A 35 yr old male presented with a 15 day history of proptosis in his right eye and pain on eye movement. There is difficulty in upward and downward gaze movements. CT scan showed a cystic lesion with a hyperdense opacity within it, located in the superior oblique muscle. What is the MOST probable diagnosis?

On the basis of physical findings and CT scan findings this patient is most likely suffering from infection with cysticercosis cellulosae the larval form of Taenia solium. Extraocular muscle and subconjunctival tissue are the two most common sites of extraocular cysticercosis. Extraocular muscle cysticercosis is the commonest form of orbital cysticercosis and patients presents with a red eye, restricted ocular motility and or proptosis. Patients develop decreased ocular motility in the direction of action of the involved muscle and a restriction of opposite movement. In the eye intraocular posterior segment such as vitreous and subretinal space are commonly affected. CT scan done in patients with orbital cysticercosis shows a cystic lesion near or within an extraocular muscle. An intraocular mass or cyst may be present. A scolex can usually be identified within the cystic lesions in nearly one half of the patients by CT or MRI and is diagnostic. Ref: Roy and Fraunfelder's Current Ocular Therapy edited by Frederick Hampton Roy, page 106-7. Imaging of the Head and Neck By Mahmood Mafee, page 59.