Absence of local sign withdrawal in chronic human spinal cord injury

Local sign withdrawal, a reflex to direct the limb away from noxious cutaneous stimuli, is thought to be indicative of a modular organization of the spinal cord. To assess the integrity of such an organization of the spinal cord in chronic human spinal cord injury (SCI), we tested the electromyogram (EMG) and joint torque responses to cutaneous stimuli applied to 6 locations of the leg in 10 SCI volunteers and 3 spinal-intact controls. The 6 locations included the medial arch of the foot, the second metatarsal, the dorsum, the region over the sural nerve at the lateral malleolus, and the anterior and posterior aspects of the lower leg. Although spinal-intact subjects demonstrated local sign withdrawal, the data from SCI subjects indicated that an invariant flexion response pattern was produced regardless of stimulus location. Ankle dorsiflexion and hip flexion were produced in all subjects at all locations and no difference in the ratio of hip:ankle torques could be detected for the 6 test locations. A windup-crossover test, employing a sequence of 6 stimuli at 1-s intervals was used to assess whether common neuronal pathways were responsible for the loss of modular organization. An additional 10 SCI volunteers were tested using stimuli in which the stimulus location was switched between the 2nd and 3rd stimulus of the test sequence. The response to the crossover stimulus more closely resembled the response to the 3rd stimulus of a windup sequence than a response without conditioning stimuli. These results indicate that increased excitability produced by windup at one stimulus site is maintained at the 2nd site. This observation suggests that deep dorsal horn neurons, typically associated with musculotopic mapping, may be reorganized in chronic spinal cord injury.