Clinical characteristics: The clinical phenotype of alpha-mannosidosis varies considerably, with a wide spectrum of clinical findings and broad variability in individual presentation. At least three clinical types have been suggested in untreated individuals: mild (clinically recognized after age ten years, with myopathy, slow progression, and absence of skeletal abnormalities); moderate (clinically recognized before age ten years, with myopathy, slow progression, and presence of skeletal abnormalities); and severe (obvious progression leading to early death from primary central nervous system involvement or infection). Core features of untreated individuals generally include early childhood-onset non-progressive hearing loss, frequent infections due to immunodeficiency, rheumatologic symptoms (especially systemic lupus erythematosus), developmental delay / intellectual disability, low tone, ataxia, spastic paraplegia, psychiatric findings, bone disease (ranging from asymptomatic osteopenia to focal lytic or sclerotic lesions and osteonecrosis), gastrointestinal dysfunction (including diarrhea, swallowing issues / aspiration, and enlarged liver and spleen), poor growth, eye issues (including tapetoretinal degeneration and optic nerve atrophy), cardiac complications in adults, and pulmonary issues (including parenchymal lung disease). However, with the advent of enzyme replacement therapy, the natural history of this condition may change. Long-term velmanase alfa (VA) treatment outcomes are still being elucidated, but may include improvement in hearing, immunologic profile, and quality of life (improved clinical outcomes for muscle strength). Similarly, affected individuals who underwent hematopoietic stem cell transplantation (HSCT) experienced improvement in development (with preservation of previously learned skills), ability to participate in activities of daily living, stabilization or improvement in skeletal abnormalities, and improvement in hearing ability, although expressive speech and hearing deficiencies remained the most significant clinical problems after HSCT.

Diagnosis/testing: The diagnosis of alpha-mannosidosis is established in a proband by identification of deficiency of lysosomal enzyme acid alpha-mannosidase (typically 5%-10% of normal activity) in leukocytes or other nucleated cells AND/OR by the identification of biallelic pathogenic variants in MAN2B1 by molecular genetic testing.

Management: Targeted therapies: Velmanase alfa (Lamzede^®) enzyme replacement therapy (ERT) has been very well tolerated and is now regarded as a standard treatment for alpha-mannosidosis; improvement in both biochemical and functional parameters have been reported in treated individuals. Hematopoietic stem cell transplantation (HSCT) has been offered as a treatment for severe alpha-mannosidosis. While HSCT carries risks, the data suggests it is a feasible therapeutic option for alpha-mannosidosis, with better outcomes achieved by performing it early before complications arise, balancing the risks and benefits.

Supportive care: Hearing aids may be helpful for those with sensorineural hearing loss, whereas pressure-equalizing tubes may be helpful for those with conductive hearing loss. Consider palmidronate (Aredia^®) monthly or zoledronic acid (Aclasta^®) once a year for osteoporosis or osteopenia. Standard treatment for immunodeficiency / recurrent infections, systemic lupus erythematosus, communicating hydrocephalus, ataxia / gait abnormalities, poor weight gain / growth issues, eye/vision issues, cardiac valve dysfunction / dilated cardiomyopathy, recurrent chest infections / respiratory dysfunction, developmental delay / intellectual disability, and psychiatric manifestations.

Surveillance: At each visit, measure weight, length/height, head circumference, and BMI; monitor growth pattern, developmental progress, and educational needs; assess for depression, including sleep disturbances, anxiety, &/or findings suggestive of psychosis; assess for new manifestations such as ataxia and gait abnormalities; evaluate for asthenia and signs/symptoms of communicating hydrocephalus; assess for muscle pain, joint aches, reduced range of motion, and bone pain; monitor for diarrhea and for size of the liver and spleen; and assess for the number and type of infections. Every six to 12 months in childhood and annually in adults, assess fine motor function, gross motor function, endurance, and muscle strength and tone by physical therapy; and assess for features of ataxia. Every one to two years, or as clinically indicated in those with hearing aids, perform an audiology evaluation. Every two to five years in children, adolescents, and adults, consider DXA bone densitometry scan to assess for osteopenia or osteoporosis; radiographs of the hips/spine may be indicated. Annually (or as clinically indicated), routine biochemical lab assessment to include liver and kidney health, blood glucose levels, fluid and electrolyte balance, and complete blood count (with platelets); consider immunlobulin levels, ESR and C-reactive protein; pulmonary function tests; and ophthalmology evaluation. At regular intervals based on clinical features, consider endocrinology evaluations, including hormonal and lipid profiles; consider assessment of liver and spleen size through ultrasound or MRI imaging; consider electrocardiogram, 24-hour electrocardiogram, and echocardiogram; consider sleep study. For those on ERT, plasma oligosaccharides to assess treatment response as clinically indicated. Post-HSCT evaluation of standard surveillance per hematologist/oncologist, which may include ongoing assessment for chimerism and enzyme activity if indicated.

Evaluation of relatives at risk: Testing of all at-risk sibs of any age (including prenatal diagnosis) is warranted to allow for early diagnosis and targeted treatment of alpha-mannosidosis. Evaluations can include molecular genetic testing if the pathogenic variants in the family are known or assay of acid alpha-mannosidase enzyme activity in leukocytes or other nucleated cells if the pathogenic variants in the family are not known.

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.