Mention the word “glaucoma” and most people immediately think “high eye pressure.” But glaucoma is more of an umbrella term than a specific disease, and it covers multiple conditions with common traits. High eye pressure is just one of those traits. When doctors examine the eyes of a person with normal tension glaucoma, they see the same physical signs of optic nerve damage as they’d find in someone with primary open-angle glaucoma.
But when they test eye pressure, they’re surprised to learn it falls in a normal range. Normal tension glaucoma still causes vision loss over time and is treated using traditional, pressure-regulating methods. One day experts will find an answer to what causes normal tension glaucoma, but for now it remains a mystery. Some say it may be related to changes in blood vessels that inhibit blood flow to the optic nerve. Others believe that in some patients, eye pressure remains normal during the day, but increases at night. Another possible explanation for normal tension glaucoma has recently gained ground—one suggesting that it’s a two-pressure disease caused by differences between intraocular pressure and cerebrospinal fluid pressure.
How Do Pressure Differences Potentially Explain Normal Tension Glaucoma?
The theory of a two-pressure disease was created by Dr. John Berdahl, an ophthalmologist specializing in glaucoma surgery. He wondered whether normal tension glaucoma might be caused by abnormal pressure outside the eye, much like the pressure inside the body is so different from pressure placed on the body while deep diving. Under this scenario, intraocular pressure could remain normal, while the optic nerve was damaged by changes in external pressure.
Berdahl’s theory was further supported by eye problems commonly experienced by astronauts due to internal and external pressure differences. About six in ten astronauts develop optic nerve damage. Although, the astronauts get swollen optic nerves that bulge forward, presumably from increased intracranial pressure from cerebral spinal fluid not draining normally. Studies following this theory support it, showing that some people with glaucoma have normal pressure inside the eye, while pressure from cerebrospinal fluid is lower than normal.
This pressure difference—called the translaminar cribrosa pressure gradient—can affect the optic nerve, causing inflammation and nerve damage like more common forms of glaucoma. Researchers are still in the early stages of exploring how this information can be turned into new glaucoma treatments. First they need to develop a noninvasive way to measure cerebrospinal fluid pressure. Then they’ll work on medical therapies to equalize the pressure differences. Both steps demand dedicated effort maintained over a significant amount of time. Innovative research is being pursued by many experts who all have one goal: to find better treatments and eventually a cure.
First posted on March 30, 2017; Reviewed on June 16, 2022