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Mild encephalitis with reversible splenial (MERS) lesion syndrome due to influenza B virus
  1. Geraldine Vanderschueren1,
  2. Katlijn Schotsmans1,
  3. Emke Maréchal1,2,
  4. Roeland Crols1,3
  1. 1Department of Neurology, ZNA Middelheim, Antwerp, Belgium
  2. 2Department of Neurology, Antwerp University Hospital (UZA), Edegem, Belgium
  3. 3Department of Neurology, ‘Heilige Familie’ Hospital, Antwerp, Belgium
  1. Correspondence to Dr Geraldine Vanderschueren, Department of Neurology, ZNA Middelheim, Antwerp 2020, Belgium; dinvds{at}


We describe a 16-year-old boy with mild encephalitis with reversible lesions in the white matter and splenium of corpus callosum as a complication of an influenza B virus infection. Although more common in Asiatic children, it can also occur in Caucasian children and adults. There are several possible causes, including metabolic disorders, hypertension and infection, and the prognosis is usually good, even without treatment.

  • mild encephalitis with reversible splenial lesions (mers)
  • reversible splenial lesion syndrome (resles)
  • influenza
  • mri
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A 16-year-old boy presented with rapidly developing lethargy while at school, having had a mild respiratory infection with fever, coughing and sore throat for 2 days. On examination, he was in a state of akinetic mutism; he was awake but unable to speak or obey commands and only occasionally spontaneously moving his limbs and head. There was no fever or neck stiffness.

Routine blood tests were normal. Electroencephalogram (EEG) showed a slowed background activity, with no epileptiform features. MR scan of the brain (figure 1) demonstrated that the entire corpus callosum and periventricular white matter showed prominent diffusion restriction, with hyperintensity on T2-fluid-attenuated inversion recovery but no gadolinium enhancement. Cerebrospinal fluid was normal. Laboratory results were consistent with seroconversion for influenza type B (IgA antibodies).

Figure 1

MRI findings at initial presentation. Diffusion-weighted images taken at the levels of the (A) mid-lateral ventricles and (B) centrum semiovale demonstrate restriction diffusion of the corpus callosum and of the white matter.

After only a few hours, his mental status spontaneously started to improve and he recovered completely within 24 hours. A neuropsychological examination was normal. Repeat MR scan of the brain 4 days later (figure 2) showed near-complete remission of the diffusion restricted lesions.

Figure 2

(A,B) MRI findings 4 days after admission. Axial diffusion-weighted images at similar levels to figure 1A,B demonstrate marked decrease in the abnormal signal in the corpus callosum.

We diagnosed influenza type B-induced encephalopathy with reversible lesions of the corpus callosum and periventricular white matter.


Mild encephalitis with reversible isolated splenial corpus callosum lesions (MERS) is a clinical-radiological syndrome1 with several possible causes, including metabolic disorders, hypertension, infection, and renal or hepatic dysfunction.2 It usually develops in children aged 0–16 years but also occasionally in adults.1

Its pathophysiology is probably through cytokine release. This activates T cells, which breach the blood–brain barrier, eventually causing inflammatory infiltrates and intramyelinic oedema.

The neurological features of MERS include reduced consciousness, seizures, irritability, agitation, disorientation and nuchal rigidity. The most common neurological symptom is deliriant behaviour with altered consciousness, in this case presenting as akinetic mutism. This altered mental state results from a disconnection between the two hemispheres and disruption of higher cortical functions.2

The diagnosis of MERS is generally confirmed by MR scanning, showing lesions with diffusion restriction that disappear after between a few days to 4 weeks. There are two radiological subtypes. Type 1 (reversible splenial lesion syndrome, or RESLES) is characterised by reversible lesions in the splenium of the corpus callosum.3 Type 2 has more extensive lesions that may also involve the bilateral subcortical white matter.2 The cerebrospinal fluid is usually normal, although sometimes shows minimal pleocytosis. EEG can be abnormal in the acute phase (with background slowing) but becomes normal with clinical recovery.

There is no consensus on its treatment. There are case reports of patients treated with antiviral medication, antibiotics and/or corticosteroids but without any clear evidence base. The prognosis is usually good, even without treatment, although one reported case was left with permanent cognitive deficits.1 Overall, patients clinically recover within 1–3 months.

Key Points

  • Mild encephalitis with reversible isolated splenial corpus callosum lesions has several possible causes and an overall good prognosis, even without treatment.

  • Type 1 is limited to the splenium of the corpus callosum (reversible splenial lesion syndrome), whereas type 2 also has bilateral involvement of the subcortical white matter.


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  • Contributors GV wrote the manuscript. KS, EM and RC revised the manuscript.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Patient consent Parental/guardian consent obtained.

  • Provenance and peer review Not commissioned; externally peer reviewed. This paper was reviewed by Aaron Berkowitz, Boston, UK.

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