ASIA CLASSIFICATION

Explanation of Muscle Groups
Ten muscle groups represent the motor innervation by the cervical and lumbosacral spinal cord. The ASIA system does not include the abdominal muscles (i.e. T10-11) because the thoracic levels are much easier to determine from sensory levels. It also excludes certain muscles (e.g. hamstrings) because the segmental levels that innervate them are already represented by other muscles.

Arm and hand muscles. C5 represents the elbow flexors (biceps), C6 the wrist extensors, C7 the elbow extensors (triceps), C8 the finger flexors, and T1 the little finger abductor (outward movement of the pinky finger).

Leg and foot muscles. The leg muscles represent the lumbar segments, i.e. L2 are the hip flexors (psoas), L3 the knee extensors (quadriceps), L4 the ankle dorsiflexors (anterior tibialis), L5 the long toe extensors (hallucis longus), S1 the ankle plantar flexors (gastrocnemius).

The anal sphincter is innervated by the S4-5 cord and represents the end of the spinal cord. The anal sphincter is a critical part of the spinal cord injury examination. If the person has any voluntary anal contraction, regardless of any other finding, that person is by definition a motor incomplete injury.

It is important to note that the muscle groups specified in the ASIA classifications represent a gross over simplication of the situation. Almost every muscle received innervation from two or more segments.

Interpretation

The ASIA Impairment Scale is follows the Frankel scale but differs from the older scale in several important respects. First, instead of no function below the injury level, ASIA A is defined as a person with no motor or sensory function preserved in the sacral segments S4-S5. This definition is clear and unambiguous. ASIA B is essentially identical to Frankel B but adds the requirement of preserved sacral S4-S5 function. It should be noted that ASIA A and B classification depend entirely on a single observation, i.e. the preservation of motor and sensory function of S4-5.

The ASIA scale also added quantitive criteria for C and D. The original Frankel scale asked clinicians to evaluate the usefulness of lower limb function. This not only introduced a subjective element to the scale but ignored arm and hand function in patients with cervical spinal cord injury. To get around this problem, ASIA stipulated that a patient would be an ASIA C if more than half of the muscles evaluated had a grade of less than 3/5. If not, the person was assigned to ASIA D.

ASIA E is of interest because it implies that somebody can have spinal cord injury without having any neurological deficits at least detectable on a neurological examination of this type. Also, the ASIA motor and sensory scoring may not be sensitive to subtle weakness, presence of spasticity, pain, and certain forms of dyesthesia that could be a result of spinal cord injury. Note that such a person would be categorized as an ASIA E.

These changes in the ASIA scale significantly improved the reliability and consistency of the classification. Although it was more logical, the new definition of "complete" injury does not necessarily mean that it better reflects injury severity. For example, is there any situation where a person could be an ASIA B and better off the ASIA C or even ASIA D?

The new ASIA A categorization turns out to be more predictive of prognosis than the previous definition where the presence of function several segments below the injury site but the absence of function below a given level could be interpreted as an "incomplete" spinal cord injury.

The ASIA committee also classified incomplete spinal cord injuries into five types. A central cord syndrome is associated with greater loss of upper limb function compared to the lower limbs. The Brown-Sequard syndrome results from a hemisection lesion of the spinal cord. Anterior cord syndrome occurs when the injury affects the anterior spinal tracts, including the vestibulospnal tract. Conus medullaris and cauda equina syndromes occur with damage to the conus or spinal roots of the cord.erson has any voluntary anal contraction, regardless of any other finding, that person is by definition a motor incomplete injury.

It is important to note that the muscle groups specified in the ASIA classifications represent a gross over simplication of the situation. Almost every muscle received innervation from two or more segments.