Spinal cord injury (SCI) classification is a standardized system that describes the severity and location of a spinal cord injury. The global standard is called the International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI), first published by the American Spinal Injury Association (ASIA) in 1982 and revised several times since. It gives clinicians a common language to describe how much function remains after an injury, which directly shapes rehabilitation goals, recovery expectations, and eligibility for clinical trials.
The Two Core Questions Classification Answers
Every SCI classification boils down to two things: where is the injury, and how severe is it? The “where” is captured by the neurological level of injury. The “how severe” is captured by a letter grade from A to E on the ASIA Impairment Scale (AIS). Together, these two pieces of information tell a medical team what functions are likely preserved, what rehabilitation targets make sense, and what kind of recovery trajectory to expect.
How the Neurological Level Is Determined
The neurological level of injury (NLI) is the lowest segment of the spinal cord where both sensation and muscle strength are completely normal on both sides of the body. To find it, clinicians test two things separately: sensory function and motor function.
For sensation, a clinician checks 28 specific points on each side of the body using light touch and a pin prick. The sensory level is the lowest point where both types of sensation are still fully normal. For motor function, the clinician tests key muscles in 10 segments on each side, grading strength on a 0 to 5 scale. The motor level is the lowest segment where the key muscle can resist gravity (a grade of 3 or higher), as long as all the muscles above it score a full 5.
Because the right and left sides of the body can differ, and sensory and motor levels can differ, up to four separate levels may be identified: right sensory, left sensory, right motor, and left motor. The single NLI reported on the classification is the highest (closest to the head) of those four. So if your right motor level is C6 and your left sensory level is C5, the NLI is C5.
The ASIA Impairment Scale: Grades A Through E
The AIS grade is the clearest shorthand for injury severity. It hinges on what function, if any, is preserved at the very lowest segments of the spinal cord (sacral segments S4 and S5), which control sensation around the anus and voluntary anal contraction. This might sound oddly specific, but these segments sit at the bottom of the cord. If signals can still reach them, the injury is by definition incomplete, meaning the cord wasn’t entirely disrupted.
- AIS A (Complete): No motor or sensory function at S4-S5. The injury has fully interrupted signals to the lowest part of the cord.
- AIS B (Sensory Incomplete): Some sensation is preserved at S4-S5, but no motor function below the injury level. You can feel but not move.
- AIS C (Motor Incomplete): Some motor function is preserved below the injury level, but more than half of the key muscles below the injury are too weak to work against gravity (graded below 3 out of 5).
- AIS D (Motor Incomplete): Motor function is preserved, and at least half of the key muscles below the injury can work against gravity (grade 3 or higher). This represents significantly more strength than AIS C.
- AIS E (Normal): Sensory and motor function are normal throughout the body. This grade is assigned when someone previously had a deficit that has fully recovered.
The jump from AIS A to AIS B is enormous in terms of prognosis. Even a small amount of preserved sensation at S4-S5 signals that the cord has some continuity, which is generally associated with better potential for recovery.
Complete vs. Incomplete Injuries
The distinction between complete and incomplete is one of the most important pieces of information after a spinal cord injury. A complete injury (AIS A) means no signals are getting through to the lowest sacral segments. An incomplete injury (AIS B, C, or D) means some pathways through the cord remain intact. In practical terms, incomplete injuries offer more potential for functional gains during rehabilitation because the surviving nerve pathways can sometimes be strengthened or retrained.
It’s worth noting that “complete” refers to the neurological exam result, not necessarily to physical severing of the cord. Many complete injuries involve bruising, swelling, or compression rather than a clean cut, but the functional result at the time of testing shows no preserved function at S4-S5.
Zone of Partial Preservation
In complete injuries, there are often segments below the NLI that still have some patchy function, even though the lowest sacral segments show nothing. This region is called the zone of partial preservation (ZPP). It’s documented as four separate values: right and left, sensory and motor.
The 2019 revision of the ISNCSCI expanded the ZPP concept beyond just complete injuries. Now, motor ZPPs can be recorded any time there’s no voluntary anal contraction, and sensory ZPPs can be recorded any time there’s no sensation at S4-S5 on a given side. When a ZPP doesn’t apply (because function is present at S4-S5), clinicians record “NA” rather than leaving it blank. This gives a more detailed picture of where residual function exists, which matters for tracking changes over time.
Incomplete Injury Syndromes
Beyond the AIS grade, incomplete injuries sometimes fall into recognizable clinical patterns based on which part of the spinal cord is damaged. Three syndromes account for roughly 44% of all incomplete injuries affecting the neck region.
Central cord syndrome is the most common, making up about 30% of incomplete neck-level injuries. It typically happens when the center of the cord is damaged, often in older adults after a fall. The hallmark is that the arms are affected more than the legs, because nerve fibers serving the arms run through the center of the cord at the neck level. Many people with central cord syndrome regain the ability to walk but have lasting difficulty with hand function.
Anterior cord syndrome accounts for about 10% of incomplete neck-level injuries. It involves damage to the front portion of the cord, which carries motor signals and pain/temperature sensation. People typically lose movement and pain sensation below the injury but retain the ability to feel vibration and body position, since those signals travel through the back of the cord.
Brown-Séquard syndrome is the least common at about 3%. It results from damage to one side of the cord, causing weakness on the injured side and loss of pain/temperature sensation on the opposite side. Of the three syndromes, Brown-Séquard generally carries the best prognosis for recovery.
How Classification Connects to Function
The injury level directly predicts what daily activities a person can perform independently. Researchers have developed functional scales that map SCI classification to real-world abilities across categories like self-care, mobility, fine motor skills, and wheelchair use.
For self-care, ability ranges from no independent self-care activities at the lowest functional level to eating, basic dressing, and bathing tasks at the next level, progressing up to full independence with all self-care. For basic mobility, the lowest level involves only limited shoulder and head movement with supported upper body, while higher levels progress through unsupported sitting, transfers between surfaces, and eventually transfers to and from surfaces of different heights.
As a general pattern, people with paraplegia (injuries at the thoracic level or below, such as T1, T12, or L4) cluster heavily in the highest functional ability group. People with tetraplegia (neck-level injuries like C5) are more likely to fall into lower functional categories, though the specific AIS grade matters enormously. A C5 AIS D injury, where significant strength is preserved, looks very different from a C5 AIS A injury in terms of daily function.
Why Classification Can Change Over Time
SCI classification is not a one-time label. Clinicians repeat the ISNCSCI exam at set intervals, particularly during the first year, because neurological recovery can shift someone from one AIS grade to another. A person initially classified as AIS A may convert to AIS B or C as swelling resolves and latent nerve pathways become measurable. These changes in classification directly influence rehabilitation goals and the functional outcomes a person works toward.
The classification also serves as the common currency for research. Clinical trials use AIS grades to define who can enroll and to measure whether a treatment produces meaningful neurological improvement. A shift from AIS B to AIS C, for example, represents a concrete change: moving from preserved sensation only to regaining some voluntary movement below the injury.