Acceleration-induced near-loss of consciousness: the "A-LOC" syndrome

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.

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.

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.

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.