CONTRIBUTION OF MRI IN SERIOUS FORMS OF ACUTE DISSEMINATED ENCEPHALOMYELITIS AND POST INFECTIOUS HERPES ENCEPHALITIS M.OMRI, W.HIZEM-HARZALLAH, Z.ABOUZARIFA, R.SALEM, M.A.JELLALI, A.ZRIG, W.MNARI, M.MAATOUK, M.GOLLI Radiology service, Fattouma Bourguiba’s Hospital, Rue 1er juin, 5004 Monastir, Tunisia NR29

INTRODUCTION: Post-infectious and inflammatory encephalomyelitis are broadly represented by the syndrome acute disseminated encephalomyelitis (ADEM). ADEM is an inflammatory demyelinating disorder of the central nervous system that is usually monophasic,which principally affects brain and spinal cord. ADEM is predominantly, though by no means exclusively, a disease of children and in particular infants. Historically it includes post infectious encephalomyelitis and post-vaccination encephalomyelitis. It usually follows an infection or vaccination. The disease is characterised by multifocal white matter lesions on neuroimaging.

It typically follows a minor infection with a latency period of 2—30 days and is thought to be immune-mediated. ADEM is clinically characterized by the acute onset of focal neurological signs and encephalopathy. Patients can require intensive care unit admission because of encephalopathy, coma, seizures or tetraplegia. Cerebrospinal fluid analysis usually shows lymphocytic pleocytosis but, unlike viral or bacterial encephalitis, no evidence of direct CNSinfection is found. There are no biologic markers of the disease and cerebral magnetic resonance imaging is essential to ADEM diagnosis, detecting diffuse or multifocal asymmetrical lesions throughout the white matter on T2- and FLAIR-weighted sequences.

OBJECTIVS: Show the contribution of MRI in the positive diagnosis in acute disseminated encephalomyelitis (ADEM) in its severe form and post infectious herpes encephalitis and also specify his interest prognosis and clinical correlation radio.

Materials and Methods Retrospective analysis of 9 clinical cases( 6 girls and 3 boys) of ADEM (5 cases) and post infectious herpes encephalitis (4 cases) explored by conventional MRI and diffusion mapping with CDA. Average age: 9 months and 12 years Recent infection (n = 6). recent vaccination (n =3 ). Reason for hospitalization: - fever (n = 7); - seizures (n = 5); -behavioral disorders (n = 4)

Physical signs: -impaired consciousness (n = 8); -meningeal syndrome (n = 5); -motor deficit (n = 3); -gait disturbance (n = 2); -achievement of cranial nerves (n = 4) PL (n = 8): - normal formula (n = 1); - and lymphocytic meningitis (n = 7) Initial brain imaging: - CT (n = 4); - MRI (n = 9). Additional spinal MRI (n = 3). Imaging control in 5 patients: -MRI (n = 5)

Results: Objectified anomalies in all cases were: Hyperintensity T2 of white matter brain: -bilateral and asymmetric (3 cases). -diffuse (2 cases). -unilateral (3cases). Hyperintensity T2 of profound gray matter (1 case). Infectious vasculitis associated with vasoplegia blood was found in one case. Diffusion was restricted with decreased ADC in 4 cases. The restriction of diffusion was associated with a severe clinical picture.

ADEM lesions are large, multiple, and asymmetric (5cas). Severe and extensive T2 lesions (2cas) contrast with a relatively small mass effect. The distribution of lesions involves the subcortical and central white matter and cortical graye white junction of both cerebral hemispheres and infratentorial areas. A pattern of diffuse demyelination is seen in one severe case with large demyelinating lesions of the white matter extending to the contralateral hemisphere.

A two-years-old previously healthy girl who developed postinfectious focal encephalitis. MRI findings: fluid-attenuated inversion recovery (FLAIR) sequences show extensive area of increased signal in left hemisphere and right basal ganglia.

HSE. T2 weighted MRI showing extensive area of increased signal in left hemisphere and right basal ganglia

Diffusion-weighted MRI (DWI) show a restrection of diffusion.

T1 with G shows cortical enhancement.

A seven-years-old previously healthy girl who developed post infectious focal encephalitis. control MRI findings. HSE. T2 weighted MRI showing an increased signal in basal ganglia

fluid-attenuated inversion recovery (FLAIR) sequences show an increased signal in basal ganglia. Diffusion est discreetly restrected.

