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Adam MP, Feldman J, Mirzaa GM, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2025.

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Free Sialic Acid Storage Disorder

, MD, PhD, , PhD, and , MD.

Author Information and Affiliations

Initial Posting: ; Last Update: June 26, 2025.

Estimated reading time: 27 minutes

Summary

Clinical characteristics.

Free sialic acid storage disorder (FSASD) is a spectrum of neurodegenerative phenotypes resulting from increased lysosomal storage of free sialic acid. Less severe FSASD (historically called Salla disease) is characterized by normal appearance and absence of neurologic findings at birth, followed by slowly progressive neurologic deterioration resulting in mild-to-moderate psychomotor delays, spasticity, athetosis, and epileptic seizures. Severe FSASD (historically referred to as infantile free sialic acid storage disease, or ISSD) is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; death usually occurs in early childhood.

Diagnosis/testing.

The diagnosis of FSASD is established in a proband by identification of biallelic pathogenic variants in SLC17A5 by molecular genetic testing.

Management.

Treatment of manifestations: Management is symptomatic and supportive: standard treatment of seizures; developmental and educational support; rehabilitation to optimize mobility; supplementation of calcium and vitamin D for low bone density; feeding therapy and provision of adequate nutrition; treatment of ophthalmologic manifestations per ophthalmologist with low vision services as needed; treatment of cardiomegaly per cardiologist; treatment of nephropathy / nephrotic syndrome per nephrologist; surgical treatment of hernia as needed; family and social support.

Surveillance: Assessment of seizures, other neurologic manifestations, development, mobility, growth, nutrition, feeding, respiratory status, and family needs at each visit. Annual ophthalmology exam in those with intermediate or severe FSASD. Annual EKG and echocardiography to assess for cardiomegaly. Annual urinalysis for proteinuria.

Genetic counseling.

FSASD is inherited in an autosomal recessive manner. If both parents are known to be heterozygous for an SLC17A5 pathogenic variant, each sib of an affected individual has at conception a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Once the SLC17A5 pathogenic variants have been identified in an affected family member, molecular genetic carrier testing and prenatal/preimplantation genetic testing are possible.

GeneReview Scope

Free Sialic Acid Storage Disorder (FSASD): Spectrum of Severity 1
  • Less severe FSASD (historically referred to as Salla disease)
  • Intermediate severe FSASD
  • Severe FSASD (historically referred to as infantile free sialic acid storage disease; ISSD)

For synonyms and outdated names see Nomenclature.

Diagnosis

There are no consensus clinical diagnostic criteria for free sialic acid storage disorder (FSASD).

Suggestive Findings

FSASD should be suspected in individuals with the following clinical, imaging, and laboratory findings [Parazzini et al 2003, Barmherzig et al 2017, Zielonka et al 2019, Huizing et al 2021].

Less Severe FSASD (including Salla disease)

Clinical findings

  • Truncal ataxia and hypotonia apparent at approximately age one year
  • Developmental delay
  • Growth deficiency (short stature)
  • Intellectual disability
  • Spasticity
  • Facial coarsening (variable and not always present)

Imaging findings on brain MRI examination

  • Hypomyelination of the basal ganglia
  • Hypoplasia of the corpus callosum

Severe FSASD (including infantile free sialic acid storage disease [ISSD])

Clinical findings

  • Nonimmune hydrops fetalis (24%)
  • Hepatosplenomegaly
  • Poor weight gain / growth deficiency
  • Severe developmental delay
  • Cardiomegaly
  • Clubfeet
  • Increasingly coarse facial features
  • Neurologic deterioration
  • Early death

Imaging findings on skeletal survey include skeletal dysostosis (e.g., irregular enlarged metaphyses, short femurs, diffuse hypomineralization with fractures, hip dysplasia, anterior beaking of the dorsal vertebrae, and hypoplasia of the distal phalanges).

Laboratory Findings (both less severe and severe forms)

Free sialic acid. Sialic acids are a family of negatively charged sugars, one of which, N-acetylneuraminic acid, is elevated in lysosomes in FSASD.

Urinary excretion of free sialic acid, measured by fluorimetric thiobarbituric acid assay, thin-layer chromatography, or mass spectrometry, is elevated about tenfold in individuals with less severe FSASD and about 100-fold in individuals with ISSD. High-performance liquid chromatography / tandem mass spectrometry is also able to detect free sialic acid in urine [Valianpour et al 2004].

Note: (1) In the thiobarbituric acid assay, interfering substances may lower the measurement and chromophores may contribute to absorbance, creating a false measurement. (2) In thin-layer chromatography, an elevation of free sialic acid may be overlooked.

Cultured fibroblasts from individuals with all forms of FSASD show increased concentration of free sialic acid [Renlund et al 1986].

Establishing the Diagnosis

The diagnosis of FSASD is established in a proband by identification of biallelic pathogenic (or likely pathogenic) variants in SLC17A5 by molecular genetic testing (see Table 1) [Verheijen et al 1999, Aula et al 2000].

