[Opsoclonus-myoclonus-ataxia syndrome associated with St Louis virus infection in Argentina]

Opsoclonus-myoclonus-ataxia syndrome is uncommon in adults. Causes are usually autoimmune, infectious, or paraneoplastic. There are cases secondary to some flaviviruses, West Nile virus, or dengue virus (1%); however, to our knowledge, there are no reports of cases in Argentina associated with the Saint Louis virus (SLEV). This virus presents with nonspecific symptoms that precede neurological manifestations. Gadolinium-enhanced brain MRI is usually normal, while cerebrospinal fluid (CSF) may show elevated protein levels and moderate pleocytosis. The diagnosis is established with the presence of IgM and IgG antibodies in serum (with seroconversion) and/or IgM or direct viral detection in CSF. Supportive treatment or immunomodulatory therapy with corticosteroids and/or immunoglobulins is described. We present the case of a 35-year-old man with no medical history who was admitted for opsoclonus, multifocal myoclonus, axial ataxia, and generalized tremor following a 10-day history of nonspecific symptoms. Neuroimaging studies were normal, CSF showed mildly elevated protein levels, multiplex encephalitis PCR was negative, and viral serology tests were negative. Samples of CSF, serum, and plasma from the patient were sent to the National Reference Laboratory for Dengue and other arboviruses, where SLEV-specific neutralizing antibodies were detected in both sera using a neutralization technique. Treatment with pulses of methylprednisolone (3 g) and gamma globulin 2 g/kg was initiated, with slight improvement. Outpatient treatment continued with a slow taper of corticosteroids, and complete resolution of symptoms occurred after 2 months of follow-up.