From OMIM
The Sanfilippo syndrome, or mucopolysaccharidosis III, is an autosomal recessive lysosomal storage disease due to impaired degradation of heparan sulfate (Esposito et al., 2000). The disorder is characterized by severe central nervous system degeneration, but only mild somatic disease. Onset of clinical features usually occurs between 2 and 6 years; severe neurologic degeneration occurs in most patients between 6 and 10 years of age, and death occurs typically during the second or third decade of life. Type A has been reported (van de Kamp et al., 1981) to be the most severe, with earlier onset and rapid progression of symptoms and shorter survival. Genetic Heterogeneity of Mucopolysaccharidosis Type III MPS III includes 4 types, each due to the deficiency of a different enzyme: heparan N-sulfatase (type A); alpha-N-acetylglucosaminidase (type B; 252920); acetyl CoA:alpha-glucosaminide acetyltransferase (type C; 252930); and N-acetylglucosamine 6-sulfatase (type D; 252940).  http://www.omim.org/entry/252900

From MedlinePlus Genetics
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.  https://medlineplus.gov/genetics/condition/mucopolysaccharidosis-type-iii