Adaptation of motor function after spinal cord injury: novel insights into spinal shock

The mechanisms underlying spinal shock have not been clearly defined. At present, clinical assessment remains the mainstay to describe progression through spinal shock following traumatic spinal cord injury. However, nerve excitability studies in combination with conventional nerve conduction and clinical assessments have the potential to investigate spinal shock at the level of the peripheral axon. Therefore, peripheral motor axon excitability was prospectively and systematically evaluated in more than 400 studies of 11 patients admitted to hospital after traumatic spinal cord injury, with cord lesions above T9 (nine cervical, two thoracic). Recordings commenced within 15 days of admission from the median nerve to abductor pollicis brevis in the upper limb and the common peroneal nerve to tibialis anterior in both lower limbs, and were continued until patient discharge from hospital. Excitability was assessed using threshold tracking techniques and recordings were compared with data from healthy controls. In addition, concurrent clinical measures of strength, serum electrolytes and nerve conduction were collected. High threshold stimulus-response relationships were apparent from the early phase of spinal shock that coincided with depolarization-like features that reached a peak on Day 16.9 (± 2.7 standard error) for the common peroneal nerve and Day 11.8 (± 2.0 standard error) for the median nerve. Overall, changes in the common peroneal nerve were of greater magnitude than for the median nerve. For both nerves, the most significant changes were in threshold electrotonus, which was 'fanned in', and during the recovery cycle superexcitability was reduced (P < 0.001). However, refractoriness was increased only for the common peroneal nerve (P < 0.05). Changes in the spinal injured cohort could not be explained on the basis of an isolated common peroneal nerve palsy. By the time patients with spinal injury were discharged from hospital between Days 68 and 215, excitability for upper and lower limbs had returned towards normative values, but not for all parameters. Electrolyte levels and results for nerve conduction studies remained within normal limits throughout the period of admission. Contrary to prevailing opinion, these data demonstrate that significant changes in peripheral motor axonal excitability occur early during spinal shock, with subsequent further deterioration in axonal function, before recovery ensues.