This prospective study found that the disparity among three glaucoma diagnostic techniques standard automated achromatic perimetry (SAP), multifocal visual evoked potential technique (mfVEP), and optical coherence tomography (OCT) – could be explained by known limitations of each technique and interindividual variability. This suggests that agreement among diagnostic tests may be better than summary statistics suggest and that the disagreements do not indicate discordance in the structure-function relationship.
One of the frustrations as a clinician is making decisions for a patient in the face of inconsistent data. This article compares discordant results from achromatic visual fields, mfEVP (which is not used frequently in a non-academic setting) and OCT.
The authors tested 69 patients (138 eyes) with glaucomatous optic neuropathy using SAP, mfVEP and OCT. Eyes with worse and better mean deviations (MDs) were analyzed separately. If the results of two tests were consistent for the presence of an abnormality in the same topographic site, that abnormality was considered a true glaucoma defect. If a third test missed that abnormality (false-negative result), the reasons for disparity were investigated. Eyes with worse MD had better agreements among tests than eyes with better MD (P < 0.01). For 94 of the 138 hemifields with abnormalities of the more advanced eye, the three tests were consistent in showing the same hemifield abnormality in 50 hemifields, and at least two tests were abnormal in 65 hemifields.
It appears intuitive that among patients with more advanced glaucomatous damage, the results demonstrated better agreement. In our day-to-day patient management, it is often more challenging to detect progression in glaucoma suspects or patients earlier in the course of disease. Having said that, the Ocular Hypertension Treatment Study clearly demonstrated that some patients will manifest functional loss first while others will manifest structural loss earlier. As technologies evolve, hopefully the gap will narrow in terms of separating normal subjects from those with early glaucoma, and earlier detection will become more realistic.
The authors give two potential explanations for the false-negative results. First, all of the techniques are aimed at detecting distinct features of glaucoma and have inherent limitations. For example, it is known that the mfVEP detects more central defects than SAP because it tests more points in the central field than a 24-2 test. On the other hand, it misses points outside of the central 10 degrees owing to sparser sampling.
The second potential explanation they give for the false-negative results is that these techniques define abnormality based on the normal distribution of values from controls. Normative databases for different technologies may not reflect the populations under investigation, as ethnicity and age, for example, are not always considered in these comparisons. The ability of a given technique to detect statistically significant abnormalities will depend on where a given patient was situated in the normal distribution before glaucoma started to progress.
The authors recommend ways to improve agreement between the tests: 1) use continuous probability scales; 2) superimpose topographic results; and 3) compare measurements in the same individual at different time points.
They conclude that by better understanding the limitations of a particular test, the physician should be able to choose the test results on which to depend when confronted with conflicting results for a particular patient.