Free sialic acid storage disorders belong to lysosomal storage disorders affecting the nervous system. There are three forms: Salla disease (SD), intermediate severe SD, and infantile free sialic acid storage disease (ISSD).
Salla Disease (SD): SD is an inherited condition characterized by a slow, progressive decline in motor and mental skills. Children with SD initially appear healthy but exhibit poor muscle tone in the first year, with delays in motor and mental skills becoming more evident over time. They develop spasticity and struggle with voluntary movements, experiencing progressive intellectual decline from the first or second year of life. Delayed language skills are common. By adulthood, individuals with SD have profound intellectual disabilities (IQs between 20 and 40), some speech ability, and the capacity to walk, though not all can. While profoundly disabled, they live normal lifespans.
Intermediate Severe Salla Disease: This severe form presents within the first six months of life, featuring extremely poor muscle tone, delayed growth, and potential seizures. Loss of motor and mental functions is rapid, potentially shortening lifespan.
Infantile Free Sialic Acid Storage Disease (ISSD): ISSD, the most severe form, leads to weak muscle tone, severe developmental delays, seizures, bone malformations, enlarged liver, spleen, and heart. Some infants may develop hydrops fetalis, accumulating excess fluid before birth. Individuals with ISSD usually live only into early childhood.
While individuals with SD have a normal lifespan, they face profound disabilities and challenges in movement. Those with ISSD have a reduced lifespan, with an exact prognosis being difficult due to the rarity of cases worldwide.
This group of conditions is caused by pathogenic (disease-causing) variants in the SLC17A5 gene and exhibits autosomal recessive inheritance. This means that both parents must be carriers to have a 25% chance to have a child with the condition. The risk of being a carrier is based on a person’s ancestry or ethnic background.
Other names for this condition are free sialic acid storage disease, N-acetylneuraminic acid storage disease, NANA storage disease and sialuria, Finnish type.