Cause:Genetic, autosomal dominant on chromosome 19
Pathophys:Abnormal insulin fasting levels and increased response to stimulation (Nejm 1967;277:837). In vitro the sarcoplasmic reticulum shows increased rapidity of calcium uptake initially and normal total uptake, suggesting fast but not slow fibers most affected (Nejm 1969;280:184) and may thus explain some of the myotonia
Sx: Onset age 20-25 yr
Si:Weak jaw, weak sternocleidomastoids, ptosis; early frontal balding; myotonia (unable to release grasp); cataracts; gonadal atrophy. Muscle percussion (thenar eminence) with reflex hammer causes localized myotonic contraction
Diabetes mellitus; flaccid esophagus with secondary reflux and strictures; billowing mitral valve (Ann IM 1976;85:18), Vtach, atrial tachycardias and heart block (Nejm 2008;358:2688, Jama 1995;274:813) contribute to 15-30% incidence of sudden death (Ann IM 1991;115:607)
r/o other myotonic syndromes (Nejm 1993;328:482) including cold myotonia (autosomal dominant), excitement myotonia (autosomal dominant), both of which respond to quinidine; "stiff man syndrome" (Tetanus, Lockjaw); hyperkalemic periodic paralysis
Lab:
Chem:Flat glucose tolerance test with normal FBS; decreased urinary ketosteroids
Noninv:EMG has a diagnostic pattern
Serol:IgG low (increased catabolism), without other globulin changes. Southern blot analysis w DNA probe detects the abnormal gene 90+% of the time (Nejm 1993;328:471)
Rx:Acetazolamide rx helps sx but causes severe muscle weakness of quads (Ann IM 1977;86:169)