Clinical
Presentation: Patient is a 30-year-old male with a three month history of headache, vertigo and neck pain. Additionally, the patient reported recent onset of nausea and vomiting.
Imaging Findings:
Figure 1: Sagittal T1 demonstrates a low signal lesion occupying the fourth ventricle.
Figure 2: Axial T2 shows a high signal cystic lesion that contains an intermediate-high signal posterior structure. Note the high signal posterior to this lesion in the cerebellum consistent with edema/inflammatory changes.
Figure 3: Axial FLAIR (superior to the T2 image) exhibits high signal surrounding the fourth ventricle lesion. Note dilation of the temporal horns consistent with hydrocephalus.
Figure 4: Coronal SPGR with contrast displays an enhancing inferior component of the cystic lesion.
Figure 5: Axial diffusion image reveals that the lesion does not demonstrate restricted diffusion.
Diagnosis: Neurocysticercosis
Discussion: Cysticercosis, infection by taenia solium (the pork tapeworm), is the most common parasitic infection in the world. Humans either ingest the eggs via contaminated food or water (i.e. fecal-oral route) or may become infected with the tapeworm itself via uncooked pork. The primary larvae (oncospheres) invade the bowel wall and disseminate to the CNS and skeletal muscle, where they develop into secondary larvae (cysticerci). It is estimated that 60-90% of infections involve the CNS.
Once in the CNS, the parasite passes through four stages over an average of 5 years. The first stage is termed “vesicular.” Here the viable parasite has not yet elicited much of a surrounding inflammatory reaction and appears as a small cyst containing clear fluid and a mural nodule. Next is the colloidal vesicular stage during which increasing inflammation surrounds a more thick-walled cyst capsule containing the degenerating larva. This is the so called “acute encephalitic” stage where infection usually first becomes clinically apparent. Upon reaching the third or granular nodular stage, the scolex (part of the degenerated larva) becomes a mineralized granule. In the final stage (nodular calcified), the entire lesion shrinks, becomes mineralized and the surrounding edema regresses.
Clinically, the period from infection to presentation is variable with the literature suggesting an approximate interval of five years. The most common presentation of neurocysticercosis is seizures. Headache is also a common presentation, attributed to hydrocephalus as the parasite has a predilection for fourth ventricle with resultant obstruction. Other signs/symptoms include syncope, dementia, visual changes, focal neurological deficits or stroke.
Laboratory serology or CSF ELISA analysis are a standard part of the workup. Of note, a positive test indicates either cysticercosis or echinococcus.
Treatment may involve albendazole (reduces the parasitic burden and seizures) and/or steroids (decreases surrounding edema and inflammatory reaction). Surgical excision may be indicated and CSF diversion is considered in the setting of obstruction.
Imaging demonstrates several important features of cysticercosis CNS infection. Classically, a cyst (1 cm on average) containing a nodule (usually 0.1-0.4 cm) is identified (i.e. “cyst with a dot”). The lesions may be solitary or multiple. Imaging characteristics depend on the larval stage and often all stages are present simultaneously. Though they may be located anywhere, lesions are predominantly found in the CSF spaces (cisterns & sulci > ventricles) and at the grey-white junction within the brain parenchyma. T1 and FLAIR sequences aid in identifying the scolex and the often more subtle intraventicular lesion(s). Depending on the larval stage, foci may exhibit either ring or nodular enhancement. Whether located in the CSF or parenchyma itself, there is commonly high T2 signal in the adjacent brain due to inflammatory reaction. The case presented here probably represents the colloid stage with surrounding edema and an enhancing focus (see figures).
The imaging differential diagnosis includes abscess, tuberculosis, neoplasm (metastatic or primary disease), arachnoid cyst, enlarged perivascular spaces or other parasitic infections.
