Myokymia as a presenting sign of congenital smooth muscle hamartoma.

We report a 4-week-old male infant with intermittent asymptomatic bouts of vermicular movements of the skin (myokymia). Myokymia started at the age of 4 days. Attacks occurred approximately every 30-60 minutes and could be elicited by gently stroking the skin. Macroscopically no abnormalities typical for smooth muscle hamartoma such as hypertrichosis or hyperpigmentation were observed. Histologic examination of a skin biopsy specimen showed bundles of hyperplastic smooth muscle tissue within the reticular dermis which were associated with hair follicles. Karyotyping did not reveal any structural or numeric chromosomal anomaly. The findings led to the diagnosis of congenital smooth muscle hamartoma. On follow-up at the age of 24 months myokymia persisted, but with a marked reduction in the number of attacks per day. The phenomenon of myokymia results from neurally induced spontaneous contraction of dermal smooth muscles. It is also known as "pseudo-Darier's sign." While most of those affected previously reported had hypertrichosis or hyperpigmentation, this patient seems to be the first to present with myokymia alone.