BLOG: How to judge visual field progression with confidence
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A vital part of glaucoma management is assessing patients for the possibility of progressive disease.
Progressive disease may be noted using structural measures (optical coherence tomography) or functional measures (automated visual field tests). A late call on glaucomatous progression results in delayed therapy and irreversible damage while calling progression pre-emptively can result in overtreatment of one’s condition. Regarding functional criteria, simply reviewing individual visual field printouts can not only be time-consuming, but also lead to inaccuracy in detection of progressive disease. Indeed, clinicians have been shown to have poor agreement when assessing for glaucomatous progression in this fashion. Fortunately, we have many software tools at our disposal to help in making this judgment.
Event-based Analysis
Event-based analysis techniques refer to comparison of the most recent testing with baseline testing. The Guided Progression Analysis (GPA; Carl Zeiss Meditec) uses this technique by comparing the individual pattern deviation values of individual points on the most recent Humphrey Visual Field (HVF, Carl Zeiss Meditec) with the average value of two baseline tests. Importantly, the GPA takes into account test-retest variability by indicating worsening of a given point only once deterioration has exceeded the 95% confidence interval for test-retest variability. If three HVF test points have been ruled as worsening on two separate tests, then then the GPA printout indicates “possible progression.” If this occurs on an additional test (three total) then “likely progression” is indicated.
Trend-based Analysis
Trend-based analyses use linear regression models and incorporate data from all of a patient’s visual field tests that have not been excluded by the clinician. The HVF device uses the global metric, Visual Field Index (VFI), to assess the rate of global visual field loss over time. Using regression models, the analysis generates a trend-line to predict future visual field loss.
Tips for Clinical Practice
Event and trend-based analyses have helped me tremendously when assessing for the possibility of glaucomatous progression using visual fields in my clinical practice. These software tools take into account expected test-retest variability, which is important since visual field sensitivity is expected to fluctuate to some degree, especially in glaucomatous individuals. The utility of these tests is amplified with increased frequency of testing. After making an initial diagnosis of glaucoma, I will often obtain at least six (or more if the diagnosis is normal-tension glaucoma) visual field tests in the first 2 years in order to leverage these tools to their greatest capacity. If a patient is deemed to have progressed, I will often aim to incrementally lower IOP using medical, laser, and/or surgical therapies. It is important to take life expectancy, rate of predicted future vision loss, and risks of these treatment modalities into account when deciding. If additional therapy has been undertaken, the clinician may choose to re-establish a baseline with which to compare future tests. Using visual field progression software in combination with structural measures offers a powerful and reasoned approach to glaucoma decision-making.
- References:
- Tanna AP, et al. Ophthalmology. doi:2011;118:60-65.
- Aref AA, et al. Ophthalmology. doi:2017;124:S51-S56.
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