Dick Land
The Schepens Eye Research Institute
LAND@VISION.ERI.harvard.edu
The human eye is a remarkable organ. It is essentially a sphere, moved by three pairs of muscles, and maintained at an internal pressure of 10-20 mm Hg above atmospheric pressure. The front of the eye is bathed in a tear film vital to the health of the cornea; the transparent external surface at the front, and the major refractive element.
There are many precautions one should take to protect the eye. In particular regular eye exams will reveal problems that may develop. Diabetes, Macular Degeneration, Cataracts, and Glaucoma are recognized as the major threats to eye health, other than accidents. In particular, Glaucoma is the elevation of the interior pressure of the eye (intraocular pressure) that may be regularly monitored by a 'puff' test, or directly by applying a pressure device to the cornea. Concern begins when the pressure is raised above 20 mm Hg. There are a variety of treatments to restore the balance of fluid influx and drainage to restore normal pressures. In general young people need not be concerned, and older people are at risk, where people after 60 years of age note slowly increasing intraocular pressure.
The major concern about increased intraocular pressure is damage to the optic nerves where the broad array across the retina comes to the 'optic disc' and makes a sharp bend to leave the eye ball. The bend normally is a risk point, but with increased pressure in the eye, the angle is made more sharp and nerve damage results. Such damage, as with all central nervous system tissue, is not recoverable. Thus the danger from Glaucoma is vision loss as the nerve fibers leaving the eye are pinched at the disc edge.
Most are familiar with closing the eye lids and pressing on the eye to see 'phosphenes', the constructs of our cortex in response to signals from the retina, but stimulated by pressure not light. Such pressure, if from a blow to the head, produces the 'seeing stars' effect commonly referred to. However, it is not recommended that this be done often or with any significant pressure. Clearly applying sustained pressure on the eye is the same as the risk from Glaucoma, and often an exterior pressure is greatly in excess of that which might be internally generated. Non repairable damage is likely to result from any significant increase of pressure on the eye. The rate of damage and extent of vision loss will be governed by eye health and age for the most part. When young the eye is resilient and pressures are absorbed more easily in other structures. With age tissues become more rigid, there is less natural protection, and the risk is increased.
It is for these reasons that any situation that brings pressure on the eye, especially for extended periods, be avoided.