Ocular Cysticercosis

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]

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.

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]

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.

Family members should be screened for infection.

The presentation of ocular cysticercosis varies based on cyst location, from asymptomatic to vision loss or disturbance and a moving sensation in the eye. Redness, photophobia, and pain may occur. Epileptiform seizure can occur if the CNS is infected.

Ocular cysticercosis may affect almost all eye tissues. The vitreous cavity, subretinal space, and subconjunctival space are common sites, while involvement of other regions (eg, extraocular muscles, optic nerve) is relatively less common.[8]

While either eye may be affected, bilateral involvement is rare.[11] Multiple cysts may develop in the same eye.[12]

Ocular manifestations may be devastating as the cysticercus enlarges. The cysticercus, if left untreated, may lead to blindness in 3-5 years.[13] Decreased vision, pain, and recurrent redness of the involved eye are common symptoms of intraocular cysticercosis.

Clinically, the cyst appears as a well-defined translucent mass with a dense white spot (scolex) at one region. When alive, the cyst appears motile due to larval invagination or evagination. The cyst measures between 1.5 and 6 disc diameters and might undulate under the examining light. This motility is absent if the larvae are dead. A live cyst can induce intraocular inflammation because of its motility. Cyst death causes the release of toxic products, which induce a severe zonal granulomatous inflammatory reaction. The cyst becomes less translucent with a surrounding inflammatory membrane.

Up to 46% of patients with ocular cysticercosis present with the cyst in the subretinal space or vitreous cavity, where it can be observed ophthalmoscopically.[14] The site of entry into the eye is the posterior ciliary arteries, from which the cyst migrates into the subretinal space, bores a hole in the retina, and enters the vitreous cavity. This passage incites inflammation, leaving behind a chorioretinal scar.[15] Other sites of entry include the ciliary and retinal blood vessels. Within the vitreous cavity, the cyst may be free-floating[16, 17] and may produce vitritis.

Apart from uveitis,[18] cysticercosis may also lead to retinal hemorrhages, proliferative vitreoretinopathy, retinal detachment, disc edema, cyclitic membrane formation, and phthisis. In individuals with intense inflammation in which the cyst cannot be clinically visualized, imaging modalities help in making the diagnosis.

Cysticerci may develop in the anterior chamber. In these instances, they may produce iridocyclitis and secondary glaucoma.[19] The cyst may be freely mobile or may be attached by inflammatory membranes to surrounding structures, such as the cornea, lens capsule, or iris.[20] The cyst may enter the anterior chamber either from the posterior ciliary arteries or from the angle. Intraocular inflammation may also cause cataract formation[21] and iris atrophy.

Subconjunctival cysticercosis usually presents as a painful, yellowish, nodular subconjunctival mass with surrounding conjunctival congestion. Spontaneous extrusion of an anterior subconjunctival cyst has been reported.[22, 23] Subconjunctival cysticercosis may even present as an eyelid nodule.

Acquired strabismus, diplopia, recurrent redness, and painful proptosis are some of the clinical signs in patients with orbital cysticercosis. One or more extraocular muscles may be simultaneously involved, although a propensity for involvement of the superior muscle complex and the lateral rectus muscles has been reported.[24, 25] Depending on the muscle involved, clinical findings of Duane retraction syndrome or Brown syndrome may be observed. Blepharoptosis may occur due to the presence of the cyst in the superior muscle complex. An intraorbital cyst may cause decreased vision due to external compression of the eyeball.[26]

Optic nerve compression by the cyst may cause decreased vision and disc edema. Optic nerve cysticercosis produces similar symptoms.[25] Lacrimal canalicular obstruction due to adnexal cysticercus has also been reported.[27]

Factors facilitating the spread of T solium infection include inadequate sanitation, breeding pigs in unsanitary conditions, and eating uncooked pork.

Risk factors include a family history of parasitic infestation, history of travel to an endemic area, or household visitors from an endemic area.

Excessive intraoperative dissection of the subconjunctival cyst may damage the extraocular muscle fibers, leading to postoperative diplopia and strabismus.

Positive test results from a serum enzyme-linked immunosorbent assay (ELISA) for anticysticercal antibodies help confirm the diagnosis; however, negative test results do not exclude cysticercosis. In fact, only 50% of ocular cysticercosis cases test positive on ELISA, whereas 80% of neurocysticercosis cases test positive.[28]

A CBC count may reveal eosinophilia; however, anterior chamber paracentesis reveals a high count of eosinophils.[1]

Imaging findings are more reliable than laboratory investigations in establishing a diagnosis.

B-scan ocular ultrasonography reveals a well-defined cystic lesion with clear contents and a hyperechoic area suggestive of a scolex.[29] This may be observed intraocularly or within the extraocular muscles. Associated retinal degeneration, if present, may also be observed. Ultrasonography is an effective and economic alternative to MRI and CT scanning to detect cysticerci in the orbit and in eyes with hazy media.[30, 31] See the image below.

