High-Pressure Neurological Syndrome

High pressure neurological syndrome is a disorder of the nervous system caused by compression to great depth, usually over about 150 m, with helium oxygen mixtures and rapid pressure rise. It causes tremor, myoclonus, cognitive change and, at extreme pressures, seizures.

Exposure. Saturation and deep experimental diving. It is rare now because commercial dives are shallower and compression is slower.

Dose relation. Symptoms depend on rate of compression and final depth. Compression at over 1 to 2 m/minute to depths over 180 m reliably produces effects, and depths over 300 m produce marked signs.

Course. Symptoms ease over days of staying at depth, then settle during decompression.

Differences from narcosis. Narcosis is caused by nitrogen and increases with depth. High pressure neurological syndrome reflects pressure itself and is seen with helium mixes.

Pathophysiology.

Pressure alters the activity of ion channels and synaptic receptors, increasing excitability of the central nervous system, especially through NMDA receptor activity.

Narcotic gases such as nitrogen or hydrogen oppose the effect, which is the basis of using a nitrogen fraction in trimix to reduce symptoms.

• Tremor of hands, then limbs, often the first sign.
• Myoclonic jerks, dizziness, nausea and vomiting, and somnolence.
• Impaired psychomotor performance, memory and sleep, and EEG slowing with theta activity.
• Seizures and loss of consciousness at extreme depth.

Red flags.

• Convulsions, loss of consciousness or persistent vomiting.
• Rapid worsening during compression.

• Clinical: onset during compression to depth with helium oxygen, with tremor, myoclonus, nausea and slowing.
• EEG changes support the diagnosis when available.
• Exclude other causes: decompression sickness, oxygen toxicity, hypoglycaemia, hypothermia and infection.

• Slow or pause compression and hold at the current depth; if symptoms are severe, reduce depth a little.
• Stop descent or compression when tremor and myoclonus appear and continue only after they stabilise.
• Add a narcotic gas (nitrogen, for example trimix) to the breathing mix, which suppresses symptoms.
• Allow time at depth for accommodation; give supportive care for nausea and sleep.
• Antiemetics for vomiting: ondansetron 4 mg intravenously or 8 mg orally every 8 hours as needed. Maximum 24 mg daily. Monitor QTc; avoid in long QT syndrome; at most 8 mg daily in severe hepatic impairment; no renal adjustment.
• No drug is proven to prevent the syndrome.

Monitoring. Tremor, reaction time and EEG during compression and for the first days at depth; decompression observed as directed by the dive supervisor.

Referral and escalation. Diving medicine physician and dive supervisor for any case; decompression chamber transfer for severe or progressive signs.