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New Visual Diagnostic Systems Combine Benefits of Visual Electrophysiology and Perimetry

ISCEV and IPS jointly published the FST guideline

The International Society for Clinical Electrophysiology (ISCEV, iscev.org) and the Imaging and Perimetry Society (IPS, perimetry.org) jointly published in January 2024 a guideline for the full-field stimulus test (FST), also known as the full-field stimulus threshold test. FST combines features from visual electrophysiology and perimetry, a development that suggests a new evolutionary path for visual diagnostic systems.

Why is FST important?

Over the last ten years, FST has been successful supporting clinical trials of gene therapies for inherited retinal diseases (IRD). We previously highlighted the BRILLANCE study as an example of a recent breakthrough. Such groundbreaking results bring us closer to a future when gene therapies are commonly prescribed to preserve or restore visual function in patients suffering from vision loss due to IRD. ISCEV and IPS wrote their guideline for FST to bring consistency to data collected in global research and in longitudinal studies. Ultimately, the goal is to help accelerate clinical translation of vision-saving gene therapies.

FST combines the strengths of visual electrophysiology and perimetry

FST was designed for subjects with advanced visual function loss. FST borrows from perimetry the sensitivity of psychophysical tests and its statistical methods to arrive at a sensitivity threshold value. Visual psychometric testing leverages the inherent photometric sensitivity of the visual system, which encompasses the retina and the brain. The visual system has demonstrated the ability to detect as little as a single photon in subjects with normal vision.

FST borrows from visual electrophysiology the strong flash stimulus and dynamic range provided by LEDs embedded in Ganzfeld stimulators. The combination of the strong full-field stimulus with the sensitivity of the functional photoreceptors remaining in the retina of subjects with advanced retinopathy has been successful in supporting studies on the efficacy of emerging gene therapies.

Adding objective tests with pupillometry

While psychophysical tests are inherently sensitive, the reliability and repeatability of the results can degrade due to several factors. These include the subject’s waning attention, fatigue, false response due to photopsia, and difficulty understanding instructions especially among young children.

Advanced pupillometry can provide an objective measure of retinal function triggered by visual stimulation, greatly reducing or eliminating factors affecting psychophysical tests. The tradeoff is that pupillometry involves a signal pathway from the retina to the pupil that is still an area of research. Therefore, pupillometry does not represent as direct a measure of retinal function as electroretinography (ERG) or visual evoked potentials (VEP). Pupillometry is a more sensitive test than ERG or VEP, and an option to consider in cases where a psychophysical test is not reliable.

The future of retinal diagnostics: more flexibility, faster speed

Through their collaboration on FST, ISCEV and IPS cut across the boundaries of visual electrophysiology and psychophysics to define a successful new test of visual function. This may be the first step on the path to diagnostic platforms that integrate these modalities in innovative ways, including with advanced pupillometry. The versatility of such multimodal diagnostic systems has the potential to accelerate research and new therapeutic breakthroughs in both ophthalmology and neurology.

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