This is a neurodegenerative disorder with progressive deterioration of muscle and central nervous system functions.  Myoclonus, mental deterioration, hepatosplenomegaly, muscle weakness and atrophy are common.  The defect in neuraminidase activity leads to abnormal amounts of sialyl-oligosaccharides in the urine.  Spinal deformities such as kyphosis are common.  Deep tendon reflexes are exaggerated.  Ataxia and hearing loss may be present.  Coarse facies, a barrel chest, and short stature are characteristic.  Hepatic cells contain numerous vacuoles and numerous inclusions.

Sialidosis types I and II are both caused by mutations in the neuroaminidase gene.  Type I is associated with milder disease than type II which has an earlier age of onset and may present in infancy or even begin in utero.  Early death within two years of age is common in the congenital or infantile forms.  There is, however, significant variability in age of onset and the course of disease among types.