Sanfilippo Syndrome: What Patients and Families Need to Know

Sanfilippo syndrome, also known as mucopolysaccharidosis type III (MPS III), is a rare, inherited metabolic disorder that primarily affects the brain and central nervous system. It belongs to a group of conditions called lysosomal storage disorders. The condition is sometimes referred to as "childhood dementia" or "childhood Alzheimer's" because it causes progressive cognitive decline in children who often develop typically at first.

What causes Sanfilippo syndrome?

Sanfilippo syndrome is caused by a deficiency in one of four enzymes needed to break down a complex sugar molecule called heparan sulfate. Without a functioning enzyme, partially broken-down heparan sulfate accumulates in the body's cells, particularly in the brain, leading to progressive damage over time. The condition is inherited in an autosomal recessive pattern, meaning a child must inherit a nonfunctioning copy of the responsible gene from both parents to develop the disease. When both parents carry one copy of the altered gene, each child has a 25% chance of being affected.

What are the different types of Sanfilippo syndrome?

There are four subtypes of Sanfilippo syndrome, each caused by a deficiency in a different enzyme:

• MPS IIIA, caused by mutations in the SGSH gene, is the most common and generally the most severe form, typically associated with earlier symptom onset and a shorter life expectancy.
• MPS IIIB, caused by mutations in the NAGLU gene, tends to have a somewhat longer life expectancy than type A.
• MPS IIIC, caused by mutations in the HGSNAT gene, is less common, and some patients may live into early adulthood.
• MPS IIID, caused by mutations in the GNS gene, is the rarest subtype.

While the underlying enzyme deficiency differs between subtypes, the clinical symptoms are largely similar across all four.

What are the symptoms of Sanfilippo syndrome?

Symptoms typically emerge in early childhood and progress over time, often described in stages:

• Early stage: Mild developmental delays, particularly in speech, may be the first noticeable signs. Some children are initially misdiagnosed with autism spectrum disorder or another developmental condition due to overlapping symptoms like language delay and impaired social communication.
• Middle stage: Behavioral changes become more pronounced, including hyperactivity, sleep disturbances and loss of previously acquired skills. Children may also experience recurrent ear, nose and throat infections, seizures, and hearing or vision impairment.
• Later stage: Progressive loss of mobility, balance difficulties and loss of verbal communication typically occur after age 10. Many children eventually lose the ability to walk and require feeding support due to swallowing difficulties.

Gastrointestinal symptoms, including chronic diarrhea, constipation and abdominal discomfort, are also common but sometimes overlooked aspects of the condition.

How is Sanfilippo syndrome diagnosed?

Diagnosis can be delayed because early symptoms often overlap with other developmental conditions, including autism spectrum disorder. When Sanfilippo syndrome is suspected, doctors typically use urine tests to detect elevated heparan sulfate levels, followed by enzyme activity testing and genetic testing to confirm the specific subtype.

Is there a cure for Sanfilippo syndrome?

There is currently no approved treatment that can halt or reverse Sanfilippo syndrome. However, several approaches are being studied or used to help manage symptoms and slow progression, including enzyme replacement therapy, gene therapy, substrate reduction therapy and stem cell transplantation. Enzyme replacement therapy has shown limited effectiveness for neurological symptoms specifically, since the blood-brain barrier can prevent the replacement enzyme from reaching the brain effectively. Gene therapy has shown early promise in clinical trials for types A and B, though more research is needed to establish long-term safety and effectiveness.

What is the life expectancy for someone with Sanfilippo syndrome?

Life expectancy varies by subtype. Individuals with MPS IIIA have historically had a mean age at death between 11 and 19 years, though outcomes for this subtype have improved somewhat in recent decades. Those with MPS IIIB or IIIC tend to have longer life expectancies, generally ranging from about 11 to 34 years. On average, life expectancy across subtypes falls within two to three decades, though this varies significantly based on subtype and individual disease progression.

What kind of care do children with Sanfilippo syndrome need?

Because Sanfilippo syndrome affects multiple body systems, care is typically multidisciplinary and evolves as the disease progresses. This can include neurology, genetics, gastroenterology, physical and occupational therapy, and dental care, since children with the condition face a higher risk of oral health complications due to movement difficulties and reduced ability to maintain hygiene. As the disease advances, many families work with palliative or supportive care teams to manage symptoms and maintain quality of life.