Tetrasomy 18p Syndrome: A Rare Case with Congenital Hypoplasia of the Depressor Angularis Oris

Introduction: Tetrasomy 18p syndrome is an extremely rare chromosomal disorder where there are four copies of chromosome 18 instead of two due to the presence of isochromosome 18p. We aim to report a case of tetrasomy 18p with unusual phenotypic presentation with congenital hypoplasia of depressor angularis oris muscle. Case Report: We present a case of a neonatal boy with tetrasomy 18p syndrome, who was initially admitted for bilateral undescended testes. This syndrome has been found to have variability in its clinical features. The patient was found to have soft dysmorphic features, feeding difficulty, congenital hypoplasia of the depressor angularis oris muscle, growth retardation, hypotonia, and jaundice. During follow-up, he was found to have strabismus, hearing loss, failure to thrive, and global developmental delay. Conclusion: We compared the phenotypic characteristics of our case with previously reported cases of this syndrome. To the best of our knowledge, congenital hypoplasia of the depressor angularis oris has not been reported previously as a phenotypic characteristic of this syndrome and might be considered a diagnostic feature if more cases are reported in the future. Additionally, it is easy to miss such cases, as some neonates do not present with typical dysmorphic features that would prompt a karyotype analysis.