Myopia and Glaucoma

I read with great interest the excellent article by K. Grodum, A. Heijl and B. Bengtsson (Acta Ophthalmol Scand 2001; 79 : 560–566), together with Stephen Drance's editorial in the same issue, on the well known and very perplexing association of myopia with glaucoma. Albrecht von Graefe already distinguished three forms of glaucoma: 1. Acute; 2. Chronic; and 3.′Amaurosis with excavation of the optic nerve′, with visual field loss and cupping but ‘absence of all other symptoms indicative of increased pressure’ (Graefe 1857). A more recent comprehensive review, and the relevant literature, may be found in standard textbooks (Duke-Elder 1970). One obstacle to clarity in the matter seems to be the lack of a common language, in particular standard physical units of measurement. The refractive power of the eye is an optical phenomenon measured in diopters (D), and refractive errors arise from discordance between this refractive power and the length of the globe measured in millimeters (mm). It is yet to be shown that the position of the optical image in front or behind the retina is causative or a risk factor in glaucomatous damage. On the other hand it is well established that the size of the globe, manifested by its axial length, plays a determining role in the disease, particularly the acute variety (Lowe 1974) where ‘Angle-closure is an anatomic disorder’(Ritch & Lowe 1996). and as Priestley Smith (Smith 1891), and Elliot after him, said (Elliot 1918) ‘small eyes are not necessarily hyperopic and hyperopic eyes are not necessarily small. After methods to measure axial length were invented, an association to tonometric values (IOP) came to light, that is, the longer the eye the lower the measured IOP. The relation was based first on the purely physical principles involved (pressure versus volume) (Phillips & Quick 1960; Sampson & Girard 1961; Szczudlowski 1979; Cahane & Bartov 1992) and secondly on clinical studies (Weekers & Grieten 1964; Leighton & Tomlison 1973; Leighton 1974; Mark et al. 2002). But although we now have the means to measure axial length, and are cognizant of its clinical import, we lack a proper name for long or short globes. ‘As the art of medicine expands it is determining such vast changes in our thinking that aconstant need for new means of expression is inevitable’ (Crookes 1981). Hyperopia is a risk factor for angle-closure glaucoma, but it is not the refractor or the lensometer which will determine it, but the A-scan. I therefore suggested that we employ the term brachophthalmos (brachophthalmia) to denote a short globe, similar to other shortened organs such as brachycephalos (short head) or brachydont (short tooth). A long globe may be termed acrophthalmos (acrophthalmia), corresponding to acrocephalos (long or pointed head) or acromphalos (protruding long naval). Since myopia may be without ophthalmoscopic changes, such as by increased refractive power in nuclear sclerosis, it would seem fitting, for example, to talk about acrophthalmic retinal degeneration rather than myopic. The distinction between a long, acrophthalmic, globe and macrophthalmos, megalophthalmos or buphthalmos, which denote a totally large globe, is of real significance, for the following reasons. Of all three-dimensional geometric forms the sphere contains the maximum volume under the minimum surface area; when the pressure inside any form, be it a pyramid, cube or ellipsoid, is increased, it will tend to become spherical, rounded. It is easier to increase the surface area, for instance by indentation or applanation, of an ellipsoid (egg-shaped) globe than one that is already spherical, because there is room for expansion. The tendency of an ellipsoid globe to become spherical with increase in iop (intrinsic or extrinsic) will manifest itself as a stretch of the posterior wall, as exaggeratedly illustrated in Fig. 1.. This mechanism may explain the temporal crescent typically seen in myopia (Curtin & Karlin 1971), and perhaps contributes to the pathogenesis of the glaucomatous cup with its attendant field loss in acrophthalmos. Routine measurement of axial length, especially in eyes suspected of glaucoma, is apt to refine our diagnostic accuracy (Mark 1998).