Hyperbaric hyperreflexia: tendon-jerk and Hoffmann reflexes in man at 43 bars.

Tendon jerk (TJR) and Hoffmann (H) reflexes of the soleus muscle were studied in two men during a 26-day simulated oxygenhelium dive to a maximum pressure of 43 bars. The amplitude of the TJR response was observed to increase markedly after compression and also at the end of decompression. This biphasic pattern of enhanced reflectivity was reproduced by synchronised but much smaller variations in H reflex response. The positive facilitatory effects of applying Jenkdrassik's manoeuvre were reduced and the negative effects increased during hyperbaric exposure. Excitability cycles (twin-pulses methods) revealed modifications in one subject similar to those observed in clinical hyperreflexia, namely a shortening of Phase III and an enhancement of Phase IV. The other subject exhibited a notable depression of Phase IV. Abnormal EMG recordings are described including randomly-triggered slow wave potentials with no mechanical effects, and fixed, long latency late responses of a reflex nature with a definite mechanical effect. The contention that alterations in vestibulo-spinal relations may result in the release from tonic presynaptic inhibitory control of myotatic reflex arc excitability is discussed as a partial explanation for these and related findings.