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Visual fields are used to assess an individual’s functional vision. Perimetry and the evaluation of the optic nerve and retinal nerve fiber layer are the keys to diagnosing and monitoring glaucoma. Perimetry helps clinicians to recognize glaucomatous loss as well as to stage the disease based upon the severity of field loss. This form of evaluation also allows physicians to monitor patients for change.
STRUCTURE VERSUS FUNCTION
Recent reports suggest that structural assessment may be of greatest importance for early glaucoma.1
In many individuals who are developing the disease, changes in the optic nerve or retinal nerve fiber layer are the initial signs of loss.2,3 Perimetric damage, when present, is thought to signal that significant structural damage has occurred.1,2,4,5
A disconnect between the amount of structural and functional damage present can occur for several reasons. It is not that functional vision is unaffected as ganglion cells are lost. Rather, perimetric devices are not capable of detecting small amounts of loss, due in part to the redundancy of the visual system and the overlap of receptor fields. Certain perimetric tests such as short-wavelength automated perimetry and frequency doubling technology perimetry were developed to recognize early damage, with varying amounts of success.6-8
Another reason for the disconnect is a scaling issue, with visual fields measured on a log scale and structural damage on a linear one.4,5 As structural damage increases over the course of the disease, with the cup getting larger and the neuroretinal rim thinner, there approaches a point at which it becomes difficult to discern further structural change (Figure 1). At this stage, a surprising amount of functional vision may remain, however, which allows physicians to monitor the patient’s condition with visual fields.
HOW TO TEST THE VISUAL FIELD
The 24-2 program with a size III stimulus is the most commonly used testing pattern and target size with the Humphrey Field Analyzer (Carl Zeiss Meditec). Classifying the visual field in regard to the extent of damage is based upon the number of points flagged, whether loss is present in one or both hemifields, and if points are depressed within the central 5º (Figure 2). For moderate loss, at least 50% of the points should be affected, with points depressed in both hemifields and/or some points within the central area depressed.
The mean deviation (MD) or visual field index (VFI) can be used to stage the condition with the caveat that the MD may be reduced due to causes other than glaucoma. The most common reason for the MD to decrease or change over time is the development or progression of cataracts. The VFI was developed to measure functional vision loss based upon glaucomatous damage only; 100% represents a full field, whereas 0% means vision is no longer present.
When most points are significantly depressed using the 24-2 test pattern and size III stimulus, the target can be enlarged to size V, thus increasing the dynamic range (Figure 3A and B). The drawback of a size V target is that there are no confidence limits for this test, so the clinician must evaluate individual test location scores over time to determine if they are getting worse. When field loss expands, the 10-2 test pattern can be used (with a size V target), and the grid spacing can be narrowed to 2º from 6º (in the 24-2), which will allow more points to be packed into a small area (Figure 3C). An MD of -15 dB is often the cut-off between moderate and severe loss. A VFI of approximately 60% correlates with this MD.
Pattern standard deviation (PSD) is another global index, and it measures local change. PSD is not useful for detecting disease progression, however, because it peaks at approximately -12 dB, when a maximum amount of local damage is present. The PSD will reverse toward 0 dB as damage worsens and spreads to both hemifields. A person with mild glaucoma and someone with advanced disease will have a similar PSD, suggesting that the metric is not useful as an indicator of change in or severity of disease.
Monitoring patients with moderate to advanced glaucoma is difficult with currently available tools, because advanced field loss renders the detection of change imprecise. As points become more depressed, variability increases. Threshold scores at any location can swing widely from visit to visit but may not be associated with true change. The Glaucoma Progression Analysis (GPA) program (Carl Zeiss Meditec) was developed to help clinicians recognize which points are truly changing and separate progression from variability. The software’s value in cases of advanced loss is limited, though, and GPA often is not available when damage is severe. In this situation, clinicians may find the overview printout useful, because they can compare a series of fields to look for change (Figure 4).
Although imperfect, visual field testing permits clinicians to monitor patients with moderate to advanced structural loss from glaucoma. Optical coherence tomography may one day replace this use of perimetry. The former collects 1 million times more data than perimetry, which should make it more sensitive to rim and nerve fiber layer loss. First, however, optical coherence tomography must become better at detecting changes from baseline measurements.
This article is reprinted with permission from the December 2013 issue of Glaucoma Today.
Murray Fingeret, OD, is the chief of the Optometry Section, Department of Veterans Affairs, New York Harbor Healthcare System, Brooklyn, New York. He is a consultant to Carl Zeiss Meditec. Dr. Fingeret may be reached at (718) 298- 8498; firstname.lastname@example.org.
- Medeiros FA, Lisboa, R, Weinreb RN, et al. Retinal ganglion cell count estimates associated with early development of visual field defects in glaucoma. Ophthalmology. 2013;120(4):736-744.
- Quigley HA, Dunkelberger GR, Green WR. Retinal ganglion cell atrophy correlated with automated perimetry in human eyes with glaucoma. Am J Ophthalmol. 1989;107:453-464.
- Medeiros FA, Alencar LM, Zangwill LM, et al. Prediction of functional loss in glaucoma from progressive optic disc damage. Arch Ophthalmol. 2009;127:1250-1256.
- Hood DC, Kardon RH. A framework for comparing structural and functional measures of glaucomatous damage. Prog Retin Eye Res. 2007;26:688-710.
- Harwerth RS, Carter-Dawson L, Smith EL III, et al. Neural losses correlated with visual losses in clinical perimetry. Invest Ophthalmol Vis Sci. 2004;45:3152-3160.
- Johnson CA, Brandt JD, Khong AM, Adams AJ. Short-wavelength automated perimetry in low-, medium- and high-risk ocular hypertensive eyes. Initial baseline results. Arch Ophthalmol. 1995;113(1):70-76.
- Demirel S, Johnson CA. Incidence and prevalence of short wavelength automated perimetry deficits in ocular hypertensive patients. Am J Ophthalmol. 2001;131(6):709-715.
- Cello KE, Nelson-Quigg JM, Johnson CA. Frequency doubling technology perimetry for detection of glaucomatous visual field loss. Am J Ophthalmol. 2000;129(3):314-322.