central nervous toxicity
Central nervous system oxygen toxicity manifests as symptoms such as visual changes (especially tunnel vision), ringing in the ears, nausea, twitching (especially of the face), irritability (personality changes, anxiety, confusion, etc.), and dizziness. This may be followed by a seizure consisting of two phases: intense muscle contraction occurs for several seconds; followed by rapid spasms of alternate muscle relaxation and contraction producing convulsive jerking. The seizure ends with a period of unconsciousness. The onset of seizure depends upon the partial pressure of oxygen in the breathing gas and exposure duration. However, exposure time before onset is unpredictable, as tests have shown a wide variation, both amongst individuals, and in the same individual from day to day. In addition, many external factors, such as underwater immersion, exposure to cold, and exercise will decrease the time to onset of central nervous system symptoms. Decrease of tolerance is closely linked to retention of carbon dioxide.