What is the most serious sign of Central Nervous System (CNS) toxicity?

The most serious sign of Central Nervous System (CNS) toxicity is convulsion. Convulsions indicate a significant disruption in normal brain function and can suggest severe levels of toxicity that can lead to further neurological damage or even life-threatening situations. This reaction may be a result of rapid changes in pressure or exposure to specific gases while diving, such as nitrogen or oxygen, at high partial pressures.

In this context, convulsions are a critical alert to divers that immediate medical attention is necessary to prevent further complications. They often require urgent intervention, as they can lead to injuries during the episode or indicate that the diver is unable to manage their own airway, which can lead to asphyxiation or other serious outcomes.

Other signs like difficulty breathing, vertigo, and nausea can certainly accompany CNS toxicity, but they do not carry the same immediate risk level or urgency as convulsions. Difficulty breathing may signal respiratory distress, vertigo might indicate disorientation or balance issues, and nausea is often a mild and self-limiting symptom compared to the acute danger posed by convulsive seizures. Hence, the presence of convulsions marks the most critical situation in terms of CNS toxicity.