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


CONCLUSIONS
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.
Shender
Barry S
BS
Drexel University, Philadelphia, PA, USA. ShenderBS@navair.navy.mil
Forster
Estrella M
EM
Hrebien
Leonid
L
Ryoo
Han Chool
HC
Cammarota
Joseph P
JP
D000328:Adult; D000337:Aerospace Medicine; D001923:Brain Chemistry; D003072:Cognition Disorders / Q000209:etiology; D004562:Electrocardiography; D004644:Emotions; D005260:Female; D006801:Humans; D018471:Hypergravity / Q000009:adverse effects*; D008297:Male; D008889:Military Personnel; D010100:Oxygen / Q000032:analysis; D010465:Perception; D013577:Syndrome; D014474:Unconsciousness / Q000503:physiopathology*
D010100:Oxygen