Pork tapeworm infection, taeniasis, results from infestation of the small intestine by the adult tapeworm Taenia solium. The larval form of T. solium is Cysticercus, the cause of cysticercosis. Humans can acquire either disease, sometimes simultaneously. When the CNS or eye is involved in cysticercosis, then neurocysticercosis exists.

T. solium is a common intestinal tapeworm, widely distributed around the world. Neurocysticercosis is the most common parasitic infection of the CNS; prevalence varies greatly according to region. Autopsy studies from Mexico suggest that up to 3.6% of the population is affected. Humans are both the definitive and intermediate hosts for T. solium. Ingestion of raw or undercooked pork infested with cysticerci leads to taeniasis. Ingested cysticerci are activated by gastric acid, penetrate the small intestine mucosa, and then develop to adulthood.162 Cysticerci have a predilection to migrate to the CNS (as well as to striated muscle and eyes).

Once situated, cysticerci evolve continuously through four stages:162

  1. edema generation
  2. development of a thin-walled cyst containing fluid and larvae
  3. inflammatory filling of the cyst with caseous material and consequent larvae destruction
  4. cyst degeneration into an inactive, calcified nodule with release of antigenic contents

Taeniasis (presence of the adult tapeworm in the small bowel) is usually asymptomatic. But cysticercosis can besymptomatic. If acute symptoms are present, fever and headache are primary.163

Neurocysticercosis can become symptomatic from 1 to 30 years after infection, with a median onset at 3 to 7 years.164Neurocysticercosis is associated with a wide variety of symptoms determined by pathogen burden, encystment location, whether encysted- parasite organisms are alive or dead, and host infection response. Differing encystment locations give rise to six clinical neurocysticercosis syndromes:

  • asymptomatic
  • parenchymal
  • subarachnoid
  • intraventricular
  • ocular
  • spinal

Mixed permutations of these syndromes can occur within a single patient.162

Parenchymal neurocysticercosis occurs when cysticerci develop within the brain, predominantly at the gray-white junction. Seizures are the most common presenting sign, affecting up to 92% of patients.13,164,165 In Mexico, parenchymal neurocysticercosis is the single most common cause of adult-onset seizures (up to 50%).165 Reports have varied regarding seizure-type frequency: In two series, partial seizures were most common (up to 72%),165 but another study reported a predominance of generalized seizures (60%).164 Generalized seizures are preceded by focal symptoms in two-thirds of cases.166 Complex partial seizures and status epilepticus are less common.166 Seizure risk can increase during therapy as the death of cysticerci causes larval antigen release, exacerbating the host’s inflammatory response.162

In subarachnoid cysticercosis (cysticercotic subarachnoiditis), patients usually present with signs of meningitis and increased intracranial pressure. Seizures are not a significant complication.

Intraventricular cysticercosis usually presents as subacute hydrocephalus and increased intracranial pressure. Again, seizures are not a frequent complication.

Diagnosis

Because symptoms can be numerous and varied, differential diagnosis is wide. Among the disorders that neurocysticercosis can mimic are:

  • tumors
  • strokes
  • hemorrhage
  • abscess
  • meningitis
  • pseudotumor cerebri
  • pheochromocytoma
  • neuropsychiatric diseases

The neurologic examination can be deceptive; one study reported normal neurologic exams in 80% of cases.13,164 Clinical suspicion based on patient history of potential exposure is key (e.g., travel to an endemic area). There are tests for cysticercotic antigen and antibody. Their sensitivity and specificity depend on the fluid tested (serum or CSF), cyst activity, and the detection method.162

Neuroimaging is central. Parenchymal neurocysticercosis can have four different appearances on CT:167

  • parenchymal calcifications or granulomas (representing inactive cysts), predominantly at the gray-white junctions
  • nonenhancing low-density rounded lesions
  • hypodense or isodense rounded masses with surrounding edema and ring or nodular enhancement
  • diffuse brain edema with small ventricles and multiple enhancing nodular lesions

MRI provides a varying amount of increased diagnostic data depending on cyst location and status. Its increased resolution identifies noncalcified cysts and helps define parenchymal cyst stage, but calcified cysts are visualized less well than on CT.

The EEG is abnormal in up to 50% of cases, demonstrating a variety of findings (diffuse slowing, focal paroxysmal activity, generalized spike waves) depending on lesion number, size, and location.164 Definitive diagnosis often requires biopsy analysis.

Treatment

Principal anticysticercotic drugs include praziquantel and albendazole. Appropriate therapy for neurocysticercosis depends on the specific clinical syndrome, degree of neurologic impairment, cysticerci location, cysticerci activity, and host immune response.162

For therapeutic decision making, it is useful to differentiate between benign and malignant neurocysticercosis.168 Benign neurocysticercosis is a chronic condition that is either asymptomatic or associated only with easily controlled seizures. Parenchymal neurocysticercosis is usually benign if there are few cysts, they are predominantly calcified, and edema is minimal. Anticysticercotic treatment, together with anticonvulsants, can reduce seizure activity by greater than 90%.169 Calcified parenchymal cysticerci neither require nor respond to anticysticercotic treatment162 but anticonvulsant treatment for epileptic symptoms is effective.

Malignant neurocysticercosis refers to the acute or subacute disease associated with arachnoiditis, encephalitis, multiple or large cysts (or both), intraventricular cysts, hydro- cephalus, increased intracranial pressure, or some combination of these conditions.168 Surgical therapy is often required in addition to medical treatment. Anticysticercotic drugs are effective, reducing cyst number by up to 90% and seizure frequency by up to 95%.170 The use of praziquantel or albendazole can exacerbate CSF pleocytosis, acutely increase intracranial pressure, or produce other neurologic sequelae in up to 60% of cases. Therefore, treatment should only be given when clinically necessary, and preferably in a hospital.171 Solitary cysts causing treatment-refractory seizures should be surgically resected.172 If seizures persist even after surgical and multidrug anticonvulsant therapy, the epileptogenic focus can sometimes be electrocorticographically ablated.172

A double-blind, placebo-controlled study reported in 2004 compared two groups of patients with viable parenchymal cysts, with seizures being treated with anticonvulsants, to see whether anticysticercotic drugs improved seizure control.202 During 30 months of follow-up, the proportion of patients having partial seizures was similar for the group who took albendazole and dexamethasone and those who took placebos, but the treatment group had significantly fewer seizures with generalization, and more of their intracranial lesions resolved. Except for abdominal pain, side effects did not differ significantly.

Adapted from: Goldstein MA and Harden CL. Infectious states. In: Ettinger AB and Devinsky O, eds. Managing epilepsy and co-existing disorders. Boston: Butterworth-Heinemann; 2002;83-133. 
With permission from Elsevier (www.elsevier.com). 

I<
Reviewed By: 
Steven C. Schachter, MD
on: 
Monday, March 1, 2004