Mucopolysaccharidosis type IIIB (MPS IIIB, also known as Sanfilippo Syndrome Type B) is a severe neurodegenerative disorder. The purpose of this study is to learn more about the health problems in patients with MPS IIIB and how to measure these problems over time. It will particularly look at how the disease develops in young children. This is an observational study, so no experimental drug will be given. The results from this study will help us design future studies to measure whether these health problems get better when we give experimental drug for MPS IIIB.

This is a multicenter, multinational, longitudinal, observational study in subjects 1 through 10 years of age who have been diagnosed with MPS IIIB. Data will be prospectively collected from 20 to 30 subjects to understand the clinical progression of MPS IIIB in terms of neurocognitive function, behavior, quality of life, imaging characteristics, genotype, and biochemical markers of disease burden. This information may help inform the design and interpretation of subsequent interventional studies.

Inclusion Criteria:

• Individuals eligible to participate in this study must meet all of the following criteria:
• Has deficient NAGLU enzyme activity at Screening. Blood for NAGLU enzyme activity will be collected and analyzed centrally.
• Is ≥ 1 and ≤ 10 years of age and has an age-equivalent of ≥ 12 months on the VABS-II
• DQ ≥ 50 (determined by BSID-III or KABC-II)
• Has presented with signs/symptoms consistent with MPS IIIB; for individuals who have not presented with signs/symptoms of disease (e.g., siblings of known patients), the determination of eligibility will be at the discretion of the BioMarin medical monitor in conjunction with the site investigator.
• Written informed consent from parent or legal guardian and assent from subject, if required
• Has the ability to comply with protocol requirements, in the opinion of the investigator

Exclusion Criteria:

• Has another neurological illness that may have caused cognitive decline (e.g., trauma, meningitis, or hemorrhage) before study entry
• Requires ventilation support, except for noninvasive support at night
• Has received stem cell, gene therapy or ERT for MPS IIIB
• Has contraindications for neurosurgery (e.g., congenital heart disease, severe respiratory impairment, or clotting abnormalities)
• Has contraindications for MRI scans (e.g., cardiac pacemaker, metal fragment or chip in the eye, or aneurysm clip in the brain)
• Has a history of poorly controlled seizure disorder
• Is prone to complications from intraventricular drug administration, including patients with hydrocephalus or ventricular shunts
• Has received any investigational medication within 30 days prior to the Baseline visit or is scheduled to receive any investigational drug during the course of the study
• Has a medical condition or extenuating circumstance that, in the opinion of the investigator, might compromise the subject's ability to comply with protocol requirements, the subject's wellbeing or safety, or the interpretability of the subject's clinical data.

Study Population: Subjects diagnosed with MPS IIIB

Males and females 1 through 10 years of age (ie, until they turn 11) with a documented diagnosis of MPS IIIB are eligible to participate in this observational study. Up to approximately 5 subjects 6 to 10 years old, inclusive, will be enrolled; the remaining subjects will be 1 to 5 years old, inclusive. Additional entry criteria follow.