Authors: Shender, Barry S.; Forster, Estrella M.; Hrebien, Leonid; Ryoo, Han Chool; Cammarota, Joseph P.

Source: Aviation, Space, and Environmental Medicine, Volume 74, Number 10, October 2003 , pp. 1021-1028(8)

Publisher: Aerospace Medical Association

Abstract:

Shender BS, Forster EM, Hrebien L, Ryoo HC, Cammarota JP Jr. Acceleration-induced near-loss of consciousness: the “A-LOC” syndrome. Aviat Space Environ Med 2003; 74:1021-8.

Background: There is an insidious phenomenon that can occur when aircrew are exposed to +Gz stress even at levels that are insufficient to cause +Gz-induced loss of consciousness (G-LOC). Under these circumstances aircrew exhibit an altered state of awareness that was termed Almost Loss of Consciousness (A-LOC) by the U.S. Navy in the late 1980's. A-LOC is a syndrome that includes a wide variety of cognitive, physical, emotional, and physiological symptoms. While A-LOC has been observed in centrifuge studies and reported in flight for over 15 yr, a definitive description of the syndrome does not exist. Methods: Nine subjects were exposed to short +6, 8, and 10 Gz pulses of increasing duration until they experienced G-LOC. Instrumentation included two channels of ECG and near infrared spectroscopy (NIRS) to measure relative cerebral tissue oxygenation (rSO₂). Subjects indicated +Gz-induced visual symptoms (light loss, LL) by pressing a switch when LL began and releasing it when total vision was restored. Short-term memory loss was assessed using a simple math task. Data analysis included a description and the time course of the physical, physiological, cognitive, and emotional responses. Results: There were 66 episodes of A-LOC that were identified out of a total of 161 +Gz pulse exposures. Many incidents of sensory abnormalities, amnesia, confusion, euphoria, difficulty in forming words, and reduced auditory acuity were documented. Often these responses occurred in multiple subjects and at different +Gz levels. One of the most common symptoms was a disconnection between cognition and the ability to act on it. There was a significant reduction in rSO₂ over baseline, greater overshoot in rSO₂ (increase in oxygenation above baseline after the +Gz exposure), faster fall in rSO₂ during +Gz stress, and prolonged recovery time associated with A-LOC as compared with +Gz exposures without symptoms. Conclusion: Evaluation of the range of symptoms associated with A-LOC can lead to a program to increase pilots' awareness of the phenomenon and further our understanding of the relationship between the outward symptoms and the underlying physiological changes.

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Keywords: acceleration; A-LOC; loss of consciousness; near infrared spectroscopy; amnesia; confusion; muscle twitching; N-LOC

Document Type: Research article

Publication date: 2003-10-01

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