29

Principles of Secondary Angle-Closure Glaucomas

David L. Epstein, MD, MMM

In Chapter 23, the mechanisms underlying the primary angle-closure glaucomas were discussed. The fundamental concept was that of pupillary block, wherein fluid pressure in the posterior chamber from the normal secretion of aqueous humor ballooned the peripheral iris forward over the trabecular meshwork and thus closed the angle (see Figure 23-2). Peripheral iridectomy, by equalizing the pressure in the posterior and anterior chambers, eliminated this fluid pressure vector from the posterior chamber acting on the iris. The key concept in such a pupillary block mechanism is that of posterior chamber aqueous humor acting as the force, resulting in the angle closure. In contrast, in plateau iris syndrome, in addition to a relative pupillary block mechanism, the ciliary process causes direct crowding of the iris itself into the angle and is another mechanism for angle closure (see Figure 27-1).

In some secondary angle-closure glaucomas, such as that due to the iridocorneal endothelial syndrome (see Chapter 33) or neovascular glaucoma (see Chapter 32), there are other abnormal local iris and angle factors acting to close the angle directly.

In other secondary angle-closure glaucomas, there is some additional force acting to close the angle. Conceptually, this additional force might act by itself to directly force the iris into the angle or by acting to move the crystalline lens forward, causing pupillary block (wherein the resulting increased fluid pressure from aqueous humor in the posterior chamber acts to force the peripheral iris over the angle). This may be termed a secondary pupillary block. The possibility for either or both of these 2 mechanisms in the secondary angle-closure glaucomas needs to be constantly kept in mind, for it is not rare for some patients in this category to respond favorably to laser iridectomy (even though primary pupillary block is not the initiating mechanism causing the disease).

Examples of the first mechanism include cysts of the ciliary body (see Chapter 35) or tumors, such as melanoma, that mechanically may push the iris into the angle (see Chapter 42); forward rotations of the ciliary body from choroidal detachment, hemorrhage, or scleral buckling that may then push the iris forward (see Chapter 34); and possible cases of ciliary body swelling, as seen in certain inflammatory syndromes and possibly after panretinal photocoagulation and in cases of transient myopia due to idiosyncratic systemic drug reaction (see Chapter 37). (In these cases, local accumulation of suprachoroidal fluid and forward rotation of the ciliary body may be an alternative explanation to that of ciliary body swelling and may also explain certain cases of so-called loose lens syndrome; see Chapter 30.) In all of these conditions, pathological processes affecting the ciliary body itself result in direct pressure of the ciliary processes on the iris to close the angle.

In other conditions, pathology involving vitreous volume, such as malignant glaucoma (see Chapter 30) and angle-closure glaucoma after vitreous hemorrhage, central retinal vein occlusion (see Chapter 36), and possibly pan-retinal photocoagulation, and certain volume-occupying massive forms of acute macular degeneration may result in pressure from the vitreous that shifts the entire lens-iris diaphragm forward (axial shallowing). Grant has observed that, in eyes with malignant glaucoma, the crystalline lens equator tends to extend behind and peripheral to the tips of the ciliary processes so that the lens equator is in a position to push the ciliary processes and the iris forward over the angle in response to this pressure from the vitreous. Likely, this relationship of the crystalline lens equator to the tips of the ciliary processes is an anatomical precondition in such eyes that have a narrowed middle segment that is accentuated by increasing lens size with increasing age. An expanded vitreous volume, as is believed to occur acutely in the various malignant glaucoma syndromes, thus results in a secondary angle-closure glaucoma. However, whether angle closure in all such cases finally results from the ciliary processes themselves, in response to this posterior pressure, pushing the iris into the angle, or whether in some cases the forward shift of the lens-iris diaphragm from the increased fluid pressure in the expanded vitreous itself directly pushes the iris into the angle is not certain. Likely, in such eyes with expanded vitreous volume and a forward shift in the lens-iris diaphragm, there is some distortion of the anatomy of the posterior chamber and the zonules (the posterior zonules normally merge into the anterior hyaloid1).

In some cases in which vitreous pressure is hypothesized to be involved, the initiating event may be an increase in retinal volume (eg, after central retinal vein occlusion, acute exudative detachment of the macula,² or after panretinal photocoagulation) that conceptually acts on a noncompressible vitreous to shift the lens-iris diaphragm forward. Alternatively, these conditions may themselves result in fluid movement into the vitreous cavity, which itself is expanded in volume as presumably occurs in syndromes involving massive vitreous hemorrhage. In either case, vitreous volume acts as a mass to shallow the anterior chamber. The question as to why in some cases a shift in the lens-iris diaphragm occurs as a result of these diseases, and in other cases does not, will be discussed in Chapter 30, but it is believed to be a result of differences in constitutive “vitreous humor permeability” to fluid flow and transfer.

Thus, these secondary angle-closure glaucomas that involve pressure from behind the iris may result from mechanical forces from the ciliary body or the vitreous. As mentioned, the forward shift of the crystalline lens represents a separate additional mechanism—that of secondary pupillary block. A third possible mechanism, at least theoretically, is for the edge of the crystalline lens equator to itself directly push the iris into the angle. This might occur with a gross mismatch of the size of the crystalline lens to that of the middle segment of the eye, as might be conceived to occur in certain congenital conditions, nanophthalmos, or perhaps with cataract formation in certain phacomorphic glaucomas. We must all remember this possibility, especially as we face new entities of secondary angle-closure glaucoma, but our suspicion is that such a postulated direct crystalline lens force acting on the iris itself (without an intervening effect of the crystalline lens on ciliary processes as proposed for malignant glaucoma) and independent of pupillary block rarely if ever occurs because the equator of the crystalline lens likely does not extend far enough peripherally to close the angle. In fact, most cases of phacomorphic glaucoma (see Chapter 48) are not due to this mechanism, but rather pupillary block³ from the enlarged crystalline lens. Thus, it is usually fluid pressure in the posterior chamber that exerts the force on the posterior iris surface to close the angle (ie, pupillary block).

All these secondary mechanisms should be kept in mind as we now discuss the various specific entities that constitute the secondary angle-closure glaucomas in the subsequent chapters. These mechanisms provide a unifying construct that explains how pressure from behind can cause the angle-closure glaucoma. However, these are usually complicated cases, and we still have much to learn about the actual mechanisms involved. New syndromes or subgroups are inevitable as we increase our understanding. These concepts likely oversimplify what is occurring in individual cases, but these constructs do allow a framework in which to initially classify these conditions to eventually increase our understanding. In the short term, this will enable effective patient management. Some of these cases only behave as if the above were the mechanisms, but if these concepts result in appropriate treatment, then this is worthwhile.

It is important to remember that these conditions of pressure from behind can cause a secondary pupillary block mechanism due to the forward movement of the crystalline lens. A diagnosis of pupillary block needs to be initially entertained as at least one contributing factor and the potential benefit of laser iridectomy contemplated. In fact, certain conditions, such as malignant glaucoma, require the presence of a patent peripheral iridectomy for both diagnosis and therapy.

REFERENCES

1.      Streeten BW, Pulaski JP. Posterior zonules and lens extraction. Arch Ophthalmol. 1978;96(1):132-138.

2.      Wood WJ, Smith TR. Senile disciform macular degeneration complicated by massive hemorrhagic retinal detachment and angle closure glaucoma. Retina. 1983;3(4):296-303.

3.      Tomey KF, al-Rajhi AA. Neodymium:YAG laser iridotomy in the initial management of phacomorphic glaucoma. Ophthalmology. 1992;99(5):660-665.