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Rasmussen encephalitis

Rasmussen encephalitis (RE), also known as chronic focal encephalitis (not to be confused with a Rasmussen aneurysm), is a chronic inflammatory disease of unknown origin, usually affecting one brain hemisphere.

Epidemiology

Most cases (85% cases) occur in children under the age of 10 years 1. However, detection in adults is increasing with routine MRI investigations for intractable seizures 5.

Clinical presentation

Patients frequently have episodes of epilepsia partialis continua or generalized status epilepticus, although this is less common. The seizures are intractable despite aggressive medical management 1.

Apart from seizures, patient may have hemiparesis and speech disturbances, as well as hemianopia (pertaining to unilateral cerebral involvement). Mental deterioration over time may also be seen, especially in patient presenting later in younghood.

Pathology

Pathologic features are similar to viral encephalitis, with lymphocytes around round cells and diffuse proliferation of microglia. However, later spongiform degeneration and cortical atrophy sets in.

The observed inflammatory changes in RE include perivascular cuffing, microglial nodules, T-lymphocytic infiltration gliosis, meningeal inflammation, and neuronal injury or loss 9,10.

An autoimmune mechanism has also been proposed describing antibodies against GluR3 subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor in a few patients 11.

The exact cause of the disease is unknown. However, various viral (SSPE-like, EBV or CMV), or inflammatory episodes have been implicated by different authors 3,4,6.

Radiographic features

CT

CT may not show any specific feature in early imaging; however, patchy hypodense attenuation areas (similar to viral encephalitis) may be seen. Late stage disease may show unilateral cortical atrophy. CT perfusion may show decreased cerebral blood flow (CBF) 1.

MRI
  • T1: unilateral cortical atrophy with ex-vacuo ventricular dilatation
  • T2: hyperintense signal areas in the affected hemisphere
  • DWI/ADC: restricted diffusion may be seen in altered signal areas
  • T1 C+ (Gd): no significant post-contrast enhancement

Treatment and prognosis

Treatment with high-dose methylprednisolone and intravenous immunoglobulin 12 has been successful further supporting the autoimmune nature of the disease.

Functional hemispherectomy is the only definitive treatment in refractory cases 12.

History and etymology

It was first described by American neurologist Theodore Brown Rasmussen (1910-2002) in 1958 2.

Differential diagnosis

General imaging differential considerations include:

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