A Treatment Extension Study of Mucopolysaccharidosis Type IIIB

Inclusion Criteria: * Must have completed 48 weeks in Part 2 of Study 250-201 and enter 250-202 within 8 weeks of study completion * 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 * Males and females who are of reproductive age should practice true abstinence, defined as no sexual activity, during the study and for 6 months after the study has been completed (or withdrawal from the study). If sexually active and not practicing true abstinence, males and females of reproductive age must use a highly effective method of contraception while participating in the study. * If female with childbearing potential, must have a negative pregnancy test at the Screening visit and be willing to have additional pregnancy tests during the study. Exclusion Criteria: * Has both (1) a cognitive AEq score ≤ 18 months, and (2) a DQ score ≤ 20 * Would not benefit from enrolling in the study in the opinion of the investigator * Has received stem cell, gene therapy or ERT (other than AX 250) for MPS IIIB * Has contraindications for neurosurgery (eg, congenital heart disease, severe respiratory impairment, or clotting abnormalities) * Has contraindications for MRI scans (eg, 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 dru…