Abstract
Idiopathic orbital myositis is a subgroup of idiopathic orbital inflammation of unknown etiology that primarily involves extraocular muscles. Orbital myositis most commonly affects young adults in the third to fourth decade of life with a female predilection. Imaging is an important diagnostic technique. Inflammatory changes in the tendon and its insertion point to idiopathic orbital myositis. Other inflammatory, endocrine, or neoplastic diseases must be ruled out. We report a case of a 12-year-old girl who presented to the ophthalmologic emergency department with spontaneously developing pain during horizontal eye movement in the left eye and nonspecific symptoms, such as light sensitivity and slightly blurred vision. Her symptoms were nonspecific and physical examination was not diagnostic. However, computed tomography and magnetic resonance imaging of the head revealed isolated, monolateral, and monomuscular lateral rectus myositis as a manifestation of idiopathic orbital inflammation. The patient rapidly responded to systemic corticosteroids. Diagnostic radiology plays a significant role in the unsuspected diagnosis of myositis. Radiologists must be aware of this rare presentation of idiopathic orbital inflammation in preteens.