Child Neurology: Reversible Dementia in an 18-Year-Old Woman Due to Undiagnosed Cobalamin-G Deficiency

Cobalamin-G deficiency is an inborn error of metabolism which disrupts the biochemical utilization of vitamin B12 to covert homocysteine to methionine in the remethylation pathway. Typically, affected patients present within the first year of life with anemia, developmental delay, and metabolic crisis. Few case reports of cobalamin-G deficiency reference a later onset phenotype primarily defined by neuropsychiatric symptoms. We report an 18-year-old woman who presented with a 4-year history of progressively worsening dementia, encephalopathy, epilepsy, and regression of adaptive functioning, with an initially normal metabolic workup. Whole-exome sequencing identified variants in the MTR gene, suspicious for cobalamin-G deficiency. Additional biochemical testing after genetic testing supported this diagnosis. Since treatment with leucovorin, betaine, and B12 injections, we have seen a gradual return to normal cognitive function. This case report expands the phenotypic range of cobalamin-G deficiency and offers rationale for genetic and metabolic testing in cases of dementia in the second decade of life.