Free sialic acid storage disorder

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

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.

The severe form of FSASD is called infantile free sialic acid storage disease or ISSD. The signs and symptoms of this form typically appear in infancy. In some cases, a condition called hydrops fetalis, in which excess fluid accumulates in the body, occurs before or soon after birth. Babies with the severe form of FSASD typically have severe developmental delays, weak muscle tone (hypotonia), and difficulty gaining weight and growing at the expected rate (faltering weight). Additional signs and symptoms may include unusual facial features that are often described as "coarse," bone malformations, an enlarged liver and spleen (hepatosplenomegaly), an enlarged heart (cardiomegaly), and seizures. Respiratory infections are common and can be life-threatening; children with the severe form of FSASD typically only survive into early childhood.

Children with the least severe form of FSASD, also called Salla disease, may not have the signs and symptoms of the condition at birth. The age at which people develop the signs and symptoms of Salla disease can vary, but hypotonia usually appears during the first year of life. People with Salla disease typically have intellectual disabilities and developmental delays, which can make it difficult for them to learn to walk and talk. Additional signs and symptoms include problems with movement and balance (ataxia); abnormal tensing of the muscles (spasticity); and involuntary slow, writhing movements of the limbs (athetosis). Signs and symptoms typically worsen over time. Although life expectancy may be shortened, individuals with Salla disease often survive into adulthood.

People with the intermediate form of FSASD, also called intermediate severe Salla disease, have signs and symptoms that are typically more severe than the signs and symptoms seen in people with Salla disease and less severe than those seen in people with ISSD.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

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

Sialin moves (transports) a molecule called free sialic acid to other parts of the cell. Sialic acid is produced when fats and proteins are broken down, and it plays an important role in cell communication and in the attachment of cells to one another (adhesion). Sialic acid is "free" when it is not attached (bound) to other molecules.

Some SLC17A5 gene variants cause cells to produce a sialin protein that does not function properly; others prevent sialin from being produced. In a few cases, sialin is produced but cannot travel to the lysosomal membrane where it is needed.

SLC17A5 gene variants that alter or eliminate sialin activity cause free sialic acid to build up in the lysosomes. It is not known exactly how this buildup causes the neurologic signs and symptoms of FSASD.

FSASD belongs to a large family of conditions called lysosomal storage disorders. Each of these disorders is caused by the deficiency of a specific lysosomal enzyme or protein.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

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

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

Fewer than 300 people with FSASD have been reported in the medical literature. Salla disease is more common among people of Finnish and Swedish descent and has been reported in approximately 150 people.

Reported clinical features and what the terms mean

The following findings are associated with this condition in Orphanet. They are not a checklist for diagnosing yourself, and they do not all occur in every affected person. Some are examination, imaging or laboratory findings that cannot be recognised at home.

The frequency labels describe how often a finding was reported among people with the condition in the source. They do not give the chance that a person with that symptom has the condition. Definitions below reproduce HPO terminology; they explain the term, not the likely severity in an individual.

Abnormal foot morphology · Very frequent (99-80%)

An abnormality of the skeleton of foot.

Abnormal pyramidal sign · Very frequent (99-80%)

Functional neurological abnormalities related to dysfunction of the pyramidal tract.

Aplasia/Hypoplasia of the abdominal wall musculature · Very frequent (99-80%)

Absence or underdevelopment of the abdominal musculature.

Ataxia · Very frequent (99-80%)

Ataxia refers to impaired coordination of voluntary muscle movement. Cerebellar ataxia refers to ataxia due to dysfunction of the cerebellum. This causes a variety of elementary neurological deficits including asynergy (lack of coordination between muscles, limbs and joints), dysmetria (lack of ability to judge distances that can lead to under- or overshoot in grasping movements), and dysdiadochokinesia (inability to perform rapid movements requiring antagonizing muscle groups to be switched on and off repeatedly).

