (a burst fracture with retropulsed bone occupying the canal). Contusion is the most common pathologic finding. The cord sustains a bruise-type injury producing central hemorrhagic necrosis of the gray matter, which has higher metabolic demand and richer vascular supply than white matter. Peripheral white matter is relatively preserved in less severe injuries. Compression is the mechanism; contusion is the finding.

Distraction is longitudinal stretching of the cord beyond its elastic tolerance, characteristic of flexion-distraction (Chance) fractures and possible without fracture in children, patients with Down syndrome, or patients with rheumatoid arthritis. Distraction also damages perfusing vessels and produces ischemia. Laceration and transection involve direct tearing by penetrating objects (knives, bullets, bone fragments). Complete anatomic transection is rare even in clinically complete injuries; most clinically complete injuries represent severe contusion with hemorrhagic necrosis. The persistent intact though dysfunctional tissue is the substrate for neuroprotective interventions. Shear involves rotational or translational forces that tear neural tissue at the gray-white matter interface, commonly accompanying fracture-dislocations.

Secondary injury is the progressive cascade of cellular, molecular, and biochemical events that extend neural damage beyond the initial mechanical insult. It begins within minutes and continues for months. The cascade is the primary therapeutic target for neuroprotective interventions, and its timeline is heavily tested.

The vascular phase comes first. Mechanical disruption of intramedullary vasculature produces petechial hemorrhages in the central gray matter within minutes; hemorrhages coalesce and progress to hemorrhagic necrosis over the ensuing hours; extent and timing vary by injury. Traumatic vasospasm of the anterior spinal artery and sulcal arteries combined with microvascular thrombosis produces ischemia in watershed zones. Autoregulation is impaired after SCI, increasing vulnerability to systemic hypotension. Traditional teaching used 85–90 mmHg for about the first week; the 2013 recommendation specified seven days. Section 3 carries the current MAP target. Reperfusion injury layers on top: when blood flow returns to ischemic tissue, reintroduced oxygen generates reactive oxygen species (ROS) that paradoxically worsen damage.

This traumatic vascular phase is arterial and microvascular disruption — worth contrasting with the Batson (vertebral venous) plexus, the valveless venous network running the length of the spine. Its valveless connections permit pressure-dependent bidirectional flow and provide a route for tumor spread to the spine without first traversing the heart and lungs. Prostate and breast tumors are the classic examples. It is one route to the spine, not the only one. Keep traumatic arterial injury physiology distinct from venous metastatic spread.

The biochemical phase is dominated by excitotoxicity. Mechanical disruption and ischemia release massive glutamate from damaged neurons and glia. Glutamate rises within minutes of injury and peaks early, at about 10 minutes in the rat compression model, then declines; magnitude and time course vary by model, location and measurement. Excess glutamate activates the NMDA, AMPA and kainate receptor families, causing ionic dysregulation and intracellular calcium overload; direct calcium permeability varies with receptor composition. Intracellular calcium overload activates calpains (proteases that degrade cytoskeletal proteins), phospholipases (which break down cell membranes), and nitric oxide synthase. It causes mitochondrial dysfunction and uncoupling of oxidative phosphorylation. Reactive oxygen species including superoxide anion, hydroxyl radical, and hydrogen peroxide overwhelm endogenous antioxidant defenses (superoxide dismutase, catalase, glutathione peroxidase), causing lipid peroxidation of cell membranes, protein oxidation, and DNA damage. The arachidonic acid cascade compounds the damage: phospholipase A2 releases arachidonic acid, cyclooxygenase produces prostaglandins and thromboxanes, and lipoxygenase produces leukotrienes, all of which generate additional free radicals and inflammatory mediators.

The inflammatory phase is double-edged. Within minutes to hours, resident microglia activate and secrete pro-inflammatory cytokines (tumor necrosis factor alpha [TNF-α], interleukin-1 beta [IL-1β], interleukin-6 [IL-6]). Neutrophils arrive first. In the human postmortem series they extravasate at 3 to 4 hours and are heaviest at 1 to 3 days, releasing proteases, ROS and myeloperoxidase that damage surrounding tissue. Microglia and macrophages follow. Human microglial activation and monocyte entry appear at 1 to 3 days, and phagocytic cells persist for months. Rat contusion gives the cleanest peaks: neutrophils around day 1, macrophages and microglia around day 7, with later increases as well. Learn the order rather than the clock, because those peaks are species- and assay-specific measurements and are not human treatment deadlines. Macrophage responses mix inflammatory and reparative phenotypes that overlap in time, and experimental contusion sustains the inflammatory phenotype rather than switching cleanly from M1 to M2. Over weeks to months, reactive astrocytes form a glial scar and deposit CSPGs. The scar limits inflammatory spread, while parts of the lesion environment, including CSPGs, inhibit regeneration. Scar-forming astrocytes can also support axon growth in experimental models, where the growth still requires an applied stimulus.

Apoptosis begins within hours and continues for weeks. In rat contusion, apoptotic cells are seen from 6 hours through 3 weeks, and the delayed wave of oligodendrocyte death in remote degenerating white-matter tracts is maximal at 7 to 8 days, with fewer labelled glia by 14 days. In human postmortem cords, apoptotic markers and caspase-3 are present in the lesion epicenter from 2 weeks through 3.5 months. The number to carry is the classic one-week delayed oligodendrocyte loss, the experimental peak measured in degenerating white-matter tracts. Loss of oligodendrocytes, the myelinating cells of the central nervous system, is particularly important. Loss of oligodendrocytes produces secondary demyelination of axons that survived the initial injury, contributing to functional deficits disproportionate to primary axonal damage. Both intrinsic (cytochrome c, caspase-9) and extrinsic (Fas ligand, caspase-8) pathways are activated; the final common pathway converges on caspase-3, the executioner caspase.