CT Findings of Toxoplasmosis and Neurocysticercosis

Brain CT Findings in Congenital Toxoplasmosis

Congenital toxoplasmosis presents with a spectrum of neuroradiological findings depending on the timing of maternal infection and the severity of fetal involvement. The classic triad of hydrocephalus, intracranial calcifications, and chorioretinitis is highly suggestive of the diagnosis.

Intracranial Calcifications

• Appearance: These typically appear as multiple, hyperdense, punctate, or curvilinear lesions without mass effect.
• Distribution: Calcifications are scattered throughout the brain parenchyma.
  • There is a specific propensity for calcifications to develop in the caudate nucleus, basal ganglia, choroid plexus, and subependyma.
  • This scattered distribution helps distinguish toxoplasmosis from Cytomegalovirus (CMV) infection, where calcifications are characteristically periventricular.
• Progression: With appropriate antimicrobial treatment during the first year of life, these calcifications may diminish or disappear. Conversely, untreated cases may show numerous calcifications associated with severe microcephaly.

Hydrocephalus and Ventricular Changes

• Hydrocephalus: This is a common finding, occurring in approximately 37% of cases.
  • It is often caused by inflammation or destruction of tissue obstructing the aqueduct of Sylvius or the foramen of Monro.
  • It may be the sole neurologic manifestation and can be present prenatally or progress during the perinatal period.
  • Severe hydrocephalus can lead to macrocephaly and requires ventriculoperitoneal shunting,.
• Ventricular Dilatation: Mild to moderate dilatation of the lateral ventricles is frequent, even in the absence of frank obstruction.
• "Stiff Ventricles": Inflammation can result in rigid ventricular walls.

Parenchymal Abnormalities

• Cerebral Atrophy: Generalized atrophy is common in severe cases, manifesting as widened sulci and encephalomalacia.
• Hypolucency: Areas of hypodensity (hypolucency) in the white matter may be observed, representing edema, necrosis, or delayed myelination.
• Porencephaly: Porencephalic cysts or cystic structures may be demonstrated, resulting from distinct areas of necrosis.
• Hydranencephaly: In massive infections, extensive destruction of the cerebral hemispheres can occur, leaving fluid-filled sacs.

Other Findings

• Microcephaly: Occurs in about 12% of cases, usually reflecting severe brain damage and associated with numerous calcifications,.

Brain CT Findings in Neurocysticercosis (NCC)

Neurocysticercosis is the most common parasitic infection of the CNS. CT findings depend heavily on the location of the cysts (parenchymal vs. extraparenchymal) and the evolutionary stage of the parasite (viable, degenerating, or dead).

Stages of Parenchymal Cysts

The appearance of parenchymal lesions evolves through four distinct stages:

• Vesicular Stage (Viable):
  • Appearance: Small (10–20 mm), rounded, fluid-filled cystic lesions.
  • Density: The cyst fluid is isodense with CSF (hypodense compared to brain parenchyma),.
  • Scolex: An eccentric mural nodule (1–3 mm) representing the invaginated scolex may be visible. This "hole-with-dot" appearance is pathognomonic for NCC,.
  • Enhancement: There is typically no contrast enhancement and no surrounding edema because the viable cyst suppresses host immunity,.
• Colloidal Stage (Degenerating):
  • Appearance: The cyst wall thickens, and the fluid becomes turbid (hyperdense compared to CSF),.
  • Enhancement: Ring-like contrast enhancement is seen due to inflammation,.
  • Edema: Significant perilesional edema is present, often causing mass effect and seizures,.
• Granular-Nodular Stage:
  • Appearance: The cyst retracts and collapses, becoming a solid nodule.
  • Enhancement: The lesion shows focal enhancement, but edema progressively subsides.
• Calcific Stage (Dead):
  • Appearance: A small (2–6 mm), hyperdense, calcified nodule.
  • Enhancement: Usually non-enhancing, though some may show perilesional edema and enhancement intermittently if antigens are released.
  • CT is superior to MRI for detecting these calcifications,.

Specific Clinical-Radiological Presentations in Children

• Single Enhancing Lesion (SEL):
  • This is the most common presentation in children (especially in India) presenting with seizures.
  • CT Findings: A solitary, small (<20 mm), round, ring-enhancing lesion with surrounding edema. It typically produces no midline shift,.
  • Differentiation: If the scolex is visible, it is NCC. If not, it is termed a "solitary cysticercus granuloma" and must be differentiated from tuberculoma,.
• Cysticercal Encephalitis:
  • Seen in children with a massive burden of cysts.
  • CT Findings: Diffuse cerebral edema with small, slit-like ventricles. Multiple enhancing lesions may give a "starry sky" appearance,.
  • Contraindication: Antiparasitic drugs are contraindicated in this stage as they worsen edema.

Extraparenchymal Neurocysticercosis

• Intraventricular Cysts:
  • Location: Often in the 3rd or 4th ventricle.
  • CT Findings: Cysts are isodense to CSF and often difficult to visualize on CT. Secondary signs like obstructive hydrocephalus or distortion of ventricular shape may be the only clues.
• Subarachnoid Cysts:
  • Racemose Cysts: Large, multilobulated cysts (giant cysticerci >5 cm) in the Sylvian fissure or basal cisterns. They lack a scolex.
  • Basilar Arachnoiditis: Abnormal enhancement of the basal meninges, often leading to communicating hydrocephalus and infarcts,.