producing an abnormal protein kinase in muscle fibers. It is an autosomal dominant disorder. weakness affects the distal limbs as well as the neck, face and jaws. The mouth often hangs open due to weak jaw closure. Myotonia is the peculiar impaired relaxation of muscle after volitional contraction. Patients may complain of difficulty letting go of a handshake or doorknob. Percssion of the thenar muscles with a reflex hammer also elicits myotonia on the clinical examination.