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).
About the Author
Andy Davis is a RAID, PADI TecRec, ANDI, BSAC and SSI qualified independent technical diving instructor who specializes in teaching advanced sidemount, trimix and wreck exploration diving courses across South East Asia. Currently residing in ‘wreck diving heaven’ at Subic Bay, Philippines, he has amassed more than 9000 open circuit and CCR dives over 27 years of diving across the globe.
Andy has published many magazine articles on technical diving, has written course materials for dive training agency syllabus, tests and reviews diving gear for major manufacturers and consults with the Philippines Underwater Archaeology Society.
He is currently writing a series of books to be published on advanced diving topics. Prior to becoming a professional technical diving educator in 2006, Andy was a commissioned officer in the Royal Air Force and has served in Iraq, Afghanistan, Belize and Cyprus.