130 S.A. Perera and T. Aung

It is logical that myopia exhibits its influence via eye size and lamina cribrosa biomechanical properties. The resulting changes in ONH biomechanics may play a role in retinal ganglion cell loss in glaucomatous optic neuropathy.74

Ocular blood flow, which has a role in the pathophysiology of glaucoma, also seems to be affected by the structural changes in myopia. Flow in the ophthalmic75 and retinal arteries76 in very high myopes with glaucoma has been found to be significantly reduced when compared with controls. This reduction may lead to an increased vulnerability to the effects of IOP.

The Influence of Myopia on the Clinical Management of the Glaucoma Patient

There are no current differences in the treatment of myopes with glaucoma compared to any other refractive error. Previously, when pilocarpine was commonplace, it was avoided in high myopes because of the risk of retinal detachment. Glaucomatous eyes with high myopia poses some additional problems, as the increased axial length, the thinner sclera, and larger intraocular volume predispose the eyes to choroidal effusions as the IOP rapidly drops post filtration surgery. The thinner sclera of myopes may collapse, leading to hypotony and shallow anterior chamber after both trabeculectomy and post-glaucoma drainage device surgeries.77 Deep sclerectomy has been reported to cause fewer of these complications with a more gradual alleviation of the high IOP, however, this must be balanced against other factors, such as the steep learning curve and the more modest IOP lowering.78

Glaucoma progression and myopia

POAG patients with myopia more severe than −6 D had a greater progression of visual field loss as revealed by logistic regression in one study. The incidence of VF loss progression was 15.1% in the group of eyes with myopia less severe than −3 D, 10.5% in the group with −3 D to −6 D, 34.4% in the group with −6 D to −9 D, and 38.9% in the group with myopia more severe than −9 D.79 This is supported by another study in Japan, which found only myopia worse than −4.0 as a major risk for progression in POAG.80 To place these findings in perspective though, it is more likely that other factors, such as elevated initial IOP, wide variations and poor control of IOP, late detection of glaucoma, and non-compliance with therapy, play a more important role in the development of blindness.81