Primary localized amyloidosis is a disease characterized by the deposition of abnormal protein fibrils in a specific tissue without systemic involvement. We report a rare case of primary amyloidosis involving the extraocular muscle. Case summary

A 51-year-old female visited our clinic due to diplopia. There was a 3 mm exophthalmos and restricted ocular motility in infraduction. Right hypertropia with 25 prism diopters was observed in the primary position. Orbital imaging scans showed fusiform enlargement of the inferior and medial rectus muscles with significant contrast enhancement in the right eye. A calcified nodule embedded in the medial rectus muscle was observed. Empirical steroid pulse therapy was conducted, but the patient showed little response. Subsequently, an incisional biopsy of inferior rectus muscle was performed. A histopathological examination showed amyloid deposits in hematoxylin and eosin staining, and a positive birefringence in Congo-red staining under polarized light, confirming amyloidosis. A systemic work up showed no evidence of systemic amyloidosis besides increased levels of rheumatoid factor. Conclusions

When infiltrative myopathy of the extraocular muscle is suspected in a patient with exophthalmos and enlargement of extraocular muscles, primary amyloidosis should be considered in the differential diagnosis.