Lysosomal storage diseases are characterized by inherited deficiencies of one or more lysosomal enzymes involved in the degradation of lipids and their products. Niemann-Pick disease (NPD) is a lysosomal storage disease caused by acid sphingomyelinase deficiency (ASMD), which catalyzes the hydrolysis of sphingomyelin (SM) to ceramide and phosphocholine. As a result, SM and its precursor lipids begin to accumulate in lysosomes, mainly in macrophages. These lipid-laden macrophages deposit in the liver, spleen, lungs, and brain causing hepatosplenomegaly, cytopenias, lung disease, and neurologic symptoms.
Traditionally, NPD is classified into four subtypes: type A, B, C, and E. Type A is known as infantile neurovisceral form with very low acid sphingomyelinase (ASM) activity and is usually fatal before the age of three. It affects younger children and results in neurological deficits and impaired growth. Type B is less severe and is characterized by variable visceral symptoms and minimal neurological involvement. The most common visceral symptoms in these phenotypes include hepatosplenomegaly, thrombocytopenia, and interstitial lung disease. Niemann-Pick disease type C (NPC) has a heterogeneous clinical presentation and includes systemic, neurologic, and psychiatric involvement. It usually affects adults but can occur during any phase of life. Early-onset NPC manifests as infantile jaundice, hepatosplenomegaly, or isolated splenomegaly, and usually, these symptoms precede neurological involvement. In about 50% of adult patients, NPC can manifest without or minimal hepatosplenomegaly, so the presence of isolated splenomegaly in patients with neurological or psychiatric illnesses favors NPC. Type E is a less common variant of NPD that develops in adulthood. The most common neurologic manifestations of NPD include cognitive or motor developmental delay in childhood-onset cases, vertical supranuclear gaze palsy, ataxia, dysarthria, dysphagia, and dystonia.
Niemann-Pick disease (NPD) is inherited in an autosomal recessive pattern, which means both copies of the gene must have mutations for the manifestation of the disease. NPD types A and B are caused by missense mutations in the sphingomyelin phosphodiesterase 1 (SMPD1) gene. Over 180 mutations in SMPD1 have been identified. NPD type C is caused by mutations in NPC1 (located on chromosome 18) and NPC2 (located on chromosome 14) genes. The mutations in these genes lead to abnormal or defective formation of proteins, which impair the movement of lipids out of the cells, leading to their accumulation within the cells.
Niemann-Pick disease (NPD) types A and B affect 1 in 250,000 individuals. The prevalence is high in Ashkenazi Jewish descent, where it affects 1 in 40,000 individuals. NPD type C affects 1 in 150,000 persons. Type C is more prevalent in French-Acadian descent in Nova Scotia.
Niemann-Pick disease types A and B are caused by mutations in the sphingomyelin phosphodiesterase 1 (SMPD1) gene, leading to a strongly decreased activity of acid sphingomyelinase (ASM). The enzyme ASM is mainly present in lysosomes and converts sphingomyelin (SM) to ceramide and phosphocholine. In ASMD, SM and its precursor lipids accumulate in lysosomes and cause cellular damage.
Niemann-Pick disease type C (NPC) is further classified as type C1 or type C2 based on the pathogenic mutations in the NPC1 or NPC2 genes, respectively. NPC1 is the predominant subtype affecting about 95% of the patient population. NPC1 and NPC2 proteins are present in late endosomes and lysosomes and are involved in the transport and intracellular mobilization of cholesterol and sterols. The loss of function of NPC1 and/or NPC2 proteins blocks cholesterol egress from lysosomes, resulting in an excessive build-up of cholesterol in lysosomes. Consequently, toxic cholesterol accumulation results in cellular and organ damage.
Due to the ineffective transport system, the affected cells become enlarged sometimes as big as 90 micrometers in diameter due to the accumulation of sphingomyelin and cholesterol. Histology shows lipid-laden macrophages in the marrow, also called foam-cells. Numerous small vacuoles of relatively uniform size are created, which give the cytoplasm a foamy appearance. Electron microscopy shows electron-opaque, concentrically laminated inclusions within the macrophage cytoplasm.
Nieman-Pick disease (NPD) type A presents in the first few months of life, usually before three months of age, as hepatosplenomegaly and growth retardation. By the age of one year, neurological symptoms appear as psychomotor retardation and regression of developmental milestones. All patients with NPD type A have a classical eye finding called cherry-red spot. These children usually do not survive past early childhood. NPD type B presents in mid-childhood and is not as severe as type A. These patients have hepatosplenomegaly, interstitial lung disease (ILD), causing recurrent lung infections, thrombocytopenia, and slowed bone growth. About one-third of patients with type B NPD have the cherry-red spot and neurological symptoms. NPD type C can present at any age but usually affects children. Affected individuals can have ataxia, dystonia, supranuclear gaze palsy (SNGP), dysphonia, dysphagia, and severe liver and lung disease.
The following findings can be expected while examining the body systems.
