Distribution and Latency of Muscle Responses to Transcranial Magnetic Stimulation of Motor Cortex After Spinal Cord Injury in Humans

Abstract

Noninvasive transcranial magnetic stimulation (TMS) of the motor cortex was used to evoke electromyographic (EMG) responses in persons with spinal cord injury (n = 97) and able-bodied subjects (n = 20, for comparative data). Our goal was to evaluate, for different levels and severity of spinal cord injury, potential differences in the distribution and latency of motor responses in a large sample of muscles affected by the injury. The spinal cord injury (SCI) population was divided into subgroups based upon injury location (cervical, thoracic, and thoracolumbar) and clinical status (motor-complete versus motor-incomplete). Cortical stimuli were delivered while subjects attempted to contract individual muscles, in order to both maximize the probability of a response to TMS and minimize the response latency. Subjects with motor-incomplete injuries to the cervical or thoracic spinal cord were more likely to demonstrate volitional and TMS-evoked contractions in muscles controlling their foot and ankle (i.e., distal lower limb muscles) compared to muscles of the thigh (i.e., proximal lower limb muscles). When TMS did evoke responses in muscles innervated at levels caudal to the spinal cord lesion, response latencies of muscles in the lower limbs were delayed equally for persons with injury to the cervical or thoracic spinal cord, suggesting normal central motor conduction velocity in motor axons caudal to the lesion. In fact, motor response distribution and latencies were essentially indistinguishable for injuries to the cervical or thoracic (at or rostral to T10) levels of the spine. In contrast, motor-incomplete SCI subjects with injuries at the thoracolumbar level showed a higher probability of preserved volitional movements and TMS-evoked contractions in proximal muscles of the lower limb, and absent responses in distal muscles. When responses to TMS were seen in this group, the latencies were not significantly longer than those of able-bodied (AB) subjects, strongly suggestive of "root sparing" as a basis for motor function in subjects with injury at or caudal to the T11 vertebral body. Both the distribution and latency of TMS-evoked responses are consistent with highly focal lesions to the spinal cord in the subjects examined. The pattern of preserved responsiveness predominating in the distal leg muscles is consistent with a greater role of corticospinal tract innervation of these muscles compared to more proximal muscles of the thigh and hip.

Abductor hallucis for monitoring lower-limb recovery after spinal cord injury in man

“…11,37 Thresholds for recruiting this muscle with single-pulse transcranial magnetic stimulation in awake, neurologically intact subjects are typically equal to or lower than those for other lower-limb muscles. 10 In persons with incomplete cervical spinal cord injurymost of whom had been injured at least 1 year prior to testing -we showed that AbH was significantly more likely to be recruited by TMS than other lower-limb muscles, including hamstring, tibialis anterior, and soleus. 10 In that study of chronic SCI, every instance in which a subject could recruit at least one lower-limb muscle in the leg included AbH as one of the muscles (or the only muscle) recruited by TMS.…”

“…10 In persons with incomplete cervical spinal cord injurymost of whom had been injured at least 1 year prior to testing -we showed that AbH was significantly more likely to be recruited by TMS than other lower-limb muscles, including hamstring, tibialis anterior, and soleus. 10 In that study of chronic SCI, every instance in which a subject could recruit at least one lower-limb muscle in the leg included AbH as one of the muscles (or the only muscle) recruited by TMS. 10 Exactly, the same situation was encountered in the present study in persons with acute SCI, using voluntary contraction as the outcome measure.…”

“…10 In that study of chronic SCI, every instance in which a subject could recruit at least one lower-limb muscle in the leg included AbH as one of the muscles (or the only muscle) recruited by TMS. 10 Exactly, the same situation was encountered in the present study in persons with acute SCI, using voluntary contraction as the outcome measure.…”

“…9 In humans, AbH is more easily recruited (ie has a lower threshold) by transcranial magnetic stimulation (TMS) in awake humans than other lower-limb muscles, 10 and the same is true in the anesthetized human when tested with transcranial electrical stimulation during spine surgery. 11 These findings point to a relatively prominent role of the AbH in cortically controlled voluntary movements.…”

Functional implications of corticospinal tract impairment on gait after spinal cord injury

“…Previous studies and clinical assessment of stroke patients indicate that the CST is involved in foot elevation during swing and that its impairment induces foot drop. [5][6][7] We have recently shown that measurement of maximum toe elevation during swing can be used as a simple and objective measurement of foot drop and reflects CST impairment in persons …”