The life cycle of T. solium involves both humans and pigs. Humans can serve as both the definitive and intermediate host, whereas pigs are only an intermediate host. The life cycle of T. solium begins after human ingestion of undercooked pork that is infected with cysticerci. After ingestion, the human develops taeniasis after the larvae evaginate in the small intestine. The larval head or scolex attaches to the mucosa, using hooks and suckers, and then begins to form segments (proglottids). Approximately 2 months after initial infection, gravid proglottids (each containing 50-60 × 103 fertile eggs) detach and are excreted in human feces. Ingestion of eggs by either humans or pigs continues the life cycle. Taeniasis usually causes mild or no symptoms but can be associated with abdominal pain, abdominal distention, nausea, and diarrhea. Most people infected will not seek medical attention. If the eggs are ingested by pigs via fecal contamination, the pig will develop porcine cysticercosis. Larvae will then form within the pig musculature continuing the life cycle after human ingestion.1
Human cysticercosis results from human ingestion of T. solium eggs via fecal contamination. Once ingested, invasive oncospheres (embryos) are liberated by gastric acids and cross the bowel wall to enter the bloodstream. They travel to terminal vessels and then encyst as cysticerci, typically within the muscles of the body and central nervous system.1,4
Outside the central nervous system, the most common manifestation is the formation of subcutaneous nodules, usually noticed in the chest and arms secondary to encysted cysticerci. The nodules are initially painless. After years, the nodules can swell and become tender and inflamed before finally disappearing. Musculoskeletal involvement can result in calcifications which appear on conventional radiographs typically as an incidental finding.1 The calcifications characteristically appear as dot-shaped or ellipsoidal, usually parallel the long axis of the muscle, and are often referred to as "rice grain calcifications" because of their morphology.5 The calcifications are most commonly seen on radiographs in the muscle bundles of the shoulders or the thighs. In one radiographic series, 75% of patients with neurocysticercosis demonstrated muscular calcifications.6 In our case, the calcifications were an incidental finding seen on the shoulder films performed because of seizure-related posterior shoulder dislocation.
Central nervous system infection (neurocysticercosis) eventually involves nearly 100% of infected patients.4,7 Seizures are the most common presentation of neurocysticercosis, occurring in 50% to 80% of patients with parenchymal cysts or calcifications. In Latin America, nearly 50% of adults with new onset of seizures will demonstrate evidence of neurocysticercosis on imaging studies.7 Other presentations include intracranial hypertension syndrome and hydrocephalus in 20% to 30% of patients.
Neurocysticercosis can involve the brain parenchyma, subarachnoid space, and ventricular system. Stages of parenchymal involvement have been termed vesicular, colloidal, granular, and nodular.1,7 Parenchymal findings on both CT and magnetic resonance imaging (MRI) depend on the stage of development of the parasites and brain reaction to the parasitic infection.
In the vesicular stage, there is little inflammatory response, secondary to active immune modulation by the parasite.7,8 Living parasites in the parenchyma at this stage have imaging characteristics of a cyst with signal and attenuation similar to cerebrospinal fluid (CSF). On MRI, the cyst contents may exhibit slightly increased signal, particularly on fluid-attenuated inversion recovery (FLAIR) sequences. Viable cysts are generally located in the gray-white matter junctions but can be found anywhere and may be solitary or multiple. Within the cyst, the scolex of the parasite will often appear as a nodule and can enhance.7,9 There is little or no surrounding brain reaction and no wall enhancement. Cysticerci may remain viable for years before degeneration and death.10
Parasite degeneration occurs when the immune modulation capabilities of the parasite diminish, and the wall and cyst contents are infiltrated by host inflammatory cells.7,8 The cyst contents change in attenuation and signal intensity, becoming denser on CT and brighter on both T1-weighted images and FLAIR sequences. The cyst wall thickens and enhances intensely (colloidal stage; Fig. 3). With continued degeneration and cyst collapse, more intense enhancement occurs, surrounding brain edema increases, and a discrete cyst is not visible (early granular stage).
With continued parasite degeneration and death, the host inflammatory response diminishes, the lesion becomes smaller, enhancement resolves, and surrounding edema disappears (late granular stage and nodular stage). The lesions calcify in the nodular stage and are best appreciated on CT or on gradient recalled echo MR sequences.7,9 Although edema and enhancement usually disappear, some reports have described some residual enhancement and edema in this stage.11,12
Cysts may also develop in the sulci and involve the subarachnoid space. Cystic lesions in the subarachnoid space follow similar patterns of evolution as the parenchymal forms. The racemose form of neurocysticercosis is considered to be a proliferative reaction of cysts without a scolex and carries the worst prognosis of the CSF forms. This form typically involves the basilar cisterns or sylvian fissure regions and presents with multilobular cysts without or with enhancement. Mass effect can be pronounced (Fig. 4).4,7,13 Basilar leptomeningitis and arachnoiditis can result in communicating hydrocephalus and vasculitis.7
Ventricular involvement occurs in 10% to 20% of patients. Ventricular cysts are usually single (but may be multiple) and tend to involve the fourth ventricle (although any ventricular location can be involved)4,7 The cysts are typically isodense and isointense to CSF on CT and MR studies and are better characterized on MRI. Slight hyperintensity may be visible on FLAIR sequences, and thin walls may be appreciated on volumetric thin-section MR acquisitions.7,14 The walls may enhance, and scolex formation may or may not be visible.7 In the vesicular stage, the cysts may migrate, causing acute obstructive hydrocephalus.15
Extraneural cysticercosis is a benign disorder and usually requires no treatment.1 Neurocysticercosis is associated with substantial morbidity and mortality, and treatment is guided by stage of disease, patient presentation, and the presence of hydrocephalus.1,4,10 The treatment of neurocysticercosis is currently the topic of heated debate and is beyond the scope of this article. Therapy should be individualized with antiepileptic therapy important in patients with seizures. Cysticidal medications (albendazole and praziquantel) are usually used in the treatment of active infection (vesicular stage) and in patients with high disease burden in the colloidal and early granular stages. Cysticidal treatment has been shown to speed clinical improvement and resolution of neuroimaging findings of active disease.10 Steroid administration is important in patients with very high parenchymal disease burden, ependymitis, meningitis, and large subarachnoid cysts because of the extensive edema that can occur before, during, and after cysticidal treatment.10 Antiparasite therapy or steroids are rarely needed in the calcified nodular stage of disease.
Hydrocephalus is typically treated surgically by shunt placement along with chronic steroid therapy to diminish the rate of delayed shunt malfunction. Intraventricular cysts are usually treated by surgical excision or endoscopic aspiration.9,10
The authors thank Robert Wissman, MD, of the Department of Radiology, University of Cincinnati, Ohio, for providing the images for Figure 1; and Momin Muzaffar, MD, of the Department of Radiology, Northwestern Memorial Hospital, Chicago, Ill, for providing the images for Figure 4.
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© 2006 Lippincott Williams & Wilkins, Inc.
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