A Study of Mucopolysaccharidosis Type IIIB (MPS IIIB)

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 int…