Summary
Free Sialic Acid Storage Disease (SSD) or Salla disease is a lysosomal storage disease. It is very rare (except in Northern Finland where an estimated 1 in 40 people are carriers). Its clinical manifestations are heterogeneous. Severe forms (Infantile Sialic Acid Storage Disease, ISSD) present in utero (with fetal hydrops and ascites) or at birth (with hypotonia, hepatosplenomegaly often associated with ascites, coarse facies, bone malformations, severe motor disorders, intellectual deficit, and seizures). Severe forms are fatal in early childhood. Individuals with the moderate form (initially described in Finland as Salla disease) present with hypotonia during the first year of life. The disease may progress to include spasticity, ataxia, psychomotor retardation, and sometimes coarse facial features. Most patients survive until adulthood but have profound intellectual deficit. The disease is caused by a defective sialic acid transporter (called sialin) in the lysosome membrane. This transporter normally ensures the efflux of free sialic acid (or N-acetylneuraminic acid) from the lysosome. The disease is transmitted in an autosomal recessive manner. The causative gene has been located at 6p14 and several mutations have been reported. Diagnosis is based on increased urinary excretion and storage of free sialic acid in fibroblasts, the trophoblast, or amniocytes. Treatment is symptomatic only. *Authors: Dr R. Froissart, Dr I. Maire (February 2005)*.
