Changes in spinal reflex excitability in brain-dead humans.

The excitability of proprio- and exteroceptive spinal reflexes was monitored electrophysiologically and clinically during the occurrence of brain death (BD) in 8 patients. After a period of total reflex unresponsiveness, the soleus H reflex attained a steady-state excitability level in 2-6 h. The recovery cycle of this response regained its normal shape at 10-20 h. The threshold of the cutaneous reflex evoked in the biceps femoris by electrical stimulation of the sural nerve had become normal in 4-13 h, although the response displayed an abnormal multi-component pattern. Digital responses to mechanical stimulation of the foot sole were evident after 6-8 h. Knee and ankle jerks were never evoked during the time of monitoring. The time-courses of the changes in excitability were not directly correlated with the fall in the blood pressure which may occur during BD. It is concluded that the human spinal cord reacts to BD with a spinal shock, characterized by sequential recovery of reflex transmission. The overall timing of this process appears to be much shorter than that previously described for the spinal shock following traumatic transection of the cord, but the latter was never studied in the earliest phases.