Orbital apocrine hidrocystoma at trochlea-tendon interstice: Surgical anatomy and clinicoradiological correlation

Orbital apocrine hidrocystoma is rare and usually found in children. Although superomedial lesions are described, this case illustrates the intricate occurrence at the tendon-trochlea interface, an anatomical niche not reported yet. Meticulous dissection while ensuring complete excision is essential to avoid complications such as iatrogenic Brown's syndrome.

A 33-year-old lady was referred for an incidental left-sided orbital lesion. Visual acuity was 20/20 in both eyes. Examination revealed a well-defined, non-tender, firm mass in the superomedial left orbit with normal ocular motility. Magnetic resonance imaging (MRI), T1-weighted fat-suppressed, axial view, demonstrated a well-circumscribed, non-enhancing, extraconal lesion (arrow), closely related to the superior oblique (SO) tendon (Panel A), appearing hyperintense on T2-weighted sequence (arrow), suggestive of an orbital cyst (Panel B). Excision biopsy was performed through upper eyelid crease incision. Intra-operatively, a thin-walled cyst (labelled C) was found nestled between the distal SO tendon laterally (asterisk) and the trochlear spine medially (circle) (Panel C and D), with clear fluid leakage at the site of adherence. The lesion was completely excised without injury to the tendon-trochlea complex (Panel E). Histopathology (H&E, ×200), revealed a cystic lesion lined by double layer of epithelium with decapitation secretions and apical snouts (arrow), consistent with apocrine hidrocystoma (Panel F).