For suspected NPD type A pr B, a blood sample is drawn, and the activity of ASM is measured in leukocytes. In the case of low enzyme activity, additional gene testing can be done to evaluate the disease further.
For NPD type C, a skin biopsy is taken and stained with a special stain 'filipin' to measure the enzyme activity. DNA testing can be done to look for genes that cause type C disease.
Once the diagnosis is confirmed, special attention should be paid to look for disease spread and manifestations in the following systems.
Liver enzymes should be measured periodically. Liver elastography or liver biopsy should be done in case of severe liver disease.
Spirometry should be done periodically. High-resolution computed tomography (HRCT) is done for interstitial lung disease (ILD).
Measure platelet counts and spleen volume.
Measure HDL-cholesterol and LDL-cholesterol.
A complete neurological exam should be performed at every visit.
Fundoscopy to look for the cherry-red spot on the macula.
Spirometry and exercise intolerance test.
Pain and Fatigue
A questionnaire should be used for grading.
Severity of Disease
For type A and B Niemann-Pick disease (NPD), there is no cure. Supportive care is the mainstay of treatment. Physicians try to keep low blood lipids levels with statins, and liver functions are monitored. If thrombocytopenia leads to bleeding episodes, transfusion of blood products may be required. For patients with ILD, oxygen is provided. Organ transplant has also been tried but with limited success. Enzyme replacement therapies and gene therapies are undergoing trials and may become the mainstay of treatment in the future.
For type C disease, supportive care is also the mainstay of treatment. Physical therapy is provided for neurological symptoms. Pain is managed with analgesics. Miglustat is a glucosylceramide synthase inhibitor and helps in Niemann-Pick disease and Gaucher disease by decreasing the production of glucocerebroside. It is approved in Europe, Canada, and Japan but is not yet approved in the United States.
Other lysosomal storage diseases should be kept in the differentials, especially Gaucher disease, Tay-Sachs disease, and Metachromatic leukodystrophy. Gaucher disease also presents with hepatosplenomegaly and cytopenias, but bone pain and lesions are more prominent. The deficient enzyme in Gaucher disease is glucocerebrosidase, which leads to the accumulation of glucocerebroside inside cells instead of sphingomyelin, which is found in Niemann-Pick disease. Tay-Sachs disease, although it does not present with hepatosplenomegaly, neurodegeneration, developmental delay, and cherry-red spot on the macula, are prominent features. The deficient enzyme here is hexosaminidase A which causes a buildup of GM2 gangliosides. Metachromatic leukodystrophy causes central and peripheral demyelination and can manifest as ataxia or other neurological symptoms.
Apart from these, other diseases affecting the liver and brain should be kept in mind based on the initial presentation.
It is almost always fatal and affected children are unlikely to live beyond 4 years of age.
These children have a slightly better prognosis than type A and may live till late childhood or early adulthood. But they develop many complications from the disease, so the life quality is not so good.
The prognosis depends on the time of the initial presentation. If it affects in infancy, the chances are very low to survive beyond 5 years of age. If it affects after 5 years, then patients may live to the age of 20 years. But each patient has a slightly different outlook depending on the severity and presentation of the disease.
Niemann-Pick disease is a progressive disease, and quite often, complications develop with time.
Although Niemann-Pick disease is fatal and often untreatable, the sooner it is recognized, the better is the chance to slow down its progression and limit the complications. Niemann-Pick disease is an autosomal recessive (AR) disease, which means both the parents must be affected or carriers of the genes for the disease to pass on to their children. Carrier persons of an AR disease do not show signs and symptoms of the disease, but they can pass the defective gene to the next generation. So if both the parents are carriers, each pregnancy has a 25% chance of producing a child with the disease. Parental counseling should be done, and genetic testing should be offered to families who may be carriers of the disease.
Niemann-Pick disease has negative repercussions for many organs, and with or without treatment, it has high morbidity and mortality. The disease is best managed by an interprofessional team that includes a hepatologist or gastroenterologist, endocrinologist, neurologist, and genetic counselor. Also, family care, nursing support, and social support are an integral part of the management. A lead consultant should be in charge of the patient care, and a nurse specialist, along with other specialists in the respective fields, should be involved to cover all the aspects of the disease. Patient education is crucial, and social worker involvement, including a geneticist, is essential. In some parts of the world, preventive strategies include prenatal screening, restrictions on issuing marriage licenses to two people with the same disease. The screening of children and pregnant women who visit clinicians is an effective strategy to limit the disease but is not cost-effective yet. The social worker should ensure that the caregivers and patients have adequate support and financial resources so that they can continue with treatment. Nurses should educate patients on the importance of treatment compliance to avoid or halt serious complications, as well as monitoring treatment progress. Pharmacists may soon play a greater role as there are new drug products to assist in gene therapy or enzyme replacement therapy that can theoretically cure the disease.Active collaboration and discussion between interprofessional team members help in the better understanding of the progression or control of the disease.
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