Safety, Tolerability and Efficacy of ICV AX 250 Treatment in MPS IIIB -OLE

Inclusion Criteria: 1. Must have completed 240 weeks of Study 250-202 and enter 250-401 within 8 weeks of dosing completion. 2. Provides written informed consent from parent or legal guardian and assent from subject, if required 3. Has the ability to comply with protocol requirements in the opinion of the investigator 4. 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: 1. Has (1) a cognitive age equivalent (AEq ) score ≤ 18 months, (2) a development quotient (DQ) score ≤ 20, and (3) no evidence of improvement during the 250-202 study in secondary or exploratory efficacy endpoints 2. Would not benefit from enrolling in the study in the opinion of the investigator 3. Has received stem cell, gene therapy or enzyme replacement therapy (other than AX 250) for MPS IIIB 4. Has a history of poorly controlled seizure disorder 5. Is prone to complications from ICV drug administration including patients with hydrocephalus or ventricular shunts 6. 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 7. 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 well-being or safety, or the interpretability of the subject's clinical data