Neurocysticercosis (NCC) - Short Note

Definition

• Harrison's Principles of Internal Medicine, 22E, p. 1181

Etiology and Epidemiology

• Causative organism: Taenia solium (pork tapeworm)
• Highly endemic in Latin America, sub-Saharan Africa, South/Southeast Asia
• Humans are the definitive host (harbor adult tapeworm); pigs are the intermediate host
• Transmission of cysticercosis: ingestion of T. solium eggs (not larvae) - from food/water contaminated with human feces, or from a tapeworm carrier via fecal-oral/auto-infection route
• The tapeworm carrier is a source of self-infection (auto-infection) and infection of close contacts
• Goldman-Cecil Medicine, p. 4268

Pathogenesis

1. Ingested ova hatch in the intestine releasing oncospheres (hexacanth embryos with 6 hooks)
2. Oncospheres penetrate the intestinal wall, enter the bloodstream, and disseminate to tissues - preferentially the CNS, skeletal muscle, subcutaneous tissue, and eye
3. In the CNS, the oncosphere matures over ~2 months into a cysticercus (fluid-filled bladder containing an invaginated scolex with suckers and hooklets)
4. When the cyst is viable (vesicular stage), the host mounts minimal inflammatory response
5. As the cyst degenerates, an intense inflammatory response is triggered - this is when seizures and other symptoms appear
6. Eventually the cyst dies, calcifies, and symptoms may resolve - or chronic epilepsy may persist

Morphology / Pathology

Gross

• Cyst: thin-walled, translucent, fluid-filled bladder, 0.5-2 cm in diameter
• Contains an invaginated scolex with 4 suckers and a crown of hooklets ("racemose" variant in subarachnoid space lacks scolex and has large lobulated cysts)
• Location: gray-white junction (parenchymal), subarachnoid space, ventricles, spinal cord, eye

Microscopic (Histopathology)

• Cyst wall (tegument): outer syncytial layer with microvilli, middle cellular layer, inner fibrous layer
• Scolex visible within: 4 suckers, rostellum with hooks (double crown)
• Host reaction varies by stage:
  • Vesicular stage: cyst intact, minimal pericystic inflammation, thin fibrous capsule
  • Colloidal stage: turbid cyst fluid, cyst wall thickened, intense pericystic inflammation - lymphocytes, plasma cells, eosinophils, histiocytes; surrounding cerebral edema
  • Granulonodular stage: scolex mineralized, granulomatous reaction with epithelioid cells and giant cells, surrounding gliosis
  • Nodular-calcified stage: dense calcification, minimal inflammation, dystrophic calcification visible on CT

Stages of NCC (Imaging Correlate - Pathological Basis)

• Harrison's Principles of Internal Medicine, 22E, p. 1181

Clinical Manifestations

• Seizures - most common presentation (partial with or without secondary generalization); result from degenerating cysts inciting inflammation at the gray-white junction
• Raised intracranial pressure - obstruction of CSF flow by ventricular or subarachnoid cysts; causes headache, vomiting, papilledema
• Focal neurologic deficits - motor or sensory, depending on location
• Hydrocephalus - communicating or obstructive
• Meningitis - from subarachnoid/basal cistern involvement (racemose NCC)
• Spinal NCC - mimics intraspinal tumor (rare)
• Ocular NCC - subretinal cysts, vitreous involvement, visual loss

Diagnosis

1. Neuroimaging (MRI preferred; CT for calcifications)
   • MRI: best for vesicular/colloidal stages and posterior fossa lesions
   • CT: best for calcified lesions; also detects scolex ("hole with dot" sign)
2. Serology: Enzyme-Linked Immunotransfer Blot (EITB) - recommended confirmatory test; ELISA less specific
3. CSF analysis: mild pleocytosis, eosinophilia, elevated protein, low glucose (in meningeal NCC)
4. Ophthalmologic exam (fundoscopy): mandatory in all suspected NCC to rule out ocular cysts before antiparasitic therapy
5. Diagnostic criteria (Del Brutto criteria): absolute, major, minor, and epidemiologic criteria combined for definitive vs. probable diagnosis

Treatment

Anti-parasitic

Adjuncts

• Corticosteroids (dexamethasone/prednisone): started before antiparasitic to dampen inflammatory reaction around degenerating cysts
• Antiepileptic drugs (AEDs): initiated at first seizure; continued for 24 months after imaging resolution with no seizures
• Shunting: for obstructive hydrocephalus due to ventricular or subarachnoid NCC
• Bradley & Daroff's Neurology in Clinical Practice, Table 79.7
• Harrison's Principles of Internal Medicine, 22E, p. 1181

Key Pathological Points to Remember (Exam High-Yield)

1. The cyst wall has a tegumental syncytial layer - a diagnostic histological feature distinguishing it from other cysts
2. Racemose form in subarachnoid space - no scolex, larger, lobulated, more destructive
3. "Hole with dot" sign on CT/MRI = cyst (hole) + scolex (dot) - pathognomonic of vesicular stage NCC
4. Calcified NCC is the most common finding on CT in endemic populations - evidence of dead parasite
5. Eosinophilic pleocytosis in CSF is a clue to parasitic CNS disease
6. Do not start antiparasitic therapy in ocular or diffuse encephalitic NCC - risk of worsening inflammation/blindness
7. NCC is the single most common cause of acquired epilepsy in developing countries