When structure and symptoms don’t align: Using ERG to clarify the glaucoma suspect

Clinicians are trained to look for alignment—between what patients report, what we observe structurally, and what we measure functionally. When those elements line up, decision-making tends to follow a clear path. When they don’t, the challenge we face is not a lack of data, but determining which findings matter most.

This case highlights that tension.

Case presentation

A 54-year-old Black man presented with a somewhat unusual complaint: He felt that certain colors—particularly yellows and oranges—looked faded. He also noted increasing sensitivity to light. His visual acuity, however, remained 20/20 in each eye.

His history added complexity. He reported a family history of glaucoma and had a diagnosis of hypertension (medically controlled), and pachymetry revealed thin corneas. On paper, this patient warrants careful observation.

Initial testing was reassuring. Intraocular pressures were within normal range by both Goldmann applanation and iCare tonometry. Optical coherence tomography (OCT) imaging showed a normal retinal nerve fiber layer, and standard automated perimetry did not reveal any visual field defects (Figure 1).

On fundus examination, the optic nerves showed mild asymmetry in cupping, although without clear focal notching or rim thinning (Figure 2). At this stage, the patient could reasonably be classified as a glaucoma suspect, despite no definitive evidence of disease.

A finding that didn’t quite fit

Because of his symptoms, the patient underwent color vision testing. The results suggested a tritan (blue-yellow) defect (Figure 3).

This result introduced some uncertainty. Tritan defects are not typically associated with early glaucoma and are more often acquired in the setting of media opacity or retinal dysfunction. At the same time, the patient’s pressures, OCT, and visual fields were all normal.

It raised a practical question that comes up more often than we might admit: Is this test helping to clarify the picture—or complicating it?

The value of objective functional testing

To better understand the source of the patient’s symptoms, electroretinography (ERG) was performed, including both flicker and photopic negative response (PhNR) protocols.

The flicker ERG showed a mild reduction in amplitude (Figure 4). The change was subtle and, in isolation, not particularly specific. Given the patient’s history of hypertension, this was interpreted cautiously, as vascular factors can influence outer and middle retinal function.

More importantly, the PhNR—revealing ganglion cell function—remained within normal limits (Figure 5). The W-ratio was preserved, indicating intact inner retinal function.

That distinction proved meaningful. If early glaucoma were present, some degree of ganglion cell dysfunction would be expected. Instead, the functional data aligned with the normal OCT and visual field findings.

Reframing the diagnosis

With no structural or functional evidence of glaucomatous damage, it became necessary to revisit the patient’s symptoms rather than relying upon the test results alone.

Further discussion revealed that his light sensitivity was most noticeable at night, with glare and haloing around lights. These descriptions were more consistent with early lens changes than with optic nerve disease. The complaint of faded color perception—initially concerning—also aligned with cataract formation, particularly given the known effect of lens yellowing on short-wavelength light transmission.

In retrospect, the color vision findings were not misleading so much as misinterpreted. They reflected a real change in visual perception, but not one originating from the optic nerve.

Clinical decision

The patient was diagnosed as a glaucoma suspect, given his risk profile, but without evidence of active disease. His symptoms were attributed primarily to early cataract formation.

Rather than initiating treatment, a decision was made to monitor via the following:

• Structural testing (OCT)
• Functional testing (visual field, ERG)
• Clinical examination over time

He was scheduled for follow-up in 6 months, with the goal of detecting any meaningful change rather than reacting to isolated findings.

Clinical insights

Several points from this case are worth carrying forward into daily practice.

First, risk factors—while important—do not establish disease. It is easy to escalate management in patients who “look concerning on paper,” even when objective testing does not support intervention.

Second, not all abnormal test results carry equal diagnostic weight. Blue-yellow (tritan) color defects, in particular, often reflect acquired changes and should be interpreted cautiously in the absence of corroborating evidence.

Third, when structural and standard functional tests are normal, additional functional testing can help localize where dysfunction is—or is not—occurring. In this case, ERG provided reassurance that ganglion cell function remained intact.

Finally, restraint is sometimes the most appropriate clinical decision. Avoiding unnecessary treatment is just as important as identifying disease early.

Conclusion

Cases like this are less about identifying pathology and more about interpreting ambiguity. The data did not point in a single direction, and the symptoms did not immediately match the test results.

By stepping back and integrating structural findings, functional testing, and patient-reported experience, it became possible to arrive at a diagnosis that fit the full story.

ERG did not replace OCT or visual fields in this case. It simply added another layer of understanding—one that helped confirm what was not happening, and in doing so, clarified what was.