Branchio-otic syndrome (also known as BOS, BOS1, BO syndrome 1 and branchiootic dysplasia) is a rare autosomal dominant disease. It manifests as abnormalities of the second branchial arch, with predominant abnormalities of the ear. No renal disease is seen, in contradistinction to its close namesake branchio-oto-renal dysplasia (BOR), with which there is broad overlap in clinical presentation, pathogenesis and management approach 1.
The incidence of branchio-oto-renal dysplasia is 1 in 40,000 births, and therefore branchio-otic syndrome is somewhat less than this 1.
The primary abnormalities are secondary to maldevelopment of the 2nd branchial arch 1.
Absence of renal abnormalities: if present then the diagnosis is branchio-oto-renal dysplasia.
Mutations in the EYA1 and SIX1 genes are found in branchio-otic syndrome 1. Penetrance is fairly high, but not complete, with both intra- and inter-family variability in its clinical severity 1.