HIGHLIGHTS FROM THE OPTOMETRIC MANAGEMENT SYMPOSIUM ON CONTEMPORARY EYE CARE

Assessing Structure and Function in Detecting Early Glaucoma

Glaucoma testing advances with complementary use of FDT and SLP.

By Richard J. Madonna, O.D., M.A., F.A.A.O. and Mitchell W. Dul, O.D., M.S., F.A.A.O.

Gone are the days when we believed that structural abnormalities always preceded functional changes in the detection of the early signs of glaucoma.

"In different cases, structural examination or functional testing may provide more definitive evidence of glaucoma. So both are needed to detect and confirm the subtle early stages of the disease."¹ This consensus statement from the World Glaucoma Association represents our current understanding of the detection of early glaucoma. Years ago, practitioners were more inclined to rely on the retinal nerve fiber layer (RNFL) and optic nerve to give clues of early signs of glaucoma. They believed that functional loss, as measured by visual fields, would occur later in the disease process. But recent clinical evidence, most notably from the Ocular Hypertensive Treatment Study (OHTS), has demonstrated that measurable structural loss doesn't necessarily precede functional loss and, especially in early glaucoma, they seldom occur together. Therefore, assessments of both structure and function are important in detecting the early stages of glaucoma and in managing its progression. In functional testing, frequency-doubling technology (FDT) has been used as an adjunct to standard automated perimetry. We have number of ways to assess structural changes related to glaucoma, including optical coherence tomography (OCT) and scanning laser ophthalmoscopy. The focus of our discussion here will be on scanning laser polarimetry (SLP).

Studying structure and function

In early glaucoma, we rarely see a correlative link between functional and structural changes. For example, some patients have a clear-cut visual field change but no associated structural change while others show structural damage, but no functional correlate. It's reassuring when science validates something you've experienced clinically for years, and that's just what the OHTS trial did.

The OHTS study found that structural changes are the first sign of conversion from ocular hypertension to glaucoma in slightly more than half of cases, while visual field changes occur first in just under half of cases. The two occurred simultaneously in a very small percentage of cases.² Structural tests and various types of perimetry showed that no single test was superior to another in detecting early glaucoma.

FDT

The Humphrey FDT and Humphrey Matrix (Carl Zeiss Meditec) test the visual field by presenting a low spatial frequency and high temporal frequency sine wave gradient in counter phase flicker on a video monitor. The target size and flicker rate are held constant while the contrast is varied. At the threshold level of contrast, it's believed that patients see approximately twice the number of stimuli in the pattern and that this target stimulates the magnocellular ganglion cell pathway.

FDT gives us rapid, effective detection of visual field loss, relying, in part, on these advantages over standard white-on-white perimetry:

■ Sensitivity. Unlike white-on-white perimetry, which isn't tuned to any particular subset of fibers in the retinal ganglion cell layer, FDT zeros in on the magnocellular ganglion cells.

■ Less refractive error. A low spatial frequency — five cycles per degree — is less influenced by refractive error.³ Consequently, you can perform FDT without corrective lenses for up to about 4.00D of myopia or hyperopia.

■ Low test-retest variability. It's very hard to deal with noise in perimetry. The Humphrey Matrix significantly reduces test-retest variability. We get a fairly consistent result from one test to the next.⁴

■ Convenience. The Humphrey FDT takes up very little space. You can move it from room to room. It offers video eye monitoring. And the display and printouts are very similar to what you're accustomed to with standard perimetry. However, this isn't a perfect test. Because the stimulus is "flickered" at such a high temporal frequency, results are influenced by the effects of cataracts and small pupils.⁵

SLP

SLP is based on the principle of polarized light changing as it passes through the RNFL, because of the RNFL's birefringence. The thicker the RNFL the greater the polarized light changes. This change is called a phase shift, or retardation.

Birefringence in the eye isn't limited to the RNFL; the cornea and lens have this property, too. The GDx SLP (Carl Zeiss Meditec) offsets these values with its variable corneal compensator. Corneal birefringent properties are individualized and so is the GDx's compensation. The result is a great advantage in the diagnosis and management of glaucoma. We have an individualized scan, and what we see is a very true representation of the RNFL thickness in the majority of patients.

Interpreting SLP

The GDx collects a tremendous amount of data and delivers its interpretation through a printout. The printout's elements are designed to deliver the data in an informative and diagnostically useful manner (Figure 1).

■ Fundus image. The high-quality fundus image tells us where the ellipse was placed on the optic nerve and gives us a sense of the size of the optic nerve.

■ RNFL thickness map. The map illustrates the geography of the RNFL, which appears thicker in the hotter colors (yellows and reds) and thinner in the blue.

■ Deviation map. The instrument takes an image, divides it into pixels, and then divides the whole image into 1,024 squares (32 × 32). It compares each of these squares to the normative database and flags those whose thickness is outside statistical norms.

■ TSNIT curve. This illustrates the RNFL thickness along a continuum from temporal, superior, nasal, inferior to temporal views around the optic nerve.

■ Parameters. Average thickness is given for the TSNIT curve, the superior 120° average and the inferior 120° average. TSNIT standard deviation is the difference in the peak-to-trough distance. Thickness is normally high in superior and inferior peaks and thinner in the nasal and temporal troughs. But as patients with glaucoma lose retinal nerve fibers, they get a much flatter peak-to-trough measurement.

■ Nerve fiber indicator (NFI). Formerly called "the number," this factor has been shown to be the single best parameter for differentiating between normal and glaucomatous eyes.⁶ The NFI recognizes the nerve fiber layers in the 500+ normative database and the abnormal glaucomatous nerve fiber layers in about 260 people. On this basis, the system examines and scores an RNFL pattern. The higher the number, the greater the chance the patient has a glaucomatous nerve fiber layer. Together, these parameters provide us with a strong basis for structural analysis.

Linked, but separate

In glaucoma, structure and function may start to change simultaneously — or they may not. Function might precede structure, or structure might precede function. There's only one real answer for us: Stay on top of both. Technologies like FDT and SLP represent major advances in doing this successfully.

Richard J. Madonna, O.D., M.A., F.A.A.O., and Mitchell W. Dul, O.D., M.S., F.A.A.O., are associate professors at SUNY State College of Optometry, New York. Dr. Madonna is chief of the ocular disease and special testing service. Dr. Dul is director of the Glaucoma Institute.