Methylmalonic acidaemia masquerading as a neurodegenerative disorder

We present the case of a toddler displaying neuroregression post-acute gastroenteritis, initially suggesting neurodegenerative disorders. Further investigations showed atypical results-neuroimaging was inconsistent with suspected disorders, while fundus evaluation, evoked potentials and nerve conduction velocity were normal. Specialised tests using gas chromatography mass spectrometry and tandem mass spectrometry identified methylmalonic acidaemia (MMA), implicating abnormal neurometabolism. Early diagnosis and comprehensive treatment are essential. A low-protein diet suitable for MMA along with a syrup containing vitamin B12 and levocarnitine were prescribed. Notable developmental improvements were seen after 18 days of hospitalisation and up to 36 months of age, with no further regression. To date, only one case of MMA mimicking Rett syndrome, an atypical neurodegenerative variant, has been reported. This case highlights the diagnostic complexity of MMA, particularly when it mimics neurodegenerative disorders.