Delivered by Ingenta to: Drexel University IP : 129.25.24.49 Fri, 22 Jun 2007 18:37:15 RESEARCH ARTICLE Acceleration-Induced Near-Loss of Consciousness: The “A-LOC” Syndrome Barry S. Shender, Estrella M. Forster, Leonid Hrebien, Han Chool Ryoo, and Joseph P. Cammarota, Jr. 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 cir- cumstances 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 (rSO 2 ). 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, physiologi- cal, cognitive, and emotional responses. Results: There were 66 epi- sodes of A-LOC that were identified out of a total of 161 +Gz pulse exposures. Many incidents of sensory abnormalities, amnesia, confu- sion, 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 rSO 2 over baseline, greater overshoot in rSO 2 (increase in oxygenation above baseline after the +Gz exposure), faster fall in rSO 2 during +Gz stress, and prolonged recovery time associated with A-LOC as compared with +Gz exposures without symptoms. Con- clusion: 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. Keywords: acceleration, A-LOC, loss of consciousness, near infrared spectroscopy, amnesia, confusion, muscle twitching, N-LOC. W HEN AIRCREW of high-performance aircraft are exposed to acceleration (+Gz) stress, a spec- trum of symptoms can occur ranging from loss of peripheral vision (light loss, LL) to blackout to +Gz- induced loss of consciousness (G-LOC). The mecha- nisms underlying these events are poorly under- stood. The focus of +Gz research over the years has been on the cardiovascular effects of acceleration, i.e., a shift of blood supply away from the head to the extremities. Whinnery (16) proposed a neurologic mechanism that may be responsible for the contin- uum of events that lead to G-LOC. He postulated that G-LOC may not be the result of injury to the central nervous system, but rather is a protective mechanism that maximizes the survivability of neurons deprived of adequate energy supplies. A mathematical model based on this theory was constructed by Cammarota (2), and was able to duplicate the loss of conscious- ness induction time (6.25 s) as measured both by Rossen et al. (10), after acute arrest of cerebral circu- lation in man, and by Beckman et al. (1), who studied G-LOC produced by rapid onset high +Gz pulses. Beckman et al. found that a high +Gz pulse of only 4.25 s duration was sufficient to cause G-LOC, which is an apparent contradiction of the acute arrest study. The model was able to resolve this discrepancy based on another study conducted by Cammarota et al. (3,4) duplicating the +Gz profiles used by Beckman et al., but with additional measurements, including visual symptoms, recovery time, cognitive function, and ce- rebral oxygenation. This study explored the tolerance of the cephalic nervous system to loss of blood flow, which upsets the balance between oxygen supply and demand. Once this balance is disturbed beyond crit- icality, nothing can prevent the subsequent loss of consciousness. This was clearly seen by Cammarota in the incidents of G-LOC that occurred after expo- sure to a high +Gz pulse was completed and the centrifuge had returned to a rest plateau. Even if the stress is insufficient to cause G-LOC, deficits in motor and cognitive function can still occur. The intent of this paper is to focus on those events in the consciousness continuum which may precede G-LOC but have not been studied in any systematic fashion. From the Naval Air Systems Command, Patuxent River, MD, (B. S. Shender, E. M. Forster); Drexel University, Philadelphia, PA (L. Hre- bien, H.C. Ryoo); EDO M. Tech, Huntingdon Valley, PA (J. P. Cam- marota, Jr.). This manuscript was received for review in October 2002. It was revised in March and May 2003. It was accepted for publication in May 2003. Address reprint requests to: Correspondence and address reprint requests to: Barry S. Shender, Ph.D., who is a Naval Air Systems Command Fellow, South Engineering Center/Crew Systems Dept., NAWCAD, BLDG 2187 SUITE 2280, 48110 Shaw Road UNIT 5, Patux- ent River, MD 20670-1906; ShenderBS@navair.navy.mil Reprint & Copyright © by Aerospace Medical Association, Alexan- dria, VA. 1021 Aviation, Space, and Environmental Medicine • Vol. 74, No. 10 • October 2003