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Main Issue July 2004

Interpreting Visual Fields

Advice on using staging systems to make informed treatment decisions.

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Much recent excellent work has focused upon the optic nerve and nerve fiber layer. Although a careful analysis of the nerve will provide valuable information about a patient’s disease, it may not provide the physician with an adequate understanding of the patient’s condition. For example, individuals who have far advanced, diffuse neuroretinal rim loss in each eye can seem to function perfectly well, whereas others with small, localized rim notches are debilitated by vision loss. How can the eyes with less rim tissue “see” better? The answer is field loss. Glaucoma affects patients, not by the nerve damage per se, but by the location of the damaged peripheral vision. Although nerve damage and field loss are nothing more than pathology and its functional manifestation, there can be a disconnection between how the physician perceives the disease’s severity (the nerve examination) and how seriously the disease is affecting the patient (the field loss).

The goals of an eye examination are identifying the problem, determining its cause, and attempting to provide a solution. The myriad ways in which glaucoma can occur make it worth bearing this framework in mind. As George Spaeth, MD, of Wills Eye Hospital in Philadelphia teaches, the optic nerve examination determines if the patient has glaucoma, gonioscopy indicates the type of glaucoma, and the history and field examination reveal how the disease is affecting the patient. In combination, these results yield information about disease severity and thus help to guide treatment decisions. This article focuses on the interpretation and staging of visual fields as means by which to determine the amount of glaucomatous damage that a patient has suffered.

GENERAL THOUGHTS
When evaluating fields, I like to follow a checklist that I learned from Donald Budenz, MD, of the Bascom Palmer Eye Institute in Miami. The list is as follows:
1. What are the patient’s demographics and clinical
characteristics?
2. What type of visual field test was performed?
3. How reliable is the visual field?
4. Is the visual field abnormal?
5. What is the pattern of the abnormality?
6. Is the field worsening?
7. Is the abnormality/worsening due to disease or artifact?

FIELD RELIABILITY
Reliability is one problem with perimetry. The Swedish Interactive Testing Algorithm (SITA) on the Humphrey Field Analyzer (Carl Zeiss Meditec Inc., Dublin, CA) measures reliability in terms of fixation losses, false positives, and false negatives. Fixation losses are measured by projecting a stimulus into the blind spot. If the patient responds, one can assume that he was not looking at the fixation target. Ideally, the number of fixation losses should be small. If there are few false positives and false negatives, however, I will accept some fixation losses—perhaps a rate as high as 50%. The gaze tracker can be used to monitor fixation on a more continuous basis (Figure 1).


Figure 1. This field shows a glaucomatous defect that would be designated as moderate according to the Hodapp-Parrish-Anderson scale or stage 4 per the Field Damage Likelihood Score. Information from the gaze tracker appears at the bottom of the printout. Vertical lines emanating from the central horizontal line indicate saccades. Few saccades in this case indicate good fixation.

False positives are defined as erroneous patient responses when no stimulus was presented. A high rate of false positives makes the field appear artificially good. To me, a rate greater than 10% to 15% indicates that the field is unreliable. False negatives occur when the patient fails to respond to a stimulus he previously saw. Some false negatives are to be expected for patients with advanced field loss, but, in general, these errors make the field look artificially bad.

INTERPRETATION
Staging Systems
After deeming the field to be reliable, one must next decide if a field abnormality is glaucomatous. If it is, staging the field will reveal the amount of field loss and help one to determine which treatments to employ. Although there is no universally agreed upon set of criteria to define glaucomatous field loss, every ophthalmologist understands the meaning of the term nasal step. I suspect that many ophthalmologists learned during their residency what a glaucomatous field “looks” like and still operate by an “I know it when I see it” or “that’s bad field loss” set of interpretational criteria. I think this approach is insufficient. Unfortunately, however, there is no universally accepted definition of what constitutes mild, moderate, or severe field loss.

Multiple staging systems have been designed, but none is in wide use. The scales for the Advanced Glaucoma Intervention Study1 and Collaborative Initial Glaucoma Treatment Study2 were designed as research tools and are too complicated to be clinically useful. Two other staging scales seek to balance ease of use (fewer stages) with sufficient detail to enable the detection of change (more stages). They are the Hodapp-Parrish-Anderson (HPA)3 and Spaeth Field Damage Likelihood Score (FDLS) staging systems.4,5

The HPA system is based on the Humphrey STATPAC printout and is easy to use, but it is probably best applied when determining baseline damage. Because it only contains three broad stages, the system is not able to detect all cases of progression. Different criteria to assess progression have therefore been developed. Table 1 presents an explanation of the staging system, slightly modified for SITA testing. One problem with the HPA system is that cataract can depress the mean deviation and thereby render the field score worse than it really is. One way to correct for the effect of cataract is to adjust the mean deviation by the foveal threshold. If the foveal threshold is depressed 3 dB, then add 3 dB to the mean deviation, thus making it less negative.

