To report an unusual case of mucosa-associated lymphoid tissue (MALT) lymphoma localized to the left inferior rectus muscle.

A 52-year-old male presented with double vision 6 months in duration, which was exacerbated in the down-gaze. On initial examination, 6 and 30 prism diopters (PDs) of left hypertropia were observed in primary gaze and down gaze, respectively. Prominently limited infraduction was also observed in his left eye. Computed tomography (CT) revealed contrast enhancing mass on the left inferior rectus muscle. He was diagnosed with suspicious orbital pseudotumor and treated with systemic steroid for 2 months. Double vision and limited infraduction was improved and the mass size was decreased on CT. After 21 months, the patient revisited the clinic with the same symptoms. In the down-gaze, 35 PDs of hypertropia were observed in his left eye along with limited infraduction. CT revealed an enlarged mass and left superior rectus muscle recession along with an incisional biopsy of the left inferior rectus muscle were performed. Infiltration by small lymphoid cells was detected with hema-toxylin-eosin and immunohistochemical stainings. Subsequently, the patient was diagnosed with MALT lymphoma localized to the left inferior rectus muscle.

This is the first description of MALT lymphoma with inferior rectus muscle invasion in a Korean patient. In a patient with strabismus and limited duction, the possibility of extraocular muscle tumor including lymphoma should be considered. Homogenously contrast-enhanced mass on orbit CT can help in to make a diagnosing extraocular muscle lymphoma.