Discussion: Acute disseminated encephalomyelitis: Post-infectious encephalomyelitis is associated with an antecedent or concomitant infection, usually viral. Most notoriously, measles virus infection is followed by ADEM in approximately 1 in 1000 cases. It is greatly reduced in incidence following the introduction of widespread measles vaccination, but still occurs. Non-specific or unidentified viral illnesses can also antecede ADEM, and this lack of a specific infectious agent should not preclude the diagnosis. There are some variations in the ADEM phenotype dependent upon the antecedent illness. Measles associated ADEM tends to produce a more clinically severe phenotype while cerebellar ataxia.

ADEM can be distinguished from acute viral encephalitis because the disease is not the result of primary tissue invasion by an infectious organism. It is thought to be immune-mediated and is characterized neuropathologically by perivenular inflammation and demyelination.Neuroimaging: Cerebral computed tomography scans performed at admission show abnormalities only in 30% of patients, essentially supratentorial readily visible diffuse or large focal hypodensities of the cerebral white matter.Cerebral computed tomography scans performed at admission show abnormalities only in 30% of patients, essentially supratentorial readily visible diffuse or large focal hypodensities of the cerebral white matter. Demyelinating lesions of ADEM are better visualised by MRI. These demyelinating lesions of ADEM usually exhibit no mass effect and can be seen scattered throughout the white matter of the posterior fossa and cerebral hemispheres

Involvement of the cerebellum and brainstem is more common in children. Characteristic lesions seen on MRI appear as patchy areas of increased signal intensity on conventional T2-weighted images and on fluid attenuated inversion recovery sequence (FLAIR). Few MRI lesions may enhance after gadolinium administration. Extensive perifocal oedema may be seen. Though white matter involvement predominates grey matter can alsobe affected, particularly basal ganglion, thalami, and brainstem. Tumour-like lesions have also been reported in a few cases. MRI lesions are identified on morphological T2-weighted and fluid- attenuated inversion recovery (FLAIR) sequences. Although no specific MRI criteria have been identified. MRI lesion patterns are generally recognized, but in all cases lesions are multifocal and involve mainly the supratentorial white matter:

-multifocal lesions of less than 5 cm. -confluent multifocal lesions of more than 5 cm. - multifocal lesions involving basal ganglia. Although patchy areas of increased signal intensity are stated to involve 50% or more of total white matter in children. In order to qualify as ADEM, lesions on MRI should be of the same age and no new lesion should appear on central nervous system imaging studies after the initial clinical attack. The corpus callosum is usually not involved in ADEM; infrequently its involvement has been reported, suggesting extensive lesion load. Corpus callosum involvement is more characteristic of multiple sclerosis.

Thalamic involvement may be seen in 40% patients of ADEM, making this finding a potentially useful discriminator. MRI changes usually appear early in the course of the disease. Although ADEM is typically a disseminated process in the central nervous system, often with impaired sensorium, a few cases are dominated by spinal pathology. DIFFERENTIAL DIAGNOSIS : Distinction between infectious and post-infectious encephalitis can be difficult and all frequent causes of infectious encephalitis must be excluded before concluding to an acute form of post-infectious inflammatory CNS disorder. The diagnosis is considered straightforward when ADEM occurs after an exanthem or immunisation. A clear cut latent period between systemic symptoms and neurological illness favours ADEM along with the typical pattern of diffuse and multifocal involvement of both the central nervous system and peripheral nervous system and the characteristic MRI appearance. The most important issue associated with the diagnosis of ADEM is can this disorder be diagnosed with certainty and differentiated from the initial manifestation of multiple sclerosis.

CONCLUSION: ADEM is a monophasic inflammatory disease affecting the central nervous system, which usually follows an infection or vaccination. Distinction between infectious and post-infectious enceph- alitis can be difficult and all frequent causes of infectious encephalitis must be excluded before concluding to an acute form of post-infectious inflammatory CNS disorder. Diffuse and focal CNS signs and peripheral nervous system involvement may be present simultaneously at physical examination. Brain and spinal cord MRI should be systematically performed at the initial phase of the disease to look for evidence of multifocal acute inflammation and demyelination.