Figure 1-1 Diagrammatic cross section of the anterior segment of the normal eye, showing the site of aqueous production (ciliary body), sites of conventional aqueous outflow (trabecular meshwork–Schlemm canal system and episcleral venous plexus), and the uveoscleral outflow pathway (green arrow). Small white arrow shows normal path of outflow and indicates that resistance in this illustration is relative, not total. (Illustration by Cyndie C. H. Wooley.)
In most individuals with glaucoma, the optic nerve and visual field changes seen with this disease are determined by both the level of the IOP and the resistance of the optic nerve to damage. Although progressive changes in the visual field and optic nerve are often related to elevated IOP, in some glaucoma patients the IOP remains within statistically normal range (see Chapter 4). However, when considering whether glaucomatous damage is truly occurring in a patient with “normal” IOP, the ophthalmologist should take into account the measurement artifact that is caused by variation in central corneal thickness and that occurs with diurnal variation in IOP. In most cases of glaucoma, it is presumed that the IOP is too high for proper functioning of the optic nerve axons and that lowering the IOP will stabilize the damage. In cases involving other pathophysiologic mechanisms that may affect the optic nerve, however, the optic nerve may continue to be damaged despite lowering the IOP.
Preperimetric glaucoma is a term that is sometimes used to denote glaucomatous changes in the optic disc in patients with normal visual fields, as determined by white-on-white perimetry. Accurate diagnosis of this condition depends on the sensitivity of the visual function test that is used. Thus, the development of new, more sensitive tests may allow earlier confirmation of this form of glaucoma, while the patient is within this preperimetric phase.
Classification
Open-Angle, Angle-Closure, Primary, and Secondary Glaucomas
Traditionally, glaucoma has been classified as open angle or closed angle and as primary or secondary (Table 1-1). Differentiating open-angle glaucoma from closed-angle glaucoma is essential from a therapeutic standpoint (Figs 1-2, 1-3), and each type of glaucoma is discussed in detail in Chapters 4 and 5. The concept of primary and secondary glaucomas is also useful, but it reflects our lack of understanding of the pathophysiologic mechanisms underlying the glaucomatous process. There are separate anatomical, gonioscopic, biochemical, molecular, and genetic views of the classification of the glaucomas, among others, each with its own merit. Traditionally, open-angle glaucoma is classified as primary when there is no identifiable underlying anatomical cause of the events that led to obstruction of aqueous outflow and subsequent elevation of IOP (see Fig 1-2). The etiology of the outflow obstruction is generally thought to be an abnormality in the extracellular matrix of the trabecular meshwork and in trabecular cells in the juxtacanalicular region, although other views exist. Trabecular cells and their surrounding extracellular matrix are understood in fairly specific terms, and the basic scientific understanding of the outflow structures is constantly increasing. Glaucoma is traditionally classified as secondary when an abnormality is identified and a putative role in the pathogenesis can be ascribed to this abnormality. As knowledge of the mechanisms underlying the causes of glaucoma has grown, the primary/secondary classification has become increasingly artificial.
Other schemes for classifying glaucoma have been proposed. Classification of the glaucomas based on initial events and classification based on mechanisms of outflow obstruction are 2 schemes that have gained increasing popularity (Table 1-2).
Ritch R, Shields MB, Krupin T, eds. The Glaucomas. 2nd ed. St Louis: Mosby; 1996:722.
Figure 1-2 Schematic of open-angle glaucoma with resistance to aqueous outflow through the trabecular meshwork– Schlemm canal system in the absence of gross anatomical obstruction. (Illustration by Cyndie C. H. Wooley.)
Figure 1-3 Schematic of angle-closure glaucoma with pupillary block leading to peripheral iris obstruction of the trabecular
meshwork. (Illustration by Cyndie C. H. Wooley.)
Table 1-1