Corticospinal function in severe brain injury assessed using magnetic stimulation of the motor cortex in man

We have assessed corticospinal function in 19 post-coma patients severely brain-injured by anoxia or physical trauma. Eleven patients were unresponsive (Category 1) and eight demonstrated minimal, non-verbal responses to simple commands (Category 2). Motor evoked potentials (MEPs) could be elicited in hand and leg muscles in nine Category 1 and all eight Category 2 patients in response to transcranial magnetic stimulation (TMS). In comparison with normal subjects, threshold to TMS was significantly elevated in Category 1 but not in Category 2. Central conduction times were within the normal range except for two patients (one in each category) in whom they were prolonged. The variability in MEP amplitude to constant TMS was not significantly different from normal in either category. The size of MEPs recorded simultaneously in different hand muscles were correlated in all three groups. The presence of H-reflexes in hand muscles was associated with an absence of MEPs or a high threshold to TMS. Variability of MEPs was substantially greater than that of H-reflexes. We conclude that brain injury of a severity that may preclude consciousness and voluntary movement does not invariably predicate a non-functional motor cortex and corticospinal system.