Mucopolysaccharidosis Type III

Mucopolysaccharidosis Type III comprises four subtypes (MPS IIIA, IIIB, IIIC, IIID) caused by deficiency of different enzymes (heparan sulfamidase, alpha-N-acetylglucosaminidase, acetyl-CoA alpha-glucosaminide N-acetyltransferase, and N-acetylglucosamine-6-sulfatase, respectively) required for sequential heparan sulfate degradation. All subtypes result in heparan sulfate accumulation, particularly in brain tissue. The neurodegeneration results from lysosomal dysfunction, neuroinflammation, oxidative stress, and toxic accumulation of partially degraded heparan sulfate intermediates. MPS III is characterized by mild somatic features (minor facial coarseness, hepatosplenomegaly, skeletal abnormalities) but severe progressive neurological disease. Children appear normal at birth and during infancy, with symptom onset typically between ages 2-6 years. Initial manifestations include behavioral problems, hyperactivity, developmental plateau or regression, and speech delay. Progressive cognitive decline is relentless, with loss of acquired skills, behavioral deterioration, and eventual severe intellectual disability. Motor skills deteriorate with loss of coordination, gait disturbance, and eventually loss of ambulation. Sleep disturbances are common and severe, particularly in MPS IIIA. Seizures develop in about 75% of patients, typically in the later stages. Progression rates vary by subtype, with MPS IIIA being most rapidly progressive and MPS IIIC being somewhat slower, though all forms eventually lead to severe disability.