Full-field electroretinography (ffERG) measures the summed electrical response in the optic nerve and the retina to a flash stimulus from an arc bowl. First introduced into ophthalmology in the mid-20th century, standard protocols for use of ffERG emerged sometime in the late 1980s. In much more recent history, tabletop and roll-cart units with smaller footprints have become available on the market, thus adding greater clinical utility.

The technical capabilities of ffERG are well established. As with any diagnostic modality, however, the relevant question about ffERG is: What does it really tell us, and how might it affect how we treat patients? The short answer is that, as an objective measure of retina function, ffERG can be useful in the clinical assessment of a patient with suspected retinal pathology. It is sensitive to early retina pathology to the point that it can aid in detecting functional deficits before structural changes become apparent on other testing methods. Also, in addition to aiding retina specialists, ffERG testing may be important for working up cataract patients prior to making a lens recommendation and for setting postoperative visual function expectations.

Earlier this year, Diopsys launched the Retina Plus electrophysiology system, which has the functionality of its other ffERG devices in a carry-case size. It offers clinicians a more convenient way to conduct ffERG testing, with the added ability to scale the system to a full visual electrophysiology suite, including multifocal, flash, and pattern ERG, as well as visual evoked potentials.

ELECTRORETINOGRAPHY IN SUSPECTED RETINAL PATHOLOGY

Electroretinography (ERG) is performed by positioning a patient in front of an arc bowl. During the test, either single-light stimulation or a series of flickers are presented to the patient’s visual system. This process is then repeated across a range of luminance under both dark- and light-adapted conditions to record the action potential of retinal cells at or near the cornea. The sum of response of neuronal components of the visual system are thus charted, providing the clinician with information regarding the functionality of the retina, as well as the rod and cone photoreceptor pathways.

ERG is often used in patients with suspected retinal pathology of unknown cause. Frequently, a review of the retina is incomplete or at least inconclusive. For instance, a few microaneurysms may be present, but such a finding is not necessarily pathognomonic of diabetic retinopathy, nor does a finding of microaneurysms automatically portend loss of visual function. Rather, like most clinical findings, microaneurysms indicate a need for further investigation. Certainly, we could use OCT to determine whether there are any relevant structural deficits in or around the areas of concern, but the imaging might still not yield answers regarding underlying retinal disease, whereas several studies have highlighted that ffERG testing is useful for discerning retinal dysfunction in eyes with diabetic retinopathy, making it a powerful tool for the diagnosis and staging of the disease.^1-5

However, ffERG is not a disease state-specific modality; it is an objective measure of global retinal function. Although several studies have emphasized its utility in managing diabetic retinopathy,^1-5 central retinal vein occlusion,^6-12 and uveitis,¹³ ffERG testing has its greatest utility in building a differential diagnosis in patients with confounding or conflicting clinical findings and in those with unexplained visual changes. In a similar vein, ERG testing can help to stage disease (ie, correlation of retinal function with other clinical parameters)^1-5 and/or gauge response to treatment (ie, changes in ffERG results over time).¹⁴ Collectively, these kinds of findings have important prognostic implications for counseling patients about outcomes, but perhaps more importantly, they are useful in discerning when to initiate appropriate treatment, how aggressively to treat, and whether or not additional treatment is likely to have a benefit.

FFERG IN CATARACT PATIENTS

Among the benefits of ffERG is its ability to test through media opacities, for example, corneal scars, hyphemia, and hemovitreous, as well in individuals with cataracts.¹⁵ It is this aspect of ERG that makes it particularly applicable in the evaluation of cataract patients, especially when selecting an intraocular lens (IOL). A dense cataract can undermine the ability to assess the health of the retina, which could have important implications for the successful implantation of a premium IOL.

In the modern era of cataract surgery, when quality refractive outcomes are increasingly stressed and expected by patients, the predictability of the outcome is more important than ever. Thus, ffERG is applicable for a variety of reasons in the evaluation of cataract patients. The first relates to the discussion above regarding suspected retinal pathology. In any individual being assessed for a cataract operation with suspected retinal pathology, it is important to understand retinal function, especially if a multifocal lens is being considered. However, performing ffERG in a patient receiving a monofocal lens who has suspected retinal issues may still be important for managing expectations for the outcome. Studies have shown that ffERG is a powerful tool for predicting visual prognosis after cataract surgery,¹⁶ especially when it is combined with visual evoked potential testing.¹⁷

Another reason that ffERG testing may be important for cataract patients is its potential role in patient selection. I do not recommend routine screening in patients receiving a standard IOL, but it is certainly useful for those with clinical suspicion of retinal pathology. In the case of multifocal IOLs, ffERG may play a role in the evaluation of any patient considering a premium lens. A way to justify the extra out-of-pocket expenditure associated with a premium lens is the use of additional testing to ensure a quality outcome. Part and parcel of this process is ensuring that the macula will support a multifocal viewing system, in which case ERG will help to rule in or out a potential candidate. A practice may have to swallow the cost of testing, but that cost may be minimal if it means avoiding a postoperative refractive surprise.

CONCLUSION

In the era of refractive cataract surgery, knowledge is power. What we can discern prior to surgery helps to confirm that the right patients are matched with the correct lens, aids in managing expectations for outcomes, and lowers the risk of postoperative surprises. ERG plays an important role in evaluating cataract patients, especially if there is suspected retinal pathology or vision loss that does not appear to be wholly attributable to the presence of a cataractous lens.

In years past, cataract patients might have been happy to simply to have vision improvement after their procedures. However, when they are paying extra for a quality outcome, it becomes incumbent on the clinician to protect that investment, both for the patient and for his or her own reputation. The data that we obtain from ffERG is another way in which we assure patients we can deliver the desired postoperative vision.

Full-field ERG is an easily administered diagnostic that can be seamlessly integrated into the clinical workflow. The learning curve for using ffERG is rather minimal, and the return for using ERG in appropriate patients is potentially enormous. RP

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