Metabolic masqueraders of paediatric and adult rheumatic diseases.

Inborn errors of metabolism comprise a clinically diverse group of conditions that arise from the decreased activity of an enzyme or metabolite transporter and subsequent blockade in a metabolic pathway. These disorders are typically considered in the differential diagnosis of critically ill neonates or young children presenting with hypoglycaemia, metabolic acidosis or hyperammonaemia. However, beyond these classic presentations, a broader group of inborn errors of metabolism can manifest more subtly, with progressive articular and multi-systemic involvement that mimics or overlaps with typical features of rheumatological disease. Consequently, these conditions might be misdiagnosed for years as rheumatological diseases, including juvenile idiopathic arthritis, systemic sclerosis, idiopathic inflammatory myopathies and systemic lupus erythematosus. Moreover, these disorders provide unique opportunities to understand the complex interplay between metabolism and immune function. With the growing availability of disease-modifying therapies for inborn errors of metabolism, rheumatologists must be able to recognize these disorders, particularly in patients with atypical features or treatment-refractory disease.