Late-onset Ornithine Transcarbamylase Deficiency in an Adolescent Male: A Fatal Case of Hyperammonemic Encephalopathy - A Case Report

Background: Ornithine transcarbamylase deficiency (OTCD) is the most common urea cycle disorder and is typically recognized in the neonatal period. However, partial enzyme deficiencies may remain clinically silent for years, resulting in late-onset presentations during periods of acute stress. Clinical Description: We report the case of a 13-year-old previously healthy boy with no significant family history who presented with acute encephalopathy following a respiratory infection. Retrospective history revealed episodic behavioral changes, hypersomnolence, and brief unexplained loss of consciousness over several years. Management and Outcome: Initial evaluation for infectious and autoimmune encephalitis was unrevealing. Serum ammonia measured 6 hours after admission was elevated (232.8 µmol/L) and continued to rise despite aggressive medical therapy, continuous renal replacement therapy, and intermittent hemodialysis with a peak value of 839.3 µmol/L. The child developed refractory raised intracranial pressure and neurogenic shock and succumbed on day 4 of illness. Subsequent metabolic evaluation revealed markedly elevated urinary orotic acid, confirming a diagnosis of late-onset OTCD. Conclusion: This case highlights the presentations of late-onset OTCD and catastrophic outcomes associated with delayed recognition. Early measurement of serum ammonia in unexplained encephalopathy is critical, as prognosis is strongly influenced by the duration of coma and peak ammonia levels.