Clinical Scenario:
A 9-year-old boy has progressive night blindness and difficulty seeing in dim light. He is obese and has postaxial polydactyly. He has learning difficulties and hypogonadism. Renal ultrasonography reveals structural renal abnormalities. Which of the following is the most likely diagnosis?
A. Alström syndrome
B. Bardet-Biedl syndrome
C. Cohen syndrome
D. Prader-Willi syndrome
E. Laurence-Moon syndrome
Correct answer & Explanation:
Answer: B. Bardet-Biedl syndrome
Discussion
Bardet-Biedl syndrome (BBS) is a genetically heterogeneous ciliopathy characterized by the combination of:
- Rod-cone dystrophy causing progressive night blindness
- Postaxial polydactyly
- Obesity
- Hypogonadism or genitourinary abnormalities
- Learning difficulties or intellectual disability
- Renal abnormalities
The combination of retinal dystrophy + postaxial polydactyly + obesity is particularly characteristic of BBS.
Alström syndrome can cause obesity and cone-rod dystrophy but typically lacks polydactyly and is often associated with hearing loss and cardiomyopathy.
Prader-Willi syndrome causes severe obesity, developmental difficulties, and hypogonadism but does not characteristically cause retinal dystrophy or polydactyly.
Cohen syndrome may cause developmental delay, hypotonia, characteristic facial features, neutropenia, and retinal dystrophy, but polydactyly is not a typical feature.
The historical distinction between Bardet-Biedl and Laurence-Moon syndromes is less useful in modern classification. Bardet-Biedl syndrome is the established diagnosis for the phenotype described here.
Exam pearl:
Obesity + postaxial polydactyly + retinal dystrophy + hypogonadism ± renal disease → Bardet-Biedl syndrome.
