Clinical Reasoning: A Toddler With Acute-Onset Hypotonia, Areflexia, and Ataxia.

This is a case report of an established genetic etiology with a unique clinical presentation and novel imaging findings that add to the clinical spectrum of this known condition. A 2-year-old male toddler presented with acute-onset weakness and ataxia after a respiratory viral illness. A previous admission was marked by similar weakness with areflexia, ataxia, and metabolic acidosis. Initial examination revealed weakness with areflexia, loss of tone, and ataxia, followed by intermittent encephalopathy. These physical findings localized to both central and peripheral etiologies. Differential diagnoses included atypical Guillain-Barre syndrome, acute disseminated encephalomyelitis, acute cerebellar ataxia, an inborn error of metabolism, or an underlying genetic etiology. Laboratory evaluation revealed a metabolic lactic acidosis with a normal lactate/pyruvate ratio but an unrevealing additional metabolic evaluation. CSF studies were significant for an albuminocytologic dissociation. MRI brain revealed symmetric diffusion restriction in the bilateral middle cerebellar peduncles extending into the central white matter of the cerebellum, with diffusion restriction also present in the central pons, which was considered to be a novel imaging finding of this condition. The definitive diagnosis was found with genetic testing with whole-exome sequencing. This case report reiterates the importance of genetic testing for conditions that may otherwise be difficult to identify based solely on the clinical findings.