Overview

Acid sphingomyelinase deficiency (ASMD) is a neurodegenerative genetic disorder caused by a deficiency of the enzyme acid sphingomyelinase. It plays an essential role in breaking down sphingomyelin, a lipid. Therefore, the lack of this enzyme results in an accumulation of sphingomyelin in various organs, including the lungs, spleen, liver, and brain, leading to neurological issues. ASMD is caused by mutations in the SMPD1 gene, which encodes for acid sphingomyelinase. 

ASMD presents as a clinical spectrum, with the severity of the condition varying widely between individuals. It also varies regarding the age of onset and specific symptoms experienced, even between members of the same family. Towards the severe end of the ASMD spectrum is Niemann-Pick disease type A – a fatal disorder presented during infancy. At the mild end of the spectrum is Niemann-Pick disease type B – affected individuals experience few/no neurological symptoms, and survival into adulthood is common. Intermediate forms of ASMD exist as well.¹

The neurological manifestations of the disease describe how the nervous system is affected. For ASMD, the neurological manifestations are variable according to the different types and stages of the disease.²

Neurological pathophysiology

The neurological features of ASMD are caused by the accumulation of sphingomyelin in the neurones of the brain. The buildup of sphingomyelin in the brain disrupts neuronal functioning and can cause neuronal death.³

The enzyme ‘acid sphingomyelinase’ is most abundant in lysosomes, and it converts sphingomyelin into ceramide and phosphocholine.⁴ In ASMD, sphingomyelin and its precursor lipids accumulate in the lysosomes instead due to an insufficient amount of acid sphingomyelinase enzyme to degrade the lipids. 

Sphingomyelin accumulation changes the composition and structural integrity of neuronal membranes, impairing cellular communication and signalling processes vital for normal brain function. 

Sphingomyelin accumulation is also often associated with inflammatory responses⁵ within the brain, which can further damage neurons and exacerbate neurological problems.

ASMD Types

Type A

Symptoms of Type A ASMD usually appear during early infancy – often within the first few months of life⁶. The condition is marked by rapidly progressing neurological symptoms, leading to severe and widespread impairments, which may include: 

• Delayed motor and cognitive development
• Failure to achieve motor and cognitive milestones at the expected age range – e.g. most healthy infants can hold up their own head by approximately 3-4 months, but this may not be the case for infants with ASMD
• Loss of motor abilities and/or language skills that have been previously acquired⁷
• Seizures, which can be severe
• Damage to intellectual abilities and adaptive skills as cognitive impairment worsens
• Difficulties in learning and processing information
• Significant functional disability (no progress before 12-month level), which may require comprehensive supportive care

Due to the severity of neurological and systemic involvement, life expectancy is usually limited, with many affected children succumbing to the disease in early childhood, with a typical life expectancy of approximately 3 years.¹⁰

Type B

Symptoms of Type B ASMD are generally less severe than Type A and present later in life, often during childhood or adulthood. The neurological involvement in Type B is typically milder and more variable compared to Type A. Unlike Type A, where severe neurological impairment is prominent, Type B ASMD primarily affects visceral organs with relatively mild and often non-progressive neurological symptoms, which may be presented as:¹¹

• Mild cognitive impairment – learning difficulties or challenges in academic performance 
• Most individuals retain relatively normal intellectual abilities, though subtle cognitive deficits might be present¹²
• Behavioural issues – e.g.attention deficits, hyperactivity, or other mild neuropsychiatric symptoms¹³ 
• Psychological distress due to managing a long-term chronic illness
• Minor motor difficulties – e.g. problems with coordination and balance
• Mild hypotonia (reduced muscle tone) may also be observed but is generally less pronounced than in Type A

Individuals with Type B ASMD often have a normal or near-normal life expectancy, with quality of life primarily affected by visceral and respiratory complications rather than neurological impairment. Most patients maintain relatively normal daily functioning, although they may require ongoing medical and supportive care for associated symptoms.¹⁴

Neurological evaluation and management

Early indicators and symptoms of ASMD which prompt neurological evaluation, vary between Type A and Type B forms of the disease but often involve a range of neurodevelopmental concerns. These symptoms arise from damage and/or death to the neurons involved in cognitive and motor processes, particularly in the basal ganglia and frontal lobe. According to the Cleveland Clinic, these include:

Developmental delay

• Delayed achievement of developmental milestones such as holding the head up, sitting without support, crawling, and walking
• Fine motor skills: difficulty executing tasks such as grasping objects and hand-to-mouth coordination.

Hypotonia (decreased muscle tone)

Infants may appear limp, with poor muscle tone, causing difficulty in supporting their own weight.

Feeding difficulties

Infants may have trouble feeding due to damage in the cranial nerves controlling the swallowing process. This causes ‘failure to thrive’ – poor growth and development due to inadequate nutrition.

Seizures

Seizures may present as early as a few months old and vary in their type and severity. One explanation for this is that the loss of neurons (due to neurodegeneration) disrupts neural pathways, which can cause abnormal electrical activity in the brain, resulting in seizures.

Cognitive impairment

• Developmental regression: infants may lose previously acquired skills, such as smiling or interactive play
• Lack of interest: Infants may show a reduced interest in their surroundings and diminished interaction with caregivers

Identifying the early signs is necessary to ensure timely diagnosis and management of the disease.

Early signs and symptoms in Type B ASMD

Indicators of type B ASMD are very similar to those present in type A ASMD, however, they are less severe. Due to the higher life expectancy of those afflicted with type B ASMD, additional symptoms are observed later in life.

Learning difficulties

• Mild cognitive impairment and learning disabilities may become apparent during early childhood
• Attention deficits: issues with attention span, hyperactivity, or other behavioral problems

Coordination problems

Difficulties with balance, coordination, and fine motor skills

Fatigue and muscle weakness

Chronic fatigue may be present, which is characterised by persistent tiredness or low energy levels

Symptomatic management

There is currently no cure for either forms of ASMD, however there are ways to manage symptoms and assist people with ASMD in navigating everyday life as seamlessly as possible.¹⁵ These include:

• Anti-seizure medications to control seizures. This is more common in type A ASMD, and each prescription is individualised depending on the severity of the seizures
• Physical therapy can be beneficial in improving muscle tone and strength, aiding in the mitigation of poor motor control
• Educational support, suited to the individual’s learning needs and cognitive abilities
• Speech therapy to assist with improving the coordination of muscles involved in speech, aiding speech production
• Enzyme replacement therapy: this aims to replace the deficient acid sphingomyelinase enzyme to reduce sphingomyelin accumulation in tissues. This works best for non-neurological symptoms
• End-of-life care: for advanced cases, palliative care focuses on comfort and support for the patient and family

Summary

• ASMD is a neurodegenerative genetic disorder caused by a deficiency of the enzyme acid sphingomyelinase
• Niemann-Pick diseases are a group of lysosomal storage disorders which include the different types of ASMD
• Niemann-pick disease A is more severe than Niemann-pick disease B
• ASMD causes neurological issues, such as impeding cognitive and motor development, resulting invarious issues such as feeding difficulties, delayed developmental milestones, learning difficulties etc.
• Many patients with ASMD experience seizures, which may vary amongst type and severity
• There is no cure, although with the aid of a multidisciplinary team of healthcare professionals, it is possible to enhance the quality of life for individuals affected by this debilitating disorder. This is coordinated through various means including: antiepileptic medications, speech therapy, enzyme replacement therapy, educational support, physical therapy and palliative care