Overview

Hepatosplenomegaly (heh-puh-toh-splee-noh-MEH-guh-lee), is a term used to describe the simultaneous enlargement of both the liver (hepatomegaly) and the spleen (splenomegaly).¹ While hepatosplenomegaly itself is not a disease, it is a significant symptom with multiple potential causes.

One associated cause is Acid Sphingomyelinase deficiency (ASMD), also known as Niemann Pick disease type A, B or C, which is a rare genetic disorder characterised by a deficiency in the enzyme acid sphingomyelinase (ASM).² This enzyme is crucial in lipid metabolism, specifically in breaking down a lipid called sphingomyelin. When this enzyme is deficient, sphingomyelin accumulates in various tissues throughout the body, specifically tissues in the liver and spleen.³ 

Niemann-Pick disease type A, the most severe form of the disorder, typically manifests in infants between 0 and 3 years of age and is commonly associated with hepatosplenomegaly. The survival rate for these infants is very low, as the condition often leads to death due to complications from enlarged organs or severe neurological deficits caused by ASMD.²

Pathophysiology of ASMD 

Role of acid sphingomyelinase

ASM is responsible for breaking down sphingomyelin into other lipids called ceramide and phosphocholine. Sphingolipids, such as sphingomyelin, are crucial components of cellular membranes; a layer of biological molecules that surround the cell.³

Cause of ASMD and the accumulation of Sphingomyelin

ASMD is caused by a mutation(s) in the gene responsible for synthesizing sphingomyelinase (SMPD1). Mutations in this gene cause the enzyme to be non-functional, resulting in an inability to properly break down sphingomyelin.² Consequently, sphingomyelin accumulates within the cells and tissue. 

Studies on mice have shown that individuals with mutant variants of the SMPD1 gene experience a lower rate of cell death compared to those without the mutations.² The combination of reduced cell death and the accumulation of sphingomyelin within cells leads to hepatosplenomegaly.

Clinical presentation of Hepatosplenomegaly in ASMD

Symptoms and signs 

The most obvious physical sign of hepatosplenomegaly in ASMD is an enlarged abdomen, resulting from the enlarged spleen and liver. However, type A ASMD is also associated with other symptoms, including:

• Ascites; major accumulation of fluid in the abdomen 
• Hypotonia; decrease in muscle tone 
• Jaundice; yellowing and discoloration of the skin 
• Frequent vomiting⁴

The above symptoms correspond to the more severe type A form of the disease. In contrast, type B, the milder and later onset form, exhibits similar but slightly different symptoms:

• Hepatosplenomegaly; this still occurs in in type B
• Neutropenia; reduced number of white blood cells 
• Thrombocytopenia; reduced number of platelets 
• Abnormal liver blood tests 
• Dyspnea; shortness of breath upon exertion⁴

Diagnostic indicators 

The first step in diagnosing hepatosplenomegaly involves a clinical evaluation and physical examination. Palpation of the abdomen is used to detect enlargement of the spleen and liver, and the individual's medical and family history is reviewed to identify any potential genetic disorders.⁵

The following laboratory tests can be conducted to confirm hepatosplenomegaly as a consequence of ASMD:

• Liver function tests: Elevated liver enzymes (AST, ALT) indicating liver dysfunction
• Complete blood count (CBC): Can show anaemia, thrombocytopenia or other hematologic abnormalities associated with spleen dysfunction 
• Lipid panel: Abnormal levels of cholesterol and other triglycerides, may indicate lipid metabolism disorders 
• Acid sphingomyelinase activity: Measurement of enzyme activity in white blood cells indicates ASMD⁶

Treatment options

Symptomatic treatment and supportive therapies

Various treatment options are available to manage symptoms, improve quality of life, and address the underlying enzyme deficiency in ASMD. For individuals with type A, ensuring proper nutritional intake is crucial. In extreme cases, a feeding tube may be necessary to maintain adequate nutrition. Nutritional intake is closely monitored through periodic assessments and it is also recommended for those who have type B ASMD to also closely monitor their diets.⁴

