Evaluating the visual field
Visual field deterioration is the final manifestation of glaucoma. Vision is first lost peripherally. Central vision loss occurs at the end stage of the disease.
An ophthalmologist will use automated static threshold perimetry to evaluate the visual field. With this technique, the patient must identify white target lights of variable brightness in different locations of a dim 1-m bowl. Various data algorithms are then employed to compare any abnormality in the visual field with patterns that are characteristic of glaucoma.9 One study reported a 97% sensitivity and 84% specificity using a certain algorithm to recognize field abnormalities due to glaucoma.15 However, automated perimetry requires 10 to 20 minutes per eye, and patient fatigue often reduces reliability of the test. Also, an optic nerve head has typically undergone considerable damage before visual field changes are detected.2
An improved test. Frequency doubling technology promises to detect glaucomatous visual defects when there has been only moderate damage to the optic nerve. With frequency doubling technology, patients must recognize patterns of alternating light and dark bars. An abnormality in recognition is thought to be indicative of the pattern of field loss in glaucoma. One study found a sensitivity and specificity each greater than 90% for identifying patients thought to have glaucoma. Another benefit is that the exam takes an average of only 6 minutes to complete in both eyes. 22
No single test result is enough
Successful screening for glaucoma should not rely solely on measuring IOP, assessment of the optic nerve, or visual field testing. These diagnostic clues are complementary and must be taken together to evaluate high-risk populations, including African Americans, those with a family history of glaucoma, and the elderly (SOR: C).
Regular follow-up
Regardless of findings, patients aged 40 to 60 years should be encouraged to have eye exams every other year, and those over age 60 should have annual eye exams (SOR: B).13 Regular ocular exams including vision check, extraocular muscle exam, papillary exam, and confrontational visual fields should be performed in these patients as well (SOR: C).
Treatment
IOP is the only risk factor for glaucoma that can be treated. Lowering IOP in randomized control trials has reduced the progression of visual field loss in OAG patients with abnormally high pressures23 as well as in NPG patients with pressures in the normal range.24
In the Early Manifest Glaucoma Trial, a 30% reduction in IOP reduced the rate of progression in the treatment group (45%) compared with the control group (62%; P=.007).25 Progression risk decreased by approximately 10% per mm Hg of IOP reduction.
Setting a target pressure. Before beginning therapy, an ophthalmologist sets a target pressure that should halt further optic nerve damage. The initial target pressure is usually 20% to 30% lower than the pretreatment pressure. If damage to the optic nerve is already substantial, the target pressure may be set even lower.2
Stepwise therapy. Topical medications are usually given first, as eye drops. A comparison of these medications is outlined in Table 2. If IOP cannot be lowered pharmacologically, argon laser trabeculoplasty (ALT) is the next step. If the pressure still cannot be lowered, filtering surgery is the final alternative (SOR: C).2
TABLE 2
Pharmacologic options for patients with open-angle glaucoma
| Medication(SOR) | % IOP reduction | % RR* | NNT to prevent visualfield loss* | Side effects | Comments |
|---|---|---|---|---|---|
| Beta-blockers (A), non-selective (timolol, carteolol, levobunolol, metipranolol) and selective (betaxolol) | 31 | 74 | 2.17 | Bradycardia, hypotension, bronchospasm (timolol, carteolol, levobunolol, metipranolol) | Cochrane review with clear recommendation as first-line treatment21 |
| Prostaglandin analogues (A) (latanoprost, travoprost, unoprostone) | 40 | 96 | 1.68 | Increased eyelash growth, iris pigmentation, muscle and joint pain | Multiple RCTs and systematic review show show clinical and statistical superiorityover beta-blockers26,27 |
| Alpha adrenergic drugs (A) (apraclonidine, brimonidine) | 23 | 55 | 2.93 | Dry nose, dry mouth, follicular conjunctivitis, hypotension (brimonidine) | Multiple RCTs support effectiveness 28-30 |
| Topical carbonic anhydrase inhibitors (A) (brinzolamide, dorzolamide) | 26 | 62 | 2.6 | GI disturbances, headache, local irritation, redness, sulfa allergies | RCT,31 Cochranereview with clear recommendation26 |
| Cholinergic agonists (A) (pilocarpine, carbachol) | 29 | 69 | 2.32 | Small, fixed pupils, induced myopia, cataracts | Consistently recommended assecond-or third-line drugs in systematic reviews and RCTs27,32 |
| *Percent decrease in risk of visual field loss and NNT to prevent visual field loss were calculated for a patient with a baseline IOP of 24 mm Hg. | |||||
| SOR, strength of recommendation; IOP, intraocular pressure; RR, relative risk; NNT, number needed to treat; RCT, randomized controlled trial. | |||||
Pharmacologic options
Medical agents work in 1 of 2 ways to lower IOP: by decreasing production of aqueous humor, or by increasing drainage of aqueous humor out of the eye. Though most glaucoma medications are given topically, severe systemic side effects can occur.2 Because the consulting ophthalmologist may not be aware of a patient’s other medical conditions, inquire about the topical ocular drops being recommended to make certain they are not contraindicated and to be alert to the potential for adverse effects (SOR: C).13