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Cysticercosis is caused by infection from the larval form of Taenia solium, is endemic to regions with poor sanitation. Human cysticercosis more commonly affects the central nervous system, with less common involvement of ocular tissues. Myocysticercosis is a subset of orbital cysticercosis and is considered a rare entity.
A 19 year old Nepalese housewife presented with left sided headache that had been present for 1 year. She had been treated for migraine headache in another hospital.
Ocular examination revealed orthotropia in primary gaze with Snellen acuity of 6/6 bilaterally. Both the Humphrey visual fields and the colour vision testing were normal in both eyes. There was no relative afferent pupillary defects detected. Ocular motility testing revealed left abduction deficit with the resulting horizontal diplopia. Hertel’s exophthalmometer reading revealed no proptosis. Her fundi examination were both normal and there were no optic disc swelling.
Magnetic resonance image (MRI) (brain/orbit) with gadolinium contrast (fig 1) showed a cystic enhancing mass measuring 1.2 cm×0.6 cm adjacent to and including left medial rectus muscle, sparing the muscle tendon. Further assessment with B-scan ultrasonography (fig 2) revealed an intramuscular cyst within the left medial rectus muscle located within mid-orbit.
Full blood count found no eosinophilia; systemic cysticercosis involvement was excluded by negative radiological findings (chest x ray and computed tomography (CT) of brain and abdomen were all normal).
The patient was prescribed treatment with albendazole 15 mg/kg per day for 8 days. Unfortunately, she had intolerable side effects (nausea, vomiting, and distressing nocturnal left eye pain) to the medication which she used for only 3 days. She was reluctant to continue with albendazole. Her symptoms settled after a short course of oral analgesic.
She has remained asymptomatic. Repeated Hess and diplopia charts B-scan ultrasonography re-evaluation at 6 months did not reveal any cysts in the muscle and her ocular motility had returned to normal.
The finding of “scolex” within the intramuscular cyst and her status of Nepalese native lend strongly to a diagnosis of myocysticercosis. Enzyme linked immunosorbent assay (ELISA) to detect the antibody to cysticercosis was unavailable in Singapore. A positive test may lend support to the diagnosis but a negative ELISA result does not rule out the diagnosis. Owing to the largely isolated and relatively mild infection of myocysticercosis, the sensitivity of ELISA is low.1 For the same reason, the absence of peripheral eosinophilia in this case is not surprising, consistent with the finding in literature.2
None of the cases in a large series of orbital hydatid cysts were found within an extraocular muscle,2 hence making this diagnosis unlikely. The location within an extraocular muscle accounted for only 1.1% to 4.1% of the total reported cases of cysticercosis.2 Statistically, medial rectus is the most commonly involved extraocular muscle,2,3 although any of them can be involved. As a general rule, the restriction of extraocular movements is greatest in the direction opposite to the involved muscle,2 as in this case (fig 2).
Broad spectrum antihelmintics such as albendazole have been proved to be effective in the treatment of myocysticercosis.4,5 Recommended duration of treatment varies from a few days to up to 6 weeks. Prolonged drug administration may not be necessary as seen in this case, in view of the drug’s potential side effects.
Among the known side effects of albendazole are gastrointestinal upsets. The nausea and vomiting in this patient settled when the medication was discontinued. On the other hand, her nocturnal left eye pain would be best explained by secondary inflammation from the release of toxins by the dying parasite.2 Concomitant administration of oral steroids has been suggested to reduce the secondary inflammation.1–4
Surgical excision of an extraocular muscle cyst had been described.6 However the potential risk of damage to adjacent tissue and adhesion from surgical exploration should not be taken lightly, particularly when effective medical therapy is available.
Stool tests should be done for all the members of the family to detect asymptomatic carrier because the treatment with systemic antihelmintic treatment is highly effective.2 It also serve to break the life cycle of the parasite.