Note: (1) Per ACMG/AMP variant interpretation guidelines, the terms "pathogenic variant" and "likely pathogenic variant" are synonymous in a clinical setting, meaning that both are considered diagnostic and can be used for clinical decision making [Richards et al 2015]. Reference to "pathogenic variants" in this GeneReview is understood to include likely pathogenic variants. (2) Identification of biallelic SLC17A5 variants of uncertain significance (or of one known SLC17A5 pathogenic variant and one SLC17A5 variant of uncertain significance) does not establish or rule out the diagnosis.

Molecular genetic testing approaches can include a combination of gene-targeted testing (single gene testing, multigene panel) and comprehensive genomic testing (exome sequencing, genome sequencing). Gene-targeted testing requires that the clinician determine which gene(s) are likely involved (see Option 1), whereas comprehensive genomic testing does not (see Option 2).

Option 1

When the phenotypic and laboratory findings suggest the diagnosis of FSASD, molecular genetic testing approaches can include single-gene testing or use of a multigene panel:

  • Single-gene testing. Sequence analysis of SLC17A5 is performed first to detect missense, nonsense, and splice site variants and small intragenic deletions/insertions. Note: Depending on the sequencing method used, single-exon, multiexon, or whole-gene deletions/duplications may not be detected. If only one or no variant is detected by the sequencing method used, the next step is to perform gene-targeted deletion/duplication analysis to detect exon and whole-gene deletions or duplications.
    Note: Targeted analysis for founder pathogenic variants can be performed first in individuals of certain ancestries. The Finnish founder variant p.Arg39Cys has been reported in homozygous or heterozygous form in ~80% of reported individuals with FSASD worldwide [Huizing et al 2021], including in some individuals of Scandinavian [Aula et al 2000] and Mennonite ancestry [Strauss et al 2005]. Other reported founder pathogenic variants are c.526-2A>G in Canadian Inuits [Lines et al 2014] and p.Gly328Glu in Israeli Bedouins [Landau et al 2004].
  • A multigene panel that includes SLC17A5 and other genes of interest (see Differential Diagnosis) is most likely to identify the genetic cause of the condition while limiting identification of pathogenic variants and variants of uncertain significance in genes that do not explain the underlying phenotype. Note: (1) The genes included in the panel and the diagnostic sensitivity of the testing used for each gene vary by laboratory and are likely to change over time. (2) Some multigene panels may include genes not associated with the condition discussed in this GeneReview. (3) In some laboratories, panel options may include a custom laboratory-designed panel and/or custom phenotype-focused exome analysis that includes genes specified by the clinician. (4) Methods used in a panel may include sequence analysis, deletion/duplication analysis, and/or other non-sequencing-based tests.
    For an introduction to multigene panels click here. More detailed information for clinicians ordering genetic tests can be found here.

Option 2

When the phenotype is indistinguishable from many other inherited disorders characterized by neurodegeneration, comprehensive genomic testing does not require the clinician to determine which gene is likely involved. Exome sequencing is most commonly used; genome sequencing is also possible.

For an introduction to comprehensive genomic testing click here. More detailed information for clinicians ordering genomic testing can be found here.

Table 1.

Molecular Genetic Testing Used in Free Sialic Acid Storage Disorder

Gene 1MethodProportion of Pathogenic Variants 2 Identified by Method
SLC17A5 Sequence analysis 390%-95% 4, 5
Gene-targeted deletion/duplication analysis 65%-10% 4, 7
1.
2.

See Molecular Genetics for information on variants detected in this gene.

3.

Sequence analysis detects variants that are benign, likely benign, of uncertain significance, likely pathogenic, or pathogenic. Variants may include missense, nonsense, and splice site variants and small intragenic deletions/insertions; typically, exon or whole-gene deletions/duplications are not detected. For issues to consider in interpretation of sequence analysis results, click here.

4.

The p.Arg39Cys variant, known as the "FIN" variant, is common in the Finnish and other Nordic populations; therefore, a higher detection rate by sequencing is expected in those populations [Aula et al 2000].

5.
6.

Gene-targeted deletion/duplication analysis detects intragenic deletions or duplications. Methods used may include a range of techniques such as quantitative PCR, long-range PCR, multiplex ligation-dependent probe amplification (MLPA), and a gene-targeted microarray designed to detect single-exon deletions or duplications.

7.

