3.1 Spinal Cord Injury Pathophysiology & ASIA Classification

Spinal Cord Injury Pathophysiology

Spinal Cord Injury (SCI) represents a complex, devastating neurological condition that produces profound physiological consequences. The pathophysiology of SCI is characterized by a biphasic process: the primary injury and the secondary injury cascade. Understanding these distinct phases is critical for the neuroscience nurse, as clinical interventions are primarily targeted at mitigating secondary injury.

Primary Injury

The primary injury is the initial mechanical disruption of axons as a result of stretch, laceration, or compression. This occurs at the moment of impact. Mechanisms include hyperflexion, hyperextension, axial loading (vertical compression), excessive rotation, and penetrating trauma. In hyperflexion injuries, commonly seen in head-on collisions, the sudden deceleration causes forced forward movement of the head, leading to tearing of the posterior ligaments and anterior compression of the spinal cord. Hyperextension injuries often result from rear-end collisions or falls where the chin is struck, causing rupture of the anterior longitudinal ligament and posterior compression. Axial loading, typical in diving accidents or falls landing on the feet/buttocks, causes vertical compression that can shatter vertebral bodies, sending bone fragments into the spinal canal. Penetrating injuries, such as gunshot or stab wounds, directly sever or damage the cord tracts.

Secondary Injury Cascade

Following the primary insult, a complex cascade of cellular and molecular events ensues, known as secondary injury. This phase can last for days to weeks and significantly exacerbates the initial neurological deficit. Key mechanisms include:

1. Vascular Compromise and Ischemia: Microhemorrhages occur within the central gray matter within minutes. Vasospasm and thrombosis further reduce blood flow, leading to profound ischemia. The disruption of the blood-spinal cord barrier contributes to vasogenic edema, which increases tissue pressure and further compromises perfusion.
2. Excitotoxicity: Ischemic cells release excessive amounts of excitatory neurotransmitters, primarily glutamate. This overstimulation of receptors leads to an influx of calcium ions into the neurons, triggering a cascade of destructive enzymes that break down cell membranes and structural proteins.
3. Oxidative Stress: The production of free radicals (reactive oxygen species) exceeds the capacity of endogenous antioxidants. Lipid peroxidation damages cell membranes, leading to cell lysis and death.
4. Inflammation: Infiltration of neutrophils and macrophages occurs, releasing inflammatory cytokines. While necessary for clearing debris, excessive inflammation can damage viable tissue.
5. Apoptosis: Programmed cell death occurs in both neurons and oligodendrocytes, contributing to ongoing demyelination and loss of function.

ASIA Impairment Scale (AIS)

The American Spinal Injury Association (ASIA) Impairment Scale is the universal standard for classifying the severity of SCI. It requires a detailed sensory and motor examination.

• Sensory Examination: Tests light touch and pinprick sensation in 28 specific dermatomes bilaterally. Scoring is 0 (absent), 1 (altered/impaired), or 2 (normal).
• Motor Examination: Tests muscle strength in 10 key myotomes bilaterally, graded on a 0-5 scale.

The Neurological Level of Injury (NLI) is determined as the most caudal (lowest) segment of the spinal cord with normal sensory and motor function on both sides of the body.

Incomplete vs. Complete Injury

A critical distinction is whether the injury is complete or incomplete. This is defined by the concept of "sacral sparing." If there is any sensory or motor function preserved at the lowest sacral segments (S4-S5), the injury is incomplete. This includes sensation at the anal mucocutaneous junction, deep anal sensation, or voluntary anal sphincter contraction.

Clinical Syndromes of Incomplete SCI

Several distinct clinical syndromes can occur with incomplete spinal cord injuries, depending on the specific tracts damaged:

• Central Cord Syndrome: The most common incomplete syndrome, typically seen in older adults with cervical spondylosis who suffer a hyperextension injury. Damage is primarily to the central portion of the cervical cord. It is characterized by disproportionately greater motor impairment in the upper extremities compared to the lower extremities, and variable sensory loss below the level of injury.
• Brown-Séquard Syndrome: Results from a hemisection (damage to one side) of the spinal cord, often due to penetrating trauma. Clinically, it presents with ipsilateral (same side) loss of motor function (corticospinal tract), proprioception, and vibratory sense (dorsal columns), combined with contralateral (opposite side) loss of pain and temperature sensation (spinothalamic tract) beginning slightly below the level of the lesion.
• Anterior Cord Syndrome: Typically results from flexion injuries or compromised blood supply to the anterior spinal artery. It involves damage to the anterior two-thirds of the cord. The defining feature is complete motor paralysis and loss of pain and temperature sensation below the injury, but preservation of proprioception, vibration, and light touch (as the posterior columns remain intact).
• Conus Medullaris Syndrome: Injury to the conus (terminal end of the cord, usually T12-L2). Presents with flaccid paralysis of the lower extremities, areflexic bowel and bladder, and saddle anesthesia.
• Cauda Equina Syndrome: Strictly speaking, a peripheral nerve injury rather than a true SCI, involving the lumbosacral nerve roots below the conus. Symptoms include asymmetrical lower extremity weakness, radicular pain, saddle anesthesia, and bowel/bladder dysfunction.