Dual Genetic Diagnosis of Prader-Willi Syndrome and TMC1-Related Severe Congenital Hearing Loss: Diagnostic Challenges and Cochlear Implant Outcomes.

Background and Clinical Significance: Prader-Willi syndrome (PWS) is an imprinting disorder not typically associated with severe congenital sensorineural hearing loss (SNHL). When profound SNHL is present in an infant with a known syndrome, an independent monogenic etiology should be considered. We report the first molecularly confirmed case of PWS co-occurring with biallelic pathogenic TMC1 variants causing congenital SNHL, outlining diagnostic challenges, cochlear implant (CI) outcomes, and implications for blended phenotypes. Case Presentation: A male infant with PWS due to a paternal 15q11.2-q13 deletion failed newborn hearing screening. Diagnostic auditory brainstem response and auditory steady-state response confirmed bilateral severe-to-profound SNHL. Temporal bone CT/MRI were normal. Comprehensive genetic testing identified compound heterozygous TMC1 variants consistent with autosomal recessive DFNB7/11 hearing loss, plus two variants of uncertain significance in SERPINB6 and EPS8L2. Sequential bilateral cochlear implantation was performed (left ear at 14 months, right at 20 months), followed by auditory-verbal therapy. Over four years, the child showed steady improvements in hearing and early-speech development. Conclusions: Early genomic evaluation is essential when clinical features appear atypical for a known syndrome. Identifying TMC1-related deafness enabled timely cochlear implantation and measurable gains. This case highlights that severe congenital SNHL in a syndromic infant may reflect a distinct monogenic disorder rather than phenotypic expansion of the primary syndrome, emphasizing the importance of recognizing blended phenotypes to guide precision-care strategies in rare disorders.