Mucopolysaccharidoses are a group of metabolic disorders caused by the absence or malfunctioning of lysosomal enzymes needed to break down molecules called glycosaminoglycans (GAGs). These long chains of sugar carbohydrates occur within the cells that help build bone, cartilage, tendons, corneas, skin and connective tissue. GAGs (formerly called mucopolysaccharides) are also found in the fluids that lubricate joints.

Individuals with mucopolysaccharidosis either do not produce enough of one of the eleven enzymes required to break down these sugar chains into simpler molecules, or they produce enzymes that do not work properly. Over time, these GAGs collect in the cells, blood and connective tissues. The result is permanent, progressive cellular damage which affects appearance, physical abilities, organ and system functioning.

The mucopolysaccharidoses are part of the lysosomal storage disease family, a group of genetic disorders that result when the lysosome organelle in animal cells malfunctions. The lysosome can be thought of as the cell's recycling center because it processes unwanted material into other substances that the cell can utilize. Lysosomes break down this unwanted matter via enzymes, highly specialized proteins essential for survival. Lysosomal disorders like mucopolysaccharidosis are triggered when a particular enzyme exists in too small an amount or is missing altogether.

The mucopolysaccharidoses share many clinical features but have varying degrees of severity. These features may not be apparent at birth but progress as storage of GAGs affects bone, skeletal structure, connective tissues, and organs. Neurological complications may include damage to neurons (which send and receive signals throughout the body) as well as pain and impaired motor function. This results from compression of nerves or nerve roots in the spinal cord or in the peripheral nervous system, the part of the nervous system that connects the brain and spinal cord to sensory organs such as the eyes and to other organs, muscles, and tissues throughout the body.

Depending on the mucopolysaccharidosis subtype, affected individuals may have normal intellect or have cognitive impairments, may experience developmental delay, or may have severe behavioral problems. Many individuals have hearing loss, either conductive (in which pressure behind the eardrum causes fluid from the lining of the middle ear to build up and eventually congeal), neurosensory (in which tiny hair cells in the inner ear are damaged), or both. Communicating hydrocephalus—in which the normal reabsorption of cerebrospinal fluid is blocked and causes increased pressure inside the head—is common in some of the mucopolysaccharidoses. Surgically inserting a shunt into the brain can drain fluid. The eye's cornea often becomes cloudy from intracellular storage, and glaucoma and degeneration of the retina also may affect the patient's vision.

Physical symptoms generally include coarse or rough facial features (including a flat nasal bridge, thick lips, and enlarged mouth and tongue), short stature with disproportionately short trunk (dwarfism), dysplasia (abnormal bone size and/or shape) and other skeletal irregularities, thickened skin, enlarged organs such as liver (hepatomegaly) or spleen (splenomegaly), hernias, and excessive body hair growth. Short and often claw-like hands, progressive joint stiffness, and carpal tunnel syndrome can restrict hand mobility and function. Recurring respiratory infections are common, as are obstructive airway disease and obstructive sleep apnea. Many affected individuals also have heart disease, often involving enlarged or diseased heart valves.

Another lysosomal storage disease often confused with the mucopolysaccharidoses is mucolipidosis. In this disorder, excessive amounts of fatty materials known as lipids (another principal component of living cells) are stored, in addition to sugars. Persons with mucolipidosis may share some of the clinical features associated with the mucopolysaccharidoses (certain facial features, bony structure abnormalities, and damage to the brain), and increased amounts of the enzymes needed to break down the lipids are found in the blood.

It is estimated that 1 in 25,000 babies born in the United States will have some form of the mucopolysaccharidoses. Approximately 1 in 100,000 newborns will experience severe mucopolysaccharidosis type I, while approximately 1 in 500,000 newborns will experience attenuated mucopolysaccharidosis type I. Most mucopolysaccharidoses are autosomal recessive disorders, meaning that only individuals inheriting the defective gene from both parents are affected. (The exception is MPS II, or Hunter syndrome, in which the mother alone passes along the defective gene to a son.) When both people in a couple have the defective gene, each pregnancy carries with it a one in four chance that the child will be affected. The parents and siblings of an affected child may have no sign of the disorder. Unaffected siblings and select relatives of a child with one of the mucopolysaccharidoses may carry the recessive gene and could pass it to their own children.