The FDLS system was developed for use with the Humphrey perimeter pattern deviation plot, but it is not specific for automated perimetry. For that reason, it can be used to track patients who may have had a variety of kinetic and static tests over the course of many years. Although simple to use, the system’s employment of eight stages (Table 2) means that movement to a different stage is triggered by smaller field changes when compared with the HPA system. The FDLS system also appears to agree with an independent assessment of field change more often than does the HPA system. It is important to recognize that the former grades the field only by the number and location of depressed points. The system does not necessarily detect deepening of an existing scotoma. Neither can it easily classify the occasional patient who has only a central island of vision, a uniformly depressed total deviation plot, and yet a relatively normal pattern deviation plot.

Despite their limitations, these two staging systems agree well with each other and provide helpful information about damage.6

Other Factors in Interpretation
Neither the HPA nor the FDLS staging system provides a complete picture of the disease’s impact upon the patient’s visual perception. Although both give the physician a good sense of an individual eye’s functional impairment, they do not reflect the fact that patients use their eyes simultaneously. Glaucomatous defects often occur in the nasal field, and the patient’s fellow eye can frequently compensate for this impairment (even in cases of advanced loss), because the two nasal fields overlap. The physician can use binocular field tests such as the Esterman in order to assess real-life impairment. This suprathreshold examination available for the Humphrey perimeter tests 150º of horizontal field. Results can be entered directly into the Physicians Desk Reference’s ophthalmic disability calculations and can provide helpful information about a patient’s ability to meet the minimal horizontal field requirements for a driver’s license. Nevertheless, binocular field status is only one determiner of glaucoma-related visual disability and quality of life.7-9

CONCLUSION
The reasons to perform visual field testing are to understand glaucoma’s effect on the patient and make informed treatment decisions. When interpreting fields, one should first assess their reliability, then determine the likelihood that they are demonstrating pathology, and, finally, identify the cause of that pathology. At that point, one may use a staging system in order to quantify the amount of glaucomatous damage. This information will be helpful in formulating a treatment strategy and establishing a baseline for monitoring the efficacy of treatment. Table 3 presents one option, but other investigators have created similar algorithms.3

At present, I do not feel it is possible to fully understand a glaucoma patient’s situation without visual field information. Certainly, fields are fraught with problems such as insufficient sensitivity to detect early glaucoma, long-term fluctuation, and poor reliability. These issues can limit the usefulness of current field technology. Aside from the medical history, however, physicians have no other way of measuring glaucoma’s impact on their patients’ lives.

Jeffrey D. Henderer, MD, is a member of the Glaucoma staff and an assistant surgeon at the Wills Eye Hospital, and he is Assistant Professor of Ophthalmology at Thomas Jefferson University School of Medicine in Philadelphia. He disclosed no financial interest in any product, technology, or company mentioned herein. Dr. Henderer may be reached at (215) 928-3272; henderer@willsglaucoma.org.

1. Advanced Glaucoma Intervention Study. 2. Visual field test scoring and reliability. Ophthalmology. 1994;101:1445-1455.
2. Gillespie BW, Musch DC, Guire KE, et al. The collaborative initial glaucoma treatment study: baseline visual field and test-retest variability. Invest Ophthalmol Vis Sci. 2003;44:2613-2620.
3. Hodapp E, Parrish II R, Anderson D. Clinical Decisions in Glaucoma. St. Louis: Mosby-Year Book, Inc.; 1993.
4. Henderer JD, Myers JS, Katz LJ. How to make better use of visual field data. Review of Ophthalmology. 2001;8:10:107-113.
5. Henderer JD, Altangerel U, Magacho L, et al. A new visual field staging system for glaucoma: observer agreement compared to three other systems. Paper presented at: The 13th Annual Meeting of the AGS; March 8, 2003; San Francisco, CA.
6. Altangerel U, Henderer JD, Magacho L, et al. Detecting glaucomatous visual field change: a comparison of the new field damage likelihood score (FDLS) with existing field staging systems. Paper presented at: The ARVO Annual Meeting; May 4, 2003; Fort Lauderdale, FL.
7. Parrish RK 2nd, Gedde SJ, Scott IU, et al. Visual function and quality of life among patients with glaucoma. Arch Ophthalmol. 1997;115:1447-1455.
8. Sherwood MB, Garcia-Siekavizza A, Meltzer MI, et al. Glaucoma’s impact on quality of life and its relation to clinical indicators. A pilot study. Ophthalmology. 1998;105:561-566.
9. Nelson P, Apinall P, Papasouliotis O, et al. Quality of life in glaucoma and its relationship with visual function. J Glaucoma. 2003;12:139-150.

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