We want to see if Zavesca (or miglustat) is safe and can be tolerated by patients with acute infantile onset GM2 gangliosidosis - classical Tay-Sachs and infantile onset Sandhoff disease. We know that miglustat inhibits the formation of GM2 ganglioside, the compound that is stored in the brains of children with Tay-Sachs and Sandhoff disease. Since it inhibits the synthesis of ganglioside, miglustat may be able to reduce or delay the onset of clinical symptoms.