What’s clear is that glaucoma begins with injury to the optic nerve as it exits the back of the eye. The damage then spreads, moving from one nerve cell to adjoining nerve cells. “In glaucoma, we’ve shown that when your retinal ganglion cells are sick, the long axons that project from the eye into the brain are also affected, resulting in changes that we can detect in the vision center of the brain,” Dr. Gupta said. The phenomenon, called transynaptic damage, occurs in Alzheimer’s and Parkinson’s disease as well.
Experts are still deciphering what causes initial injury to the optic nerve. Although elevated intraocular pressure clearly increases the danger, some researchers suspect that steep fluctuations in pressure may be even more damaging.
“A structure in the optic nerve called the lamina cribosa is designed to act like a trampoline, going up and down in response to normal changing pressure,” said Dr. Rohit Varma, director of the glaucoma service at Keck School of Medicine at the University of Southern California. “But if those fluctuations become extreme enough, they may end up injuring the optic nerve.”
Another culprit may be perfusion pressure, or the difference between pressure within the eye and overall blood pressure. Low perfusion pressure occurs when pressure within the eye is high and systemic blood pressure is low. “When perfusion pressure drops, there’s not enough blood flow getting to the optic nerve and the retina,” Dr. Varma said. Lack of adequate blood flow may damage not only the optic nerve but also supporting tissues around it.
Then again, some people may have optic nerves that are simply more or less vulnerable to a variety of stresses, experts say.