Acid Sphingomyelinase Deficiency

In: GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993.
[updated ].


Clinical characteristics: The phenotype of acid sphingomyelinase deficiency (ASMD) occurs along a continuum. Individuals with the severe early-onset form, infantile neurovisceral ASMD, were historically diagnosed with Niemann-Pick disease type A (NPD-A). The later-onset, chronic visceral form of ASMD is also referred to as Niemann-Pick disease type B (NPD-B). A phenotype with intermediate severity is also known as chronic neurovisceral ASMD (NPD-A/B). Enzyme replacement therapy (ERT) is currently FDA approved for the non-central nervous system manifestations of ASMD, regardless of type. As more affected individuals are treated with ERT for longer periods of time, the natural history of ASMD is likely to change. The most common presenting symptom in untreated NPD-A is hepatosplenomegaly, usually detectable by age three months; over time the liver and spleen become massive in size. Growth failure typically becomes evident by the second year of life. Psychomotor development progresses no further than the 12-month level, after which neurologic deterioration is relentless. This feature may not be amenable to ERT. A classic cherry-red spot of the macula of the retina, which may not be present in the first few months, is eventually present in all affected children, although it is unclear if ERT will have an impact on this. Interstitial lung disease caused by storage of sphingomyelin in pulmonary macrophages results in frequent respiratory infections and often respiratory failure. Most untreated children succumb before the third year of life. NPD-B generally presents later than NPD-A, and the manifestations are less severe. NPD-B is characterized in untreated individuals by progressive hepatosplenomegaly, gradual deterioration in liver and pulmonary function, osteopenia, and atherogenic lipid profile. No central nervous system manifestations occur. Individuals with NPD-A/B have symptoms that are intermediate between NPD-A and NPD-B. The presentation in individuals with NPD-A/B varies greatly, although all are characterized by the presence of some central nervous system manifestations. Survival to adulthood can occur in individuals with NPD-B and NPD-A/B, even when untreated.

Diagnosis/testing: The diagnosis of ASMD is established by detection of biallelic pathogenic variants in SMPD1 by molecular genetic testing and residual acid sphingomyelinase enzyme activity that is less than 10% of controls (in peripheral blood lymphocytes or cultured skin fibroblasts).

Management: Targeted therapies: Olipudase alfa (Xenpozyme®) enzyme replacement therapy (ERT) helps to reduce the accumulation of sphingomyelin in the lung, liver, spleen, and other non-central nervous system organs. It does not impact the central nervous system and therefore does not impact the neurocognitive issues seen in individuals with NPD-A or NPD-A/B. Hematopoietic stem cell transplantation (HSCT) can correct the metabolic defect, improve blood counts, and reduce increased liver and spleen volumes but does not stabilize neurologic disease. The morbidity and mortality associated with HSCT limit its use, and it is likely to become obsolete now that enzyme replacement therapy is available.

Supportive care: Feeding therapy and/or feeding tube as needed for nutritional support; supportive management of coagulopathy and end-stage liver disease manifestations; transfusion of blood products for life-threatening bleeding; partial splenectomy may be considered for individuals with severe hypersplenism; supplemental oxygen for symptomatic pulmonary disease; physical and occupational therapy to maximize function and to prevent contractures; early intervention and developmental support for those with developmental issues; treatment of hyperlipidemia in adults; calcium and vitamin D for osteopenia/osteoporosis; sedatives for irritability and sleep disturbance as indicated.

Prevention of secondary complications: Monitor liver function in individuals receiving hepatotoxic medications (e.g., statins for hypercholesterolemia).

Surveillance: Periodic assessments of nutritional status; annual EKG; echocardiogram every two to four years; liver transaminases (ALT, AST), albumin, clotting factors, and platelet count at least annually; assess for fatigue, abdominal pain, and/or increased bleeding at least annually; radiologic measurements of liver and spleen size as needed; assess for shortness of breath at each visit; annual pulmonary function testing; chest radiograph every two to four years; assess neurologic function and frequency of headaches at least annually; monitor developmental progress, educational needs, and occupational and physical therapy needs at each visit; fasting lipid profile at least annually; assess for extremity pain at each visit; bone density assessment every two to four years; assess need for family support and resources at each visit.

Agents/circumstances to avoid: Contact sports in those who have splenomegaly.

Pregnancy management: For pregnant women with ASMD, prenatal care by a high-risk obstetrician is indicated to ensure appropriate monitoring of pulmonary function and hematologic status. Olipudase alfa ERT has not been studied in pregnant women, but animal studies have identified a potential impact on fetal development. Therefore, ERT is not recommended during pregnancy.

Genetic counseling: All forms of ASMD (NPD-A, NPD-A/B, and NPD-B) are inherited in an autosomal recessive manner. If both parents are known to be heterozygous for an SMPD1 pathogenic variant, each sib of an affected individual has at conception a 25% chance of being affected, a 50% chance of being a carrier, and a 25% chance of being unaffected and not a carrier. Once the SMPD1 pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives and prenatal/preimplantation genetic testing are possible. Biochemical prenatal diagnosis for a pregnancy at 25% risk is also possible by testing of acid sphingomyelinase enzyme activity.

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