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Editors Selection IGR 15-4

Miscellaneous: IOP and intracranial pressure in zero-gravity

Comment by John Liu on:

51279 The translaminar pressure gradient in sustained zero gravity, idiopathic intracranial hypertension, and glaucoma, Berdahl JP; Yu DY; Morgan WH, Medical Hypotheses, 2012; 79: 719-724

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Recent ophthalmic evaluations of American astronauts after their six month missions to the International Space Station (ISS) revealed unexpected visual abnormalities. Among several possible explanations, elevated intracranial pressure (ICP) due to head-ward body fluid shift under microgravity is considered the leading cause. This spaceflight-related problem was named visual impairment/intracranial pressure (VIIP) syndrome. Recent clinical observations also indicated that low ICP can play a role in glaucoma pathogenesis. Based on these complimentary discoveries, Berdahl, Yu, and Morgan have put forth a unified hypothesis that translaminar pressure difference at the optic nerve head, defined as IOP minus ICP, is critical for maintaining ocular structure and function. Since translaminar pressure difference is dynamic due to daily changes of body position, the VIIP syndrome and glaucoma must involve abnormally low or high translaminar pressure difference for an extensive period of time. While a high percentage of ISS onboard astronauts are developing visual abnormalities, there is no strategy to prevent the damage. Without a fundamental understanding of the VIIP syndrome, our future space exploration including a manned mission to Mars that requires spaceflight of a few years will be in jeopardy. Therefore, a timely evaluation of related hypotheses is essential. The current hypothesis can be evaluated in ground-based experiments using head-down body tilt to alter translaminar pressure difference. Anticipated new data from the ISS should also be helpful. Tonopen was recently delivered to the ISS and NASA will send a spectral-domain OCT in the near future. IOP data and ocular imaging including optic nerve head and retinal nerve fiber layer will be monitored. If the current hypothesis is correct, a physiological or pharmacological IOP increase to counter the VIIP syndrome is possible. In addition, spaceflight-related ICP information may benefit research in glaucoma for providing a new dimension of studying this challenging disease on earth.

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