Neurologic mysteries: Mild Encephalopathy with Reversible Splenium lesion (MERS)

MERSMERS is not a so rare clinical entity even if epidemiological data are scanty; the clinical presentation is variable such as the etiologies of this syndrome. However, although the splenium lesion is usually quite big, these patients do not show signs of posterior callosal disconnection (such as alien hand, left hand agraphia or apraxia, tactile aphasia or anomia, agraphia without alexia). They show some form of encephalopathy (delirium, confusion, transient consciousness alteration) and few cases have been reported with relatively pure psychotic features.
Causes include metabolic or toxic conditions such as epileptic drug withdrawal, epileptic status, hyponatremia and/or hypoglycemia, vitamin deficiency or malnutrition, hemolytic uremic syndrome, thyroiditis, neuroleptic malignant syndrome, EBV or influentia virus encephalitis, Kawasaki disease, Marchiafava-Bignami’s disease (in this condition demyelination usually occurs in the central part of the corpus callosum).
The brain MRI establishes the diagnosis showing the typically large isolated spherical lesion in the splenium of the corpus callosum. This lesion is usually hyperintense in T2, FLAIR and DWI images, with low ADC values and does not show contrast enhancement. In circumstances that are more exceptional other similar lesions could be present on the periventricular regions in the sémiovale center.
Although MRI feature of MERS are specific, some differential diagnosis should be thought, such as lymphoma, multiple sclerosis, and stroke.
EEG does not provide further elements to diagnosis and it is often normal. LCR examination would be indicated in most cases especially if the patient is still symtpomatic.
There are several hypotheses on pathogenesis: transient intramyelinic edema after generalized seizure, hyponatremia axonal damage, oxidative stress, AED toxicity and vasogenic mechanisms. Actually, the splenium of the callosum body probably receives blood from multiple arteries and the hypothesis of watershed mechanisms is less probable. The multitude of causes producing this unique lesion suggest a common but not specific pathogenic mechanism. This mechanism should have some similarity, I suppose, with the one at the origin of the subcortical occipital MRI abnormalities observed with posterior reversible encephalopathy.
Although some authors advanced the possibility of IgG or steroid pulse therapies, there is no specific therapy for MERS as the condition is usually fully reversible.

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