New insights into ocular blood flow at very high altitudes

1 Department of Ophthalmology, University Hospital Zurich, Zurich; 2 Department of Intensive Care Medicine, Inselspital, Bern University Hospital and University of Bern, Bern; 3 Institut de Recherche en Ophtalmologie, Sion; 4 Pulmonary Division, University Hospital Zurich, Zurich; 5 Medical Intensive Care Unit, University Hospital Zurich, Zurich; 6 Department of Internal Medicine, State Hospital Winterthur, Winterthur; 7 Department of Internal Medicine, State Hospital St. Gallen, St. Gallen; 8 Department of Surgery, State Hospital Liestal, Liestal; 9 Department of Laboratory Medicine, State Hospital Aarau, Aarau; and 10 Center for Integrative Human Physiology, University of Zurich, Zurich, Switzerland Submitted 14 July 2008 ; accepted in final form 3 December 2008 Little is known about the ocular and cerebral blood flow during exposure to increasingly hypoxic conditions at high altitudes. There is evidence that an increase in cerebral blood flow resulting from altered autoregulation constitutes a risk factor for acute mountain sickness (AMS) and high-altitude cerebral edema (HACE) by leading to capillary overperfusion and vasogenic cerebral edema. The retina represents the only part of the central nervous system where capillary blood flow is visible and can be measured by noninvasive means. In this study we aimed to gain insights into retinal and choroidal autoregulatory properties during hypoxia and to correlate circulatory changes to symptoms of AMS and clinical signs of HACE. This observational study was performed within the scope of a high-altitude medical research expedition to Mount Muztagh Ata (7,546 m). Twenty seven participants underwent general and ophthalmic examinations up to a maximal height of 6,800 m. Examinations included fundus photography and measurements of retinal and choroidal blood flow, as well as measurement of arterial oxygen saturation and hematocrit. The initial increase in retinal blood velocity was followed by a decrease despite further ascent, whereas choroidal flow increase occurred later, at even higher altitudes. The sum of all adaptational mechanisms resulted in a stable oxygen delivery to the retina and the choroid. Parameters reflecting the retinal circulation and optic disc swelling correlated well with the occurrence of AMS-related symptoms. We demonstrate that sojourns at high altitudes trigger distinct behavior of retinal and choroidal blood flow. Increase in retinal but not in choroidal blood flow correlated with th