Late onset arginase deficiency presenting with encephalopathy and midbrain hyperintensity

Urea cycle disorders (UCD) are very rare metabolic disorders that present with encephalopathy and hyperammonemia. Of the UCDs, Arginase deficiency (ARD) is the rarest and presents in childhood with a progressive spastic diplegia or seizures. Acute presentation in adulthood is extremely unusual.[1] We present the first case of adult onset ARD presenting with encephalopathy and diffusion weighted MRI findings that resembled a moustache in the midbrain.