Clinical Characteristics

Clinical Description

Free sialic acid storage disorder (FSASD) comprises a spectrum of neurodegenerative phenotypes resulting from increased lysosomal storage of free sialic acid [Huizing et al 2021]. Historically, FSASD was divided into separate allelic disorders: Salla disease, intermediate severe Salla disease, and infantile free sialic acid storage disease (ISSD). Salla disease was named for a municipality in Finnish Lapland where a specific founder variant is relatively prevalent. However, the term "Salla" has been used in the literature to refer to less severe FSASD in general. Less severe FSASD is characterized by normal appearance and neurologic findings at birth followed by slowly progressive neurologic deterioration resulting in mild-to-moderate psychomotor delay, spasticity, athetosis, and epileptic seizures. More severe FSASD, also known as ISSD, is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; death usually occurs in early childhood. To date, approximately 260 individuals have been reported worldwide with biallelic pathogenic variants in SLC17A5 [Aula et al 2000, Alajoki et al 2004, Zielonka et al 2019, Huizing et al 2021]; an additional 50 individuals with FSASD are reported in the literature without molecular data. The following description of the phenotypic features associated with this condition is based on these reports.

Table 2.

Free Sialic Acid Storage Disorder: Frequency of Select Features

Feature 1% of Persons w/Feature
Developmental delay / cognitive impairment75%
Facial dysmorphism / coarse facies 250%-68% 2
Hepatosplenomegaly54%
Truncal hypotonia54%
Skeletal abnormalities50%
Spasticity48%
Ataxia44%
Poor weight gain42%
Short stature27%
Hydrops fetalis24%
Epilepsy22%
Neurodegenerative course20%
Neonatal ascites19%
Cardiomegaly19%
Hernias19%
Microcephaly18%
Recurrent airway infections16%
Nystagmus12%
Nephropathy10%
Optic atrophy7%
Athetosis6%
Ptosis3%
Hoarse voice2%
Corneal clouding1%
Brain MRI findingsBrain atrophy23%
Hypomyelination22% 3
Hypoplasia of the corpus callosum16%
1.

Includes features reported in entire spectrum of phenotypes (less severe to severe FSASD)

2.

Coarse facies are more frequent in severe FSASD.

3.

This was likely underascertained in this cohort. Severe hypomyelination and delayed brain myelination is a consistent hallmark of FSASD [D Adams, M Huizing, & M Wasserstein, unpublished data]

Less Severe FSASD (including Salla disease)

Salla disease, which serves as a model for less severe FSASD, has the mildest phenotype [Varho et al 2002]. It is characterized by a normal appearance and normal neurologic findings at birth followed by slowly progressive neurologic deterioration resulting in mild-to-moderate psychomotor delay [Renlund et al 1983, Alajoki et al 2004]. Muscular hypotonia is often first recognized at approximately age six months. One third of affected children learn to walk. Expressive language development can be limited to single words, but receptive speech is good. Slow developmental progress often continues until the third decade, after which regression can occur.

Some individuals with Salla disease present later in life with spasticity, athetosis, and epileptic seizures, becoming nonambulatory and nonverbal. Affected individuals are characterized as good-humored and sociable [Varho et al 2002].

T2-weighted bright cerebral white matter changes on brain MRI are typical but variable. Abnormal myelination of the basal ganglia and hypoplasia of the corpus callosum are constant and early findings [Sonninen et al 1999]. Cerebellar white matter changes are also present and can explain the ataxia [Linnankivi et al 2003, Biancheri et al 2004]. In addition to the central dysmyelination, a peripheral dysmyelination with the clinical picture of a polyneuropathy occurs with variable neurologic presentations [Varho et al 2000, Varho et al 2002].

Life expectancy appears to be shortened, although affected individuals up to age 72 years have been observed.

Intermediate Severe FSASD

Since the advent of molecular studies, phenotypes with a severity between those of Salla disease and ISSD [Aula & Gahl 2001] have been attributed to compound heterozygosity for the Salla disease-causing common pathogenic variant p.Arg39Cys and another SLC17A5 pathogenic variant [Kleta et al 2003]. Thus, the term "intermediate severe Salla disease" was proposed [Aula et al 2000].

Two sisters described as having intermediate severe Salla disease with p.Lys136Glu and c.819+1G>A SLC17A5 pathogenic variants had disease onset within the first six months of life. Initial symptoms included hypotonia and developmental delay. One of the two sibs was described as dysmorphic with synophrys, epicanthal folds, a low hairline, and a high-arched palate. Both sibs demonstrated coarsening facial features over time. Loss of head control and cognitive skills were noted in the second decade of life. Cerebellar signs were noted during the second year of life in both, but dystonia was discordant (onset at age 19 months in one and age 14 years in the other) [Chapleau et al 2023].

Severe FSASD (including infantile free sialic acid storage disease [ISSD])

ISSD, the most severe phenotype, is characterized by severe developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly. Additional reported features include early truncal hypotonia with later spasticity and ataxia, skeletal abnormalities, and seizures (see Table 2). No single feature occurs in all individuals.

ISSD can present prenatally and in the neonatal period with nonimmune hydrops fetalis (24% of individuals) [Lemyre et al 1999, Stone & Sidransky 1999, Froissart et al 2005, Zielonka et al 2019, Huizing et al 2021]. Some affected infants are born prematurely. Other affected infants appear normal at birth but lose developmental milestones during infancy [Kleta et al 2003, Kleta et al 2004].

