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    What astronauts can teach us about glaucoma

     

    Intracranial pressure

    Intracranial pressure (ICP) also affects the optic nerve—when there is high IOP, there is low ICP, which is typical on Earth. But for those traveling into space, the opposite is true, he said

    Further, ICP decreases with age, with CSF pressure dropping from about 12 to 13 mm Hg at birth to barely 6 mm Hg once someone has reached their 90s. Vascular diseases have a rapid onset (think heart attack, he said), adding to his belief that glaucoma is not a vascular disease.

    Related: New glaucoma drug delivery devices home in on compliance

    “If we think of IOP as a transcorneal pressure difference, it’s a surrogate for the trans-laminar pressure difference,” Dr. Berdahl said.

    Cupping is caused by a posteriorly directed force generated by the translaminar pressure difference, and field loss occurs when ganglion cell death is caused by an inadequate axonal transport, he said.

    From ASCRS 2016: How cosmetic iris implants are associated with glaucoma

    And interestingly, as the intracranial pressure decreases with age, the likelihood of developing glaucoma increases.

    “So, we need to think of glaucoma as IOP minus the intracranial pressure, divided by the lamina cribosa difference and multiplied by time . . . and maybe some other aspects as well,” he said.

    In normal eyes, the absolute pressure difference between atmospheric and intraocular pressures (transcorneal pressure) is 16 mm Hg, and the difference between intraocular and intracranial pressures (translaminar pressure) is 4 mm Hg, he said.

    Related: Exploring balance of controlling inflammation, IOP in uveitis

    But in a glaucomatous eye, the transcorneal pressure differences rise to 22 mm Hg and the translaminar differences increase to 13 mm Hg.

    Treating glaucoma, however, results in a lower local atmospheric pressure, and lowered absolute intracranial pressure, dropping the translaminar differences to a normalized 3 mm Hg.

    So what does that mean for astronauts and others involved in space travel? NASA’s Vision Impairment and Intracranial Pressure (VIIP) project was started after NASA realized many astronauts return from space with vision problems, and these problems may last for years after mission completion.

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