Background Respiratory syncytial virus (RSV) causes every year millions of respiratory infections worldwide. Common complications are pneumonia and middle ear infection. Neurological complications such as seizures, encephalopathy and encephalitis are very rare, but have been reported especially in the paediatric population, more often affecting children younger than two years of age.

Case description We describe the case of a previously healthy 9-year-old boy, who presented to our emergency department with a 2 days history of fever, cough and vomiting. After 3 days of acute flu-like symptoms, the boy became afebrile, but developed an acute loss of consciousness, slurred speech, lethargy, weakness and photophobia. No neck stiffness. Peripheral blood count didn’t show leukocytosis and CRP was negative. Cerebral spinal fluid (CSF) analysis showed mild pleocytosis with no protein and glucose abnormalities. Neurotropic viruses, including HSV, were tested negative in CSF. Electroencephalography (EEG) showed continuous generalised slow activity without epileptiform discharges. Brain magnetic resonance imaging (MRI) showed basal ganglia involvement. By missing clinical, radiological and EEG suspicion for an HSV associated encephalitis, no antiviral treatment was initiated.

View the associated respiratory symptoms, an analysis for M. pneumoniae and respiratory virus in nasopharyngeal swab was performed, showing a positive result only for RSV. CSF was also tested with RT-PCR for RSV and resulted negative. Nevertheless, the clinical symptoms are highly suggestive of a RSV associated acute encephalitis.

The patient showed a significant improvement in clinical and neurological symptoms within 7 days, with normal EEG findings. The patient was discharged with a close clinical and neuroradiological follow-up.

Discussion The clinical presentation with respiratory symptoms followed by acute neurologic symptoms and detection of RSV as viral pathogen, led us to the diagnosis of RSV associated acute encephalitis. The negative RT-PCR on CSF doesn’t exclude our diagnosis, since in the literature only up to 50% of patients with RSV infection and neurological symptoms had a positive PCR in CSF. The cerebral MRI images showed basal ganglia involvement, which have also been described in the literature.

Fortunately the recovery is quick and no specific treatment is required. Little is known about the long-term neurodevelopmental outcomes of children developing RSV associated acute encephalitis, so a prolonged period of neurologic follow up can be recommended.