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PubMed This is a summary of 22 peer-reviewed journal articles Updated
Medical Genetics

Understanding Free Sialic Acid Storage Disease (FSASD)

At a Glance

Free sialic acid storage disease (FSASD) is an inherited lysosomal disorder caused by two pathogenic SLC17A5 variants. It traps free sialic acid inside cells and can cause symptoms ranging from severe infantile disease to slower neurological problems; genetic testing confirms the diagnosis.

Receiving a diagnosis of Free Sialic Acid Storage Disease (FSASD) can feel like being dropped into an unfamiliar world with no map. Because this condition is an ultra-rare disease—with only about 200 cases historically reported in medical literature worldwide—you may find that many local doctors have never encountered it before [1][2]. It is important to note that these published case counts are not exact prevalence estimates, and the disease is likely underdiagnosed because its early symptoms are non-specific.

It is normal to feel overwhelmed, isolated, or even a sense of grief for the path you imagined [3]. You may also find yourself quickly becoming an expert, taking on the roles of medical navigator and advocate [4]. This page is designed to help you understand the biological foundation of FSASD and give you the language to discuss it with your medical team.

The Biological Mechanism: A “Recycling” Problem

To understand FSASD, it helps to look at the lysosome, which is often called the cell’s “recycling center.” Inside our cells, lysosomes break down various substances so the body can reuse their building blocks. One of these building blocks is a sugar called sialic acid (specifically N-acetylneuraminic acid, or Neu5Ac) [5][6].

Under normal circumstances, a protein called sialin acts like a door or a pump. It moves the “free” (unbound) sialic acid out of the lysosome and back into the rest of the cell to be reused [1][7].

In FSASD, the instructions for making this protein—located in the SLC17A5 gene—contain an error (pathogenic variant). This results in a sialin protein that doesn’t work correctly or isn’t there at all [5]. Because the “door” is broken or missing, the sialic acid cannot escape. It builds up inside the lysosomes, eventually causing the cell to malfunction [1].

Autosomal Recessive Inheritance

FSASD is inherited in an autosomal recessive pattern. This means an individual must inherit two pathogenic variants (one from each parent) in the SLC17A5 gene to develop the condition. The parents are usually healthy “carriers” who have one working copy of the gene and one non-working copy. When both parents are carriers, there is a 25% chance in each pregnancy of having a child with FSASD. A genetic counselor can discuss these sibling risks, carrier testing for extended family, and reproductive planning options [1].

The Spectrum of FSASD

Doctors view FSASD as a continuous spectrum rather than several distinct diseases, but you may hear different names depending on the severity and age of onset:

  • Infantile Free Sialic Acid Storage Disease (ISSD): The most severe form, often visible at birth or in early infancy. It can involve significant hypotonia (low muscle tone), an enlarged liver or spleen (hepatosplenomegaly), and distinctive facial features [8][9].
  • Salla Disease: Generally a milder, more slowly progressive form. It typically presents in the first year of life with delays in motor skills, ataxia (problems with coordination and balance), and involuntary eye movements (nystagmus) [10][11].
  • Intermediate Form: Some fall between these two ends of the spectrum, with symptoms beginning in early childhood followed by a slow decline in neurological function over time [8][12].

Navigating the Diagnosis

Because FSASD is so rare, the road to diagnosis can be long and complicated [13]. Standard tests may not always catch it. For example, while many individuals with FSASD have high levels of free sialic acid in their urine, some with the Salla form may have results that are only slightly elevated or even normal [8][14].

If clinical symptoms suggest the disease, doctors often look for characteristic patterns on a brain MRI. These can include a thin corpus callosum (the bridge between the two sides of the brain) or hypomyelination (an impairment or persistent delay in the development of the protective coating on nerve fibers) [1][10]. These imaging findings are supportive, but genetic testing for the SLC17A5 gene is required to confirm the diagnosis [1].

Caring for the Caregiver

Managing an ultra-rare diagnosis is an immense responsibility. Families often report that the constant need to coordinate specialists, navigate insurance, and monitor health takes a toll on every aspect of life [15][16].

  • Validation is Vital: It is common to feel a sense of “unmet needs” because local resources may not be equipped for such a rare condition [3][17].
  • Seek Specialized Care: Many families find more effective support when they connect with specialized metabolic clinics or academic medical centers that focus on lysosomal storage disorders [18][19].
  • Connect with Others: Although the disease is rare, patient advocacy groups and online communities can provide invaluable peer mentoring, practical advice, and a sense of belonging [20][21].

