Sialidosis, also known as mucolipidosis type I, is a rare inherited metabolic disorder characterized by a deficiency of the enzyme neuraminidase (sometimes referred to as sialidase). Deficiency of neuraminidase results in the abnormal accumulation of toxic materials in the body. Sialidosis is divided into two types (i.e., type I and type II). Sialidosis type I usually becomes apparent during the second decade of life with the development of sudden involuntary muscle contractions (myoclonus), distinctive red spots (cherry-red macules) in the eyes, and sometimes additional neurological findings. Sialidosis type II is usually more severe than sialidosis type I. Type II often begins during infancy or later during childhood and is characterized by cherry-red macules, mildly coarse facial features, skeletal malformations and mild cognitive impairment. Sialidosis is inherited as an autosomal recessive trait.
Sialidosis belongs to a group of diseases known as the lysosomal storage disorders (LSDs). Lysosomes are particles bound in membranes within cells that function as the primary digestive units within cells. Enzymes within lysosomes break down or digest particular nutrients, such as complex molecules composed of a sugar attached to a protein (glycoproteins). In sialidosis patients, low levels or inactivity of the neuraminidase enzyme leads to the abnormal accumulation these compounds in the cells with unwanted consequences. Sialidosis is also classified as one of the mucolipidoses, a subgroup of the LSDs.