Abstract:
Background: Opsoclonus-myoclonus ataxia syndrome (OMAS) is a rare, immune-mediated neurological disorder of childhood, frequently associated with neuroblastoma. Its clinical presentation may overlap considerably with infectious or autoimmune encephalitis, particularly during the early stages of disease. Recognition of the characteristic movement disorder is essential, as delayed diagnosis may result in prolonged neurological morbidity.
Case Presentation: A 15-month-old girl presented with fever, persistent vomiting, convulsions, bulbar manifestations including choking and swallowing difficulties, followed by progressive inability to walk. Cerebrospinal fluid (CSF) examination showed elevated protein (295 mg/dL), glucose of 68 mg/dL, and fewer than five lymphocytes, with negative culture. The initial CSF IgG index was 0.17. Brain magnetic resonance imaging and electroencephalography were normal. Based on the initial presentation, treatment for viral encephalitis was initiated; however, there was no clinical improvement. On reassessment, the CSF IgG index had increased to 0.62, raising suspicion of an immune-mediated process. N-methyl-D-aspartate receptor (NMDAR) antibodies were negative, and EEG remained normal. The patient subsequently received treatment directed toward autoimmune encephalitis, again without significant improvement.
During hospitalization, careful clinical observation revealed characteristic abnormal eye and limb movements. The eye movements were spontaneous, involuntary, conjugate, multidirectional, arrhythmic saccades occurring in all directions of gaze and persisting during sleep, consistent with opsoclonus. Nonepileptic lightning-like limb movements consistent with myoclonus were also observed. These findings were accompanied by marked irritability and behavioral changes, including excessive crying and self-biting. The combination of opsoclonus, myoclonus, ataxia, and behavioral disturbance shifted the diagnostic consideration toward OMAS.
Investigations and Outcome: Extensive evaluation for an underlying neuroblastoma, including spinal MRI, abdominal ultrasonography, PET-CT, and urinary vanillylmandelic acid and homovanillic acid measurements, was negative. Neuroblastoma and other malignancy were therefore excluded. Treatment was subsequently initiated according to an OMAS-directed regimen. The patient showed marked clinical improvement with ACTH immunotherapy. After six months of treatment, she had returned to an essentially normal neurological state, with only a slight residual cognitive deficit.
Conclusion: This case highlights the diagnostic challenge posed by OMAS when its initial presentation mimics encephalitis. Normal neuroimaging and EEG, persistent ataxia, poor response to encephalitis-directed therapy, and the later emergence of characteristic opsoclonus and myoclonus should prompt reconsideration of the diagnosis. Careful longitudinal neurological examination is crucial, as the characteristic movement disorder may appear late and can be missed initially. Early recognition of OMAS and appropriate evaluation for an underlying neuroblastoma are essential for timely treatment and improved neurological outcome.
