Case Report: Serial Analysis to Detect Early Glaucoma Using the GDx

Optometric Management, Jul 2006 by Choplin, Neil T

Primary open-angle glaucoma (POAG) is no longer considered a disease of intraocular pressure (IOP). The classic triad of findings in glaucoma - elevated IOP, optic nerve cupping and visual field loss - no longer completely defines all cases of POAG.

Studies and clinical experience have shown that IOP has very poor sensitivity and specificity for diagnosing glaucoma. IOP measurements may be confounded by how much the central corneal thickness deviates from the average assumed by the Goldmann applanation tonometer, and IOP must therefore be interpreted in this context. An observed increase in the size of the cup over time, corresponding to thinning of the rim with loss of retinal nerve fibers, is definitive for POAG. We also now know that visual field testing, at least on standard white-on-white automated perimetry, detects glaucoma damage relatively late in the disease process. Some studies estimate 30% to 40% of retinal ganglion cells and their axons, the tissue affected in POAG, can be lost before a reproducible defect is detectable on standard automated perimetry.

Glaucoma redefined

POAG is now considered a "multifactorial" optic neuropathy, characterized by progressive loss of retinal ganglion cells and their axons which are contained in the retinal nerve fiber layer (RNFL). IOP is considered a risk factor for glaucoma, and the diagnosis and monitoring of glaucoma for progression requires evaluation of the optic nerve and the RNFL. In fact, evaluation of the RNFL, particularly with the GDx using scanning laser polarimetry, often allows detection of structural defects before functional defects occur, allowing earlier intervention.

Case study

Consider the following clinical case. This is a 45-year-old Hispanic woman first seen in 2003. Intraocular pressures in both eyes were consistently elevated to the upper 20s, with central corneal thickness of 580 microns in each eye. She had no family history of glaucoma or other risk factors for glaucoma. Visual acuity was 20/20 in both eyes, with normal anterior segments and open angles in both eyes. Figure 1 shows her visual fields and optic nerve photographs; both were considered clinically normal in both eyes.

Scanning laser polarimetry was obtained with the GDx. and the most recent results are shown in Figure 2. In the third in an annual series of GDx exams, the right eye clearly demonstrates progressive loss of RNFL, with "thinning" evident on the thickness map, increasing number of abnormal parameters, increasing NFI and increasing numbers of flagged pixels on the deviation map. The right eye demonstrates a number of abnormalities, evidenced by a diffuse reduction in retardation values seen on the "thickness" map, a flattening of the TSNIT plot, and numerous statistically significant deviations from normal, superiorly greater than inferiorly, seen on the deviation map. This, coupled with the clinical history, is sufficient to make a diagnosis of early (i.e. preperimetric) glaucoma.

GDx and progressive loss

The Ocular Hypertension Treatment Study suggested that patients with elevated IOPs (and normal discs and visual fields), particularly with thick corneas (> 588 microns), are at relatively low risk for developing glaucoma. This patient with elevated IOPs and thick corneas was considered to be at low risk for developing glaucoma, and she did not have it at the time of initial presentation based upon optic disc and visual field criteria.

Her initial GDx exam showed a limited number of abnormalities in the right eye, which were considered suspicious but not diagnostic. Serial GDx exams, obtained annually and displayed on the serial analysis, nicely illustrate progressive loss of RNFL in this patient's right eye over a 2-year period while her disc still appears "normal" and her visual field shows no glaucoma defect. Scanning laser polarimetry with the GDx shows the first evidence of glaucoma damage in this patient at risk and demonstrates the ability of the GDx to detect change over time.

Clinical evidence for SLP

Recent studies have supported the early detection of glaucoma by SLP: SLP abnormalities can be found in both eyes when only one eye has visual field defects (Ophthalmology. 2004;111:2199-2203), SLP abnormalities are evident in both the superior and inferior hemifield when only one hemifield has visual field loss (Jpn J Ophthalmol. 2003;47:22-27), and SLP abnormalities prediet visual field loss in patients with ocular hypertension (Am J Ophthalmol. 2004;138:592-601).

The diagnosis and monitoring of glaucoma, therefore, requires initial and ongoing evaluation of the RNFL. Early detection of glaucomatous damage and institution of therapy is most likely to prevent functional blindness. As illustrated in this case, the GDx, using SLP, can detect early glaucoma damage as well as change over time.

Neil T. Choplin, M.D.

Glaucoma Specialist, Eye Care of San Diego