Diagnosis: Rasmussen Encephalitis
Discussion: Rasmussen Encephalitis or chronic focal encephalitis (new terminology) is characterized by chronic, progressive unilateral encephalitis of undetermined etiology. Researchers speculate that a viral trigger in individuals with a genetic predisposition may contribute to this rare form of encephalitis. Approximately 50% of cases are preceded by a viral infection such as upper respiratory infection or tonsillitis.
Mean age of onset is 6-8 years old. Over time, focal motor seizures progress to ipsilateral motor function loss with cognitive deficits and refractory seizure activity. CT and MRI are often normal in early phases during childhood. Encephalitis starts with cortical swelling, leading to blurring of the cortical ribbon on T1WI and patchy white matter hyperintense signal on T2WI. Abnormalities eventually become diffuse and are typically lobar, occasionally involving the entire hemisphere. Small areas of hyperintensity increase over time. Abnormal regions of enhancement are usually not present. End-stage disease is characterized by advanced hemispheric atrophy, which may involve the basal ganglia and hippocampi. The frontal and frontotemporal regions are usually most affected.
Additionally, SPECT imaging can be used to demonstrate large regions of hemispheric decreased perfusion. Secondary to disruption of the corticopontocerebellar system, crossed cerebellar diaschisis can sometimes be demonstrated secondary to decreased perfusion of the contralateral cerebellar hemisphere. MRS is not specific, but demonstrates decreased NAA, increased myoinositol, Cho, glutamine, and glutamate. PET imaging shows unilateral decreased metabolism. Combination of imaging features and clinical history are required to make the diagnosis.
Histopathology demonstrates typical characteristics of encephalitis. Perivascular lymphocytic infiltration with cuffing, gliosis and microglial nodules has been demonstrated. In later stages, neuronal loss is present, without persistent inflammatory cells.
In this particular case, other entities such as Dyke-Davidoff-Masson and Sturge-Weber syndrome could be considered, but Rasmussen Encephalitis is strongly favored. Dyke-Davidoff-Masson (hemispheric infarction) demonstrates unilateral hemispheric atrophy, which is present in this case, but other typical findings such as compensatory calvarial thickening, elevation of the petrous ridge and hyperaeration of the ipsilateral paranasal sinuses are absent. Sturge-Webber syndrome is unlikely as the typical cortical calcifications, port wine facial nevus and enhancement of a pial angioma is absent.
Prognosis for such patients is poor. Hemiplegia eventually occurs, with or without treatment attempts. Cognitive decline and motor loss is typical. If onset occurs at an older age, a protracted course of the disease may occur, but the end result is the same.
Treatment options for patients with Rasmussen Encephalitis are limited. Antiepileptic medications can be attempted, but the disease is usually refractory. Transient improvement has been seen with plasma exchange and steroids. Surgical options such as hemispherectomy have also been used.