Skeletal abnormalities can include irregular metaphyses, diffuse hypomineralization, clubfeet, short femurs, enlarged metaphyses, fractures, hip dysplasia, anterior beaking of the dorsal vertebrae, and hypoplasia of the distal phalanges [Froissart et al 2005].

Dysmorphic facial features are nonspecific and generally fall into the spectrum of "coarsened" features (e.g., epicanthal folds, ptosis, anteverted nose, gum hypertrophy).

Reported ocular findings include nystagmus, exotropia, optic atrophy, and albinoid fundi. Corneal clouding has been rarely reported.

Additional reported features include nephropathy and/or nephrotic syndrome and hernias [Lemyre et al 1999, Ishiwari et al 2004].

Death usually occurs in early childhood, typically from recurrent respiratory infections.

Genotype-Phenotype Correlations

Correlations between the type of SLC17A5 pathogenic variant and the severity of FSASD have been identified [Aula et al 2000, Varho et al 2000, Kleta et al 2003, Barmherzig et al 2017, Zielonka et al 2019, Huizing et al 2021]:

Variable phenotypic expression has been observed among affected family members [Varho et al 2002, Landau et al 2004], suggesting involvement of additional genetic or environmental factors.

Penetrance

FSASD appears to be fully penetrant. However, two individuals homozygous for SLC17A5 pathogenic variant p.Lys136Glu had no detectable urinary sialic acid abnormality and elevated cerebrospinal fluid free sialic acid, suggesting that penetrance based on urinary studies alone may be reduced [Mochel et al 2009].

Nomenclature

Reference to FSASD by historically defined terms such as Salla disease, intermediate severe Salla disease, and ISSD has resulted in confusion for clinicians, affected individuals, researchers, diagnostic laboratories, disease databases, and the pharmaceutical rare disease industry. The designation "free sialic acid storage disorder" (FSASD) was proposed as an encompassing term for the entire spectrum of disease severity in order to improve worldwide disease awareness and to facilitate diagnosis, estimation of disease prevalence, and therapeutic research [Huizing et al 2021].

Prevalence

The prevalence of FSASD was estimated to be 1-3 in 1,000,000 individuals worldwide using population databases of genetic variants [Huizing et al 2021]. Higher estimated prevalence rates of ~35 in 1,000,000 occur in Finland, due to the increased carrier frequency of the SLC17A5 p.Arg39Cys founder variant [Aula et al 2000, Huizing et al 2021].

To date, there are about 260 individuals reported worldwide with biallelic pathogenic variants in SLC17A5 [Aula et al 2000, Zielonka et al 2019, Huizing et al 2021], of which ~80% have the p.Arg39Cys variant in homozygous or heterozygous form [Huizing et al 2021]. Homozygosity for p.Arg39Cys was reported in the Old Order Mennonite population [Strauss et al 2005]. A variety of other SLC17A5 pathogenic variants are reported in more than 70 individuals worldwide, including Canadian Inuit (homozygous for c.526-2A>G) [Lines et al 2014], Israeli Bedouin (homozygous for p.Gly328Glu) [Landau et al 2004], Dominican, French, Italian, Japanese, Kurdish, Polish, and Saudi Arabian [Verheijen et al 1999, Ishiwari et al 2004, Biancheri et al 2005, Froissart et al 2005, Matsuura et al 2018, Mochel et al 2009, Tarailo-Graovac et al 2017, Zielonka et al 2019].

Differential Diagnosis

Biochemical Findings

Increased urinary and cellular free sialic acid. The only disorders in which significantly elevated urinary and cellular free sialic acid is known to occur are sialuria (OMIM 269921), N-acetylneuraminate pyruvate lyase (NPL) deficiency, and free sialic acid storage disorder (FSASD) [Huizing et al 2021]. Of note, in both sialuria and NPL deficiency, elevated free sialic acid is localized in the cytoplasm rather than the lysosome as in FSASD. The clinical course of sialuria involves developmental delay and hepatomegaly. NPL deficiency is associated with progressive cardiac myopathy and mild skeletal myopathy. Neither disorder is associated with the severe neurologic involvement that occurs in FSASD.

Based on clinical suspicion and the finding of elevated free sialic acid in urine, one of two steps is taken to distinguish these conditions:

  • The cellular (cytoplasmic versus lysosomal) localization of free sialic acid can be documented; a predominantly lysosomal localization indicates FSASD.
  • Molecular genetic testing of SLC17A5 (for FSASD), GNE (for sialuria), or NPL (for NPL deficiency) can be performed.

Note: Other causes of mild elevation in urinary free sialic acid may exist.

