A three-year-old girl with subacute encephalopathy, myoclonus, and regression

This previously healthy three-year-old girl presented with two months of developmental regression, progressive fatigue, and jerking movements in the setting of failure to thrive and macrocytic anemia. She previously spoke in full sentences but began using only one-to-two-word phrases. She would collapse after walking short distances or going up stairs. Examination revealed lethargy, irritability, myoclonic jerks of trunk and limbs, diffuse hyperreflexia with clonus, and wide-based unsteady gait. Labs showed vitamin B₁₂ < 200 pg/mL (lowest reportable), methylmalonic acid 14 μmol/L (ref. 0.0–0.4 μmol/L), homocysteine 224 μmol/L (ref. 0–15 μmol/L), and mean corpuscular volume 110 (ref. 76–87 fL). Magnetic resonance imaging revealed no brain abnormalities, but spinal imaging showed extensive T2 hyperintensity of the dorsal columns (Figure 1). She was diagnosed with spinal cord degeneration secondary to vitamin B₁₂ deficiency. Parenteral B₁₂ supplementation led to rapid clinical improvement. However, without continued parenteral supplementation, she had declining B₁₂ levels and is undergoing further investigation for a malabsorptive or genetic-metabolic etiology.

Cobalamin, or vitamin B₁₂, is important in myelin metabolism and brain development. Pediatric B₁₂ deficiency is most commonly due to nutritional insufficiency, malabsorption, or more rarely, genetic-metabolic syndromes.^1-3 Infants and toddlers often present with developmental regression, lethargy, hypotonia, and abnormal movements such as myoclonus or chorea.^{1, 4} Spinal cord degeneration secondary to B₁₂ deficiency is exceedingly rare in children in developed countries.^{1, 2} In terms of treatment, oral B₁₂ replacement is sufficient in cases of nutritional deficiency. Malabsorptive or genetic-metabolic etiologies, on the other hand, are likely to require ongoing parenteral supplementation.³ Prognosis is dependent on age of onset and duration of uncorrected B₁₂ deficiency.¹

Shermila Pia: Conceptualization; investigation; writing—original draft. Ryan Carrier: Writing—review & editing. David Bearden: Supervision; writing—review & editing.

DB is an associate editor for Annals of the Child Neurology Society. SP and RC declare no conflicts of interest.