Benign familial disease with muscle mounding and rippling

Four members of a family in three generations exhibited unusual clinical features of localised transient swelling of muscle induced by percussion (muscle mounding or myoedema) and were able, voluntarily, to induce rhythmic waves of contraction in certain muscles (muscle rippling or rolling). All had raised serum creatine kinase activity. Muscle biopsy in two members showed no specific abnormality. Experimental studies performed on excised intercostal muscle showed that abnormal 'after-contractions' and increased sarcolemmal excitability could be demonstrated in vitro.