myotonic muscle disorders

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  • DR. SUMIT S. KAMBLE

    SENIOR RESIDENT

    DEPT. OF NEUROLOGY

    GMC, KOTA

  • Myotonia is a phenomena of delayed relaxation after forceful voluntary contractionIs due to repetitive depolarization of the muscle membrane. Myotonia is due to increased excitability of the muscle membrane often caused by dysfunction of muscle ion channels.
  • As a clinical sign, myotonia is often best appreciated in the hand and fingers.To elicit grip myotonia, the patient is instructed to grip the examiner's fingers firmly and then to let go rapidly; in the presence of myotonia, the relaxation of the fingers is delayed.
  • To demonstrate percussion myotonia, the examiner firmly percusses the thenar eminence (specifically the abductor pollicis brevis); in the presence of myotonia, the thumb will abduct and then relax slowly. Alternately, the examiner percusses the extensor digitorum; in the presence of myotonia, the third digit will extend and then relax slowly. Percussion myotonia can be elicited in other muscles as well.
  • Electrical myotonia appears as repetitive muscle fiber potential discharges (eg, positive waves or fibrillation potentials) with waxing and waning frequency and amplitude with a firing rate between 20 and 80 Hz. When played over the audio, myotonic discharges have a characteristic sound of a dive bomber, or an accelerating and decelerating motorcycle engine.
  • Muscular dystrophies: Myotonic dystrophy type 1 and 2 Myofibrillar myopathies Chondrodystrophic myotonia(Schwartz Jampel syndome)Muscle channelopathies: Nondystrophic myotonia (myotonia congenita, paramyotonia congenita, sodium channel myotonia) Hyperkalemic periodic paralysis
  • Metabolic myopathies Acid maltase deficiency, Debrancher deficiency, McArdle disease (myophosphorylase deficiency)Toxic myopathiesChloroquine/hydroxychloroquine myopathy Statin myopathy Colchicine myopathy
  • Inflammatory myopathies Polymyositis DermatomyositisCongenital myotubular/centronuclear Electrical myotonia without clinical myotonia
  • The most common;Age at onset late teensInheritance autosomal dominant Gene defect expansion of a CTG trinucleotide repeat in the 3'-untranslated region of the dystrophia myotonica protein kinase gene (DMPK gene) on chromosome 19q 13.3
  • The myotonia mainly distal demonstrated with grip and percussion myotonia can be elicited most easily over the thenar muscles and long finger extensors.

    Mild weakness & wasting distal & bifacial, neck flexor muscles the distal proximal,

    Muscle stiffness that improves with repeated contractions.

  • There is a characteristic fascial appearance

    Hatchet face

    Frontal balding, a narrow, elongated face and horizontal smile

    Ptosis

    Atrophy of sternocleidomastoid

  • Extra muscular involvement

    Cataracts, Slitlamp examination reveals posterior capsular cataracts, which early on have a characteristic multicolored pattern.Cardiac Conduction defect as arythmia Pulmonary Defects excessive daytime sleepiness due to weakness of the diaphragm and intercostal msEndocrine Dysfunction, insulin resistance Testicular Atrophy, Gynecologic Problems, In Some Patients, Mild To Moderate Cognitive Impairment.
  • Approximately 10%, characterized by severe weakness and hypotonia at birth mental retardation.Upper lip forms inverted V or shark mouth maternally inherited. In many cases, the mother may be so minimally affected that her diagnosis is not made until the infant is born with severe hypotonia and a myopathic facies.
  • Creatine kinase (CK) levels may be mildly to moderately elevated.Electrodiagnostic Testing -Widespread electrical myotonia on needle EMG and tends to bewaxing and waningResembles the sound of diving propeller airplane called as dive bomber or motorcycle potentials.
  • Muscle biopsy- variation in fiber size,rounded atrophic fibers, increased central nuclei, increased connective tissue,and fatty replacement of muscle. DM1patients show preferential atrophy of type1 fibers.DNA Analysis- definative diagnostic test.
  • Age at onset: Teens to early adult Inheritance it is an autosomal dominantGene defect: Zinc finger protein-9, chromosome 3q21, this mutation are expanded CCTG repeats in intron 1.
  • myotonia in the setting of grip and percussion myotonia. Bifacial weakness, ptosis, progressive weakness, predominantly proximal (PROMM) although proximal and distal weakness is seen,And some patients may have muscle hypertrophy. Frontal balding
  • Posterior capsular cataracts,Testicular atrophy, andCardiac conduction defects.Many patients have an intermittent pain syndrome in the thighs, arms, or back.
  • Treatment-SupportiveDrugs for myotonia- Quinine, phenytoin, procainamide, mexiletine,and acetazolamide.Modafinil improves hypersomnolanceGenetic screening of family members
  • Two major forms- Autosomal dominant and recessive.Both associated with abnormalities in chloride channel, CLCN1 on chromosome 7q35.
  • Myotonia CongenitaThomsen diseaseMyotonia Congenita Becker Age of onset infant early childhoodinheritancedominantrecessive. Gene chloride channel chromosome 7qMyotonia generalized painless myotonia non progressiveWeakness lack of weakness weakness and wasting of distal muscles Muscle hypertrophy the proximal arms, thighs, and calves.CK levels Muscle biopsy may be slightly elevated may show a lack of type IIB fibersmoderately elevated Extramuscular nonoProvocativecoldcoldAlleviatingexerciseexercise
  • Associated with mutations in the SCNA4A gene that encodes the alpha subunit of sodium channel.Hyperkalemic periodic paralysisParamyotonia congenitaPotassium aggraveted myotonia
  • characterized by skeletal deformities, muscle stiffness, and myotonia.distal weakness and atrophy, with proximal upper and lower extremity muscle hypertrophy. Characteristic facial and physical appearance include short stature, short neck, and multiple facial anomalies (micrognathia, low-set ears,pursed lips, prominent eyebrows, upward slanting eyes, blepharophimosis, exotropia, and microcornea) 20% with cognitive impairment.
  • 1. Needle EMG proximal and distal muscles of one upper and lower extremity, as well as facial and paraspinal muscles.abnormal spontaneous activity, including myotonic discharges, complex repetitive discharges, fibrillation potentials and positive waves MUAP low amp, short duration and polyphasic
  • A myotonic discharge:

