1. Exercise Testing in Channelopathies

2. Indications Periodic Paralysis Myotonic Disorders
Dystrophic
Nondystrophic
Paramyotonia
Neuromuscular junction Disorders
Myasthenia gravis
Eaton-Lambert
Botulism
Congenital myasthenic syndromes

3. Scientific Background
Resting State
Resting membrane potential mV
Na+ and Ca+ concentrations higher extracellularly than intracellularly
K+ and Cl- concentrations higher intracellularly than extracellularly
Constant balance between high intracellular K+ and low intracellular Na+ by active transport through specific ion channels

4. Scientific Background
Ion Channels
Ion specific channels are macromolecular proteins spanning lipid bilayer of cells and can be opened or closed.
Ions can flow through open channels
Flow depends on: Electrochemical gradient
Relative permeability
Relative selectivity of the channel
Voltage depend gating: gating is mediated by channels sensitivity to electrical potential changes of the membrane.
Other channel types: some do not require a change in voltage to open or close e.g. ligand gated channels, exchangers, transporters etc

5. Scientific Background
Depolarization
Threshold is reached.
Voltage gated Na channels activated, leading to increased Na conductance and thus fast Na influx into cell.

6. Scientific Background
Repolarization
With change in polarity, inactivation of Na channel occurs, K and Cl conductance rises
Therefore, K out and Cl in, reestablishing negative resting membrane potential.

7. Model for membrane excitability disorders
Membrane excitability is a balance between excitation and inhibition.

8. Model for membrane excitability disorders
Excitatory influences (ex)
Inhibitory influences (iy)
Glutamate receptors
Na channels
Ca channels
GABA receptors
K channels
Cl channels
E= (e1+e2+e3)- (i1+i2+i3)

9. Model for membrane excitability disorders
Physiological effects of SCN4A and ClCN1 mutations e.g. hyperkalemic periodic paralysis, paramyotonia congenita, Myotonia congenita.

10. Diseases of Ion Channels
Ion channels found in:
muscle: periodic paralysis, myotonia
brain: epilepsy, familial hemiplegic migraine, episodic ataxia
heart: long QT syndrome, Andersen-Tawil syndrome

11. Protocol for Evaluation of Myotonia & Periodic Paralysis
1. Routine motor & sensory nerve conduction studies in upper & lower limbs.
2. EMG: include distal, proximal, facial and paraspinal muscles.
3. Muscle cooling if suspicion of  paramyotonia congenita.
a. Wrap arm in plastic bag, immerse in ice water for min till skin temperature is 20°C. Remove hand if weakness develops.
b. EMG hand muscle.
c. Allow muscle to rewarm to precooling temp & continue to record EMG (may take >1 hour)
4. Short exercise test if above test does not yield definitive result.
a. Immobilize hand. Record ADM CMAP (Make sure immobilize hand).
b. Record CMAP every minute for 5 minutes with muscle at rest to ensure stable baseline.
c Perform max voluntary contraction for 10 sc.
d. Record CMAP immediately. If a decrement in CMAP is seen, continue to record CMAP every 10 seconds till returns to baseline (usually 1-2 mins).
e. In cases where decrement is seen after exercise, repeat same procedure several times to see if reduction in CMAP continues or habituates.

12. Protocol for Evaluation of Myotonia & Periodic Paralysis
5. Prolonged exercise test if above do not provide diagnostic clues.
a. Immobilize hand. Record ADM CMAP (Make sure immobilize hand).
b. Record CMAP every minute for 5 minutes with muscle at rest to ensure stable baseline.
c. Perform max voluntary contraction for 5 minutes: resting every 15 sec for 3 seconds.
d. Ask patient to relax completely.
e. Record CMAP immediately, then every 2 minutes for 60 minutes afterwards or until there is no further decline (maybe >1 hour).
Decrement = (highest CMAP postexercise - smallest CMAP postexercise)/(highest CMAP postexercise) X 100.
Any decrement >40% is abnormal.
f. NB: Immediately after exercise, CMAP maybe larger before slow decline in amplitude.
Comments:
CMAP often increases with cooling in NORMAL individuals.
1. Myotonia congenita may have an initial but transient decrement after exercise that quickly resolves. Not worsened by cooling, and, as in normality, may increase with cooling.
2. Paramyotonia congenita have a prolonged decrement with combination of exercise and cooling that is still often apparent on rewarming.