Sialic acid bound to glycoproteins or glycolipids. If sialic acid bound to glycoproteins or glycolipids is stored, disorders such as sialidosis caused by sialidase (neuraminidase) deficiency (OMIM 256550) and galactosialidosis (OMIM 256540) caused by combined sialidase and galactosidase deficiency should be considered (see Table 3). These enzyme deficiencies involve lysosomal storage of sialic acid-containing glycoconjugates. Neuraminidase deficiency and galactosialidosis both have features typical of lysosomal storage diseases but vary widely in their manifestations.

Clinical Findings

See Table 3 for other lysosomal storage disorders that are associated with the clinical manifestations of coarse facial features and developmental delays as well as other causes of nonimmune hydrops fetalis.

Table 3.

Other Genes of Interest in the Differential Diagnosis of Free Sialic Acid Storage Disorder

Clinical Finding(s) Overlapping w/FSASDGeneDisorderMOIAssociated Enzyme
Coarse facial features & developmental delays AGA Aspartylglucosaminuria ARN(4)-(beta-N-acetylglucosaminyl)-L-asparaginase
ARSB MPS VI (OMIM 253200)ARArylsulfastase B
FUCA1 Fucosidosis (OMIM 230000)ARTissue alpha-L-fucosidase
GLB1 GM1 gangliosidosis (See GLB1-Related Disorders.)ARBeta-galactosidase
GNPTAB Mucolipidosis II (I-cell disease) (See GNPTAB-Related Disorders.)ARN-acetylglucosamine-1-phosphotransferase subunits alpha/beta
IDS MPS II XLIduronate 2-sulfatase
IDUA MPS I ARAlpha-L-iduronidase
MAN2B1 Alpha-mannosidosis ARLysosomal alpha-mannosidase
NEU1 Sialidosis type II (OMIM 256550)ARSialidase-1
Nonimmune hydrops fetalis CTSA Galactosialidosis (OMIM 256540)ARLysosomal protective protein
GALC Krabbe disease ARGalactocerebrosidase
GBA1 Gaucher disease ARLysosomal acid glucosylceramidase
GBA2 Beta-glucosidase deficiency (OMIM 614409)ARNon-lysosomal glucosylceramidase
GNPTAB Mucolipidosis II (I-cell disease) (See GNPTAB-Related Disorders)ARN-acetylglucosamine-1-phosphotransferase subunits alpha/beta
GUSB MPS VII ARBeta-glucuronidase
IDUA MPS I ARAlpha-L-iduronidase
LIPA Lysosomal acid lipase deficiency ARLysosomal acid lipase / cholesteryl ester hydrolase
NEU1 Sialidase deficiency (OMIM 256550)ARSialidase-1
SMPD1 Acid sphingomyelinase deficiency ARSphingomyelin phosphodiesterase

From Saudubray & Charpentier, Chapter 86, Table 42, Online Metabolic and Molecular Bases of Inherited Disease. Accessed 8-23-22 (Registration required).

AR = autosomal recessive; FSASD = free sialic acid storage disorder; MOI = mode of inheritance; MPS = mucopolysaccharidosis; XL = X-linked

Management

No clinical practice guidelines for free sialic acid storage disorder (FSASD) have been published. In the absence of published guidelines, the following recommendations are based on the authors' personal experience managing individuals with this disorder.

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with FSASD, the evaluations summarized in Table 4 (if not performed as part of the evaluation that led to the diagnosis) are recommended.

Table 4.

Free Sialic Acid Storage Disorder: Recommended Evaluations Following Initial Diagnosis

System/ConcernEvaluationComment
Neurologic Neurologic eval
  • To incl brain MRI
  • Consider EEG if seizures are a concern.
Developmental Developmental assessment
  • To incl motor, adaptive, cognitive, & speech-language eval
  • Eval for early intervention / special education
Musculoskeletal Orthopedics / physical medicine & rehab / PT & OT evalTo incl assessment of:
  • Gross motor & fine motor skills
  • Mobility, activities of daily living, & need for adaptive devices
  • Need for PT (to improve gross motor skills) &/or OT (to improve fine motor skills)
Gastrointestinal/
Feeding
  • Gastroenterology / nutrition / feeding team eval
  • Assess for hernia in those w/intermediate severe or severe FSASD.
  • To incl eval of aspiration risk & nutritional status
  • Consider eval for gastrostomy tube placement in those w/dysphagia &/or aspiration risk.
Eyes Ophthalmologic eval in those w/intermediate severe or severe FSASDTo assess for reduced vision, abnormal ocular movement, best corrected visual acuity, refractive errors, & strabismus
Cardiomegaly EKG & echocardiographyTo assess for cardiomegaly
Nephropathy / Nephrotic syndrome UrinalysisTo assess for proteinuria
Genetic counseling By genetics professionals 1To obtain a pedigree & inform affected persons & their families re nature, MOI, & implications of FSASD to facilitate medical & personal decision making
Family support
& resources 		By clinicians, wider care team, & family support organizationsAssessment of family & social structure to determine need for:

FSASD = free sialic acid storage disorder; MOI = mode of inheritance; OT = occupational therapy; PT = physical therapy

1.

