Reversible Regression of Milestones and Hypotonia in an Infant: A Case of B12 Deficiency

Introduction: Regression of milestones and hypotonia in an infant are disconcerting chief complaints, as they are often caused by irreversible diagnoses including genetic and metabolic disorders. It is important, however, to consider reversible etiologies when investigating the cause of these symptoms.

Case Description: We present a previously healthy 6-month-old exclusively breastfed female admitted for regression of milestones, weight loss, and hypotonia over the previous month. Initial exam revealed a weak cry with loss of social smile, significant head lag, and decreased truncal tone. Extensive workup demonstrated anemia (Hgb 8.5 g/dL, MCV 87 fL), neutropenia, elevated LDH, and concern for hemolysis on blood smear. MRI brain revealed diffuse brain volume loss and thinning of the corpus callosum, which was initially concerning for either a neurodegenerative genetic or metabolic etiology (Figure 1). Further examination revealed glossitis and hyperpigmentation of her distal phalanges, which in the setting of hypotonia and anemia, raised the concern for B12 deficiency (Figure 2). Additional testing was obtained which supported the diagnosis of B12 deficiency, which was later confirmed by metabolic testing. Intramuscular B12 therapy was promptly initiated with subsequent marked improvement in neurologic status. Maternal B12 levels were found to be low which was deemed to be the primary cause of our patient’s B12 deficiency. On follow-up visit, our patient was noted to have regained her social smile with improved tone and strength.

Discussion: Symptoms of B12 deficiency in infants are often non-specific but can present with neurologic deficits, poor growth, and gastrointestinal complaints. Vitamin B12 in infants is exclusively obtained through neonatal stores at birth and through diet. If exclusively breastfeeding mothers are deficient in B12, then infants may be at risk for B12 deficiency as well. Macrocytic anemia is often not a reliable indicator of B12 deficiency; Vitamin B12 deficiency is best measured through methylmalonic acid and homocysteine levels. Treatment of B12 deficiency can reverse the majority of symptoms rapidly.

Conclusion: This case highlights the importance of recognizing B12 deficiency in infants, as treatment can both prevent and reverse the significant neurologic deficits that are seen with this condition.