A progressive neurodegenerative disorder that begins with muscle weakness, then progresses to startle reaction, retardation and seizures. It is characterized by the accumulation of G(M2) GANGLIOSIDE in neurons that is caused by a lack of G(M2) ACTIVATOR PROTEIN function. The AB variant designation refers to the increase of both HEXOSAMINIDASE A and HEXOSAMINIDASE B in tissues that lack of G(M2) activator protein.