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.
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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 the latter is less common. The seizures are often intractable despite aggressive medical management 1.
Apart from seizures, the patient may have hemiparesis, speech disturbances, and hemianopia, each pertaining to unilateral cerebral involvement. 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.
An autoimmune mechanism has also been proposed describing antibodies against the GluR3 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.
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 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 high-dose methylprednisolone and intravenous immunoglobulin has been positive, further supporting the autoimmune nature of the disease 16,17. Anti-seizure medications have a limited symptomatic role 17.
Hemispherectomy or functional hemispherectomy are the only definitive treatments in 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
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