Mucopolysaccharidosis type III

What it is, symptoms and effects

From: MedlinePlus Genetics, National Library of Medicine

Mucopolysaccharidosis type III (MPS III), also known as Sanfilippo syndrome, is a disorder that primarily affects the brain and spinal cord (central nervous system). It is characterized by deterioration of neurological function (neurodegeneration), resulting in many of the features of the condition. Other body systems can also be involved, although the physical features are usually mild in the early stages.

People with MPS III generally do not display any features of the condition at birth, but they begin to show signs and symptoms of the disorder during early childhood. Early signs and symptoms of MPS III can include frequent ear and throat infections or bowel problems, though most common are mild developmental delay or delayed speech. Behavioral problems often worsen with affected children becoming restless, hyperactive, destructive, anxious, impulsive, fearless, or aggressive. Some affected children display features of autism spectrum disorder, which is a condition characterized by difficulty with social interactions and communication. Children with MPS III may have an increased tendency to chew on objects or put things in their mouth (be hyperoral). Sleep disturbances are also very common in children with MPS III. This condition causes progressive intellectual disability and the loss of previously acquired skills (developmental regression or dementia). In later stages of the disorder, people with MPS III may develop seizures, loss of mobility, and movement disorders.

The physical features of MPS III are less pronounced than those of other types of mucopolysaccharidosis. Individuals with MPS III typically have mildly "coarse" facial features, a prominent forehead, a large head (macrocephaly), and thick hair and eyebrows. Some people with MPS III have short stature, joint stiffness, or mild dysostosis multiplex, which refers to multiple skeletal abnormalities seen on x-ray.

People with MPS III often have a slightly enlarged liver (mild hepatomegaly) or spleen (mild splenomegaly), and a soft out-pouching around the belly-button (umbilical hernia) or lower abdomen (inguinal hernia). Cardiac abnormalities may also occur in this condition, including weakening of the heart muscle (cardiomyopathy), disruption of the heart's normal rhythm (arrhythmia), or problems with the heart's valves. Affected individuals often experience chronic diarrhea and recurrent upper respiratory and ear infections. People with MPS III may also have hearing loss and vision problems.

MPS III is divided into types IIIA, IIIB, IIIC, and IIID, which are distinguished by their genetic cause. The different types of MPS III have similar signs and symptoms, although the features of MPS IIIA typically appear earlier in life and progress more rapidly. People with MPS III usually live into adolescence or early to mid-adulthood.

Causes and biological mechanisms

From: MedlinePlus Genetics, National Library of Medicine

Variants (also called mutations) in the GNS, HGSNAT, NAGLU, and SGSH genes cause MPS III. These genes provide instructions for making enzymes involved in the breakdown of large sugar molecules called glycosaminoglycans (GAGs). GAGs were originally called mucopolysaccharides, which is where this condition gets its name. The GNS, HGSNAT, NAGLU, and SGSH enzymes are involved in the step-wise breakdown of a subset of GAGs called heparan sulfate.

MPS IIIA is caused by variants in the SGSH gene, and MPS IIIB is caused by NAGLU gene variants. Variants in the HGSNAT gene result in MPS IIIC, and GNS gene mutations cause MPS IIID. Variants in these genes reduce or eliminate enzyme function. A lack of any one of these enzymes disrupts the breakdown of heparan sulfate. As a result, partially broken down heparan sulfate accumulates within cells, specifically inside the lysosomes. Lysosomes are compartments in the cell that digest and recycle different types of molecules. Conditions such as MPS III that cause molecules to build up inside the lysosomes are called lysosomal storage disorders. Researchers believe that the accumulation of GAGs interferes with the functions of other proteins inside the lysosomes and disrupts the normal functions of cells. It is unknown why the buildup of heparan sulfate mostly affects the central nervous system in MPS III.

Inheritance and family implications

From: MedlinePlus Genetics, National Library of Medicine

This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have variants. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition.

How common is it?

From: MedlinePlus Genetics, National Library of Medicine

MPS III is the most common form of mucopolysaccharidosis; the estimated incidence of all four types combined is 1 in 70,000 newborns. MPS IIIA and MPS IIIB are much more common than MPS IIIC and MPS IIID.

Reported clinical features and what the terms mean

The following findings are associated with this condition in Orphanet. They are not a checklist for diagnosing yourself, and they do not all occur in every affected person. Some are examination, imaging or laboratory findings that cannot be recognised at home.

