Why this family study matters
Bardet-Biedl syndrome (BBS) is a genetically diverse condition in which changes in genes involved in cilia—the tiny cell structures that help cells sense and respond to their environment—can affect vision and other parts of the body. Retinal degeneration is a central concern for many people with BBS, but the combination and severity of features can vary substantially, even among relatives.
A 2026 BMC Ophthalmology case series offers an important close look at this variability. Researchers described four Palestinian siblings from a consanguineous family who share the same disease-causing genetic finding in CFAP418. All four siblings have a shared molecular and retinal-disease context, yet they did not have identical features. Most notably, only the male twins had congenital postaxial polydactyly—extra fingers or toes on the outer side of the hands or feet.
For patients and families, this study reinforces a practical message: a shared genetic diagnosis does not guarantee an identical clinical experience. Care needs to be guided by each person’s own vision, symptoms, functional needs, and broader health assessment.
The shared genetic finding
The siblings were found to have two copies of the same CFAP418 splice-donor variant: NM_177965.3:c.155+1G>A. A splice variant can disrupt how a gene’s instructions are processed, potentially changing or reducing the protein made from that gene. This variant had previously been reported in the family in a 2018 research letter. A 2019 clinical laboratory report confirmed the variant in the older brother and sister and showed that both parents carried one copy.
At the time histories were collected in March 2026, the male twins reported being 27 years old, their older brother was 30, and their sister was 26. The twins were described by the family as monozygotic, although independent testing to confirm identical-twin status had not been performed.
The paper’s careful handling of evidence is a strength. It distinguishes between dated clinical records, information recalled by participants, and historical summaries. That distinction matters because clinical measurements taken at the time of an eye examination do not carry the same weight as memories of past symptoms or diagnoses.
What the eye findings showed
The most detailed ophthalmic records were available for the twins. These records documented severe visual impairment alongside features of retinal degeneration, including pigmentary changes in the retina and abnormalities of the macula’s outer retinal layers. They also had posterior subcapsular cataracts, a type of lens clouding located toward the back of the lens.
Testing showed that their visual fields were markedly constricted on Goldmann perimetry. In everyday terms, this means their remaining vision was limited to a small central area, which can make navigating unfamiliar spaces, detecting obstacles, and moving safely more difficult.
Electroretinography (ERG), which measures the retina’s electrical response to light, found virtually absent rod and mixed responses, while some cone responses remained. Rod cells support vision in dim light and peripheral vision; cone cells support central, color, and detail vision. The findings fit with a severe retinal disorder affecting night and side vision, with some remaining cone-related function.
Visual acuity differed between visits and testing conditions. This is an important reminder that a single vision measurement may not fully capture how retinal disease affects someone over time or in different real-world environments.
Contemporary standardized eye data were not available for the older brother and sister. However, participants across the family reported photophobia, progressive difficulty with vision, and major effects on mobility, education, everyday tasks, and family responsibilities.
Different features within the same family
The study’s clearest intrafamilial contrast was polydactyly. The twins had congenital postaxial polydactyly, while their older brother and sister did not. This difference occurred despite the siblings’ shared CFAP418 variant and shared retinal disease context.
The authors note that the 2026 genotype-informed BBS diagnostic algorithm supports a syndromic interpretation across the sibling group. At the same time, incomplete assessment of systemic features limits what can be concluded about involvement of other organs. This is especially relevant in BBS, where features beyond the eyes can be important to identify and monitor.
The report also cautions against overinterpreting recalled symptoms. Memories of changing vision do not, on their own, establish a measured shift between different retinal diagnoses or an electrophysiological transition. Continued standardized testing is essential for understanding disease course.
Implications for care and future research
No measured treatment benefit was established in this report. Documented care included ophthalmic follow-up, low-vision services, recommendations for optical aids, and supportive prescriptions. These supports remain meaningful because they can help people adapt to changes in vision and maintain independence in daily life.
For this family and others with CFAP418-related disease, the findings support regular standardized retinal follow-up, low-vision care, genetic counselling, and multidisciplinary surveillance. Systematic assessments can clarify which features are present, identify support needs, and create reliable baseline information for future research.
More broadly, studies that examine several relatives with the same genetic variant are valuable for the BBS field. They show why researchers cannot assume that one person’s features predict another’s, even within a single family. As genetic knowledge expands, detailed natural-history studies—combining repeated eye testing with thorough assessment of non-eye features—will be crucial for defining meaningful clinical outcomes and preparing for future treatment research.