32

(Aung et al., 2001; Foster et al., 2002). Primary angle closure (PAC) is present when there are features indicating that trabecular meshwork obstruction by the peripheral iris has occurred with consequences in the eye such as peripheral anterior synechiae (PAS), increased intraocular pressure (IOP), iris whorling, glaucomfleken, lens opacities, or excessive pigment deposition on the trabecular meshwork. At this stage, the optic disc does not have signs of glaucomatous damage. PACG is PAC with evidence of glaucomatous optic neuropathy (GON). Table 1 summarizes the definitions.

A patient with acute primary angle closure (APAC) usually has the following symptoms: ocular or periocular pain, nausea and/or vomiting, a history of intermittent blurring of vision with haloes, IOP W 21 mmHg: and the following signs: conjunctival injection, corneal epithelial edema, mid-dilated unreactive pupil, shallow anterior chamber, and the presence of an occludable angle.

Mechanism

The mechanism responsible for angle closure is also important, especially in planning clinical management. Pupil-block, anterior nonpupil-block (plateau iris and peripheral iris crowding), lens related and retrolenticular mechanisms have been suggested as the four main mechanisms of angle closure, though they may coexist. When assessing a case of angle closure both staging and mechanism should be taken into account (Ritch and Lowe, 1996a).

Ritch et al. described the mechanisms of angle closure resulting in iris blocking aqueous outflow

Table 1. Classification of primary angle closure (PAC)

through the trabecular meshwork. This is caused by forces acting at four anatomic levels: the iris, ciliary body, the lens, and vectors posterior to the lens. It should be noted that each level of the block may have a component of the preceding levels and a combination of mechanisms may coexist in the same patient. Therefore, treatment can become more complex for each level of the block as the lower levels of block may also require treatment (Ritch and Lowe, 1996a).

Level 1 — iris and pupil

Pupillary block is the most common mechanism of angle closure (Nolan et al., 2000), the majority of other causes of angle closure will have an element of pupil block. In East Asians, the mechanism is predominantly mixed (He et al., 2006). In pupillary block, there is resistance to aqueous flow through the pupil in the area of iridolenticular contact. This causes a limitation of aqueous flow from the nonpigmented ciliary epithelium (where it is produced) in the posterior chamber to the anterior chamber. This creates an increased pressure gradient between the anterior and posterior chambers causing anterior bowing of the iris, narrowing of the angle, and acute or chronic iridotrabecular contact. Usually the anterior segment structures appear normal; however, occasionally, there may abnormalities of the iris architecture (thickness, orientation, muscle tone) that may be contributing factors. Laser iridotomy relieves the pressure difference between the anterior and posterior chambers. This reduces the iris convexity, the iris