From OMIM
Sialic acid storage diseases are autosomal recessive neurodegenerative disorders that may present as a severe infantile form (ISSD) or as a slowly progressive adult form that is prevalent in Finland (Salla disease). The main symptoms are hypotonia, cerebellar ataxia, and mental retardation; visceromegaly and coarse features are also present in the infantile cases. Progressive cerebellar atrophy and dysmyelination have been documented by MRI. Enlarged lysosomes are seen on electron microscopic studies, and patients excrete large amounts of free sialic acid in the urine (Verheijen et al., 1999).  http://www.omim.org/entry/604369

From MedlinePlus Genetics
Salla disease is a less severe form of sialic acid storage disease. Babies with Salla disease usually begin exhibiting hypotonia during the first year of life and go on to experience progressive neurological problems. Signs and symptoms of Salla disease include intellectual disability and developmental delay, seizures, problems with movement and balance (ataxia), abnormal tensing of the muscles (spasticity), and involuntary slow, sinuous movements of the limbs (athetosis). Individuals with Salla disease usually survive into adulthood.

People with intermediate severe Salla disease have signs and symptoms that fall between those of ISSD and Salla disease in severity.

Infantile free sialic acid storage disease (ISSD) is the most severe form of this disorder. Babies with this condition have severe developmental delay, weak muscle tone (hypotonia), and failure to gain weight and grow at the expected rate (failure to thrive). They may have unusual facial features that are often described as "coarse," seizures, bone malformations, an enlarged liver and spleen (hepatosplenomegaly), and an enlarged heart (cardiomegaly). The abdomen may be swollen due to the enlarged organs and an abnormal buildup of fluid in the abdominal cavity (ascites). Affected infants may have a condition called hydrops fetalis in which excess fluid accumulates in the body before birth. Children with this severe form of the condition usually live only into early childhood.

Sialic acid storage disease is an inherited disorder that primarily affects the nervous system. People with sialic acid storage disease have signs and symptoms that may vary widely in severity. This disorder is generally classified into one of three forms: infantile free sialic acid storage disease, Salla disease, and intermediate severe Salla disease.  https://medlineplus.gov/genetics/condition/sialic-acid-storage-disease