Update on high altitude cerebral edema including recent work on the eye.

This review summarizes recent research on high altitude cerebral edema (HACE) and on the eye with focus on the retina and optic nerve as visible brain tissue at high altitude. Hemosiderin deposits in the corpus callosum have been characterized as rather specific long-lasting footprints of HACE, indicating a leak of the blood-brain barrier (BBB) and resulting in microhemorrhages. These are compatible with the concept of increased capillary pressure due to venous outflow limitation as suggested by Wilson et al. There are no human data on the role of vascular permeability in HACE, while animal models of uncertain relevance for human HACE suggest that an impaired integrity of the BBB through VEGF and ROS is more important than hemodynamic changes. Examinations by ultrasound show an inconsistent increase of the optic nerve sheath diameter, whereas unequivocal optic disc swelling (ODS), increased retinal vessel diameter, as well as retinal vessel leakage occur at high altitude. However, whether these morphological changes correlate with symptoms of AMS as a possible precursor of HACE or high altitude headache supporting the concept of venous outflow limitation remains questionable and is discussed in detail in this article.