The American Spinal Injury Association (ASIA) Impairment Scale is the standardized tool used globally by medical professionals to classify the extent of damage following a spinal cord injury (SCI). Officially known as the International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI), this system provides a common language for describing a patient’s neurological status. The systematic assessment determines both the location and the severity of the damage, which guides treatment and predicts potential recovery.
The assessment results assign a specific ASIA Impairment Scale (AIS) Grade, ranging from A to E. This grade concisely summarizes the completeness of the injury, indicating whether the loss of function is total or if some neural connections remain intact below the lesion site. Applying these standards ensures consistency in patient evaluation, allowing clinicians to track changes and communicate effectively with the rehabilitation team.
The Assessment Procedure: Sensory and Motor Testing
The ASIA classification relies on a standardized physical examination focused on sensory and motor function. This procedure tests specific points corresponding to distinct spinal nerve segments: dermatomes for sensation and myotomes for muscle strength. This systematic approach ensures the data collected is objective and repeatable.
Sensory Examination
The sensory examination assesses two primary modalities: light touch and pinprick sensation at 28 key points on both sides of the body. Clinicians score each point on a three-point scale: 0 for absent sensation, 1 for impaired sensation, and 2 for normal sensation. This detailed mapping helps pinpoint the extent of spinal cord damage.
Motor Examination
The motor examination focuses on ten key muscle groups, five in the upper extremities and five in the lower extremities, each representing a specific myotome. Muscle strength is graded using a six-point scale from 0 (total paralysis) to 5 (active movement against full resistance). A muscle grade of 3, where the patient can move the joint against gravity, is significant in the classification process.
The scores from both components are meticulously recorded on a standardized worksheet. This raw data, which includes a total sensory score (out of 224) and a total motor score (out of 100), is then used to calculate the Neurological Level of Injury and the final ASIA Impairment Scale Grade.
Determining the Neurological Level of Injury
The Neurological Level of Injury (NLI) is a specific, localized measure distinct from the overall severity grade. The NLI is defined as the most caudal (lowest) spinal cord segment that exhibits normal sensory and normal motor function on both the right and left sides of the body. The NLI essentially marks the boundary between preserved function above the injury site and impaired function below it.
To determine the NLI, clinicians identify the sensory level and the motor level for each side separately. The motor level is the lowest key muscle with a strength grade of at least 3, provided all muscles above it are fully intact (grade 5). The sensory level is the lowest dermatome scoring a normal grade of 2 for both pinprick and light touch.
The single NLI is assigned as the most rostral (highest) of these four levels (right sensory, left sensory, right motor, left motor). For example, if the right sensory level is C5 and the other three levels are C6, the NLI is C5. This precise location, such as C5, T10, or L3, is crucial because it indicates which parts of the body are directly affected by the injury, providing a clear anatomical reference for the patient’s paralysis or loss of sensation.
Understanding the ASIA Impairment Scale Grades
The ASIA Impairment Scale (AIS) Grade is a five-point system (A through E) classifying the severity and completeness of the spinal cord injury. This grade is determined based on the presence or absence of sensory or motor function in the lowest sacral segments, S4-S5. Assessing this area is necessary to determine injury completeness.
- Grade A (Complete Injury): There is no sensory or motor function preserved in the sacral segments S4-S5. This signifies a total loss of function below the level of injury.
- Grade B (Sensory Incomplete): Sensory function, but not motor function, is preserved in the sacral segments S4-S5. The patient has sensation in the anal area, but no voluntary motor function below the Neurological Level of Injury (NLI).
- Grade C (Motor Incomplete): Motor function is preserved below the NLI, but most key muscles in that region have a muscle grade less than 3 (cannot move against gravity).
- Grade D (Motor Incomplete): Motor function is preserved below the NLI, and at least half of the key muscles below the NLI have a muscle grade of 3 or greater, indicating functionally useful strength.
- Grade E: Sensory and motor function has recovered to a normal level on the ISNCSCI exam, though the patient may have had a prior SCI.
Any preservation of function in the S4-S5 segments results in an Incomplete classification (Grades B, C, or D). The distinction between Complete (Grade A) and Incomplete injuries is important for prognosis, as it suggests the potential for the return of function.
Key Concepts for Refining Prognosis
Clinicians use two specific concepts to refine the prediction of a patient’s potential for recovery beyond the AIS grade. These indicators provide insight into residual neural connections not fully captured by the NLI or the main AIS grade.
Sacral Sparing
Sacral Sparing refers to the preservation of sensation or voluntary movement in the most caudal sacral segments (S4/S5). The presence of minor sensation (light touch or pinprick in the perianal area) or the ability to voluntarily contract the external anal sphincter is a significant finding. Sacral sparing is the defining feature of an incomplete injury (AIS Grades B, C, or D) and correlates with a higher likelihood of regaining walking function compared to a Grade A injury.
Zone of Partial Preservation (ZPP)
The Zone of Partial Preservation (ZPP) is applied exclusively to Grade A (Complete) injuries. The ZPP describes the spinal cord segments immediately below the NLI that still have some detectable sensory or motor function, even though the S4/S5 segments are completely impaired. It is documented as the most caudal segment with any function, and tracking the ZPP helps monitor the potential for neurological recovery adjacent to the primary injury site.