Specific Imaging Findings

Rasmussen encephalitis is an inflammatory process affecting one cerebral hemisphere. The initial scans are normal or may in some cases show mild swelling and T2 hyperintensity of the involved cortical and subcortical areas, commonly in the frontal and temporal lobes. This is followed by progressive deterioration with gradual development of atrophy, predominantly in the affected hemisphere, but can also occur in the contralateral hemisphere. CT usually demonstrates asymmetrical atrophy, with or without brain infarcts. MRI can better demonstrate the atrophy and the abnormal tissue, before the formation of encephalomalacia on CT images. The cortical gray matter and subcortical white matter of the affected hemisphere are of abnormal high T2 signal. The basal ganglia may also be involved with progressive atrophy. There is typically no contrast enhancement and no hemorrhage. Diffusion tensor imaging shows increased ADC values and decreased fractional anisotropy (FA) in the affected hemisphere; there is also decreased perfusion as well as decreased metabolic activity on FDG-PET scans. MR spectroscopy typically shows a low NAA level, whereas elevated choline peak and lactate may also be found. Glutamine and glutamate (Glx peak) are usually increased, likely due to seizures. Atrophy and hypoperfusion of the contralateral cerebellar hemisphere (crossed cerebellar diaschisis) may also be present.

Pertinent Clinical Information

The age of onset is in childhood, usually between 1 and 14 years of age. An adult form has been recognized recently. The initial presentation is with seizures, which keep increasing in frequency and evolve into intractable epilepsia partialis continua, typically accompanied by progressive hemiparesis/hemiplegia and cognitive decline. Hemiparesis usually becomes fixed months to years after the onset of seizures. Patients may develop a variety of involuntary movements. The prognosis is poor, with cognitive and neurological deterioration, but relatively better in older individuals. In some patients, immunoglobulin or corticosteroids reduce seizure frequency in the short term. Biopsy is performed for definitive diagnosis. Seizures are refractory to antiepileptic medications and currently the only treatment is surgical. Functional hemispherectomy (or possibly cortical resection) may lead to seizure control and prevent further progression of neurological impairment and cognitive deterioration.