Young adult with mild encephalitis/encephalopathy with reversible splenial lesion (MERS)

A 20-year-old woman presented with aphasia following a history of flu-like illness. The aphasia resolved in hospital, lasting less than 24 hours. Bloods tests revealed a c-reactive protein of 37 but normal white cell count (WCC). Lumbar puncture results were all within normal range (glucose 2.9mmol/L, protein 0.4g/L, WCC 2 cells/mm³).

The first MRI was done 2 days after the onset of aphasia. This MRI brain showed prominent restricted diffusion (high signal on DWI, low signal on ADC maps) in the splenium of the corpus callosum and the posterior corona radiata bilaterally. Milder but still restricted diffusion was seen in the rest of the corpus callosum. These areas showed mild high T2 signal, more noticeable posteriorly. T1-weighted imaging showed no abnormality.  There was no enhancement post gadolinium and MRI whole spine was normal.

Repeat MRI brain 2 months later showed significant improvement, with resolution of the previous restricted diffusion and T2 signal change. 

A.    Background
Mild encephalitis/encephalopathy with reversible splenial lesion (MERS) was first identified as a concept by Tada et al. in 2004 [1]. It is a rare clinic-radiological entity characterised by the magnetic resonance imaging (MRI) finding focal restricted diffusion lesion in the splenium of corpus callosum [2] which reverses, usually within a week [3], and sometimes involving the anterior corpus callosum and symmetrical white matters [4].
B.    Clinical Perspective
Patients tend to present with mild neurological symptoms, such as headache, disturbed consciousness and seizures. These patients recover completely within a month [2]. MERS has been reported more commonly in children [4], and is associated with various infections [5][6].
The diagnosis and description of MERS overlaps with posterior reversible encephalopathy (PRES). Both can present with seizures, headache, visual disturbances and reduced consciousness, although PRES symptoms can be more severe including patients being comatose, or in status epilepticus. Also over 70% of patients with PRES are hypertensive [7].

C.    Imaging Perspective
On MRI, MERS typically involves a transient lesion in the midline of the splenium of the corpus callosum that is slightly hyperintense on T2- and isointense to mildly hypointense on T1-weighted images. It shows restricted diffusion without contrast enhancement post-gadolinium. It may also involve posterior and symmetrical cerebral white matter and/or rest of the corpus callosum with similar signal; occasionally the cerebellum is involved [8, 9, 10].
There is an overlap in the presentations of MERS and PRES. PRES classically involves the posterior cerebral white matter symmetrically with the changes usually not showing restricted diffusion on MRI. Involvement of corpus callosum seen in 10% [11]. Like the imaging description of MERS, reversible lesions of the splenium of the corpus callosum with restricted diffusion have also been reported in PRES [11, 12, 13]. These abnormal areas also reverse on MRI although approximately 30% of PRES will not show complete reversal [11].

D.    Outcome
This patient received no treatment as her symptoms resolved. There is no consensus on the treatment of MERS. Some evidence recommends a combined antiviral and steroid pulse therapy [14]. Other evidence recommends treating patients with infectious encephalopathy with methylprednisolone pulse therapy and intravenous immunoglobulin [15]. Most cases make a full recovery within one week [16].

E.    Take Home Message / Teaching Points
Not all restricted diffusion signifies infarction or permanent parenchymal damage. Consider a benign outcome if it involves the splenium of the corpus callosum and the patient has a mild encephalopathy.

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