Clinical geneticist, certified genetic counselor, certified genetic nurse, genetics advanced practice provider (nurse practitioner or physician assistant)

Treatment of Manifestations

The medical and psychosocial management of individuals with FSASD is symptomatic and supportive.

Table 5.

Free Sialic Acid Storage Disorder: Treatment of Manifestations

Manifestation/
Concern		TreatmentConsiderations/Other
Epilepsy Standardized treatment w/ASM by experienced neurologist
  • Many ASMs may be effective; none has been demonstrated effective specifically for FSASD
  • Education of parents/caregivers 1
Developmental delay / Intellectual disability See Developmental Delay / Intellectual Disability Management Issues.
Spasticity Orthopedics / physical medicine & rehab / PT & OT incl stretching to help avoid contractures & fallsConsider need for positioning & mobility devices, disability parking placard.
Low bone density / Fractures Supplementation as necessary to provide adequate calcium & vitamin D intake
Poor weight gain
  • Feeding therapy
  • Gastrostomy tube placement may be required for persistent feeding issues.
Low threshold for clinical feeding eval &/or radiographic swallowing study when showing clinical signs or symptoms of dysphagia
Ophthalmologic manifestations
  • Treatment per ophthalmologist
  • Low vision services as needed
Cardiomegaly Treatment per cardiologist
Nephropathy / Nephrotic syndrome Treatment per nephrologist
Hernia Surgical treatment as needed
Family/
Community
  • Ensure appropriate social work involvement to connect families w/local resources, respite, & support.
  • Coordinate care to manage multiple subspecialty appointments, equipment, medications, & supplies.
  • Ongoing assessment of need for palliative care involvement &/or home nursing
  • Consider involvement in adaptive sports or Special Olympics.

ASM = anti-seizure medication; FSASD = free sialic acid storage disorder; OT = occupational therapy; PT = physical therapy

1.

Education of parents/caregivers regarding common seizure presentations is appropriate. For information on non-medical interventions and coping strategies for children diagnosed with epilepsy, see Epilepsy Foundation Toolbox.

Developmental Delay / Intellectual Disability Management Issues

The following information represents typical management recommendations for individuals with developmental delay / intellectual disability in the United States; standard recommendations may vary from country to country.

Ages 0-3 years. Referral to an early intervention program is recommended for access to occupational, physical, speech, and feeding therapy as well as infant mental health services, special educators, and sensory impairment specialists. In the US, early intervention is a federally funded program available in all states that provides in-home services to target individual therapy needs.

Ages 3-5 years. In the US, developmental preschool through the local public school district is recommended. Before placement, an evaluation is made to determine needed services and therapies and an individualized education plan (IEP) is developed for those who qualify based on established motor, language, social, or cognitive delay. The early intervention program typically assists with this transition. Developmental preschool is center based; for children too medically unstable to attend, home-based services are provided.

All ages. Consultation with a developmental pediatrician is recommended to ensure the involvement of appropriate community, state, and educational agencies (US) and to support parents in maximizing quality of life. Some issues to consider:

  • IEP services:
    • An IEP provides specially designed instruction and related services to children who qualify.
    • IEP services will be reviewed annually to determine whether any changes are needed.
    • Special education law requires that children participating in an IEP be in the least restrictive environment feasible at school and included in general education as much as possible, when and where appropriate.
    • Vision consultants should be a part of the child’s IEP team to support access to academic material.
    • PT, OT, and speech services will be provided in the IEP to the extent that the need affects the child’s access to academic material. Beyond that, private supportive therapies based on the affected individual’s needs may be considered. Specific recommendations regarding type of therapy can be made by a developmental pediatrician.
    • As a child enters the teen years, a transition plan should be discussed and incorporated in the IEP. For those receiving IEP services, the public school district is required to provide services until age 21.
  • A 504 plan (Section 504: a US federal statute that prohibits discrimination based on disability) can be considered for those who require accommodations or modifications such as front-of-class seating, assistive technology devices, classroom scribes, extra time between classes, modified assignments, and enlarged text.
  • Developmental Disabilities Administration (DDA) enrollment is recommended. DDA is a US public agency that provides services and support to qualified individuals. Eligibility differs by state but is typically determined by diagnosis and/or associated cognitive/adaptive disabilities.
  • Families with limited income and resources may also qualify for supplemental security income (SSI) for their child with a disability.

Motor Dysfunction

Gross motor dysfunction

  • Physical therapy is recommended to maximize mobility and to reduce the risk for later-onset orthopedic complications (e.g., contractures, scoliosis, hip dislocation).
  • Consider use of durable medical equipment and positioning devices as needed (e.g., wheelchairs, walkers, bath chairs, orthotics, adaptive strollers).
  • For muscle tone abnormalities including hypertonia or dystonia, consider involving appropriate specialists to aid in management of baclofen, tizanidine, Botox®, anti-parkinsonian medications, or orthopedic procedures.

