Restrictive limitation of sursumduction caused by an anomalous muscular structure.

We report a peculiar case of congenitally restricted sursumduction caused by an anomalous band-like structure within the right orbit which was completely distinct from the extraocular muscles. Imaging (static and dynamic ultrasound studies of the orbit and globe, CT and MRI) revealed the origin of the anomalous structure in Zinn's ring, its intraconal course and bulbar insertion. The ultrasound characteristics were similar to those of the extraocular muscles and optic nerve, and contractile activity appeared to occur during eye movements. The seemingly muscular nature of this structure and its total autonomy with respect to the normal extraocular muscles distinguish this case from previously reported cases of congenital restrictive strabismus.