Ocular dimensions in low tension glaucoma compared with open-angle glaucoma and the normal.

Pathological excavation of the optic disc in the apparent absence of raised ocular tension was first described by von Graefe (1857), and since then it has aroused interest among ophthalmologists; various names have been given to it and these have reflected ideas about its aetiology. Schnabel (I885) described a cavernous optic atrophy which he believed had resulted from progressive diminution of blood supply to the optic disc, a view supported by Wolff (I947). Sjogren (1946) considered that open-angle glaucoma and low tension glaucoma were one and the same condition. Lyle (I957) suggested that the terms pseudo-glaucoma and low tension glaucoma should be replaced by arterosclerotic optic atrophy. Duke-Elder (I949) has stated that low tension glaucoma tends to occur in myopic eyes. Perkins (1959) and Winstanley (1959) commented on a high incidence of myopia (36 per cent., ofwhom a quarter were > -4 D sph.) in cases of low tension glaucoma. This may indicate that large eyeballs are unduly prone to glaucomatous field defects. Tomlinson and Phillips (I969) reported a correlation between cup-to-(optic) disc ratio and eyeball axial length, large eyeballs tending to have a high cup-to-disc ratio, i.e. a relatively large optic cup. Armaly and Sayegh (I969) suggested that eyes with large optic cups (i.e. a high cupto-disc ratio) appear to be more susceptible to the development of visual field defects than those with small optic cups. It also seems likely that a large disc rather than a large cup is more susceptible to a raised intraocular pressure. This may explain the high prevalence of myopia in cases of low tension glaucoma. Armaly (I963) has shown that patients with open-angle glaucoma and low tension glaucoma share a similar ocular hypertensive response after 6 weeks' dexamethasone drops, suggesting that the two conditions are related. The purpose of this investigation was to find out whether the eye in low tension glaucoma differed in its major dimensions as well as in its applanation tension from either the normal eye or the eye in open-angle glaucoma.