Update on High Altitude Cerebral Edema Including Recent Work on the Eye

Abstract

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.

Acute high-altitude sickness

“…Most MRI studies in AMS found volume changes of <10 mL between 6 and 16 h of hypoxia with no correlation to AMS symptom scores [43]. These studies could not distinguish between oedema and changes in blood volume, but most of them found decreased water mobility compatible with intracellular swelling associated with AMS.…”

High Altitude Retinal Hemorrhage

“…This is thought to be the producer of tissue edema causing optic disc edema as the swollen of the disc in HACE is not related to elevation of the intracranial pressure (ICP). Hypoxia may be the cause of vessel filtration in the periphery of the retina [11]. Also this may be the causative agent of damage of the wall cells of the vessels leading to the production of hemorrhages [11].…”

“…Hypoxia may be the cause of vessel filtration in the periphery of the retina [11]. Also this may be the causative agent of damage of the wall cells of the vessels leading to the production of hemorrhages [11]. Daniel Barthelmes et al mentioned that higher hematocrit, may contribute to the damage of the endothelial cells [12].…”

Acute high-altitude illness

“…Research into the use of point-ofcare ultrasound scans of the lung and optic nerve sheath diameter is a promising new modality for screening, prediction and early diagnosis of HAPE and HACE respectively, but is still at the experimental stage. [25,26] …”