Glaucoma: new rules apply to an old disease

In the past, eye-care providers diagnosed glaucoma based solely on raised IOP or visible or distinct damage to the optic nerve. Those rules have changed, thanks to more information about the risk factors for glaucoma including thin central corneas, elevated IOP, cup-to-disc ratios, and retinal nerve fiber layer defects.

Key Points

In the past, eye-care providers diagnosed glaucoma based solely on raised IOP or visible or distinct damage to the optic nerve. However, these rules have changed, thanks to more information about risk factors for glaucoma-thin central corneas, elevated IOP, cup-to-disc ratios, and retinal nerve fiber layer (RNFL) defects, said Dr. Semes, a professor of optometry at University of Alabama at Birmingham School of Optometry.

In his lecture, "Glaucoma, the Rules Have Changed," Dr. Semes discussed how new paradigms for identifying glaucoma early and models for risk calculation in ocular hypertension can aid in this quest.

Researchers from the Early Manifest Glaucoma Trial compared the ability of Fast Swedish interactive threshold algorithm (SITA) short-wavelength automated perimetry (SWAP), full-threshold SWAP, and standard automated perimetry (SAP) using the SITA-Fast program to detect early glaucomatous visual field loss.1 When testing 100 patients with ocular hypertension, patients suspected of having glaucoma, and patients with early manifest glaucoma, they found no significant difference in the number of significantly depressed test point locations (i.e., glaucomatous visual field loss) between the three programs.

The SITA SWAP identified at least as much glaucomatous visual field loss as the older full-threshold SWAP. One difference: Test time was considerably less with SITA SWAP. Conventional SAP using SITA Fast was not significantly less sensitive than either of the two SWAP programs.

Six-step versus continuous classification. Practitioners traditionally have used the Glaucoma Staging System (GSS) to look for evidence of progression. The GSS, based on Humphrey visual field printouts, consists of six stages based on the mean deviation (MD) index and the number of depressed points. At stage 0, the set of defects does not meet the minimum qualifications for stage 1 (early defect). The following stages are moderate defect (stage 2), advanced defect (stage 3), severe defect (stage 4) and end-stage disease (stage 5).

The Glaucoma Progression Index (GPI) is now incorporated into the Humphrey visual field analyzer. The GPI, which uses pattern deviation probability maps, is a continuous scale that shows a linear progression of glaucomatous damage. Other advantages: It is less affected by cataract than MD, and it applies a weighting scheme that favors central retinal sensitivity. The GPI also suggests future progression patterns.

One versus multiple visual fields. Along these same lines, Dr. Semes said, research suggests that one visual field exam per year is not sufficient to detect progression. Previously, there was a lack of evidence-based guidance on how many visual field exams are needed to identify clinically meaningful rates of change in glaucoma. However, researchers from Halifax, Nova Scotia, say that three examinations per year are required to identify an overall change in MD of 4dB over 2 years in a patient with average visual field variability.2

"The significance of this is most third-party payers, particularly Medicare, limit the patient to one visual field per year," Dr. Semes said. "Based on this approach, we should probably be doing them much more often."

Cup-to-disc versus structural evaluation. Glaucoma researchers now believe that the concept of cup-to-disc ratio is not a reliable approach for assessing glaucomatous disc progression.3 A better approach: digital evaluation of the neuroretinal rim and the RNFL and for the presence of hemorrhage.3

In one study of patients suspected of having glaucoma who were followed an average of 4 years, those who had thinner RNFL at baseline, were more likely to show glaucomatous changes of the follow-up years than those who had a thicker RNFL at baseline.2

Another study found that most defects occur superotemporally or inferotemporally.4 RNFL measurement should vary no more than 10 µm in a stable patient between examinations.

Ocular perfusion pressure. Researchers have speculated that sufficient blood pressure is necessary to nourish and maintain the health of the optic nerve. Several studies have confirmed a greater incidence of primary open-angle glaucoma (POAG) in patients with lower diastolic ocular perfusion pressure (DOPP).5-8

More recent research has found that reduced perfusion during sleep or use of topical and oral beta-blockers can contribute to DOPP. Patients using topical beta-blockers, such as timolol, should not be dosed at night if there is any suspicion of nerve head perfusion during sleep, Dr. Semes said.

References

1. Bengtsson B, Heijl A. Diagnostic sensitivity of fast blue-yellow and standard automated perimetry in early glaucoma: a comparison between different test programs. Ophthalmology. 2006 Jul;113:1092-1097.

2. Chauhan BC, Garway-Heath DF, Goñi FJ, et al. Practical recommendations for measuring rates of visual field change in glaucoma. Br J Ophthalmol. 2008 Apr;92:569-573.

3. Caprioli J, Garway-Heath DF, International Glaucoma Think Tank. A critical reevaluation of current glaucoma management: International Glaucoma Think Tank, July 27-29, 2006, Taormina, Sicily. Ophthalmology. 2007 Nov;114(11 Suppl):S1-S41.

4. Kim TW, Park UC, Park KH, Kim DM. Ability of Stratus OCT to identify localized retinal nerve fiber layer defects in patients with normal standard automated perimetry results. Invest Ophthalmol Vis Sci. 2007 April;48:635-634.

5. Tielsch JM, Katz J, Sommer A, et al. Hypertension, perfusion pressure, and primary open-angle glaucoma. A population-based assessment. Arch Ophthalmol. 1995 Feb;113:216-221.

6. Bonomi L, Marchini G, Marraffa M, et al. Vascular risk factors for primary open angle glaucoma: the Egna-Neumarkt Study. Ophthalmology. 2000 Jul;107:1287-1293.

7. Leske MC, Wu SY, Nemesure B, Hennis A. Incident open-angle glaucoma and blood pressure. Arch Ophthalmol. 2002 Jul;120:954-959.

8. Quigley HA, West SK, Rodriguez J, et al. The prevalence of glaucoma in a population-based study of Hispanic subjects: Proyecto VER. Arch Ophthalmol. 2001 Dec;119:1819-1826.