Background
Cysticercus cellulosae, the larval form of the pork tapeworm Taenia solium, is the causative organism of cysticercosis, in which humans are the intermediate hosts in the life cycle. Cysticercus cellulosae may become encysted in various bodily tissues, usually the eyes, central nervous system (CNS), and subcutaneous tissues. An immunologic reaction with fairly intense inflammatory signs and symptoms may be produced, and the surrounding structures may be compressed.
Ocular cysticercosis may be extraocular (in the subconjunctival or orbital tissues) or intraocular (in the vitreous, subretinal space, or anterior chamber). [1, 2, 3, 4, 5, 6, 7]
Pathophysiology
Humans are the intermediate hosts for T solium, and pigs are the definitive hosts. [8] A tapeworm larval cyst (cysticercus) is ingested with poorly-cooked infected pork; the larva escapes the cyst and passes to the small intestine, where it attaches to the mucosa with scolex suckers. Egg-containing proglottids develop as the worm matures in 3-4 months. The adult worm may live in the small intestine for as long as 25 years without symptoms (taeniasis) and pass gravid proglottids intermittently with the feces. Eggs extruded from the proglottid contaminate and persist on vegetation, where pigs consume them. T solium embryos penetrate the gastrointestinal mucosa of the animal host and are then hematogenously disseminated to peripheral tissues with the resultant formation of larval cysts (cysticerci).
Human cysticercosis occurs when T solium eggs are ingested via fecal-oral transmission from a tapeworm host. The human then becomes an accidental intermediate host. These oncospheres (primary larvae) penetrate the intestinal mucosa and enter the circulatory system. Hematogenous spread to neural, muscular, and ocular tissues occurs. Within these tissues, the oncospheres develop into secondary larvae (ie, the cysticerci).
The incubation period may vary from months to years. The host inflammatory response to cysticerci depends on the parasite's ability to evade host immunity; therefore, inflammation is restricted to degenerating cysts whose ability to evade host defenses is faltering. Lack of inflammation occurs with both healthy cysticerci (active disease) and those that have involuted (inactive disease). Upon involution, cysts undergo granulomatous change and exhibit calcification.
Epidemiology
Frequency
United States
The incidence in the United States is increasing secondary to increased immigration from endemic areas, increased travel to endemic areas, and improved serologic testing and availability of diagnostic imaging. An estimated 1000 new cases per year are diagnosed in the United States.
International
Cysticercosis affects an estimated 50 million people worldwide. Ocular cysticercosis is endemic in tropical areas, such as sub-Saharan Africa, India, and East Asia. Other endemic areas include Mexico and Latin America. The reported incidence of ocular involvement varies from 10-30% in endemic areas. Some European studies report a higher incidence of ocular cysticercosis than neurocysticercosis. In Western countries, [9] the most common site of involvement in ocular cysticercosis is subretinal. In India, both intraocular cysticercosis and extraocular cysticercosis is observed with almost equal frequency.
Mortality/Morbidity
Cysticercosis may cause significant visual loss, especially if the cyst is located intraocularly or is compressing the optic nerve.
Race
The incidence is more common in the Asian and Latin American populations, in which cysticercosis is endemic.
Sex
There is no specific sex predilection.
Age
Orbital cysticercosis most frequently affects individuals aged 10-30 years. [10]
Prognosis
With adequate treatment, the prognosis is good in most individuals with orbital cysticercosis as the cyst resolves. Medical therapy, consisting of oral albendazole and corticosteroids, can arrest recurrent inflammation and improve ocular motility.
Intraocular cysticercosis may lead to blindness due to the severity of induced inflammation. Cysticerci compressing the optic nerve may also cause blindness.
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CT scanning of the orbit demonstrating cysticercus within the medial rectus muscle observed as a hypodense area with a central hyperdensity suggestive of the scolex.
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B-scan ocular ultrasonography demonstrating cysticercus within an extraocular muscle observed as a well-defined cystic mass with a central hyperechoic area.
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MRI of the orbit demonstrating deep orbital cysticercus within superior muscle complex observed as a hypointense area with central hyperintensity compressing the optic nerve posteriorly.
