For people with Leber congenital amaurosis (LCA), measuring vision can be difficult—especially when vision is severely reduced. Many familiar eye tests depend on seeing letters, identifying shapes, or keeping the eyes fixed on a target. These demands may make conventional visual-field testing unreliable for some people with advanced inherited retinal disease.
A 2026 review in the Journal of Clinical Medicine examines full-field stimulus threshold (FST) testing, a method designed to measure remaining light sensitivity across the retina. The review highlights why FST has become increasingly important in inherited retinal dystrophies, including LCA and retinitis pigmentosa (RP), and why it is being used as a functional outcome measure in clinical trials of emerging treatments such as gene therapy.
What Is Full-Field Stimulus Threshold Testing?
FST is a psychophysical test, meaning it records a person’s response to a visual stimulus. Instead of asking someone to look at a small spot in a specific location, FST presents flashes of light across the full visual field. The brightness of the flash is adjusted to identify the dimmest light a person can reliably detect—their threshold.
In simple terms, FST asks: how bright does a full-field flash need to be before it can be seen? A person who can detect a dimmer flash has greater residual light sensitivity than someone who requires a much brighter flash.
This approach is particularly valuable in LCA, a group of inherited retinal disorders that typically causes severe vision impairment from infancy or early childhood. In some forms of LCA, central vision, fixation, eye movements, or the ability to perform standard vision tests may be substantially affected. Because FST does not require precise fixation on a target, it can provide information when other tests are difficult or impossible to complete reliably.
Why Residual Vision Matters
Severe visual impairment does not necessarily mean that all retinal light sensitivity has been lost. A retina may retain enough functioning cells to detect light even when detailed vision is very limited. FST can help capture this residual function.
The review describes FST as a global measure of retinal function. Rather than mapping sensitivity at many individual retinal locations, it assesses the overall ability of the retina to respond to light. This can be especially useful when retinal disease is widespread, as it often is in advanced IRDs.
For patients and families, this distinction matters. Standard measures such as visual acuity do not always reflect changes in light perception or broad retinal sensitivity. A treatment might improve the ability to detect dim light without immediately changing letter-chart vision. FST offers a way to measure that type of functional change.
A Growing Role in LCA and Other IRD Clinical Trials
The review identifies FST as an important outcome measure in clinical trials, particularly trials evaluating novel gene therapies for IRDs. Gene therapies aim to address disease-causing genetic changes by delivering a functional genetic instruction to retinal cells. To understand whether a treatment is helping, researchers need measures that can detect meaningful changes in vision across a wide range of disease severity.
FST is well suited to this challenge because it can identify residual visual function in people with profound impairment and does not depend on stable fixation. In trials involving LCA and RP, this makes it possible to assess whether participants become more sensitive to light or maintain light sensitivity over time.
FST may also help researchers follow disease progression. If thresholds worsen over repeated visits, that may indicate declining retinal sensitivity. If thresholds improve or remain more stable after treatment, the findings can contribute to the overall picture of treatment effect. Importantly, FST is one part of a broader assessment: clinical studies may combine functional testing with other measures of vision and retinal health.
The Need for Consistent Testing Standards
Despite its promise, FST is not a perfect or fully uniform measure. The review emphasizes ongoing challenges involving standardization and test variability. Results can be influenced by the details of the testing protocol, including how the test is performed and how responses are collected. For FST to serve as a dependable metric across clinics and trials, researchers need consistent procedures and carefully optimized protocols.
This work is especially important for rare diseases such as LCA. Small clinical trials may involve participants at different sites, sometimes in different countries. Standardized FST methods can help ensure that a change in light sensitivity is interpreted consistently, whether it is measured at one visit, another clinic, or another study.
Looking Ahead
The growing use of FST reflects a broader shift in inherited retinal disease research: finding outcome measures that match the lived reality of severe vision loss. For people with LCA, the ability to detect light may be a meaningful remaining function and a relevant sign of retinal activity.
As protocols become more standardized, FST may become an increasingly established tool in both care and research. Its fixation-independent design and ability to detect residual function make it particularly valuable as gene therapy and other treatment approaches continue to advance. Better measurement will help researchers more clearly identify when a therapy is preserving or improving retinal function—and help ensure that changes important to people with LCA are not overlooked.