Topographic electroencephalographic studies in a hyperbaric environment--specific reference to high pressure nervous syndrome.

Hyperbaric chamber dives at various equivalent depths below sea level, i.e. 7, 14, 19 and 31 atmosphere absolute (ATA) with helium-oxygen or helium-nitrogen-oxygen have been performed at the Japan Marine Science and Technology Center. A two-dimensional (topographic) display of the scalp EEG was used during simulated underwater experiments to determine; 1) Whether there are any characteristic EEG patterns in high pressure nervous syndrome (HPNS), 2) the relationship between the EEG changes and the compression rate, and 3) the relationship between the EEG changes and the characteristic signs and symptoms of HPNS. A two-way analysis of variance and a distribution analysis technique revealed that the topographic brain patterns depended on the diving depth and indicated the most affected brain areas during compression and decompression. Significant correlations between the diving depth and the EEG potentials were observed at different brain locations. Alpha waves showed a diffuse cortical distribution. Theta wave activity was more localized in the frontal midline region. These waves developed paroxysmally in relatively brief bursts supplanting or intermixing with normal background EEG rhythms. In our subjects, frontal midline theta activity was associated mostly with some of the characteristic features of HPNS, such as a transient episode of laughter or euphoria at depths greater than 21 ATA. An intimate correlation between frontal midline theta wave and laughter was observed. Frontal midline theta waves may be related to emotional activities induced by helium under high pressure. There were significant individual variations in susceptibility and subjective signs and symptoms. The EEG is of great value in studying man's physiological reactions in an undersea environment and also very important in selecting divers who are relatively more tolerant of a severe hyperbaric environment.