WHAT IS MASTICATORY MYOSITIS? Masticatory myositis (MM) is a disease that causes an inability to open the mouth. Other names for this are Masticatory Muscle Myositis, MM, Trismus or Triple M. German Shepherds, Labrador Retrievers, Golden Retrievers, Doberman Pinschers, and Cavalier King Charles Spaniels are the most commonly affected breeds, but any breed or mix…