Clinical and imaging features of congenital and acquired isolated inferior rectus muscle hypofunction.

BACKGROUND: Inferior rectus (IR) underaction may arise from various causes that are distinguishable through imaging. We investigated clinical and imaging characteristics of congenital and acquired causes of IR underaction. METHODS: Cases of IR underaction were selected from data prospectively collected in a study of orbital imaging in strabismic patients. RESULTS: Review identified 3 cases of congenital IR underaction (2 with bilateral IR aplasia and 1 with unilateral IR hypoplasia), 12 acquired cases, including 4 due to denervation (2 idiopathic, 1 after multiple strabismus surgeries, 1 after head trauma), and 8 cases of direct IR damage (5 with orbital trauma and 3 with previous surgery, including 2 sinus surgery and 1 laser blepharoplasty). Of the 23 cases, 11 adults had high-resolution magnetic resonance imaging, and 2 children had computed tomography. Imaging identified the anatomic diagnosis in congenital cases; in acquired cases, imaging helped to identify atrophy and exclude alternative orbital causes; and in direct mechanical damage, imaging clarified the mechanism of underaction, extent of IR damaged, and the degree of retained contractility. Patients with congenital IR absence or hypoplasia exhibited A pattern exotropia that was typically absent in isolated acquired denervation or direct IR damage. CONCLUSIONS: Orbital imaging demonstrates a variety of abnormalities in patients with congenital or acquired IR hypofunction, helping to clarify the underlying mechanism and guide management.