Rasmussen syndrome is a very rare form of chronic and debilitating encephalitis that arises due to unihemispheric T-cell mediated inflammatory changes. Three clinical stages are described - a prodromal phase in which minor symptoms are seen, the active phase where drug-resistant seizures and a decrease in motor and cognitive function are prominent, and a residual stage in which neurological symptoms persist without further progression. Clinical criteria, electroencephalography (EEG), and magnetic resonance imaging (MRI) are necessary in order to make the diagnosis.
Rasmussen syndrome (commonly referred to as Rasmussen encephalitis) is a rare disorder of the central nervous system (CNS)    . The exact pathogenesis remains unclear, but it is assumed that T-cell mediated reactions play a key role in inflammation and subsequent atrophy of one of the brain hemispheres . The clinical presentation typically starts around the age of 6, and is divided into three distinct stages   :
Given the rare occurrence of Rasmussen syndrome in practice (a prevalence of approximately 0.18 per 100,000 individuals, and an incidence of 2.4 per 10 million individuals younger than 18 years of age), the diagnosis may be difficult to attain  , which is why physicians must obtain a detailed patient history and perform a thorough physical examination. The parents of the patient (especially if younger children are affected) could provide key information regarding the onset of symptoms and their severity. An additional challenge can be the highly variable course of Rasmussen syndrome, ranging from insidious and slowly progressive to rapidly ensuing and severely debilitating . After an extensive neurological exam, electroencephalography (EEG) and magnetic resonance imaging (MRI) of the endocranium are recommended. EEG will show a unilateral pattern of slowed activity, whereas focal cortical atrophy, accompanied either by hyperintense signaling on T2-weighed or FLAIR images of the gray or white matter, or atrophy of the ipsilateral caudate head are main findings on MRI   . Unilateral ventriculomegaly is another hallmark of Rasmussen syndrome in the acute stage .