Fine motor dysfunction. Occupational therapy is recommended for difficulty with fine motor skills that affect adaptive function such as feeding, grooming, dressing, and writing.

Oral motor dysfunction should be assessed at each visit and clinical feeding evaluations and/or radiographic swallowing studies should be obtained for choking/gagging during feeds, poor weight gain, frequent respiratory illnesses, or feeding refusal that is not otherwise explained. Assuming that the child is safe to eat by mouth, feeding therapy (typically by an occupational or speech therapist) is recommended to help improve coordination or sensory-related feeding issues. Feeds can be thickened or chilled for safety. When feeding dysfunction is severe, an NG-tube or G-tube may be necessary.

Communication issues. Consider evaluation for alternative means of communication (e.g., augmentative and alternative communication [AAC]) for individuals who have expressive language difficulties. An AAC evaluation can be completed by a speech-language pathologist who has expertise in the area. The evaluation will consider cognitive abilities and sensory impairments to determine the most appropriate form of communication. AAC devices can range from low-tech, such as picture exchange communication, to high-tech, such as voice-generating devices. Contrary to popular belief, AAC devices do not hinder verbal development of speech, but rather support optimal speech and language development.

Surveillance

To monitor existing manifestations, the individual's response to supportive care, and the emergence of new manifestations, the evaluations summarized in Table 6 are recommended.

Table 6.

Free Sialic Acid Storage Disorder: Recommended Surveillance

System/ConcernEvaluationFrequency
Neurologic
  • Monitor those w/seizures as clinically indicated.
  • Assess for new manifestations (e.g., seizures, changes in tone, movement disorders).
At each visit
Developmental Monitor developmental progress & educational needs.
Musculoskeletal
  • Physical medicine &/or rehab medicine & OT/PT assessment of mobility & self-help skills
  • Assess bone health incl vitamin D intake.
Feeding
  • Measurement of growth parameters
  • Eval of nutritional status & safety of oral intake
Respiratory Monitor for evidence of aspiration, respiratory insufficiency.
Eyes Ophthalmologic evalAnnually or per ophthalmologist in those w/intermediate severe or severe FSASD
Cardiomegaly EKG & echocardiographyAnnually for signs of cardiomegaly
Nephropathy / Nephrotic syndrome Urinalysis for proteinuriaAnnually
Family/Community Assess family need for social work support (e.g., palliative/respite care, home nursing, other local resources), care coordination, or follow-up genetic counseling if new questions arise (e.g., family planning).At each visit

FSASD = free sialic acid storage disorder; OT = occupational therapy; PT = physical therapy

Evaluation of Relatives at Risk

No routine testing of apparently asymptomatic at-risk family members is recommended because adult presentations are unusual, and no early interventions are available.

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

Search ClinicalTrials.gov in the US and EU Clinical Trials Register in Europe for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.

Genetic Counseling

Genetic counseling is the process of providing individuals and families with information on the nature, mode(s) of inheritance, and implications of genetic disorders to help them make informed medical and personal decisions. The following section deals with genetic risk assessment and the use of family history and genetic testing to clarify genetic status for family members; it is not meant to address all personal, cultural, or ethical issues that may arise or to substitute for consultation with a genetics professional. —ED.

Mode of Inheritance

Free sialic acid storage disorder (FSASD) is inherited in an autosomal recessive manner.

Risk to Family Members

Parents of a proband

  • The parents of an affected child are presumed to be heterozygous for an SLC17A5 pathogenic variant.
  • Molecular genetic testing is recommended for the parents of a proband to confirm that both parents are heterozygous for an SLC17A5 pathogenic variant and to allow reliable recurrence risk assessment.
  • If a pathogenic variant is detected in only one parent and parental identity testing has confirmed biological maternity and paternity, it is possible that one of the pathogenic variants identified in the proband occurred as a de novo event in the proband or as a postzygotic de novo event in a mosaic parent [Jónsson et al 2017]. If the proband appears to have homozygous pathogenic variants (i.e., the same two pathogenic variants), additional possibilities to consider include:
  • Heterozygotes (carriers) are asymptomatic and are not at risk of developing the disorder.

Sibs of a proband

  • If both parents are known to be heterozygous for an SLC17A5 pathogenic variant, each sib of an affected individual has at conception a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
  • Variable expression has been observed among affected sibs [Landau et al 2004].
  • Heterozygotes are asymptomatic and are not at risk of developing the disorder.

Offspring of a proband. To date, individuals with FSASD are not known to reproduce.

Other family members. Each sib of the proband's parents is at a 50% risk of being a carrier of an SLC17A5 pathogenic variant.

Carrier Detection

Molecular genetic carrier testing for at-risk relatives requires prior identification of the SLC17A5 pathogenic variants in the family.

Biochemically based carrier testing is not feasible.

