Open-Angle, Angle-Closure, Primary, and Secondary Glaucomas
Adult forms of glaucoma are classified as open angle or angle closure and as primary or secondary (Table 1-1). Pediatric forms of glaucoma are described in Chapter 11. Distinguishing open-angle glaucoma from angle-closure disease is essential from a therapeutic standpoint.
In open-angle glaucoma (OAG), no obstruction of the trabecular meshwork is visible on gonioscopic examination of the anterior chamber angle. The condition is further classified as primary open-angle glaucoma (POAG) when no underlying abnormality known to cause IOP elevation is seen on clinical examination; and IOP elevation, if present, cannot be attributed to the use of corticosteroids. OAG is classified as secondary when an abnormality is identified that has a putative role in the pathogenesis of the glaucoma.
Table 1-1 Classification of Glaucoma and Related Conditions
In eyes that have POAG with elevated IOP, the etiology of the outflow obstruction is thought to be an abnormality in the extracellular matrix of the trabecular meshwork and in trabecular cells in the juxtacanalicular region or an abnormality in the function of the endothelial cells lining the inner wall of Schlemm canal (see Chapter 2). The term normal-tension glaucoma is often used for eyes with POAG without known IOP elevation. The conceptual basis for this distinction and for the terminology itself is controversial and is discussed in greater detail in Chapter 7.
In angle closure, the peripheral iris partially or completely obstructs the trabecular meshwork. The obstruction may be caused either by appositional iridotrabecular contact or by adhesions, known as peripheral anterior synechiae (PAS), between the iris and the trabecular meshwork. When angle closure is present in association with glaucomatous optic neuropathy, it is known as angle-closure glaucoma. Angle closure is classified as primary in the absence of an underlying disorder to explain the mechanism of iridotrabecular contact or secondary when the angle closure can be attributed to certain disease processes (see Table 1-1). Globally, primary angle closure is a major public health problem.
Normal aqueous humor flow in the anterior segment is illustrated in Figure 1-2A. In primary angle closure (and in some forms of secondary angle closure), the flow of aqueous humor from the posterior to the anterior chamber is obstructed at the pupil (Fig 1-2B). The resulting pressure gradient pushes the peripheral iris forward into the anterior chamber angle. This is known as pupillary block angle closure.
In some forms of secondary angle closure and in plateau iris, the peripheral iris or the entire lens–iris interface is pushed forward, narrowing the iridocorneal angle (see Chapter 9 in this volume). This can result from an abnormality of the ciliary body, posterior segment tumors, hemorrhage, or other causes described in Chapter 10. In other forms of secondary angle closure, the peripheral iris is pulled forward, typically by contraction of a cellular, fibrovascular, or inflammatory membrane. These conditions are called non–pupillary block angle closure.
Figure 1-2 Aqueous humor flow. A, Normal flow of aqueous humor from the posterior chamber, through the pupil, and into the anterior chamber. Aqueous humor exits the eye through 2 pathways in the iridocorneal angle: the trabecular meshwork and the uveoscleral pathway. B, In primary angle closure due to pupillary block, the flow of aqueous through the pupil is obstructed, resulting in a positive pressure gradient between the posterior and anterior chambers, anterior displacement of the peripheral iris, and closure of the anterior chamber angle.
(Illustration by Mark Miller.)
Combined-mechanism glaucoma refers to the condition in which an eye with glaucomatous optic neuropathy that has undergone successful treatment for angle closure with either laser iridotomy or removal of the crystalline lens continues to demonstrate reduced outflow facility and elevated IOP in the absence of PAS. This term can also be used when secondary causes of glaucoma play a role in the disease process in an eye with previously diagnosed POAG or primary angle-closure glaucoma, for example, when a patient with POAG develops a central retinal vein occlusion and subsequent neovascular glaucoma. IOP elevation associated with uveitis may occur due to a combination of mechanisms, including inflammation of the trabecular meshwork, increased viscosity of aqueous humor due to the presence of cells and protein, corticosteroid use, and the presence of PAS.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.