Aplasia of the depressor anguli oris muscle: a rare cause of congenital lower lip palsy?

Congenital unilateral lower lip palsy (CULLP) with or without additional malformations is a well-known limited variation of congenital unilateral facial palsy. Some electromyographical studies referred to a hypoplasia or an aplasia of the depressor anguli oris muscle. However, no attempt has been made to investigate the cause for this mimical disorder by using imaging procedures. We examined the occurrence of the depressor anguli oris muscle in 7 patients presenting with congenital lower lip palsy by using B-scan sonography. In 6 of the patients, the muscle was well-developed on the affected side, but only in one patient the muscle seemed to be completely absent. Thus, in the majority of cases, hypoplasia or aplasia of the depressor anguli oris muscle is obviously not the reason for this mimical disorder. This observation may be important with regard to a possible therapeutic management.