Rasmussen encephalitis

Rasmussen encephalitis, also known as chronic focal encephalitis, is a chronic inflammatory disease of unknown origin, usually affecting one brain hemisphere.

It is not to be confused with a Rasmussen aneurysm.

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 ofClinically, the Rasmussen encephalitis can be considered in three stages ¹⁷:

• prodromal stage

  • infrequent seizures, usually focal seizures

  • mild focal neurological deficits, e.g. subtle hemiparesis

• acute stage

  • frequent seizures due to drug-refractory focal epilepsy, including epilepsia partialis continua or generalised status epilepticusand focal-to-bilateral tonic-clonic seizures

  • progressive focal neurological deficits, although the latter is less common. The seizures are often intractable despite aggressive medical management ¹.

    Apart from seizurese.g. hemiparesis, the patientdysphasia (if dominant hemisphere), homonymous hemianopia

  • cognitive impairment may have hemiparesis, speech disturbances,develop

• residual stage

  • ongoing drug-refractory focal epilepsy

  • permanent and hemianopia, each pertaining to unilateral cerebral involvement. Cognitivestable focal neurological deficits and cognitive impairment over time may also be seen, especially in a patient presenting later in adolescence.

Pathology

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

The observed inflammatory changes in Rasmussen encephalitis include perivascular cuffing, microglial nodules, T-lymphocytic infiltration gliosis, meningeal inflammation, and neuronal injury or loss ^9,10. In the later phases, spongiform degeneration and cortical atrophy manifest.

AnThe 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. Additionally, an autoimmune mechanism has also been proposed describing antibodies against theGluR3 subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic 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 ^3,4,6,17.

Radiographic features

Radiographic features are usually isolated to a single hemisphere; however, bilateral Rasmussen encephalitis has also been described in numerous case reports ^12-14. 

CT

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

MRI

Atrophy is typically earliest appreciated in the ipsilateral caudate nucleus, before atrophy and signal changes are appreciated more generally in the hemisphere. Signal characteristics include:

• T1: unilateral cortical atrophy with ex vacuo ventricular dilatation ¹⁵

• T2: hyperintense signal areas in the cortical and subcortical regions of the affected hemisphere ¹⁵

• DWI/ADC: restricted diffusion may be seen in altered signal areas ¹⁵

• T1 C+ (Gd): no significant post-contrast enhancement ¹⁵

Treatment and prognosis

Treatment with immunosuppression (e.g. high-dose methylprednisolone and, intravenous immunoglobulin, plasma exchange) has been positive in the acute stage, further supporting the autoimmune nature of the disease ^16,17.

Anti-seizure medications have a limited symptomatic role, but epilepsy due to Rasmussen encephalitis tends to be drug-refractory in nature ¹⁷.

Hemispherectomy or functional hemispherectomy are the only definitive treatments for epilepsy in refractorydrug-refractory cases, which resolves seizure activity or significantly decreases seizure frequency in most patients ¹⁶.

History and etymology

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

Differential diagnosis

General imaging differential considerations include:

• Dyke-Davidoff-Masson syndrome

• Sturge Weber syndrome

• unilateral megalencephaly

• hemiconvulsion-hemiplegia epilepsy (HEE) syndrome

  • HHE syndrome usually shows a seizure-free interval of months to years from initial presentation ^7,8