While the medical team manages the clinical details, your observations and advocacy are essential tools in the care journey [22][18].

Common questions in this guide

What is free sialic acid storage disease (FSASD)?
Free sialic acid storage disease is an ultra-rare inherited disorder in which cells cannot move free sialic acid out of lysosomes, the cell structures that recycle materials. The buildup can interfere with cell function and may affect development and the nervous system.
How does a child inherit FSASD?
FSASD develops when a child inherits two pathogenic variants in the SLC17A5 gene, one from each parent. Parents with one altered copy are usually healthy carriers; when both parents are carriers, each pregnancy has a 25% chance of FSASD.
What are the different forms of FSASD?
FSASD is a spectrum that includes infantile free sialic acid storage disease, the most severe form, Salla disease, which generally progresses more slowly, and intermediate presentations. The form depends largely on the age symptoms begin and how severely neurological function is affected.
What symptoms can free sialic acid storage disease cause?
Symptoms vary by form and age of onset. Severe infantile disease may involve low muscle tone, an enlarged liver or spleen, and distinctive facial features, while Salla disease may cause motor delays, coordination and balance problems, involuntary eye movements, and gradual neurological decline.
How is FSASD diagnosed if a urine test is normal?
Doctors may measure free sialic acid in urine and use brain MRI to look for findings such as a thin corpus callosum or delayed development of myelin. Urine levels can be only slightly high or normal in some people with Salla disease, so genetic testing that identifies pathogenic SLC17A5 variants is needed to confirm FSASD.
Which specialists can help a family after an FSASD diagnosis?
Families may benefit from a specialized metabolic clinic or academic medical center familiar with lysosomal storage disorders. A genetic counselor and multidisciplinary team can help interpret testing, discuss family risks, coordinate care, and connect families with rare-disease support.

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References

References (22)
  1. 1

    Free sialic acid storage disorder: Progress and promise.

    Huizing M, Hackbarth ME, Adams DR, et al.

    Neuroscience letters 2021; (755()):135896 doi:10.1016/j.neulet.2021.135896.

    PMID: 33862140
  2. 2

    A cross-sectional quantitative analysis of the natural history of free sialic acid storage disease-an ultra-orphan multisystemic lysosomal storage disorder.

    Zielonka M, Garbade SF, Kölker S, et al.

    Genetics in medicine : official journal of the American College of Medical Genetics 2019; (21(2)):347-352 doi:10.1038/s41436-018-0051-3.

    PMID: 29875421
  3. 3

    A systematic review and integrative sequential explanatory narrative synthesis: The psychosocial impact of parenting a child with a lysosomal storage disorder.

    Hassall S, Smith DM, Rust S, Wittkowski A

    Journal of inherited metabolic disease 2022; (45(3)):406-416 doi:10.1002/jimd.12482.

    PMID: 35124835
  4. 4

    Alone in a Crowd? Parents of Children with Rare Diseases' Experiences of Navigating the Healthcare System.

    Baumbusch J, Mayer S, Sloan-Yip I

    Journal of genetic counseling 2018; doi:10.1007/s10897-018-0294-9.

    PMID: 30128673
  5. 5

    Clinical and Genetic Characteristics of Free Sialic Acid Storage Disorder.

    Wolfenson Z, Grois G, Hailemeskel RF, et al.

    Journal of inherited metabolic disease 2026; (49(2)):e70148 doi:10.1002/jimd.70148.

    PMID: 41879269
  6. 6

    Investigating the Utility of Leukocyte Sialic Acid Measurements in Lysosomal Free Sialic Acid Storage Disorder.

    Sabir MS, Pollard L, Wolfe L, et al.

    JIMD reports 2025; (66(4)):e70029 doi:10.1002/jmd2.70029.

    PMID: 40529477
  7. 7

    The molecular mechanism of sialic acid transport mediated by Sialin.

    Hu W, Chi C, Song K, Zheng H

    Science advances 2023; (9(3)):eade8346 doi:10.1126/sciadv.ade8346.

    PMID: 36662855
  8. 8

    A New Patient With Intermediate Severe Salla Disease With Hypomyelination: A Literature Review for Salla Disease.

    Barmherzig R, Bullivant G, Cordeiro D, et al.

    Pediatric neurology 2017; (74()):87-91.e2 doi:10.1016/j.pediatrneurol.2017.05.022.

    PMID: 28662915
  9. 9

    Biochemical and molecular analyses of infantile sialic acid storage disease in a patient with nonimmune hydrops fetalis.

    Kang E, Kim YM, Heo SH, et al.

