Spinal Reflexes

Spinal reflex evaluation

This is an important part of the evaluation of gait and postural reactions testing. It is not a separate entity. The spinal cord can be divided into four regions: cranial cervical [C1 to C5], cervico-thoracic [C6 to T2], thoraco-lumbar [T3 to L3] and lumbo-sacral [L4 to S3]. The LMN (lower motor neuron) cell bodies for the thoracic limbs are located in the grey matter of the cervico-thoracic intumescence (segments C1 to C5) and for the pelvic limbs in the lumbo-sacral intumescence (segments L4 to S3).

After gait and postural reactions testing, the clinician can narrow down the lesion localization to being cranial to T3 spinal cord segments, caudal to T3 spinal cord segments, or peripheral nervous system (peripheral nerve, neuromuscular junction or muscles) (table X). Spinal reflex evaluation further narrows down the lesion localization by testing the integrity of the C6 – T2 and L4 – S3 intumescences, as well as the respective segmental sensory and motor nerve (LMN) that form the peripheral nerve and the muscles innervated.

Spinal reflexes are segmental and only evaluate the spinal segment(s) within the intumescences corresponding to the stimulated nerve. They do not require consciousness. Lesions at the level of these intumescences or those affecting the peripheral nervous system result in the loss of segmental spinal reflexes, as well as reduced muscle tone and size. Lesions cranial to the intumescence (UMN dysfunction) will result in normal to increased segmental spinal reflexes (release of the inhibitory modulatory effect of the UMN on the LMN).

It is worth mentioning that animals with severe peracute transverse thoracolumbar spinal cord lesions usually show severe pelvic limb hypotonia and depressed spinal reflexes for a few days after the onset. Despite many spinal reflexes being described, the most reliable are the withdrawal reflex in the thoracic limb and the patellar and withdrawal reflexes in the pelvic limbs. Other spinal reflexes (triceps, biceps, extensor carpal radialis, and gastrocnemius) are more difficult to perform and interpret.