SCI Terminology & the ASIA Impairment Scale

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Understanding the ASIA Impairment Scale 

ASIA Examination
• Sensory level
• Motor level
• Neurological level of injury (NLI)
• Complete vs. Incomplete injury
• Sacral sparing
• Zone of Partial Preservation (complete injuries)

Dermatome:
This term refers to the area of the skin innervated by the sensory axons within each segmental nerve (root). Dermatomes are clinically important and necessary for assessing and diagnosing the level of spinal cord injury in the American Spinal Injury Association (ASIA) Impairment scale.

The spinal cord has 31 segments, each with a pair (right and left) of ventral (anterior) and dorsal (posterior) nerve roots that innervate motor and sensory function, respectively. The anterior and posterior nerve roots combine on each side to form the spinal nerves as they exit the vertebral canal through the intervertebral foramina or neuroforamina. The 31 spine segments on each side give rise to 31 spinal nerves, which are composed of 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal spinal nerve. Dermatomes exist for each of these spinal nerves, except the first cervical spinal nerve. Sensory information from a specific dermatome is transmitted by the sensory nerve fibers to the spinal nerve of a specific segment of the spinal cord.

This list details the nerve roots and the approximate area of skin that they provide innervation to. The dermatomes of the head are supplied by branches V1, V2 and V3 of the trigeminal nerve.

• Cranial nerve 5 (trigeminal)
o V1 – ophthalmic branch – top of the head
o V2 – maxillary branch – nares to the top lip
o V3 – mandibular branch – bottom lip to jaw

• C2 – occipital protuberance
• C3 – supraclavicular fossa
• C4 – acromioclavicular joint
• C5 – lateral antecubital fossa
• C6 – thumb
• C7 – middle finger
• C8 – little finger
• T1 – medial antecubital fossa
• T2 – the apex of the axilla
• T3- T12 – bands down the torso
• L1 – upper anterior thigh
• L2 – mid anterior thigh
• L3 – medial femoral condyle
• L4 – medial malleolus
• L5 – dorsum of 3rd metatarsal phalangeal joint
• S1 – lateral heel
• S2 – popliteal fossa
• S3 – ischial tuberosity
• S4-5 – perianal area

Myotome:
The anatomical term myotome refers to the muscles served by a spinal nerve root. A myotome is, therefore, a set of muscles innervated by a specific, single spinal nerve. Ten muscle groups represent the motor innervation to the cervical and lumbosacral spinal cord.

Muscle strength in a particular myotome helps in identifying at which level a nerve root is compromised.
This list details which movement is most strongly associated with each myotome:
• C5 – Elbow flexion
• C6 – Wrist extension
• C7 – Elbow extension
• C8 – Finger flexion
• T1 – Finger abduction
• L2 – Hip flexion
• L3 – Knee extension
• L4 – Ankle dorsiflexion
• L5 – Great toe extension
• S1 – Ankle plantarflexion

Motor level:
The motor level is determined by examining a key muscle function within each of the 10 myotomes on each side of the body, and is defined by the lowest key muscle function that has a grade of at least 3 (on manual muscle testing- MMT) in the supine position, providing the key muscle functions represented by segments above that level are judged to be intact (graded as a 5 on MMT).This may be different for the right and left side of the body.

Motor scores:
This term refers to a numerical summary score of motor function. There is a maximum score of 25 for each extremity, totaling 50 for the upper limbs and 50 for the lower limbs. This score can reflect the degree of neurological impairment associated with the SCI.

Sensory level:
Defined as the most caudal, intact dermatome for both pin prick and light touch sensation. The sensory level is determined by performing an examination of the key sensory points within each of the 28 dermatomes on each side of the body (right and left) and is the most caudal, normally innervated dermatome for both pin prick (sharp/dull discrimination) and light touch sensation. This may be different for the right and left side of the body.

Sensory scores:
This term refers to a numerical summary score of sensory function. There is a maximum total of 56 points each for light touch and pin prick (sharp/dull discrimination) modalities, for a total of 112 points per side of the body. This can reflect the degree of neurological impairment associated with the SCI.

Skeletal level: 
This term has been used to denote the level at which, by radiographic examination, the greatest vertebral damage is found.

Neurological Level of Injury (NLI):
The NLI refers to the most caudal segment of the spinal cord with normal sensory and antigravity motor function on both sides of the body, provided that there is normal (intact) sensory and motor function rostrally.

– The sensory level refers to the most caudal, intact dermatome for both light touch and pinprick sensation.
– The motor level refers to the most caudal myotome with a key muscle function of at least grade 3.

If there is a discrepancy between the most caudal intact section between the four possible levels of right sensory level, left sensory level, right motor level, or left motor level, the neurological level of injury is considered the most rostral segment of these four levels. Thus, up to 4 different segments may be identified in determining the neurological level, i.e., R(ight)-sensory, L(eft)-sensory, R-motor, L-motor. The single NLI is the most rostral of these levels.

Complete injury:
This term is used when there is an absence of sensory and motor function in the lowest sacral segments (S4-S5) (i.e. no sacral sparing).

Incomplete injury:
This term is used when there is preservation of any sensory and/or motor function below the neurological level that includes the lowest sacral segments S4-S5 (i.e. presence of “sacral sparing”). Sensory sacral sparing includes sensation preservation (intact or impaired) at the anal mucocutaneous junction (S4-5 dermatome) on one or both sides for light touch or pin prick, or deep anal pressure. Motor sacral sparing includes the presence of voluntary contraction of the external anal sphincter upon digital rectal examination. 

ASIA Impairment Scale- used in grading the degree of impairment

The ASIA Impairment Scale is a system used to classify the severity of spinal cord injury. It was developed by the American Spinal Injury Association in 2006 and is the most widely used classification system today.