Open-angle glaucoma: tips for earlier detection and treatment selection

Journal of Family Practice, Feb, 2005 by Ahmad A. Aref, Brian P. Schmitt

Practice recommendations

* Screen persons older than 60 years, African Americans of any age, and those with a family history of open-angle glaucoma (C). Further evaluation by an ophthalmologist is warranted if optic nerve damage is suspected or if a patient reports decreasing vision.

* Elevated intraocular pressure (IOP) is not necessary for open-angle glaucoma to occur. Assess optic nerve status and visual field in those at risk. (C)

* Inquire about topical ocular drops recommended by an ophthalmologist, to be certain they are not contraindicated for other conditions the patient might have, and to be alert to the potential for adverse effects. (C)

Evaluate for open-angle glaucoma (OAG) when a patient reports decreased vision, or when a patient even with good eyesight is found to be at high risk for the disease. Most patients with early glaucoma are unaware of the initial decrease in peripheral vision. (1)

A relatively new diagnostic technique can detect even moderate damage to the optic nerve, and the procedure is brief. Ophthalmologists can choose from among several topical medications to reduce intraocular pressure. Your knowledge of the patient's medical history is critical to avoiding potential drug-drug interactions.

Laser surgery and trabeculectomy may be indicated as first-line therapy for select patients.

* WHOM TO SCREEN

Persons aged older than 60 years, African Americans of any age, and those with a family history of OAG are at particularly high risk, and all risk factors should be fully assessed (SOR: B). (2) (See Open-angle glaucoma: The scope of the problem.)

In the Caucasian population aged 40 to 49 years with no family history of OAG, disease prevalence is just 0.18%. Prevalence is 4 times greater in African Americans of the same age range. Caucasians aged 60 to 69 years have a prevalence of OAG 4 times greater than patients aged 40 to 49. For African Americans older than 80 years, prevalence exceeds 11%. (3)

For persons with a first-degree relative with OAG, risk was found to be 9.2 times greater than for those without such a history. (8)

Ask specifically about decreased vision, loss of peripheral vision, difficulty seeing in the dark, and difficulty reading (SOR: B).

Before referring high-risk patients for a full ophthalmologic examination, examine the optic nerve with direct ophthalmoscopy (SOR: B).

Determining optic nerve status

Examination of the optic nerve head provides clues as to whether structural damage has occurred. Cup-disc ratio is used to assess risk of glaucoma development. The probability of abnormality increases dramatically for values above 0.5. (9)

The standard clinical technique used by primary care clinicians is with the direct ophthalmoscope. Sensitivity and specificity for a cupdisc ratio greater than 0.6 have been reported to be 64% and 96%, respectively, using direct ophthalmoscopy. (10)

Ophthalmologists use stereoscopic fundus photography to visualize the optic nerve. With this technique, sensitivity and specificity for a cup-disc ratio greater than 0.5 have been found to be 48% and 89%, respectively. (11) Studies, however, have reported a high interobserver variation in measurement of the cup-disc ratio even among experts in the field. (12)

What to look for. Characteristic changes include narrowing or notching of the neuroretinal rim, or characteristic visual field loss, such as arcuate defects and nasal loss. (13) Describe an abnormal optic disc in terms of its cup-disc ratio, and report visual loss to the ophthalmologist as a defect in a respective field quadrant as detected on confrontational visual field testing or as an afferent pupillary defect in a given eye.

Referral. A final diagnosis of open-angle glaucoma can be made only after characteristic damage to the optic nerve has been confirmed by an ophthalmologist (SOR: B). Therefore, patients at high risk of developing OAG (age >60 years, African American race, positive family history) should be referred for an eye examination.

Other key diagnostic tests include measurement of intraocular pressure and visual field testing. (2) The accuracy of these tests is outlined in Table 1.

Intraocular pressure: Caveats

Intraocular pressure (IOP) is measured by a tonometer. The eye is subjected to a force that flattens the cornea. This force is then related to the pressure in the eye, or IOP. The standard instrument for measuring IOP is the Goldman applanation tonometer. Handheld versions (tonopen) are useful for screening by the primary care clinician. (11) Studies of IOP distribution show the normal range of IOP values to be less than 21 mm Hg with a slight skew towards higher values. (16)

The altering effect of corneal thickness. IOP measurement may vary with the thickness of one's cornea. A corneal thickness greater than 555 [micro]m can produce falsely high readings, and a corneal thickness less than 540 [micro]m can produce falsely low readings. (2,17,18) Thus, central corneal thickness (CCT) is a factor that may affect the accuracy of an IOP reading. Central corneal thickness is measured with a pachymeter, and an ophthalmologist must take this measurement into account when assessing a patient's IOP.

Pressure may not be elevated in OAG. A number of population-based studies have documented an increase in the prevalence of OAG with an increase in IOP. (7,19,20) However, these same studies have also concluded that many patients with OAG have IOP levels in the normal range. These patients are deemed to have normal pressure glaucoma (NPG), a subtype of OAG. (21)