Myopia has been found to be associated with an increased prevalence and progression (OHT to POAG) (POAG progression) of glaucomatous optic neuropathy. The Blue Mountain Eye Study revealed that patients with moderate to high myopia (equal or greater than -3.00 Diopters) were over 3 times more likely to have visual field loss and optic disc cupping that was characteristic of glaucoma. Although intra-ocular pressure (IOP) was found to be on average slightly higher in myopes (0.5mmHg) this was not statistically significant.
The Beijing Eye Study similarly revealed that Chinese high myopes (greater than -6.00 Diopters) were over 7 times more likely to have glaucomatous appearing visual field loss and optic discs (compared to emmetropes -0.5 to +2Diopters). In this study, IOP was not found to be associated with myopia. Importantly, neither of these two studies however looked at progression.
There seemed to be clues that myopia associated glaucomatous optic neuropathy was pressure independent.
The Malmo Eye Survey done in Sweden showed that the association between myopia and glaucoma was greatest at low IOP and is perhaps a form of Normal Tension Glaucoma (NTG).
The big question is: “What is the target pressure for these high myopes who present with a glaucomatous appearing optic neuropathy?”
The answer seems to depend on whether or not you are Chinese!
In the Collaborative NTG Study Group, Asians with NTG were discovered to have a very low rate of progression compared with Caucasians and Blacks. The refractive differences between the racial groups was not discussed in this study.
In a recent publication from San Francisco, ophthalmologists discovered an interesting pattern of stability within a specific demographic subset of their glaucoma suspect patients. These 16 patients were young Chinese males, most with myopia and tilted optic discs, and were followed without progression over an average of 7 years. It is believed that they may have developed this glaucomatous appearing visual field loss during the growth phase of their myopia. After the end of this axial growth, their risk of further progression would therefore be expected to be low.
We must remain cautious however in assuming that these patterns are similar in other Asian populations. A study done of a Japanese population over 10 years ago showed that severe myopia (greater or equal to -4.00 Diopters) was associated with a higher risk (1.5 OR) of progressive field loss.
Myopia and associated eye problems:
High myopia can be defined as near sightedness requiring stronger than -6.00 Diopters of prescription glasses to see clearly at distance. Most people with high myopia develop the condition as a result of excessive eye growth during their teens and early twenties (axial myopia). Why young Asians are more likely to have this excessive growth is a long discussion and is still being researched. What is alarming however is that besides the obvious need to wear thicker glasses, there are more serious long term ramifications for our eyes, including a higher incidence of cataracts, retinal detachments, macular thinning, retinal degenerations and optic nerve damage.
What is the optic nerve and optic disc and how does the disc become tilted?
The optic nerve can be thought of as the cable that connects the the eye to the brain. The optic disc is the 1.5mm opening where the nerves of the eye exit to the optic nerve, and can be seen by the ophthalmologist from the inside of the eye by looking through the pupil. Ophthalmologists evaluate the optic disc appearance and thickness of the nerve fiber layer to assess for damage to the optic nerve, such as that caused by glaucoma. Tilted optic discs can be found in high myopes, or near sighted people with glasses prescriptions greater that -6.00 Diopters. The optic nerve normally exits the eye at a slight angle, but in eyes that are especially long, it is as if the nerve is exiting from the side of the eye. When looking at the optic disc from the perspective of the pupil, the disc appears tilted.