What astronauts can teach us about glaucoma

New Orleans-Researchers and clinicians may not know what glaucoma is, or what causes cupping and field loss, but it is well accepted that IOP plays a role, said John Berdahl, MD. 

Speaking during the Innovators General Session at ASCRS 2016, Dr. Berdahl said glaucoma is much like the dark side of the moon.

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“We’ve only looked at one side of glaucoma (it being a one-pressure disease) much like we’ve only seen one side of the moon,” he said.

“It’s commonly believed that glaucoma is a pressure disease, and it’s likely true,” Dr. Berdahl said.

What remains unknown, however, is why IOP matters.

Why IOP matters

The “intra,” portion of the “IOP” term, however, is a misnomer, Dr. Berdahl said-noting clinicians should be referring to these pressure differences in relation to how they act-across two areas of the eye, not within them.

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Philosopher Herbert Spencer has been quoted as saying “how often misused words generate misleading thoughts,” and Dr. Berdahl said that is the case with IOP.

“Really, IOP is the transcorneal pressure difference,” he said. “This is important because it betrays how we think of IOP as the absolute pressure in the eye, but really it a measurement of how much higher the pressure is inside the eye compared to the pressure outside of the eye.”

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Atmospheric pressure in Denver is about 620 mm Hg, at sea level about 760 mm Hg, and 30 feet under water (average scuba dive) it is closer to 1,520 mm Hg, “and all those pressure differences affect the eye.”

But scuba divers don’t get glaucoma, he said.

Newton’s Third Law of Motion says equal and opposite forces cancel each other out, but “if one force is bigger than another (i.e., IOP is greater than cerebrospinal fluid, or CSF), then a net force is generated and movement (or cupping) could occur,” he said.

Intracranial pressure

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.

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“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.

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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.

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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.

Balance goggles

According to NASA, vision problems range from hyperopic shift and choroidal folds to globe flattening and papilledema.

Working with other ophthalmologists as part of the Vision to Mars program, Dr. Berdahl is developing “balance goggles” that might be able to address the VIIP concerns.

In essence, these goggles maintain a “regular” pressure when being worn, reducing the potential VIIP symptoms.

“In space without gravity, the CSF at eye level is higher, which explains why astronauts get VIIP,” he said.

A manned mission to Mars-which NASA hopes to achieve by 2030-will take about 400 days. With VIIP occurring in people who have spent 6 months at the space station, ocular health and vision is a key concern before sending astronauts off to Mars, he said.

Absolute IOP plummets in a vacuum to local atmospheric pressure, Dr. Berdahl said.

Only after the vacuum is removed does the pressure normalize, but MRI images of astronauts have clearly shown swelling around the optic nerve.

“When the first person steps on Mars it will be the defining moment of our generation,” he said, and added the progress with the Balance Goggles may alleviate at least one health concern.

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Dr. Berdahl reported a financial disclosure with Equinox.