CNS Imaging - Types and Symptoms

Introduction

Cysticercosis is a parasitic disease caused by the tapeworm Taenia solium that affects the brain, muscles, and other tissues. Cysticercosis is caused by the eggs of the Taenia solium found in the feces of a human who ate undercooked or infected pork containing larval cysts (cysticerci). It is spread through contaminated foods and water. Cysticercosis affecting the central nervous system is called neurocysticercosis (NCC). About ninety percent of the patients with cysticercosis have CNS (central nervous system) involvement. In addition, about one to three percent of the affected patients have spinal cord cysts, and five percent of patients have eye involvement.

What Are the Clinical Signs and Symptoms of Neurocysticercosis?

Usually, neurocysticercosis takes about one to thirty years to onset the symptoms from the point of infection. Some of the common symptoms are:

• Seizures (abnormal electrical impulses of the brain cells that cause twitching or stiffness of muscles).

• Headache.

• Hydrocephalus (collection of fluid inside the brain cavity).

• Altered mental status.

• Bruns syndrome (presence of intraventricular mass causes rapid onset of headache, vertigo, and vomiting).

• Arteritis (swelling of the arteries).

What Are the Different Types of Neurocysticercosis and Its Radiological Findings?

Neurocysticercosis is classified into three types based on its location. They are:

• Parenchymal neurocysticercosis.

• Intraventricular neurocysticercosis.

• Spinal neurocysticercosis.

1. Parenchymal Neurocysticercosis: Parenchymal cysticercosis affects the brain's parenchymal tissues and occurs commonly in the cerebrum. However, sometimes it may be located in the basal ganglia, cerebellum, and brain stem. CT (computed tomography) and MRI (magnetic resonance imaging) scan is the standard imaging technique for neurocysticercosis which is performed after the intravenous administration of contrast materials (a special dye). Escobar's classification of stages of parenchymal neurocysticercosis is currently used for imaging.

• Vesicular Stage (Active Stage): When humans ingest the larvae of Tenia solium, they reach the brain's blood vessels through the bloodstream. After two to three months, they develop into viable cysts in the brain parenchyma. During this stage, the cyst is not visible in imaging. After two to three months in the brain parenchyma, the larva becomes visible as nodules invade the fluid-filled cyst. The diameter of the cyst at this stage is approximately five to twenty millimeters; hence it does cause any symptoms.

• CT and MRI Findings: During this stage, CT and MRI clearly show the cyst's margin. The scolex (head or front end of the tapeworm) of the tapeworm present in the center of the cyst is indicative of the vesicular stage. The fluid density found inside the cyst is similar to the cerebrospinal fluid density (CSF). The scolex appears as an isointense or hyperintense structure on both T1 and T2 weighted images of MRI (magnetic resonance imaging). T1 MRI images are obtained using short TR (repetition times) and TE (time to echo) times. T2 images are obtained through longer TE and TR times. As there is no inflammatory response during this stage, no edematous reaction is visible during imaging. However, multiple larvae in the brain parenchyma produce a “Swiss cheese appearance” on imaging. This stage lasts for about months to years.

• Colloidal Vesicular Stage (Active Stage): The larvae begin to degenerate during this stage, and their size gradually reduces. As a result of degeneration, the cystic fluid becomes proteinaceous, and the cystic wall becomes thicker due to the inflammatory reaction.

• CT and MRI Findings: The cystic fluid appears hyperdense in a CT (computed tomography) scan. On T1 weighted images of MRI, the fluid seems slightly hypodense and hyperdense on T2 weighted images. The scolex degenerates and gradually disappears on imaging.

• Granular Nodular Stage (Active Stage): During this stage, the size of the cyst becomes smaller, and it forms a granulomatous nodular lesion. The cyst appears nodular, small, ring-like enhanced structure with mild edema (swelling) surrounding the cyst. Pericystic gliosis is a significant pathology during this stage.

• Nodular Calcified Stage (Nonactive Stage): The nodular calcified stage is the last stage where the cyst almost shrinks half of its original size and is mineralized completely. No visible edema surrounding the cyst.

• CT and MRI Findings: CT scan can clearly show mineralization of the cyst comparable to MRI. However, it is a nonactive stage that sometimes causes recurrent seizures. Seizures might be an inflammatory response to cystic remnants. If the calcified mass inside the hippocampus causes seizure, it can be treated with hippocampal resection (surgical removal of the hippocampus).

2. Intraventricular Neurocysticercosis: Intraventricular cysticercosis is the second most common type and accounts for about twenty-two percent of neurocysticercosis. Usually, it is found isolated; about twenty percent of cases of intraventricular cysts are associated with the parenchymal cyst. This cyst is often located in the fourth ventricle, sometimes in the third ventricle, lateral ventricle, and cerebral aqueduct of Sylvius. The intraventricular cyst can cause obstructive hydrocephalus, which is lethal.

• CT and MRI Findings: In CT and MRI, the radiodensity and signal intensity of cystic fluid are the same as that of cerebrospinal fluid. MRI is superior in imaging the cyst and visualizing the cystic wall, scolex, and cystic fluid. FLAIR (fluid-attenuated inversion recovery) sequence MRI with 100 percent supplemental O2 and three-dimensional constructive interfaces in steady-state (3D-CISS) helps differentiate cystic fluid from CSF. Similar to the parenchymal cyst, the intraventricular cyst has a thick wall, ring-like enhanced structure, and edema surrounding the cyst, but it does not have calcification. The ring-like lesion should be differentiated from the neoplasm by correlating the clinical examination and laboratory tests.

3. Spinal Neurocysticercosis: Spinal neurocysticercosis is found inside the spinal cord parenchyma. This cyst occurs in two forms: intramedullary form and leptomeningeal form. The leptomeningeal form is more common and found along with intracranial disease. This cyst showed a widening of the spinal cord in imaging.

Conclusion

Neurocysticercosis is the most common parasitic infection affecting the central nervous system. As the diagnosis of neurocysticercosis is complex, the clinical examination, lab results, and imaging results are correlated to obtaining the proper diagnosis. Patients with single-enhancing lesions will need an MRI scan every six months till the cystic components resolve. MRI (magnetic resonance imaging) is useful for diagnosing hydrocephalus and seizures, and a CT (computed tomography) scan is useful in the diagnosis of a parenchymal neurocysticercosis cyst. Ultrasonography is useful in the diagnosis and guiding of the treatment of ocular neurocysticercosis.