Fibroblastic tumour in the medial rectus muscle presenting as a large-angle exotropia

Objective: The aim is to report a very rare case of a fibroblastic tumour in the medial rectus muscle of the right eye in a young adult. Methods: A 28-year-old patient presented to the strabismus clinic with large-angle exotropia in the right eye. The patient had previously undergone strabismus surgery in both eyes, and the old photos showed increasing exotropia from childhood to adulthood. On examination, the patient had a best corrected visual acuity of 20/20, 85 PD of exotropia with a -4 limitation of adduction, and retinal evaluation was unremarkable. In view of the chronicity of the deviation, increasing exotropia over time, and limitation of the ocular motility in adduction, a magnetic resonance imaging was performed. Results: Orbital magnetic resonance imaging (MRI) undertaken to identify the extraocular muscle position revealed a bulky and thickened right medial rectus muscle with post-contrast enhancement and without fat stranding and orbital extension. A muscle biopsy was performed, and a series of histopathological stains revealed that the tumour cells were positive for CD34 and vimentin. Immuno histochemistry and histopathology suggested a benign fibroblastic tumour (superficial CD34+ fibroblastic tumour) with low malignant potential. Conclusion: This report describes a fibroblastic tumour of the eye muscle identified on neuroimaging and confirmed on histopathology. Such a presentation is very rare and to the best of our knowledge has not been previously reported in ophthalmic literature.