Clinical characteristics: Alpha-mannosidosis encompasses a continuum of clinical findings from mild to severe. Three major clinical subtypes have been suggested:
A mild form recognized after age ten years with absence of skeletal abnormalities, myopathy, and slow progression (type 1)
A moderate form recognized before age ten years with presence of skeletal abnormalities, myopathy, and slow progression (type 2)
A severe form manifested as prenatal loss or early death from progressive central nervous system involvement or infection (type 3)
Individuals with a milder phenotype have mild-to-moderate intellectual disability, impaired hearing, characteristic coarse features, clinical or radiographic skeletal abnormalities, immunodeficiency, and primary central nervous system disease – mainly cerebellar involvement causing ataxia. Periods of psychiatric symptoms are common. Associated medical problems can include corneal opacities, hepatosplenomegaly, aseptic destructive arthritis, and metabolic myopathy. Alpha-mannosidosis is insidiously progressive; some individuals may live into the sixth decade.
Diagnosis/testing: The diagnosis of alpha-mannosidosis is established in a proband by identification of deficient acid alpha-mannosidase enzyme activity in peripheral blood leukocytes or other nucleated cells such as fibroblasts. Identification of biallelic pathogenic variants in MAN2B1 by molecular genetic testing can confirm the diagnosis and allow for family studies.
Management: Treatment of manifestations: The enzyme replacement therapy velmanase alfa has been approved in some European countries for treatment of alpha-mannosidosis. Treatments aimed at preventing complications and optimizing quality of life also 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 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.
Prevention of secondary complications: Prophylactic vaccinations because of immunodeficiency.
Surveillance: Annual or semiannual medical history and physical examination and specific otolaryngology, audiometry, ophthalmologic, neuropsychological, and skeletal examinations, along with blood tests and CT scans of the brain.
Evaluation of relatives at risk: Test at-risk newborn sibs to permit early intervention for affected children.
Genetic counseling: Alpha-mannosidosis is inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk relatives is possible if the pathogenic variants in the family are known. Prenatal testing for a pregnancy at increased risk is possible by assay of acid alpha-mannosidase enzymatic activity or molecular genetic testing once the pathogenic variants have been identified in the family.
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