Individuals with the severe form of type A may also require physical and occupational therapy. Physical therapy helps maintain mobility and muscle strength, while occupational therapy assists with daily living activities and improves quality of life. Additionally, psychological support is essential for patients and their families to cope with the emotional and psychological impact of the disease.⁴

Enzyme replacement therapy

Enzyme replacement therapy (ERT), is a special type of therapy given to individuals who suffer from enzyme deficiencies. In the case of ASMD a medication: Olipudase alpha is a synthetic form of deficient sphingomyelinase and is given to sufferers to reduce the accumulation of sphingomyelin in cells.⁴ Importantly it reduces hepatosplenomegaly by reducing the size of the spleen and liver.⁷

Emerging therapies 

One approach currently being studied as a potential therapy for ASMD is gene therapy. ASMD, like many other rare genetic disorders, could potentially be cured by replacing the defective gene with a functional one in the affected tissue.⁴ An example of a genetic disorder successfully treated by gene therapy is severe combined immunodeficiency (SCID). Although the side effects were severe, clinical trials demonstrated that gene therapy could effectively treat the underlying condition.⁸

Prognosis and Outcomes

ASMD disease progression 

The progression of ASMD varies depending on the type: A or B. Type A, as mentioned before, is the more severe and tends to present earlier compared to type B.⁴ Below is a detailed description of the disease progression in the absence of treatment:

Type A

• Early infancy (0 - 6 months): Early infants often experience poor feeding habits and can exhibit developmental delays 
• Infancy (6 - 12 months): Infants suffer from progressive hepatosplenomegaly, persistent feeding issues and muscle weakness
• Late infancy (12 - 24 months): At this point, there is severe enlargement of the spleen and liver, progressive loss of motor and cognitive function and increased muscle stiffness
• Toddlerhood (2 - 3 years): By toddlerhood, there is a marked decline in neurological and physical functions; most children by this stage, unfortunately, do not make it due to complications in liver function and infection⁴

Type B

• Childhood (0 - 10 years): Children experience mild hepatosplenomegaly, but, mostly normal growth and development
• Adolescence (10 - 20 years): Adolescents now experience progressive hepatosplenomegaly, have reduced lung function and increased susceptibility to infections 
• Adulthood (20+ years): As adults, there is further enlargement of the liver and spleen, chronic lung disease and bone/ joint pain are also common symptoms which are experienced^4,9

Research and Future Directions

Ongoing research for ASMD continues, as no definitive cure has yet been identified. Current efforts focus on both improving existing therapies, such as enzyme replacement therapy (ERT) and exploring novel treatments, including gene therapies.

Advancements in ERT

Olipudase alfa, currently used in enzyme replacement therapy (ERT), has proven to be an effective treatment for ASMD, but it comes with limitations. Its efficacy is partial, and it can negatively impact other organs, such as the lungs.¹⁰ ERT is a symptomatic treatment rather than a cure, and its long-term safety remains to be clinically validated. Ongoing research aims to address these issues associated with ERT for ASMD.

Gene therapies

Gene therapy for ASMD is currently being tested to correct the mutations in the SMPD1 gene, enabling the production of the functional enzyme. Although it has not yet proven successful in humans, studies in mice have shown promising results. Injecting a synthetic functional version of the gene has prevented motor and memory impairment and reduced hepatosplenomegaly.¹¹

Summary

Hepatosplenomegaly, the simultaneous enlargement of the liver and spleen, is a hallmark of the rare genetic disorder Acid Sphingomyelinase Deficiency (ASMD), also known as Niemann-Pick disease types A and B. This disorder results from mutations in the SMPD1 gene, leading to a deficiency in acid sphingomyelinase and an accumulation of the lipid sphingomyelin in the liver and spleen cells.

ASMD is categorized into Type A and Type B; Type A is the more severe form, manifesting early in infancy and being almost universally fatal. Type B presents later in life with milder symptoms.

Diagnosis involves clinical evaluation, family history assessment, and laboratory tests. Treatments focus on managing symptoms, such as enzyme replacement therapy (ERT), and improving the quality of life for sufferers through various supportive therapies. While the ERT medication Olipudase alfa has shown success, it has limitations, prompting ongoing research to enhance current treatments and develop new ones. Future strategies include gene therapy as a potential cure for the disease.