What Are Free Sialic Acid Storage Disorders?
Free sialic acid storage disorders, caused by mutations in the SLC17A5 gene, belong to a group of diseases known as lysosomal storage disorders that affect the nervous system. There are three forms, which vary in severity: Salla disease (SD), intermediate severe SD, and infantile free sialic acid storage disease (ISSD).
SD is an inherited condition causing a slow, progressive decline in motor and mental skills. Children with SD appear healthy at birth but show poor muscle tone in the first year of life. Delays in their motor and mental skills become more obvious with age. They become spastic and have difficulty coordinating their voluntary movements. Loss of intellect is progressive over time, beginning in the first or second year of life. Children with SD typically have delayed language skills. By adulthood, all individuals with SD have profound intellectual and developmental disabilities, with IQs between 20 and 40. Most can speak some words in short sentences. Most will be able to walk in adulthood, though some cannot. Adults with the disease are profoundly disabled but live normal lifespans.
Intermediate Severe Salla Disease
In a more severe form of SD, also called intermediate severe Salla disease, symptoms appear in the first six months of age. These infants have extremely poor muscle tone, have delayed growth, and may have seizures. Losses of motor and mental functions are more rapid with this form of the condition, and lifespan may be shortened.
Infantile Free Sialic Acid Storage Disease
The most severe form of this disorder is infantile free sialic acid storage disease (ISSD). Infants born with this condition do not grow at the expected rate, and they have weak muscle tone and severe developmental delay. They may have seizures, bone malformations, an enlarged liver and spleen, and an enlarged heart. Infants may have a condition called hydrops fetalis in which excess fluid accumulates in the body before birth. Individuals with ISSD usually live only into early childhood.
How Common Are Free Sialic Acid Storage Disorders?
Free sialic acid storage disorders are rare except in Northern Finland, where 1 in 50 individuals are carriers. Only 30 cases of intermediate severe SD have been documented outside of Finland.
How Are Free Sialic Acid Storage Disorders Treated?
There is no effective treatment for free sialic acid storage diseases other than to address symptoms as they arise. Special education or physical, occupational, or speech therapy may be helpful.
What Is the Prognosis for an Individual with a Free Sialic Acid Storage Disorder?
Individuals with SD have normal lifespans, but all will be profoundly disabled and will have difficulty with movement. Most will be able to walk, but some will not. Those with ISSD will have a reduced lifespan. The small number of cases known worldwide make an exact prognosis difficult. Individuals with ISSD usually live only into early childhood.