Review articleTwo newly proposed infectious encephalitis/encephalopathy syndromes
Introduction
Encephalitis, defined as inflammation of the brain, can present a wide range of central nervous system manifestations. In clinical practice the diagnosis is frequently made on the basis of neurological manifestations, epidemiologic information, and inflammatory changes, such as pleocytosis of cerebrospinal fluid (CSF). Acute encephalopathy is generic term for acute brain dysfunction caused by various agents, such as infection, metabolic disease, hepatic or renal dysfunctions, hypertension, etc. The pathologic substrate of acute encephalopathy is diffuse or widespread, non-inflammatory brain edema. Thus, inflammatory cells are not usually found in the brain or CSF, as described in the diagnostic criteria of Reye syndrome or acute necrotizing encephalopathy (ANE) [1]. In Japan, acute encephalopathy is usually preceded by infection, most often by influenza virus or human herpes virus (HHV) 6 and 7, and its incidence is highest in infancy and early childhood [1], [2], [3].
Magnetic resonance imaging (MRI) is accepted as a more sensitive technique than CT for the diagnosis of encephalitis/encephalopathy; diffusion-weighted images (DWI) is particularly useful for detecting early changes in the brain. Recently, some encephalitis/encephalopathy syndromes, such as ANE have been established based on clinicoradiological features [1]. In this article, two newly proposed infectious encephalitis/encephalopathy syndromes, in which MRI is also essential for the diagnosis, have been reviewed.
Section snippets
Acute encephalopathy with biphasic seizures and late reduced diffusion (AESD, Fig. 1)
AESD is clinically characterized by biphasic seizures, i.e., a febrile seizure (usually lasting longer than 30 min) as the initial neurological symptom on day 1, followed by secondary seizures (most often in a cluster of complex partial seizures) at day 4 to 6 [1], [2]. MRI shows no acute abnormality during the first two days; but reveals subcortical reduced diffusion during days 3 to 9.
Clinically mild encephalitis/encephalopathy with a reversible splenial lesion (MERS, Fig. 4)
MERS is clinically mild encephalitis/encephalopathy characterized by the MRI finding of a reversible lesion with transiently reduced diffusion in the corpus callosum (at least involving the splenium of the corpus callosum [SCC]), sometimes associated with symmetrical white matter lesions [14], [15], [16], [17].
Acknowledgements
This paper was presented at the 3rd German-Japanese Symposium of Pediatric Neurology on September 2008, Munich, Germany.
This study was supported in part by the Research Grant (20A-14) for Nervous and Mental Disorders from the Ministry of Health, Labor and Welfare of Japan; and by the Japan Epilepsy Research Foundation.
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