Related Genetic Counseling Issues

Family planning

  • The optimal time for determination of genetic risk and discussion of the availability of prenatal/preimplantation genetic testing is before pregnancy.
  • It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are carriers or are at risk of being carriers.
  • Carrier testing should be considered for the reproductive partners of known carriers, particularly if both partners are of the same ancestry. The carrier frequency of the SLC17A5 pathogenic variant p.Arg39Cys is in the range of 1:100 in the founder region of northeastern Finland [Aula et al 2000]. Founder variants have also been identified in the Inuit and Mennonite populations (see Prevalence).

Prenatal Testing and Preimplantation Genetic Testing

Molecular genetic testing. Once the SLC17A5 pathogenic variants have been identified in an affected family member, prenatal and preimplantation genetic testing are possible.

Differences in perspective may exist among medical professionals and within families regarding the use of prenatal and preimplantation genetic testing. While most health care professionals would consider use of prenatal and preimplantation genetic testing to be a personal decision, discussion of these issues may be helpful.

Resources

GeneReviews staff has selected the following disease-specific and/or umbrella support organizations and/or registries for the benefit of individuals with this disorder and their families. GeneReviews is not responsible for the information provided by other organizations. For information on selection criteria, click here.

Molecular Genetics

Information in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED.

Table A.

Free Sialic Acid Storage Disorder: Genes and Databases

GeneChromosome LocusProteinHGMDClinVar
SLC17A5 6q13 Sialin SLC17A5 SLC17A5

Data are compiled from the following standard references: gene from HGNC; chromosome locus from OMIM; protein from UniProt. For a description of databases (Locus Specific, HGMD, ClinVar) to which links are provided, click here.

Table B.

OMIM Entries for Free Sialic Acid Storage Disorder (View All in OMIM)

269920INFANTILE SIALIC ACID STORAGE DISEASE; ISSD
604322SOLUTE CARRIER FAMILY 17 (ACIDIC SUGAR TRANSPORTER), MEMBER 5; SLC17A5
604369SALLA DISEASE; SD

Molecular Pathogenesis

The gene product of SLC17A5, sialin, is an integral lysosomal membrane transporter that exports free sialic acid from lysosomes [Mancini et al 1991]. Deficient or defective sialin results in excessive lysosomal storage of the free sialic acid produced by lysosomal degradation of glycoproteins and glycolipids. How elevated intralysosomal free sialic acid causes pathology is not understood. Expression of sialin in the brain may explain part of the neurologic sequelae of free sialic acid storage disorder (FSASD) [Aula et al 2004].

Mechanism of disease causation. Loss of function

Table 7.

SLC17A5 Pathogenic Variants Referenced in This GeneReview

Reference SequencesDNA Nucleotide ChangePredicted Protein ChangeComment [Reference]
NM_012434​.4
NP_036566​.1 		c.115C>Tp.Arg39CysFounder variant in Finnish & other Nordic populations [Aula et al 2000] as well as Mennonite population [Strauss et al 2005]
c.983G>Ap.Gly328GluFounder variant in Israeli Bedouin population [Landau et al 2004]
c.406A>Gp.Lys136GluPenetrance may be reduced in some homozygous persons [Mochel et al 2009]
NM_012434​.4 c.526-2A>G
(IVS3-2A>G)		--Founder variant in Inuit population [Lines et al 2014]

Variants listed in the table have been provided by the authors. GeneReviews staff have not independently verified the classification of variants.

GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen​.hgvs.org). See Quick Reference for an explanation of nomenclature.

Chapter Notes

Author Notes

David Adams, MD, PhD, is a pediatrician, medical geneticist, and biochemical geneticist who performs clinical and basic research on rare diseases at the National Institutes of Health.

Drs David Adams, Marjan Huizing, and Melissa Wasserstein are actively involved in clinical and laboratory research regarding free sialic acid storage disorder (FSASD), although there are no open clinical protocols at the time of this update. Future studies will be registered with ClinicalTrials.gov. The authors would be happy to communicate with persons who have questions regarding diagnosis of FSASD or other consideration. Dr Adams is also interested in hearing from clinicians treating families affected by FSASD in whom no causative variants have been identified through molecular genetic testing.

Author History

David Adams, MD, PhD (2008-present)
William A Gahl, MD, PhD; National Human Genome Research Institute (2003-2020)
Marjan Huizing, PhD (2025-present)
Robert Kleta, MD, PhD; National Human Genome Research Institute (2003-2008)
Melissa Wasserstein, MD (2020-present)

Revision History

  • 26 June 2025 (sw) Comprehensive update posted live
  • 23 January 2020 (sw) Comprehensive update posted live
  • 6 June 2013 (me) Comprehensive update posted live
  • 3 July 2008 (me) Comprehensive update posted live
  • 4 October 2005 (me) Comprehensive update posted live
  • 13 June 2003 (ca) Review posted live
  • 28 February 2003 (wg) Original submission

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