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Free sialic acid storage disorder
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Condition

Free sialic acid storage disorder

Free sialic acid storage disorder (FSASD) is an inherited condition that primarily affects the brain and spinal cord, characterized by the buildup of free sialic acid within cells.

Overview

Free sialic acid storage disorder (FSASD) is an inherited condition that primarily affects the brain and spinal cord (central nervous system). The signs and symptoms of FSASD can vary from person to person, and the condition is often divided into three forms based on the severity of these features.

Severe Form (ISSD)

The severe form of FSASD is called infantile free sialic acid storage disease (ISSD). Signs and symptoms typically appear in infancy:

  • Hydrops fetalis (excess fluid accumulation) occurring before or soon after birth.
  • Severe developmental delays and weak muscle tone (hypotonia).
  • Failure to thrive (inability to gain weight and grow at the expected rate).
  • "Coarse" facial features and bone malformations.
  • Enlarged liver and spleen (hepatosplenomegaly) and enlarged heart (cardiomegaly).
  • Seizures.
  • Life-threatening respiratory infections; children typically only survive into early childhood.

Salla Disease

Children with the least severe form, Salla disease, may not show symptoms at birth. Hypotonia usually appears during the first year of life. Symptoms include:

  • Intellectual disabilities and developmental delays.
  • Problems with movement and balance (ataxia).
  • Abnormal tensing of the muscles (spasticity).
  • Involuntary slow, writhing movements of the limbs (athetosis).

Signs and symptoms typically worsen over time. While life expectancy may be shortened, individuals with Salla disease often survive into adulthood.

Intermediate Form

People with the intermediate form, also called intermediate severe Salla disease, have signs and symptoms that are more severe than Salla disease but less severe than ISSD.

Causes

Variants in the SLC17A5 gene cause all forms of FSASD. This gene provides instructions for producing a protein called sialin, which is located on the membranes of lysosomes—compartments in the cell that digest and recycle molecules.

Sialin transports free sialic acid to other parts of the cell. Sialic acid is produced when fats and proteins are broken down and is essential for cell communication and adhesion. When sialin activity is altered or eliminated, free sialic acid builds up in the lysosomes. FSASD belongs to a family of conditions called lysosomal storage disorders.

Inheritance

This condition is inherited in an autosomal recessive pattern, meaning both copies of the gene in each cell must have a variant to cause the disorder. Parents of an individual with an autosomal recessive condition each carry one copy of the altered gene but typically do not show signs or symptoms.

Frequently Asked Questions

Differentiation relies on the severity and progression of symptoms. The severe infantile form (ISSD) presents with hydrops fetalis, severe hypotonia, and rapid deterioration, often leading to death in early childhood. The intermediate form presents with symptoms more severe than Salla disease but milder than ISSD; these children may survive longer and show a slower progression of neurological decline. Genetic testing for the SLC17A5 gene can confirm the diagnosis, but the specific clinical phenotype helps determine the expected prognosis and care plan. What specific daily care strategies are recommended to manage the movement disorders and spasticity associated with Salla disease?::Management of Salla disease focuses on supportive care to maintain quality of life. Physical and occupational therapy are essential to prevent joint contractures and maintain mobility. Medications such as muscle relaxants or antispasmodics may be prescribed to manage spasticity and involuntary movements. Additionally, speech therapy can help address communication difficulties, and assistive devices may be needed to support balance and prevent falls as ataxia progresses. Can genetic testing identify the specific variant in the SLC17A5 gene to predict which form of the disorder a child will develop?::Genetic testing can identify the specific variants in the SLC17A5 gene, which is necessary for a definitive diagnosis. However, the relationship between specific genetic variants and the severity of the clinical phenotype (severe, intermediate, or Salla disease) is not always straightforward. While some variants are strongly associated with severe forms, clinicians primarily rely on the clinical presentation and progression of symptoms to classify the disorder rather than predicting severity solely from the genetic code. What are the primary causes of mortality in individuals with the severe infantile form of free sialic acid storage disorder?::In the severe infantile form (ISSD), mortality is typically caused by life-threatening respiratory infections and complications related to severe developmental delays and organ involvement. The condition leads to significant weakness and immune system vulnerabilities, making it difficult for the body to fight off infections. Additionally, the progressive neurological decline and potential cardiac or hepatic issues contribute to the poor prognosis, with most children not surviving beyond early childhood.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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