In 1990, the American Spinal Cord Injury Association (ASIA) proposed a uniform classification system that had five categories, defined in Table 1. Motor level is defined as the level at which the key muscle innervated by the segment has at least 3/5 of its normal strength.
Sensory level is defined as the lowest spinal cord level that still has normal pinprick and touch sensation. If there is a spinal cord level below which there is no voluntary motor or conscious sensory function, the person is called a “complete” spinal cord injury. Since the S5 is the lowest spinal cord level that innervates the anal sphincter, a person that has no voluntary anal sphincter control or sensation is defined as a “complete” spinal cord injury. A person who has any anal control or sensation is an “incomplete” spinal cord injury.
Some people may have a “complete” spinal cord injury but still has preserved motor or sensory function between the injury level and S5. This is called the “zone of partial preservation”. Usually, the spinal cord injury level and severity is classified between 72 hours and 7 days after injury. Note that some people have neurological loss at a given spinal cord level but partially preserved function for several or even many segments; this is called the zone of partial preservation (ZPP).
Neurological Classification of Spinal Cord Injury
A No motor or sensory function preserved in the lowest sacral segments
B Sensory but not motor function preserved in the lowest sacral segments
C Motor function present below the injury but most key muscles are <3/5
D Motor function present below the injury but most key muscles are >3/5
E Motor and sensory function normal in key muscles and dermatomes
F Some patterns of spinal cord injury have special names.
• In the “Central Cord Syndrome”, arm function is affected more than the legs. This paradoxical condition is attributed to damage to the central part of the spinal cord. Recent studies of central cord syndrome suggest that the syndrome may be associated with destruction of the lateral spinal tracts.
• “Brown-Sequard Syndrome” refers to injuries limited to one side of the cord. People have weakness and loss of touch sense in one leg but loss of pain and temperature sensation in the other side.
• “Anterior Cord Syndrome” refers to the condition when sensation is preserved but motor function is absent below the injury site.
• “Posterior Cord Syndrome” refers to the condition when motor function is preserved in the absence of sensation.
• “Conus Medullaire” refers to injury of the conus or lower tip of the spinal cord. This damages the lower lumbar and sacral spinal cord segments.
• “Cauda Equina Injury” refers to the condition when the damage is limited to the spinal roots below L1.