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

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 cup-disc 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.

TABLE 1
Characteristics of diagnostic tests for open-angle glaucoma

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 μm can produce falsely high readings, and a corneal thickness less than 540 μm 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

Likewise, many patients with elevated IOP have no demonstrable optic nerve damage;^19,20 this condition has been termed ocular hypertension (OHT).

A proper perspective. So, although an elevated IOP is associated with glaucoma, it is important to note that OAG is not defined by the presence of an elevated IOP. Optic nerve atrophy can occur in the absence of an increased IOP. ²¹ These findings, taken together with the variance of IOP with CCT, are reflected in the modest sensitivity and specificity for IOP readings greater than 21 mm Hg—47.1% and 92.4%, respectively. ¹⁴ Patients with a high IOP (>21 mm Hg) are at higher risk for developing OAG, but further ancillary studies and tests are necessary to confirm the diagnosis. ¹³