Clinical, magnetic resonance imaging, and neuropathological features of suspected inborn methylmalonic aciduria in a domestic shorthair kitten.

Methylmalonic aciduria should be considered in kittens with progressive neurological signs and hypocobalaminemia.

This case report describes a 10-month-old domestic shorthair kitten presenting with progressive neurological signs including ataxia, episodic opisthotonus, impaired vision, dysphagia, and seizures. Clinicopathological evaluation revealed marked hypocobalaminemia alongside abnormal urinary concentrations of methylmalonic acid, methylcitrate, and propionylglycine, consistent with a diagnosis of methylmalonic aciduria (MMA). MRI findings demonstrated generalized cerebral and cerebellar atrophy with mild ventriculomegaly. Treatment comprising dietary modification, cobalamin supplementation, and antiepileptic therapy failed to halt neurological deterioration, and the kitten was euthanized due to refractory seizures. Post-mortem neuropathological examination identified multiorgan involvement and central nervous system changes including neuronal degeneration and necrosis, cortical atrophy, globular eosinophilic intracytoplasmic neuronal inclusions, and gliosis. MMA is a rare inherited metabolic disorder arising from defects in cobalamin metabolism or methylmalonyl-CoA mutase activity. This case highlights MMA as a differential diagnosis in young cats presenting with progressive neurological deterioration and hypocobalaminemia, underscoring the importance of urine organic acid profiling in the diagnostic workup. The poor prognosis despite aggressive management reflects the severity of the underlying metabolic defect and associated neuropathology in this species.

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