CHOCO ENCUENTRA UNA MAMÁ

The final common pathway in the development of primary angle-closure is the formation of irreversible synechial adhesions between the peripheral iris and the uveal surface of the trabecular meshwork. In most cases, this is probably

preceded by intermittent appositional contact, although the natural history is poorly understood.

1.7.4.1 Pupil-block

This is the underlying mechanism in the majority of cases of primary angle- closure, at least in Europeans. There is believed to be variable resistance to aqueous flow from posterior to anterior chamber that is accentuated in certain circumstances. The steeply convex iris seen in cases of pupil block suggests a substantial pressure differential exists between anterior and posterior chambers. In such patients with narrow drainage angles, iridotomy will result in a marked flattening of iris contour^ This report emphasised that the axial depth of the anterior chamber did not change.

Vector models of the forces acting on the pupil margin have been derived from anterior segment photographs. These suggest that pupil-blocking force is related to pupil size and the angle of the iris to the plane of the pupil One of the cardinal clinical signs of primary angle-closure is an unreactive, mid-dilated pupil. The pharmacological provocation test (pilocarpine-phenylephrine test, see section

1.7.5.2) induces co-contraction of sphincter and dilator muscles, and may precipitate a rise in pressure from angle-closure in some individuals. It seems likely that co-contraction of dilator and sphincter muscles, giving a small resultant force perpendicular to the lens surface, may be one of the mechanisms inducing

As aqueous production continues when relative pupil block is present, posterior chamber pressure rises. The peripheral iris is bowed forward, coming into contact with trabecular meshwork. This may then result in partial or complete obstruction of aqueous outflow by the trabecular route.

There is some evidence that individuals with PAC may have a generalised

autonomic dysfunction, in some cases possibly due to diabetes offering one possible explanation for poor co-ordination of iris muscle function. This is an attractive explanation for the catastrophic pressure rise which is characteristic of the acute, symptomatic form of PAC. However, it is not entirely satisfactory when considering “relative” pupil block in the majority of cases with the chronic,

asymptomatic disease.

1.7.4.2 Non pupil-block mechanisms

This category accounts for almost all other cases of primary angle-closure, given our current understanding of the condition. In contradistinction to pupil block PAC, it is probably a purely anatomical abnormality that may be divided into two

categories.

1.7.4.2.1 Peripheral iris crowding

In eyes with a low-volume anterior chamber, the iris occupies a larger proportion of the available space. This is even more pronounced in eyes with dark brown, thick

irises. As the pupil dilates, the iris may be thrown into circumferential folds that come into contact with trabecular meshwork. These eyes typically have a relatively anterior iris insertion (for example- at the level of the scleral spur). These characteristics have previously been described as a “prominent last iris roll”. It is difficult to exclude a component of pupil-block in the mechanism of cases such as this.

1.7.4.2.2 Plateau iris

Plateau iris configuration is the term given to the gonioscopic appearance of the iris root rising from the anterior surface of the ciliary body and then angulating sharply away from the corneo-scleral coat toward the visual axis. Typically these eyes have a relatively deep central anterior chamber. Lowe suggested that this

appearance was attributable to crowding of the cilio-lenticular space resulting from the combination of a thick lens that was positioned more anteriorly than usual. He believed this would cause the ciliary body to be rotated anteriorly, narrowing the drainage angle The advent of ultrasound biomicroscopy has confirmed this supposition. The peripheral iris is draped over anteriorly-rotated ciliary processes, causing the angulated iris configuration The cilio-lenticular space is often very narrow in these cases. Varying degrees of plateau configuration are seen in the population, both in terms of the height of the plateau, and the point of insertion of the iris In cases where the iris inserts into the apex of the ciliary body, a wide “gutter” can be seen between face of the trabecular meshwork and the peripheral iris.

The term plateau iris syndrome refers to cases of primary angle-closure with a plateau iris configuration that have a patent iridotomy but subsequently suffer spontaneous or mydriatic-induced rises in intraocular pressure associated with gonioscopically-confirmed angle-closure.

1.7.4.3 Cilio-lenticular block

The dynamics of the cilio-lenticular space are not well understood. Cases of primary cilio-lenticular block are rare, and are believed to occur as a result of mis­ direction of aqueous into the posterior segment. This probably occurs a

consequence of a crowded cilio-lenticular space being further compromised by constriction of the ciliary ring and anterior movement of the lens. This may result from the use of strong miotics in the management of angle-closure. Progressing, asymmetrical shallowing of the axial anterior chamber is the hall-mark of this condition. It is important to realise that the abnormality lies at the ciliary ring (in the initial stages) and in the vitreous cavity (in advanced cases), and it is toward these regions that management must be directed.

1.7.4.4 “Creeping” angle-closure

Lowe suggested this term following a visit to Singapore. He believed it was particularly applicable to Asians in whom peripheral anterior synechiae “creep” progressively in an anterior direction across the ciliary face, as well as in a circumferential direction”'”'®’''’'®. Anecdotally, this pattern is not universally

acknowledged by glaucoma specialists in East and Southeast Asia. Recognition of this pattern still requires classification under one of the three mechanisms listed above.