    On emg the spontaneous discharge of a muscle fiber Waxes and wanes in both amplitude and frequency. Myotonic potential may have either a positive wave or a brief spike morphology.Produce a dive bomber sound .
  • 2. Muscle cooling : paramyotonia congenita.

    A. Wrap the limb in a plastic bag, submerge in ice water for about 10 to 20 minutes to bring skin temperature to 20C. Remove the patient's hand from water. B. Needle EMG of a distal forearm or hand muscle is performed, noting the presence of abnormal spontaneous activity (fibrillation potentials, myotonic bursts)
  • 3. Short exercise test.

    Record at abductor digiti minimi stimulating ulnar nerve at the wrist. Supra maximal CMAPs are recorded at baseline and then following 10 seconds of sustained contraction of the ADM. Additional CMAPs are recorded 2 seconds after exercise and then every 10 seconds for a total of 60 seconds; this protocol is repeated 3 times.
  • The short-exercise protocol has a high sensitivity of 100% (83%100%) in PC, 83% (53%100%) in MC, and 60% in SCM. Three patterns are observed in patients with NDM, Fournier I, II, and III Patients with MC commonly exhibit Fournier pattern II with an initial postexercise CMAP decrement, which repairs by 60 seconds subset of MC patients (primarily dominant MC) exhibit Fournier pattern III Patients with PC exhibit Fournier pattern I
  • Fig. 2. Short exercise test. (A) Transient decrease in CMAP amplitude in myotonia congenita. (B) Fournier pattern III: initial increment in CMAP with return to baseline. No change with subsequent tests. (C) Fournier pattern II: initial CMAP decrement postexercise with return to baseline on subsequent trials. (D) Fournier pattern I: decrement of CMAP amplitude postex- ercise, which worsens on subsequent trials. (Modified from Fournier E, Arzel M, Sternberg D, et al. Electromyography guides toward subgroups of mutations in muscle channelopathies. Ann Neurol 2004;56:65061; with permission. Copyright with Wiley InterScience.)

  • 5. Prolonged exercise:

    a. Have the patient perform maximal voluntary muscle contraction for 2 to 5 minutes, resting every 15 seconds for 3 to 4 seconds. then relax completely.c. Record the CMAP immediately, then every 1 to 2 minutes for 40 to 60 minutes afterward or until there is no further decline observed in the CMAP.Decrement is calculated as follows: (Highest CMAP amplitude after exercise - Smallest CMAP amplitude after exercise)/(Highest CMAP amplitude after exercise x 100). Any decrement >40% abnormal.
  • LET has a sensitivity of 80% to 90% in both hypokalemic PP and hyperkalemic PP Patients with a sodium channel mutation often exhibit Fournier pattern IV manifest by an increment in CMAP amplitude/area with exercise followed by a decrement in amplitude/area 40% to 80% of baseline; maximal decrement is observed between 30 and 45 minutes postexercise.Fournier pattern V manifest by a maximal decrement in area/amplitude after 20 to 40 minutes is more typical of calcium channel-related PP;
  • Fig. 3. Long exercise test. Example of long exercise test (Fournier pattern IV) in a patient with hypokalemic PP believed due to SCN4A mutation. (A) CMAPs evoked (5 mV/division) from the ADM during brief pauses (every 1 minute) in a 3-minute exercise with a transient increase in amplitude (17%) and area (81%). (B and C) Aft

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