Central Nervous System Oxygen Toxicity

Central nervous system oxygen toxicity is seizure activity and neurological symptoms caused by breathing oxygen at partial pressures usually over 1.4 atmospheres. It is a main hazard in oxygen rebreather diving, technical diving and hyperbaric oxygen therapy.

Exposure limits. Working limits for diving are usually a partial pressure of 1.4 atmospheres for work and 1.6 atmospheres for rest or decompression, with time limits. Pure oxygen at 2.4 to 2.8 atmospheres is used in treatment chambers with air breaks.

Epidemiology. Seizures occur in a small fraction of hyperbaric treatments (about 1 in 2000 to 1 in 10000 sessions at 2.4 atmospheres, higher at greater pressure). Underwater convulsion carries a high risk of drowning.

Variability. Susceptibility varies between people and from day to day in one person, so no single limit is safe for all.

Pathophysiology.

Reactive oxygen species and nitric oxide impair GABA synthesis and ion channel function, and increase neuronal excitability. Cerebral vasodilation with hypercapnia worsens the effect.

Seizure results from synchronised neuronal discharge. The attack is self-limited and rarely causes brain injury.

• Warning signs: visual disturbance (tunnel vision), tinnitus, nausea, muscle twitching especially of the face and lips, irritability, dizziness, and a sense of unease.
• Tonic phase (about 30 seconds), clonic phase (1 to 2 minutes), then postictal drowsiness.
• Symptoms can be absent before convulsion.

Red flags.

• Seizure underwater or during a rebreather dive.
• Seizure that continues over 5 minutes, or aspiration, injury or drowning.

• Clinical, based on the oxygen partial pressure, time, exercise, carbon dioxide and symptoms.
• Check blood glucose, oxygen saturation and neurological state afterwards.
• Review gas analysis, equipment (rebreather faults and scrubber failure) and dive profile.
• Brain imaging and EEG are not needed after a single typical event with full recovery; investigate when atypical or recurrent.

Differential diagnosis. Hypoxia, carbon dioxide toxicity, carbon monoxide poisoning, nitrogen narcosis, high pressure neurological syndrome, decompression sickness, arterial gas embolism, primary epilepsy, hypoglycaemia, panic.

• In a chamber: remove the oxygen mask or reduce partial pressure at once and breathe air; protect the head and airway; do not restrain; continue the chamber depth during the convulsion, then ascend slowly once it ends.
• Underwater: do not ascend during the tonic or clonic phase because of the risk of lung overexpansion; support the diver, keep the regulator in position if possible, and ascend after the convulsion ends, with an open airway and continuous ascent.
• Lorazepam 4 mg intravenously over 2 minutes if seizure persists beyond 5 minutes or recurs, repeated once after 10 minutes. Maximum 8 mg. Monitor breathing and blood pressure; caution in respiratory depression and hepatic impairment; no renal adjustment.
• After a seizure: airway, breathing and circulation, oxygen by a safe regimen, and observation for aspiration, drowning and decompression illness.
• Prevention: keep inside partial pressure limits, avoid heavy exercise and carbon dioxide retention, check the gas before each dive, and use air breaks in hyperbaric therapy.
• Return to diving after an event is decided by a diving medicine physician, and an in water seizure usually needs review before any return.

Monitoring. Neurological observation for at least 24 hours after an event with any aspiration or injury; review at 2 weeks.

Referral and escalation. Diving medicine physician for every event; emergency transfer for prolonged seizure, aspiration, drowning or suspected embolism; neurology if events recur.