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 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.