The frequency labels describe how often a finding was reported among people with the condition in the source. They do not give the chance that a person with that symptom has the condition. Definitions below reproduce HPO terminology; they explain the term, not the likely severity in an individual.

Chronic otitis media · Very frequent (99-80%)

Chronic otitis media refers to fluid, swelling, or infection of the middle ear that does not heal and may cause permanent damage to the ear.

Coarse hair · Very frequent (99-80%)

Hair shafts are rough in texture.

Delayed speech and language development · Very frequent (99-80%)

A degree of language development that is significantly below the norm for a child of a specified age.

Generalized hirsutism · Very frequent (99-80%)

Abnormally increased hair growth over much of the entire body.

Intellectual disability, severe · Very frequent (99-80%)

Severe intellectual disability (ID) is defined as a type of ID characterized by severely sub-average adaptive functioning and intellectual functioning, with an intelligence quotient (IQ) the range of 20-34.

Malabsorption · Very frequent (99-80%)

Impaired ability to absorb one or more nutrients from the intestine.

Mucopolysacchariduria · Very frequent (99-80%)

Excessive amounts of mucopolysaccharide in the urine.

Sleep abnormality · Very frequent (99-80%)

An abnormal pattern in the quality, quantity, or characteristics of sleep.

Other findings in the same source

From: Orphanet

Additional reported features include Urinary glycosaminoglycan excretion (Very frequent (99-80%)); Central nervous system degeneration (Very frequent (99-80%)); Intellectual disability, progressive (Very frequent (99-80%)); Progressive neurologic deterioration (Very frequent (99-80%)); Abnormal facial shape (Frequent (79-30%)); Abnormal form of the vertebral bodies (Frequent (79-30%)); Abnormal rib morphology (Frequent (79-30%)); Abnormality of skeletal morphology (Frequent (79-30%)); Abnormality of the clavicle (Frequent (79-30%)); Adenoiditis (Frequent (79-30%)). This is a selected summary, not a complete description of the condition.

Which doctor should you see?

The suggested department for discussing Mucopolysaccharidosis type III is Metabolic Medicine, with a metabolic specialist / clinical geneticist as the relevant type of clinician. Paediatrician (children) or physician (adults), with metabolic specialist / clinical geneticist referral.

This is an editorial referral starting point. The appropriate clinic depends on the person’s age, symptoms, previous diagnosis and local services. The first clinician can decide whether another specialty or a team is needed; a department label does not confirm the diagnosis.

How to prepare for an assessment

Bring a short timeline of the main symptoms: when they first appeared, whether they are constant or episodic, what seems to change them, and how they affect daily activities. Include previous reports, discharge summaries, current medicines and supplements, allergies, and any relevant family history. A dated record is more useful than trying to match every feature in an online article.

Ask the clinician what is already established and what remains uncertain. If a test is suggested, ask what question it answers, what its limitations are and how the result would change the next step. The information here is not an instruction to arrange every possible test. In children, bring growth, developmental and school information if it is relevant to the concern.

• Is a biochemical or molecular result needed to clarify the diagnosis?
• Does this condition require an individual plan for illness or reduced food intake?
• Should nutrition advice come from a specialist metabolic dietitian?

Treatment discussions and follow-up

The material gathered for this draft does not provide a complete condition-specific treatment pathway for Mucopolysaccharidosis type III. That gap does not mean that treatment is unavailable. A clinician needs to establish the diagnosis and review current guidance before recommending medicines, procedures, rehabilitation or other support.

Before leaving the appointment, clarify the next review date, who will communicate results, and whom to contact if the situation changes. Discuss difficulties with sleep, work, school, mobility, eating or emotional wellbeing when these are relevant. Practical support may require coordination between the treating clinician and other services.

The collected references do not establish a complete prevention or long-term outlook section for this entry. Missing information should not be interpreted as proof that prevention is impossible or that a particular outcome is inevitable. Ask what is known for the exact subtype, stage and personal circumstances, and which uncertainties remain.

When to seek emergency help

Severe breathing difficulty, collapse, new stroke-like symptoms, a seizure that is prolonged or repeated without recovery, uncontrolled major bleeding, or an immediate risk of self-harm require emergency help. In India, call 112 or reach the nearest emergency department. This is a general, non-exhaustive warning list; it is not a condition-specific triage tool.