Background A de novo mutation of the ACTA2 gene encoding the smooth muscle cell α-actin has been established in patients with multisystemic smooth muscle dysfunction syndrome associated with patent ductus arteriosus and mydriasis present at birth.

Objective To describe the structural ocular findings in three Danish children with this new syndrome and evaluate the possible functional consequences for visual development of the poorer imaging condition.

Results Unresponsive mydriatic pupils with scalloping wisps of persistent pupillary membrane from the iris collarette were an early indicator of this rare genetic disorder in all three cases. Tortuousity of retinal arterioles was the main posterior pole finding, apparent during the first year of life and with a tendency to increase with age. In one case, it progressed to an aneurysmal-like state with breakdown of the blood–retinal barrier.

Conclusions Congenital mydriasis is an extremely rare pupil anomaly and is the feature for the early diagnosis of this new syndrome. The ophthalmologist should act in close collaboration with other specialists owing to the risk of aortic and cerebrovascular diseases and other complications associated with this disorder.