Vitamin B12 Deficiency in Infants: Clinical Manifestations, Neurological Consequences, and Therapeutic Management

Vitamin B12 deficiency is a well-recognised cause of neurodevelopmental disorders and psychomotor regression in pediatric patients. In developed countries, infant B12 deficiency most commonly occurs in exclusively breastfed infants whose mothers have subclinical or overt B12 deficiency. Beyond its clinical consequences, vitamin B12 deficiency in infancy also carries important public health implications. This comprehensive review aims to synthesise current knowledge regarding the pathophysiology, clinical manifestations, diagnostic approaches, and therapeutic management of vitamin B12 deficiency in infants. It predominantly affects exclusively breastfed infants of mothers with cobalamin deficiency, whether related to nutritional insufficiency, vegetarian or vegan diets, or maternal malabsorption syndromes. The condition typically becomes apparent between 2 and 12 months of age and is characterised by vomiting, lethargy, growth failure, muscular hypotonia, and developmental arrest or regression. Biochemically, it is associated with elevated urinary levels of methylmalonic acid and homocysteine, reflecting impaired cellular metabolism. Although neurological manifestations of vitamin B12 deficiency have been documented for nearly two centuries, the underlying biological mechanisms remain only partially elucidated. Early recognition is crucial, as parenteral vitamin B12 supplementation leads to rapid clinical improvement and prevents long-term sequelae. Furthermore, preventive maternal supplementation during pregnancy and lactation has proven effective in avoiding neurological impairment in infants. Given the essential role of vitamin B12 in fetal and neonatal neurodevelopment, maternal awareness is critical. In particular, vegetarian and vegan mothers should be informed of the risks associated with inadequate intake during pregnancy and breastfeeding, since untreated deficiency can result in severe and potentially irreversible neurological damage in their offspring. This article reports the case of an 8-month-old infant presenting with psychomotor regression, hypotonia, and lethargy secondary to vitamin B12 deficiency, underscoring the importance of early diagnosis and maternal nutritional vigilance.