ASIA Classification for Spinal Cord Injury
The American Spinal Injury Association (ASIA) Impairment Scale is the gold standard classification system for spinal cord injury, consisting of five grades (A through E) that categorize injury severity based on motor and sensory function, with particular emphasis on sacral sparing. 1, 2
The Five ASIA Grades
The ASIA system assigns grades from A (most severe) through E (normal function) 1:
ASIA A (Complete): No motor or sensory function preserved in sacral segments S4-S5, representing complete injury with no sacral sparing 1
ASIA B (Sensory Incomplete): Sensory function preserved below the neurological level including sacral segments S4-S5, but no motor function preserved more than three levels below the motor level 1
ASIA C (Motor Incomplete): Motor function preserved below the neurological level, with more than half of key muscle groups below the neurological level having a muscle grade less than 3/5 1
ASIA D (Motor Incomplete): Motor function preserved below the neurological level, with at least half of key muscle groups below the neurological level having a muscle grade of 3/5 or greater 1
ASIA E (Normal): Normal sensory and motor function in all segments 1
Key Advantages Over Older Systems
The ASIA system provides more accurate definition of complete SCI and improved methods for determining motor and sensory scores compared to older classification systems like the Frankel scale. 1 The system generates continuous numerical scores for both motor and sensory function, allowing assessment of small increments in recovery that may not change the overall grade 1, 2. This is critical because the motor and sensory scores can detect meaningful neurological improvements even when the AIS grade remains unchanged 1, 2.
Essential Examination Components
Trained examiners must perform standardized assessments to achieve high interrater reliability 1:
Motor strength testing: Bilateral assessment of all major muscle groups using the 0-5 grading scale 3
Sensory testing: Light touch and pin prick sensation at each dermatome level 1
Sacral examination: This is the most critical component, as sacral sensation preservation is a critical prognostic indicator 1, 3. Specifically test perianal sensation, rectal tone, and voluntary anal sphincter contraction 3. The presence or absence of sacral sparing determines whether an injury is complete (ASIA A) or incomplete (ASIA B-D) 1
Critical Limitations and Pitfalls
A major disadvantage of the ASIA system is the ceiling effect in Grades C and D, where patients may experience significant functional recovery without changing to the next grade. 1 For example, research shows that when AIS remained unchanged between assessment points, there was no change in the number of muscles graded 3 or more in 73% of transitions, yet patients still experienced meaningful functional improvements 4. This highlights why additional outcome measures beyond the AIS grade are essential 1.
Principal investigators should never serve as neurological examiners to minimize bias 1. Studies demonstrate that incomplete paraplegia cases are particularly difficult to classify accurately, with correct classification rates as low as 16-21% for motor levels even among trained professionals 5.
Conversion Rates and Prognostic Value
Entry ASIA Impairment Scale grade is the strongest predictor of functional outcomes. 1, 2 However, spontaneous conversion rates are substantial and must be considered when designing clinical trials 4, 6:
Approximately 70% of patients initially diagnosed as ASIA A do not convert, while 90% of ASIA D patients remain stable 4
Conversion rates from complete to incomplete injury have been increasing over time, particularly for tetraplegia (from 17.6% in 1995-1997 to 50% in 2013-2015) 6
Changes in sacral segments account for 40% of AIS conversions, motor improvement 31%, sensory improvement 19%, and changes in neurological level 10% 4
Neurological examination at 72 hours post-injury should be obtained as the baseline for comparison with subsequent assessments 7
Integration with Functional Outcome Measures
While the ASIA system excels at neurological classification, functional outcome measures such as the Functional Independence Measure (FIM), Spinal Cord Injury Measure, and Walking Index in Spinal Cord Injury must be incorporated to assess clinically meaningful recovery. 1, 2 These measures address patient-prioritized outcomes including bowel, bladder, sexual function, and ambulation that the ASIA scale does not directly capture 1, 2.
For injuries at or above T6, the ASIA grade determines both neurological severity and risk of autonomic complications, particularly autonomic dysreflexia 1. Voluntary external anal/urethral sphincter contraction correlates significantly with bladder function recovery and should be specifically documented 1, 3.