CLINICAL ASSESSMENT IN GLAUCOMA

Tonometry

Tonometry is measurement of intraocular pressure. The most widely used instrument is the Goldmann applanation tonometer, which is attached to the slitlamp and measures the force required to flatten a fixed area of the cornea. Other applanation tonometers are the Perkins tonometer and the Tono-Pen, both of which are portable; and the pneumatotonometer, which is useful when the cornea has an irregular surface and can be used with a soft contact lens in place. The Schiotz tonometer is portable and measures the corneal indentation produced by a known weight. (For further discussion of tonometry, see Chapter 2; for tonometer disinfection techniques, see Chapter 21).

The normal range of intraocular pressure is 10-24 mm Hg. A single normal reading does not rule out glaucoma. In primary open-angle glaucoma, many affected individuals will have a normal intraocular pressure when first measured. Conversely, isolated raised intraocular pressure does not necessarily mean that the patient has primary open-angle glaucoma, since other evidence in the form of a glaucomatous optic disk or visual field changes is necessary for diagnosis. If the intraocular pressure is consistently elevated in the presence of normal optic disks and visual fields (ocular hypertension), the patient may be observed periodically as a glaucoma suspect.

Gonioscopy (See also Chapter 2)

The anterior chamber angle is formed by the junction of the peripheral cornea and the iris, between which lies the trabecular meshwork (  Figure 11-3). The configuration of this angle-ie, whether it is wide (open), narrow, or closed-has an important bearing on the outflow of aqueous. The anterior chamber angle width can be estimated by oblique illumination with a penlight (Figure 11-4) or by slitlamp observation of the depth of the peripheral anterior chamber, but it is best determined by gonioscopy, which allows direct visualization of the angle structures (  Figure 11-3). If it is possible to visualize the full extent of the trabecular meshwork, the scleral spur, and the iris processes, the angle is open. Being able to see only Schwalbe's line or a small portion of the trabecular meshwork means that the angle is narrow. Being unable to see Schwalbe's line means that the angle is closed.

Large myopic eyes have wide angles, and small hyperopic eyes have narrow angles. Enlargement of the lens with age narrows the angle and accounts for some cases of angle-closure glaucoma.

Race is also a factor. The angles of Southeast Asians are much narrower than those of Caucasians.

Optic Disk Assessment

The normal optic disk has a central depression-the physiologic cup-whose size depends on the bulk of the fibers that form the optic nerve relative to the size of the scleral opening through which they must pass. In hyperopic eyes, the scleral opening is small, and thus the optic cup is small; the reverse is true in myopic eyes. Glaucomatous optic atrophy produces specific disk changes characterized chiefly by loss of disk substance-detectable as enlargement of the optic disk cup-associated with disk pallor in the area of cupping. Other forms of optic atrophy cause widespread pallor without increased disk cupping.

In glaucoma, there may be concentric enlargement of the optic cup or preferential superior and inferior cupping with focal notching of the rim of the optic disk. The optic cup also increases in depth as the lamina cribrosa is displaced backward. As cupping develops, the retinal vessels on the disk are displaced nasally (Figure 11-5). The end result of glaucomatous cupping is the so-called "bean pot" cup in which no neural rim tissue is apparent (Figures 11-6 and 11-7).

The "cup-disk ratio" is a useful way of recording the size of the optic disk in glaucoma patients. It is the ratio of cup size to disk diameter, eg, a small cup is 0.1 and a large cup 0.9. In the presence of elevated intraocular pressure, a cup-disk ratio greater than 0.5 or significant asymmetry between the two eyes is highly suggestive of glaucomatous atrophy.

Clinical assessment of the optic disk can be performed by direct ophthalmoscopy or by examination with the 70-diopter lens, the Hruby lens, or special corneal contact lenses that give a three-dimensional view.

Other clinical evidence of neuronal damage in glaucoma is atrophy of the nerve fiber layer. This is detectable (Hoyt's sign) by ophthalmoscopy-particularly when red-free light is used-and precedes the development of optic disk changes.

Visual Field Examination

Regular visual field examination is essential to the diagnosis and follow-up of glaucoma. Glaucomatous field loss is not in itself specific, since it consists of nerve fiber bundle defects that may be seen in other forms of optic nerve disease; but the pattern of field loss, the nature of its progression, and the correlation with changes in the optic disk are characteristic of the disease.

Glaucomatous field loss involves mainly the central 30 degrees of field (  Figure 11-8). The earliest change is baring of the blind spot. Contiguous extension into Bjerrum's area of the visual field-at 15 degrees from fixation-produces a Bjerrum scotoma and then an arcuate scotoma. Focal areas of more pronounced loss within Bjerrum's area are known as Seidel scotomas. Double arcuate scotomas-above and below the horizontal meridian-are often accompanied by a nasal step (of Roenne) because of differences in size of the two arcuate defects. Peripheral field loss tends to start in the nasal periphery as a constriction of the isopters. Subsequently, there may be connection to an arcuate defect, producing peripheral breakthrough. The temporal peripheral field and the central 5-10 degrees are affected late in the disease. Central visual acuity is not a reliable index of progress of the disease. In end-stage disease, there may be normal central acuity but only 5 degrees of visual field in each eye. In advanced glaucoma, the patient may have 20/20 visual acuity and be legally blind.

Various ways of testing the visual fields in glaucoma include the automated perimeter, the Goldmann perimeter, the Friedman field analyzer, and the tangent screen. (For technique and other details, see Chapter 2)

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