Human β‐mannosidosis: a 3‐year‐old boy with speech impairment and emotional instability

β‐mannosidosis is a recently described inherited disorder with predominantly neurological signs and symptoms as the major manifestations of the disorder. The heterogeneous manifestations of the disease have been presented in seven previous patients. We describe a further case of European descent with an infantile onset of the disease, with the features of speech impairment as the first symptom, β‐mannosidase activity was completely deficient in the patient and a heterozygote level was found in the parents. In addition, mannosyl‐N‐acetylglucosamine was identified in the patient's urine in keeping with the diagnosis of β‐mannosidosis.