Provided herein are methods and compositions for treating a subject suffering from a deficiency in ±-L-Iduronidase in the CNS. The methods include systemic administration of a bifunctional fusion antibody comprising an antibody to a human insulin receptor and an ±-L-Iduronidase. A therapeutically effective systemic dose is based on the specific CNS uptake characteristics of human insulin receptor antibody-±-L-Iduronidase fusion antibodies as described herein.