Sialic acid storage disorders:

Infantile free (ISSD), Salla disease and intermediate severe Salla disease


Autosomal recessive

Inheritance from both parents will result in the disease

Age range at onset:


Specialists you may see:


Infantile free Sialic acid Storage Disorder (ISSD) is the most severe form. Infants with ISSD have severe developmental delay, coarse facial features and hypotonia. These distinctive facial features are a flat nasal bridge, anteverted (upturned) nostrils, micrognathism and median facial dysplasia; in which mid facial features are deficient and a cleft lip is present, with or without a cleft palate. They suffer from seizures and bone malformations as well as hepatosplenomegaly and cardiomegaly. Hydrops fetalis, accumulation of fluid in the body before birth, affects some with ISSD. Those with ISSD are not expected to survive past early childhood.

Salla disease is a less severe form of this. In the first year of life, those with Salla disease develop hypotonia and progressive neurological problems. Seizures, ataxia, spasticity, intellectual disability and athetosis are common. Those with Salla disease usually survive into adulthood.

Intermediate severe Salla disease falls between the conditions above in terms of severity.


All forms of Sialic acid storage disorder are caused by mutations in the SLC17A5 gene. This gene creates a protein called sialin, which moves free sialic acid around the cell – it may also have other functions which are not yet understood.

When the gene is mutated, sialin either is not produced or does not function normally. This creates a build up of free sialic acid, which is not being moved by the sialin. It is not yet known how this causes the described symptoms.