Major Features
Cone-rod dystrophy. Early macular involvement typically presents first with night blindness, followed by progressive peripheral vision loss, diminution of color discrimination, and overall loss of visual acuity [Weihbrecht et al 2017]. These symptoms are often what brings individuals to medical attention and obtain the diagnosis of BBS, usually in the first decade of life.
Electroretinography (ERG) is more likely to show significant findings after age five years.
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Individuals often become legally blind by the second to third decade of life.
Retinal disease is the most penetrant feature in BBS, affecting up to 100% of individuals in some studies [Denniston et al 2014].
Central (truncal) obesity develops in the first year of life; birth weight is usually normal. Other features commonly associated with obesity are considered minor features in BBS. Mean body mass index has been reported to be 35.7±8.0 kg/m2 [Mujahid et al 2018].
Postaxial polydactyly are additional digits usually found on the ulnar side of the hand and/or fibular side of the foot. Mesoaxial polydactyly is also reported in individuals with pathogenic variants in LZTFL1 (BBS17) [Schaefer et al 2014].
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Cognitive impairment. Historical reports describing significant intellectual disability as a prominent feature did not account for visual impairments.
Kerr et al [2016] evaluated the cognitive, adaptive, and behavioral function of 24 individuals with molecularly confirmed BBS and visual acuity ≥20/400. Visual fields and visual acuity were not significantly correlated with verbal comprehension index or any of the other cognitive measures. While only 20%-25% of individuals met criteria for a diagnosis of intellectual disability, mean intellectual functioning of all participants was 1.5 SD below the mean. Individuals also had impairments in verbal fluency (22%-44%), perceptual reasoning (53%), attention capacity (69%), and functional independence (74%). Features associated with autism spectrum disorders were present in 77%.
Hypogonadism and genitourinary malformations. Hypogonadism with delay in onset of secondary sexual characteristics may not be apparent until puberty.
Males can have micropenis and/or small-volume testes. Cryptorchidism is present in 9% of males with BBS. On endocrinologic assessment in one study, 19.5% of males were hypogonadal [Mujahid et al 2018].
Females can have anatomic anomalies including hypoplastic or duplex uterus, hypoplastic fallopian tubes and/or ovaries, septate vagina, partial or complete vaginal atresia, absent vaginal and/or urethral orifice, hydrocolpos or hydrometrocolpos, persistent urogenital sinus, and vesico-vaginal fistula [Deveault et al 2011].
Infertility is common, but both sexes are known to have been able to have biological children.
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Kidney disease. The renal phenotype of BBS is highly variable and can include structural anomalies, hydronephrosis and vesicoureteral reflux, and progressive renal parenchymal disease that is commonly associated with urinary concentration defects (symptoms of polyuria and polydipsia) [Putoux et al 2012].
Structural kidney disease includes developmental anomalies such as horseshoe, ectopic, duplex, or absent kidneys; or dysplastic cystic disease ranging from single unilateral to multiple bilateral cysts.
Urologic complications including neurogenic bladder and bladder outflow obstruction have been reported in 5%-10% of adults [Forsythe et al 2017].
Chronic kidney disease (CKD) is a major contributor of morbidity and mortality in individuals with BBS. In a recent study, CKD was present in 31% of children and 42% of adults; 6% of children and 8% of adults developed end-stage kidney disease requiring dialysis and/or transplantation [Forsythe et al 2017].
In the majority of children with BBS with advanced (Stage 4-5) chronic kidney disease, the initial diagnosis of renal disease was made within the first year of life and almost all were diagnosed by age five years [Forsythe et al 2017].
Comorbidities including hypertension (present in about one third of individuals with BBS) and type 2 diabetes mellitus (T2DM) may affect progression of CKD.
Favorable long-term outcomes of renal transplantation have been reported [Haws et al 2016].
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