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/269920

From MedlinePlus Genetics
Free sialic acid storage disorder (FSASD) is an inherited condition that primarily affects the brain and spinal cord (central nervous system). The signs and symptoms of FSASD can vary from person to person, and the condition is often divided into three forms based on the severity of these features. 

The severe form of FSASD is called infantile free sialic acid storage disease or ISSD. The signs and symptoms of this form typically appear in infancy. In some cases, a condition called hydrops fetalis, in which excess fluid accumulates in the body, occurs before or soon after birth. Babies with the severe form of FSASD typically have severe developmental delays, weak muscle tone (hypotonia), and an inability to gain weight and grow at the expected rate (failure to thrive). Additional signs and symptoms may include unusual facial features that are often described as "coarse," bone malformations, an enlarged liver and spleen (hepatosplenomegaly), an enlarged heart (cardiomegaly), and seizures. Respiratory infections are common and can be life-threatening; children with the severe form of FSASD typically only survive into early childhood.

Children with the least severe form of FSASD, also called Salla disease, may not have the signs and symptoms of the condition at birth. The age at which people develop the signs and symptoms of Salla disease can vary, but hypotonia usually appears during the first year of life. People with Salla disease typically have intellectual disabilities and developmental delays, which can make it difficult for them to learn to walk and talk. Additional signs and symptoms include problems with movement and balance (ataxia); abnormal tensing of the muscles (spasticity); and involuntary slow, writhing movements of the limbs (athetosis). Signs and symptoms typically worsen over time. Although life expectancy may be shortened, individuals with Salla disease often survive into adulthood.

People with the intermediate form of FSASD, also called intermediate severe Salla disease, have signs and symptoms that are typically more severe than the signs and symptoms seen in people with Salla disease and less severe than those seen in people with ISSD.  https://medlineplus.gov/genetics/condition/free-sialic-acid-storage-disorder