Alpha-mannosidosis is a rare inherited lysosomal storage disorder characterized by a deficiency of the enzyme alpha-D-mannosidase. Alpha-mannosidosis is divided into three subtypes: a mild, slowly progressive form (type 1); a moderate form (type 2); and a severe, often rapidly progressive and potentially life-threatening form (type 3). Symptoms range widely in their onset and severity. Symptoms may include distinctive facial features, skeletal abnormalities such as osteopenia, calvaria, vertebrae, bowed legs or knock knees, and deterioration of the bones and joints, hearing loss, intellectual disability, and dysfunction of the immune system, such as lupus erythemadoses, pancytopenia, hypothyroidism or primary biliary cirrhosis. Prevalence is estimated to be 1 in 500,000 live births.

Diagnosis of the condition is based on measuring acid alpha-mannosidase activity in leukocytes or other nucleated cells, and it can be confirmed by genetic testing for the MAN2B1 gene. Management should be proactive, preventing complications and optimizing quality of life, and may include early use of antibiotics for bacterial infections, hearing aids for hearing loss, insertion of pressure-equalizing tubes if fluid accumulates in the middle ear, glasses to correct refractive error, physiotherapy, use of a wheelchair, orthopedic intervention, and shunting as needed for hydrocephalus. Educational considerations may include use of sign language for individuals with hearing loss, early educational intervention for development of social skills, speech therapy, and special education to maximize learning.