CT scanning of the orbits is a reliable technique to help establish a diagnosis of ocular cysticercosis. Compared with MRI, CT scanning is less expensive and less time-consuming. The characteristic feature is a hypodense mass with a central hyperdensity suggestive of the scolex. Usually, a solitary cyst with wall enhancement is observed.[32] Adjacent soft-tissue inflammation may be present. The scolex may not be visible if the cyst is dead or ruptured and has surrounding inflammation. Concurrent neurocysticercosis may be present and should be excluded.[33] See the image below.

MRI reveals a hypointense cystic lesion and hyperintense scolex within the extraocular muscle. See the image below.

Fine-needle aspiration of the cyst may be obtained to confirm the diagnosis.

Tissue reaction is less or minimal when the cyst is alive. Scolex with hooklets may be identified within the cyst. Gradual absorption of the dead parasite results in a violent tissue reaction. The fibrous cyst wall may be surrounded by a florid inflammatory response consisting of eosinophils, polymorphs with granulomatous reaction, and giant cell formation without caseation.

Surgical removal is mandatory in individuals with intraocular cysts. Medical therapy, other than the use of corticosteroids, is not part of treatment. In individuals with uveitis, perioperative corticosteroid administration is recommended.

Surgical excision of orbital cysts was considered the ideal treatment modality. However, deep orbital dissection and difficulty in completely excising the cyst because of the surrounding inflammatory response increase the chances of postoperative complications (eg, decreased vision, diplopia). The treatment of extraocular muscle cysticercosis has undergone a radical change with the successful results of medical alternatives to surgical excision of the cysts. Anthelminthics (eg, albendazole, praziquantel) and oral corticosteroids have been found to be effective in extraocular disease.[25]

Cysts deep within the orbit are best treated conservatively with a 4-week regimen of oral albendazole (15 mg/kg/d) in conjunction with oral steroids (1.5 mg/kg/d) in a tapering dose over a 1-month period. Treatment may increase inflammation as the cyst involutes, leading to worsening clinical states. Thus, concomitant administration of corticosteroids is recommended to avert an inflammatory response.[34, 29]

Resolution of the cyst may take from a few days to months depending on the density of the surrounding inflammation. Based on the individual’s response to medical therapy, another course of medication may be required.

Destruction of the larvae in situ by photocoagulation,[30] cryotherapy, and diathermy has been attempted with some success. However, as intraocular cysticercosis may lead to severe inflammation following larval death, early surgical removal of the cyst is the treatment of choice.[13]

In the treatment of intraocular cysticercosis, the most common surgical approach is removal through the pars plana route following vitrectomy. Subretinal cysts anterior to the equator may be removed transsclerally, whereas subretinal cysts posterior to the equator and intravitreal cysts are best removed transvitreally.[13] Ideally, the cyst should be removed in toto. Complete surgical removal of the intact cyst results in good functional recovery in eyes with intraocular cysticercosis.[32]

The treatment of anterior chamber cysticercosis is essentially surgical. The different modalities used to remove the cyst include paracentesis, extraction with capsule forceps, cryo-extraction, erysiphake extraction, and viscoexpression.[33, 35] Viscoexpression allows removal of an intact cyst through a small limbal incision.

Anterior subconjunctival cysts may be treated with excision biopsy.[29] As the cyst is usually adherent to the adjacent muscle, excision may be difficult. Care must be taken to keep the extraocular muscle intact during dissection.

A neurology consultation may be required in individuals with concurrent neurocysticercosis.

Cysticercosis can be prevented through practicing good hygiene measures, such as washing hands frequently, washing raw vegetables and fruits well before consumption to prevent fecal-oral transmission, and avoiding consumption of raw or undercooked pork and other meat.

Serial B-scan ocular ultrasonography, CT scanning, or MRI of the orbit helps to follow the resolution of the cyst, which is recognized by the disappearance of the scolex.

Class Summary

Parasite biochemical pathways are different from those of the human host; thus, toxicity is directed to the parasite, egg, or larvae.

Albendazole (Albenza)

Broad-spectrum anthelmintic. It is a cysticidal drug and destroys approximately 85% of cysts with a single course.

Praziquantel (Biltricide)

Antihelminthic that induces rapid contraction of schistosomes by a specific effect on permeability of the cell membrane. Drug further causes vacuolization and disintegration of the schistosome tegument. Destroys approximately 75% of cysts with a single course.

Class Summary

Have anti-inflammatory properties and cause profound and varied metabolic effects. In addition, these agents modify the body's immune response to diverse stimuli.

Prednisone (Deltasone, Meticorten, Orasone)

Anti-inflammatory glucocorticoid; causes profound and varied metabolic effects and modifies the body's immune responses to diverse stimuli.

Dexamethasone (Decadron, AK-Dex)

Anti-inflammatory glucocorticoid; causes profound and varied metabolic effects and modifies the body's immune responses to diverse stimuli. Use in patients with cysticercal encephalitis or in patients with severe mass effect, edema, or vasculitis if preferred over prednisone.