Accessory Extraocular Muscle With Inflammatory Enlargement: A Rare Presentation

A 63-year-old man presented with progressive left-sided proptosis and restrictive strabismus. Initial orbital MRI (Panel A), a fat-saturated axial T2-weighted sequence, demonstrated diffuse enlargement and edema of the left extraocular muscles, most prominently the superior rectus. This diffuse muscle enlargement and enhancement raised suspicion for idiopathic orbital inflammation. An irregular muscular structure was also seen between the optic nerve and the lateral rectus (red arrow, Panel A), showing signal intensity similar to the other extraocular muscles. Retrospective review of these early images revealed a suspicious muscular structure in the inferolateral orbit, raising the possibility of an accessory extraocular muscle. A new follow-up orbital MRI was performed, and identified a rare accessory extraocular muscle arising from the common tendinous ring and coursing parallel to the lateral rectus. The axial T2-weighted sequence (red arrow, Panel B) showed interval reduction in extraocular muscle thickness and edema, and the accessory muscle demonstrated a marked decrease in volume while maintaining signal intensity similar to the other left orbital muscles, allowing the diagnosis. Notably, this supernumerary muscle enlarged during the active phase of idiopathic orbital inflammation, mirroring the behavior of the primary extraocular muscles, and subsequently underwent marked volume reduction following clinical and radiologic improvement. This dynamic inflammatory enlargement masked its congenital origin and contributed to diagnostic uncertainty. Recognition of this very rare anatomic variant was only possible once the inflammatory process subsided. The patient remains clinically and radiologically stable.FIG.