Deep Diving Maladies – High-Pressure Nervous Syndrome

by Andy Davis

HPNS or High-Pressure Nervous Syndrome (also called High-Pressure Neurological Syndrome)  can onset beyond a depth of 150m/500ft when using heliox or trimix breathing gasses. The severity of High-Pressure Nervous Syndrome is determined by the speed of descent (rate of compression), depth attained (absolute pressure) and the percentage of helium inspired by the diver. There is also a varying level of individual susceptibility, but an individual diver is likely to see no variation in susceptibility on different dives..

High-Pressure Nervous Syndrome is a neurological/physiological disorder, that was first described using the term “helium tremors” by the navy physiologist Peter Bennett in 1965 (they were first discovered by Russian scientist G. L. Zal’tsman in 1961, but this report was not made public until 1967). Numerous studies have been completed since that time but, despite much research, the physiological/neurological causes of High-Pressure Nervous Syndrome are still unknown.

The symptoms of the high-pressure nervous syndrome are; tremors, seizures, twitches or jerks, somnolence (strong desire to sleep), visual disturbances, nausea, dizziness, and decreased mental performance.  On dives up to 300m/1000ft, these symptoms may subside after some time stabilizing at depth. However, beyond 300m/1000ft, the symptoms typically persist, regardless of time.

To avoid High-Pressure Nervous Syndrome, the following advice is given:

• Refrain from diving  Heliox (O2/He) beyond 122m/400ft.
• Refrain from diving Trimix (O2/He/N2) beyond  180m/600ft
• Descend at slower rates (less than 30cm/1ft per minute can mitigate HPNS, but is impractical for technical diving).