    Clinica chimica acta; international journal of clinical chemistry 2018; (482()):199-202 doi:10.1016/j.cca.2018.04.016.

    PMID: 29654786
  10. 10

    Identification of a large intronic transposal insertion in SLC17A5 causing sialic acid storage disease.

    Tarailo-Graovac M, Drögemöller BI, Wasserman WW, et al.

    Orphanet journal of rare diseases 2017; (12(1)):28 doi:10.1186/s13023-017-0584-6.

    PMID: 28187749
  11. 11

    A 13-year follow-up of Finnish patients with Salla disease.

    Paavola LE, Remes AM, Harila MJ, et al.

    Journal of neurodevelopmental disorders 2015; (7(1)):20 doi:10.1186/s11689-015-9116-7.

    PMID: 26171070
  12. 12

    Longitudinal Characterization of the Clinical Course of Intermediate-Severe Salla Disease.

    Chapleau A, Mirchi A, Tran LT, et al.

    Pediatric neurology 2023; (148()):133-137 doi:10.1016/j.pediatrneurol.2023.08.013.

    PMID: 37713976
  13. 13

    Patient and family experiences of lysosomal storage diseases in Canada: A qualitative interview study.

    Awada N, Holcik M

    JIMD reports 2024; (65(2)):63-84 doi:10.1002/jmd2.12403.

    PMID: 38444574
  14. 14

    Atypical free sialic acid storage disorder associated with tissue specific mosaicism of SLC17A5.

    Shinawi M, Wegner DJ, Paul AJ, et al.

    Molecular genetics and metabolism 2025; (144(1)):109004 doi:10.1016/j.ymgme.2024.109004.

    PMID: 39742826
  15. 15

    Assessing the impact on caregivers caring for patients with rare pediatric lysosomal storage diseases: development of the Caregiver Impact Questionnaire.

    Harrington M, Hareendran A, Skalicky A, et al.

    Journal of patient-reported outcomes 2019; (3(1)):44 doi:10.1186/s41687-019-0140-3.

    PMID: 31338630
  16. 16

    Impact of Mucopolysaccharidosis Type II in Young Children from the Caregiver's Perspective: A Qualitative Study.

    Baldwin J, Engmann N, Stanczyk S, et al.

    Advances in therapy 2026; (43(9)):4060-4078 doi:10.1007/s12325-026-03643-y.

    PMID: 42283955
  17. 17

    "Why them, why me, why us?" The experiences of parents of children with lysosomal acid lipase deficiency: an interpretative phenomenological analysis study.

    Hassall S, Smith DM, Rust S, et al.

    Orphanet journal of rare diseases 2022; (17(1)):193 doi:10.1186/s13023-022-02335-4.

    PMID: 35550173
  18. 18

    "It is like a jungle gym, and everything is under construction": The parent's perspective of caring for a child with a rare disease.

    Currie G, Szabo J

    Child: care, health and development 2019; (45(1)):96-103 doi:10.1111/cch.12628.

    PMID: 30370696
  19. 19

    Experiences of caregivers of children with inherited metabolic diseases: a qualitative study.

    Siddiq S, Wilson BJ, Graham ID, et al.

    Orphanet journal of rare diseases 2016; (11(1)):168 doi:10.1186/s13023-016-0548-2.

    PMID: 27927250
  20. 20

    "We are ambassadors, we are advocates": rare disease patient advocacy groups as knowledge brokers across health and social systems-a qualitative study from Poland.

    Domaradzki J

    Frontiers in public health 2026; (14()):1743598 doi:10.3389/fpubh.2026.1743598.

    PMID: 41835439
  21. 21

    "It Is a Whole Different Life from the Life I Used to Live": Assessing Parents' Support Needs in Paediatric Palliative Care.

    Aoun SM, Stegmann R, Deleuil R, et al.

    Children (Basel, Switzerland) 2022; (9(3)) doi:10.3390/children9030322.

    PMID: 35327694
  22. 22

    Exploring the experience of communication in healthcare settings with parents of children with a rare genetic condition: "It's the more negative ones that you remember".

    Burbury L, Ayres S, Boyle J, et al.

    Journal of genetic counseling 2025; (34(4)):e70102 doi:10.1002/jgc4.70102.

    PMID: 40831303

This page is for informational purposes only and does not constitute medical advice about Free Sialic Acid Storage Disease. A metabolic specialist, geneticist, or neurologist can interpret your family’s symptoms and test results and discuss next steps.

Guide contents Free sialic acid storage disease
Research & Literature
02 The FSASD Spectrum