A 12-year-old boy from rural Maharashtra presents with progressive hearing loss over the past 5 years, now requiring hearing aids. Audiometry shows bilateral symmetric sensorineural hearing loss most marked at high frequencies, as marked **B** in the diagram. He also reports progressive visual loss with photophobia since age 3, truncal obesity, and recent diagnosis of type 2 diabetes mellitus with acanthosis nigricans. His parents are consanguineous. Which of the following best explains the underlying pathophysiology of the hearing loss pattern marked **B**?
A. Phytanic acid accumulation causing demyelination of the vestibulocochlear nerve
B. Mutations in GJB2 gene causing connexin-26 gap junction defects in the cochlea
C. Loss-of-function mutations in ALMS1 gene disrupting centrosomal and basal body function, impairing ciliary trafficking in cochlear sensory epithelium
D. Rod photoreceptor degeneration leading to secondary cochlear dysfunction via shared retinal-cochlear metabolic pathway
Explanation
Why option 1 is correct
The hearing loss pattern marked B — bilateral, symmetric, progressive sensorineural hearing loss with high-frequency predominance in early childhood — is a cardinal feature of Alström syndrome. The clinical constellation (cone-rod dystrophy with early photophobia, childhood obesity, early-onset type 2 diabetes with insulin resistance, and acanthosis nigricans) is pathognomonic for Alström syndrome. The underlying mechanism is biallelic loss-of-function mutations in the ALMS1 gene on chromosome 2p13. ALMS1 protein localizes to the centrosome and basal body of primary cilia, and its loss disrupts ciliary trafficking and signal transduction. This ciliary dysfunction affects multiple tissues including the cochlear sensory epithelium, resulting in the characteristic progressive sensorineural hearing loss. The high-frequency predominance reflects the vulnerability of basal cochlear regions where ciliary dysfunction is most pronounced.
Why each distractor is wrong
Option 2 (Rod photoreceptor degeneration): While Alström does feature cone-rod dystrophy, the hearing loss is not secondary to photoreceptor loss. The SNHL results directly from ALMS1 dysfunction in cochlear cilia, not from retinal pathology. Moreover, the cone > rod pattern in Alström is opposite to retinitis pigmentosa (rod-cone), and Usher syndrome (which combines SNHL + retinitis pigmentosa) does not feature obesity, diabetes, or cardiomyopathy.
Option 3 (GJB2/Connexin-26 mutations): GJB2 mutations cause non-syndromic hearing loss or syndromic forms like Keratitis-Ichthyosis-Deafness (KID) syndrome. This patient's multi-system phenotype (obesity, diabetes, cardiomyopathy, visual loss) is incompatible with isolated connexin-26 defects and does not explain the systemic ciliary dysfunction.
Option 4 (Phytanic acid accumulation): This describes Refsum disease, a peroxisomal disorder characterized by peripheral neuropathy, ataxia, retinitis pigmentosa, and elevated plasma phytanic acid. Refsum disease does not feature cone-rod dystrophy, childhood obesity, insulin resistance, or cardiomyopathy, making it a key differential diagnosis but not the correct answer here.
