Case report on CNS oxygen toxicity

A 42-year-old male is an experienced technical diver with no history of significant medical problems. He was not taking any medications or recreational drugs.

He and his buddy had dived in Indonesian sea waters on air to 50m for 30 minutes before beginning their ascent. The latter involved stops at 33m (on air), 24m (air), 15 msw (EAN50), 12m (EAN50), 9m (EAN 50) before ascending to the 6m stop. At 6m they both switched to 100% oxygen to complete their decompression.

After breathing oxygen at 6m for almost 20 minutes, the diver developed strong muscle contractions in his diaphragm which made breathing difficult. He had not noticed any other symptoms at all prior to this. After about 10-15 seconds, the contractions abated for a short time and he realised that he should change to a lower oxygen mix.

However, before he could make the switch, the contractions re-commenced and he soon became unconscious.

His buddy who was decompressing (at 4.5m) noticed his friend rising towards the surface, convulsing, with his regulator out of his mouth. The buddy abandoned his remaining decompression, grabbed his friend, brought him to the surface and to the nearby boat. The diver was placed in the recovery position continuing to convulse for a short time. When the convulsion ceased he was breathing, was left on his side and given oxygen first aid on the way to hospital.

He regained consciousness after about 30 minutes but remained very confused. In hospital, he was examined, given a chest X-ray and discharged (too soon!).

Unfortunately, the breathing gas was not examined after the incident. It is highly likely that the diver suffered seizures as a result of Central Nervous System (CNS) Oxygen Toxicity.

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What is oxygen toxicity and how can it affect a diver?

When oxygen is breathed at partial pressures (ppO2) usually greater than about 1.6 ATA, it can cause toxic effects on the brain which can occur rapidly and is comparatively unpredictable. There is a wide variation in the amount of oxygen different individuals can tolerate before showing effects, and also within an individual from day to day.
The most dramatic direct result of CNS oxygen toxicity is a seizure that may occur with, or without warning. Minor symptoms such as nausea, muscle twitches (particularly lips), vertigo, light-headedness, visual disturbances (e.g. tunnel vision), irritability and numbness may precede the convulsion but these are often not noticed.

Some of the factors known to increase susceptibility include submersion, exertion, high temperatures, high levels of carbon dioxide, increased levels of adrenaline, and the presence of certain drugs (e.g. amphetamines and other stimulants).

Many divers routinely decompress at 6 msw on 100% oxygen, usually without incident. However, problems can still occur and technical divers need to be well aware of this.

Cases such as this reminds us, the need for care, and highlights the importance of a vigilant dive buddy.

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