The majority of patients is initially screened by enzyme assay, which is the most efficient method to arrive at a definitive diagnosis. In some families where the disease-causing mutations are known and in certain genetic isolates, mutation analysis may be performed. In addition, after a diagnosis is made by biochemical means, mutation analysis may be performed for certain disorders.

Diagnosis often can be made through clinical examination and urine tests (excess mucopolysaccharides are excreted in the urine). Enzyme assays (testing a variety of cells or body fluids in culture for enzyme deficiency) are also used to provide definitive diagnosis of one of the mucopolysaccharidoses. Prenatal diagnosis using amniocentesis and chorionic villus sampling can verify if a fetus either carries a copy of the defective gene or is affected with the disorder. Genetic counseling can help parents who have a family history of the mucopolysaccharidoses determine if they are carrying the mutated gene that causes the disorders.

The diagnosis of ML is based on clinical symptoms, a complete medical history, and certain laboratory tests.

Diagnostic measures can include the following.

Before birth:

- Abnormally low levels of UDP-N-acetylglucoseamine-1-phosphodiesterase enzyme activity in amniotic fluid cells or chronic villi

In infants:

- Elevated plasma lysosomal enzyme concentration

- Decreased concentration of lysosomal enzymes in cultured fibroblasts

- Presence of inclusion bodies and peripheral blood lymphocytes

- Low levels of UDP-N-acetylglucoseamine-1-phosphotransferase enzyme activity as measured in white blood cells

Seven distinct clinical types and numerous subtypes of the mucopolysaccharidoses have been identified. Although each mucopolysaccharidosis (MPS) differs clinically, most patients generally experience a period of normal development followed by a decline in physical and/or mental function. (Note: MPS-V and MPS-VIII are no longer in use as designations for any disease.)

The symptoms of LSD vary, depending on the particular disorder and other variables such as the age of onset, and can be mild to severe. They can include developmental delay, movement disorders, seizures, dementia, deafness, and/or blindness. Some people with LSDhave enlarged livers (hepatomegaly) and enlarged spleens (splenomegaly), pulmonary and cardiac problems, and bones that grow abnormally.

A cure does not exist for I-Cell disease/Mucolipidosis II disease. Treatment is limited to controlling or reducing the symptoms that are associated with this disorder. Nutritional supplements, particularly iron and vitamin B12, are often recommended for individuals with I-Cell disease. Physical therapy to improve motor delays and speech therapy to improve language acquisition are treatment options. Surgery can remove the thin layer of corneal clouding to temporarily improve the complication. It is possible that bone marrow transplant may be helpful in delaying or correcting the neurological deterioration that occurs with I-Cell disease.. Even though there is no existing treatment, the Yash Gandhi Foundation is a 501(c)(3) non-profit organization focused on funding research for I-Cell disease

There is no cure for Pseudo-Hurler Polydystrophy/Mucolipidosis IIIA. Treatment is limited to controlling or reducing symptoms associated with this disorder. Physio-therapy, particularly hydrotherapy has proven effective at relieving muscle stiffness and increasing mobility. The use of crutches, a wheelchair or scooters are treatment options as the metabolic bone disease progresses. The insertion of rods in the spine to stabilize the vulnerable areas can treat scoliosis. Heart valve replacement surgery may be necessary as this disorder progresses.

There is no specific treatment to this disorder. However, several symptoms may be alleviated. For instance, anemia is treated by iron supplements. Some of the movement deficiencies may be corrected with orthopedic intervention. The corneal clouding can be, at least, temporarily corrected by corneal transplantation.

"See the equivalent section in the main mucolipidosis article.

Other diseases that result from a deficiency in the sialidase enzyme are categorized in a broader group known as sialidoses. Because ML I is classified as a sialidosis, it is sometimes referred to as sialidosis type II.

A rarer form of sialidosis – sialidosis type 1– occurs in children and adolescents and is often referred to as the juvenile form of the disorder. Children usually begin to show symptoms during the second decade of life, and myoclonus and cherry-red macules are often the initial symptoms. Patients usually develop seizures and progressive deterioration of coordinated muscular and mental activities.

Mucolipidosis type IV is severely under-diagnosed. It is often misdiagnosed as cerebral palsy. In the Ashkenazi Jewish population there are two severe mutations with a higher carrier frequency of 1:90 to 1:100.

The other two types are closely related.

Mucolipidosis types II and III (ML II and ML III) result from a deficiency of the enzyme N-acetylglucosamine-1-phosphotransferase, which phosphorylates target carbohydrate residues on N-linked glycoproteins. Without this phosphorylation, the glycoproteins are not destined for lysosomes, and they escape outside the cell.

Mucolipidosis type I (ML I) or sialidosis is an inherited lysosomal storage disease that results from a deficiency of the enzyme alpha-N -acetyl neuraminidase (sialidase). The lack of this enzyme results in an abnormal accumulation of complex carbohydrates known as mucopolysaccharides, and of fatty substances known as mucolipids. Both of these substances accumulate in bodily tissues.

Pseudo-Hurler polydystrophy, also referred to as mucolipidosis III (ML III), is a lysosomal storage disease closely related to I-cell disease (ML II). This disorder is called Pseudo-Hurler because it resembles a mild form of Hurler syndrome, one of the mucopolysaccharide (MPS) diseases.