Hypersarcosinemia presenting as acute leukoencephalopathy with restricted diffusion in a young child

Hypersarcosinemia, resulting from sarcosine dehydrogenase (SARDH) gene mutation, is a rare autosomal recessive disorder with variable, often nonspecific clinical presentations, leading to diagnostic difficulty and low clinical awareness.

A 6-year-old boy presented with clinical features suggestive of viral encephalitis, including headache, vomiting, intermittent fever, and lethargy. Initial MRI revealed cytotoxic edema in the subcortical white matter and splenium of the corpus callosum. Although symptoms improved transiently with steroid therapy, persistent imaging abnormalities prompted metabolic and genetic evaluations. Metabolic and genetic investigations confirmed a diagnosis of hypersarcosinemia, with markedly elevating sarcosine levels and compounding heterozygous SARDH mutations. Genetic testing identified compound heterozygous mutations in the SARDH gene (c.293G > C and c.679 C > T), confirming hypersarcosinemia. Following initiation of folic acid and mecobalamin, partial radiological improvement was observed, although a causal relationship could not be established.

This case highlights the diagnostic challenge of hypersarcosinemia and its potential mimicry of acquired encephalitis. To our knowledge, this is the first report describing an acute encephalitis-like presentation accompanied by persistent cytotoxic edema on MRI, thereby suggesting a possible expansion of the known clinical and neuroimaging spectrum of this disorder, although this observation requires confirmation in additional cases. However, given the rarity of hypersarcosinemia and the possibility of underreporting, the absence of prior similar reports should be interpreted with caution. In this case, genetic testing was essential for establishing the diagnosis, although the necessity of genetic testing in all cases of unexplained white matter changes cannot be determined from a single report. The temporal association of partial radiological improvement with folic acid and mecobalamin supplementation is hypothesis-generating only and requires further investigation ; no causal or therapeutic conclusion can be drawn from this single case.