[oliverx0]

The treatment works on the child without symptoms because in most cases neurological affections are irreversible. Since the child had not developed them, the therapy would essentially fix the body before it caused them.

In the case of the child who already shows neurological symptoms, the treatment can help stop the progression of the disease, but most likely not reverse the already existing damage. However, there are already treatments approved to help stop the progression of the disease, including enzyme replacement therapy (provide the enzyme in charge of degrading the sustrate through the bloodstream every so often).

Some would argue that if treatments with similar outcomes already exist and are approved, it could be unethical to test this experimental treatment on a patient (I disagree).