Chronic Idiopathic Recurrent Orbital Myositis in a Patient With Concomitant Late-Onset Hashimoto's Thyroid Eye Disease.

A 38-year-old female presented to a tertiary centre with a nine-year history of chronic recurrent isolated lateral rectus orbital myositis (OM). Initially, her condition responded to short courses of oral corticosteroids and orbital steroid injections, but it later became refractory, with an inability to taper off steroids. Concurrently, the patient developed a low-grade clinical activity score for bilateral thyroid eye disease (TED) due to Hashimoto's thyroiditis (HT). Although the active phase of TED resolved without significant sequelae, she continued to experience unilateral pain and extraocular muscle (EOM) motility restriction consistent with lateral rectus myositis. This report describes a rare case of chronic recurrent idiopathic OM overlapping with a concomitant bilateral TED associated with HT, which was successfully managed with rituximab immunosuppression.