External beam radiotherapy in the management of subfoveal choroidal neovascular membranes of the eye: a new treatment for an old disease.

The exudative form of Age-Related Macular Degeneration (ARMD) is associated with a particularly poor visual prognosis and accounts for 88% of ARMD sufferers who are registered blind.' In this disease, characteristic changes occur in the posterior pole of the eye within the confines of the macula which is the region of the retina responsible for central vision. When visual loss results from progressive degeneration of the retinal pigment epithelium and photoreceptors the condition is known as dry macular degeneration. In a significant proportion of eyes there is invasion of the sub-pigment epithelial and sub-retinal spaces by new blood vessels originating from the choroid. The infiltrating vessels are enmeshed in fibrous inflammatory tissue constituting a choroidal neovascular membrane (CNVM).2 The natural history of a CNVM is one of rapid expansion with increasing leakage of fluid and blood associated with progressive severe visual loss. The neovascular channels within the membrane eventually stop perfusing and involute with the development of a fibrous disciform scar. The morphogenesis of the scarring process destroys the retinal pigment epithelium and the photoreceptors and is incompatible with normal central visual function. Laser photocoagulation has been used for many years to ablate those choroidal neovascular membranes which are located outside the foveal avascular zone.3 Laser photocoagulation of the foveal avascular zone will of necessity destroy the foveal photoreceptors resulting in complete loss of the central 5° of visual field and an immediate fall in visual acuity. In 1991 the Macular Photocoagulation Study Group (MPS) showed that well-defined sub-foveal membranes no larger than 3.5 disc areas benefit from laser photocoagulation.4 In this study, patients treated with laser photocoagulation showed on average a 3.3 line fall in acuity from baseline while controls showed a 3.7 line drop at 12 months; a difference of 0.4 lines. With continuing follow up, at 18 months and at 24 months, treated patients had visual acuities which were one line or two lines of vision better than that which was found in the no treatment group.4 However as already stated, treatment destroys the fovea and there is an immediate marked fall in visual acuity (on average a fall of 3 lines on the Snellen chart).5 It is therefore questionable whether a long-term marginal visual benefit at the cost of an immediate significant deterioration in vision is worthwhile. Understandably alternative treatment modalities are being sought. It has been known for a long time that ionising radiation can …