Gait disturbance · Very frequent (99-80%)

The term gait disturbance can refer to any disruption of the ability to walk.

Global developmental delay · Very frequent (99-80%)

A delay in the achievement of motor or mental milestones in the domains of development of a child, including motor skills, speech and language, cognitive skills, and social and emotional skills. This term should only be used to describe children younger than five years of age.

Hypotonia · Very frequent (99-80%)

Hypotonia is an abnormally low muscle tone (the amount of tension or resistance to movement in a muscle). Even when relaxed, muscles have a continuous and passive partial contraction which provides some resistance to passive stretching. Hypotonia thus manifests as diminished resistance to passive stretching. Hypotonia is not the same as muscle weakness, although the two conditions can co-exist.

Intellectual disability · Very frequent (99-80%)

The term intellectual disability or intellectual developmental disorder is used to describe significantly sub-average intellectual and adaptive functioning based on clinical assessment and as measured by individually administered, appropriately normed, standardized and validated tests of intellectual functioning and adaptive behavior, with onset during the developmental period from infancy through adolescence.

Other findings in the same source

From: Orphanet

Additional reported features include Nystagmus (Very frequent (99-80%)); Spasticity (Very frequent (99-80%)); Abnormal facial shape (Frequent (79-30%)); Abnormality of skin pigmentation (Frequent (79-30%)); Abnormality of speech or vocalization (Frequent (79-30%)); Abnormality of the upper limb (Frequent (79-30%)); Ascites (Frequent (79-30%)); Athetosis (Frequent (79-30%)); Dysarthria (Frequent (79-30%)); Hydrops fetalis (Frequent (79-30%)). This is a selected summary, not a complete description of the condition.

Which doctor should you see?

The suggested department for discussing Free sialic acid storage disorder is Metabolic Medicine, with a metabolic specialist / clinical geneticist as the relevant type of clinician. Paediatrician (children) or physician (adults), with metabolic specialist / clinical geneticist referral.

This is an editorial referral starting point. The appropriate clinic depends on the person’s age, symptoms, previous diagnosis and local services. The first clinician can decide whether another specialty or a team is needed; a department label does not confirm the diagnosis.

How to prepare for an assessment

Bring a short timeline of the main symptoms: when they first appeared, whether they are constant or episodic, what seems to change them, and how they affect daily activities. Include previous reports, discharge summaries, current medicines and supplements, allergies, and any relevant family history. A dated record is more useful than trying to match every feature in an online article.

Ask the clinician what is already established and what remains uncertain. If a test is suggested, ask what question it answers, what its limitations are and how the result would change the next step. The information here is not an instruction to arrange every possible test. In children, bring growth, developmental and school information if it is relevant to the concern.

• Is a biochemical or molecular result needed to clarify the diagnosis?
• Does this condition require an individual plan for illness or reduced food intake?
• Should nutrition advice come from a specialist metabolic dietitian?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for Free sialic acid storage disorder. That gap does not mean that treatment is unavailable. A clinician needs to establish the diagnosis and review current guidance before recommending medicines, procedures, rehabilitation or other support.

Before leaving the appointment, clarify the next review date, who will communicate results, and whom to contact if the situation changes. Discuss difficulties with sleep, work, school, mobility, eating or emotional wellbeing when these are relevant. Practical support may require coordination between the treating clinician and other services.

The collected references do not establish a complete prevention or long-term outlook section for this entry. Missing information should not be interpreted as proof that prevention is impossible or that a particular outcome is inevitable. Ask what is known for the exact subtype, stage and personal circumstances, and which uncertainties remain.

When to seek emergency help

Severe breathing difficulty, collapse, new stroke-like symptoms, a seizure that is prolonged or repeated without recovery, uncontrolled major bleeding, or an immediate risk of self-harm require emergency help. In India, call 112 or reach the nearest emergency department. This is a general, non-exhaustive warning list; it is not a condition-specific triage tool.