13. Calculations Specificity = 97.8% Increment
(highest CMAP postexercise – CMAP before exercise) X 100 /
(CMAP before exercise)
Any increment > 30% is abnormal for CMAP.
Any increment > 25% is abnormal for CMAP area.
Decrement
(highest CMAP postexercise – smallest CMAP postexercise) X 100 /
(CMAP postexercise)
Any decrement > 40% is abnormal for CMAP.
Any decrement > 45% is abnormal for CMAP area.
Specificity = 97.8%

14. Common Channel Diseases

15. Electrophysiologic Testing

16. Electrophysiologic Testing in Channelopathies (Part 2)

17. Sensitivity / Specificity
Mean sensitivity 63%
Primary periodic paralysis 81%
Chloride channelopathies 17%
Thyrotoxic periodic paralysis 75%
Specificity = 97.8%

18. Hypokalemic Periodic Paralysis: Exercise Test

19. Exercise Test: Amplitude increase

20. Exercise Test: Amplitude decrease

21. Myotonic dystrophy
AD, trinucleotide (CTG) disorder on Chr 19q affecting myotonin gene.
Usually present in late teens with distal weakness and delayed relaxation (milkmaid grip, percussion Myotonia)
Stiffness that improves with repeated contractions.
Classical appearance of bifacial weakness, temporal weakness (triangular facies) , frontal balding, ptosis, neck flexor weakness, distal muscle wasting and weakness.
Other clinical features include:
Cataracts (posterior capsule, multicolor pattern))
Cardiac conduction abnormalities
Pulmonary defects (pectus excavatum)
Endocrine dysfunction (diabetes, testicular atrophy, hypogonadism)
Hypersomina (sleep apnea syndrome)
Mild cognitive impairment
CK: mild to moderate elevation
Muscle biopsy: atrophy Type I fibres, increase in central nucleation, ring fibres, occasional small angulated fibres.
10% congenital
Anticipation more profound when inherited from mother

22. Myotonic dystrophy
A three-generation family affected with myotonic dystrophy.  The degree of severity increases in each generation.  The grandmother (right) is only slightly affected, but the mother (left) has a characteristic narrow face and somewhat limited facial expression.  The baby is more severely affected and has the facial features of children with neonatal-onset myotonic dystrophy, including an open, triangular-shaped mouth.  The infant has more than 1000 copies of the trinucleotide repeat, whereas the mother and grandmother each have approximately 100 repeats.

23. Myotonic dystrophy Electrophysiology
Low CMAP maybe secondary to myopathy.
Myotonia most prominent in distal hand, forearm extensor, tibialis anterior, facial muscle but not usually found in proximal muscles.
MUAP analysis difficult if profound myotonia.
Muscle cooling ahs no effect.
Short exercise test produces drop in CMAP immediately after exercise. If CMAP recorded every 10 sec up to 2 min, it recovers to baseline. If short exercise test repeated, the decremental responses habituates.

24. Proximal Myotonic Myopathy
AD, trinucleotide (CCTG) disorder on Chr 3q
Weakness is mostly proximal and may have muscle hypertrophy.
Peculiar intermittent pain syndrome in thigh, arms or back.
Phenotype otherwise is very similar to myotonic dystrophy.

25. Proximal Myotonic Myopathy
Electrophysiology
Effects of cooling, short and long exercise test not well documented.

26. Myotonia congenita
Lack of weakness in most patients and absence of extramuscular abnormalities.
AD (Thomsen’s disease): Julius Thomsen (1876) had the disease
AR(Becker’s disease): first described by Becker
Both arise from ClCN1 abnormality on Chr7q
Minor wasting and weakness in Becker and may also have transient periods of weakness.
Onset in infancy or early childhood.
Usually present with nonprogressive muscle stiffness.
Muscle hypertrophy is common (Herculean).
Stiffness worsens after rest, cold and diminishes with exercise (warm up period).
Grip and percussion myotonia.
CK slightly elevated in AD form, moderately elevated in AR form.
Biopsy: may show lack of Type IIB fibres.

27. Myotonia congenita
Young child with percussion myotonia of tongue.

28. Myotonia congenita Electrophysiology
Widespread myotonic discharges, rare myopathic units in AR form.
Muscle cooling in AD form may produce myotonic bursts of longer duration that maybe more easily elicited than at room temperature.
The short exercise test produces a CMAP immediately after exercise which recovers over 1-2 min with repeated CMAP recording every 10 seconds.
This is unlike paramyotonia congenita in which a decremental response recovers very slowly over many minutes.
RNS causes decrement  15% (up to 50%) similar to MG
Repairs with brief exercise, but CMAP remains 10-15% lower
In ensuring 30 sec, decrement reappears.
These responses to RNS and brief exercise characteristic of congenital myotonia.
Rapid RNS (>10Hz) decrement >25%

29. Myotonia Congenita (Na channel)
New syndrome that is K sensitive, SCN4A mutation on Chr17q(same abnormality as hyperkalemic periodic paralysis), see end of presentation for new syndromes.

30. Myotonia Fluctuans (K+ Aggravated Myotonia)
SCN4A mutation, AD
Symptoms occur after period of rest following exercise
Stiffness often painful especially around chest.
Marked variation in intensity of myotonia
Myotonia aggravated by oral potassium or carbonic anhydrase inhibitors.

31. Myotonia Permanens SCN4A mutation, AD
Similar to myotonia fluctuans except persistent rather than episodic
Symptoms occur after period of rest following exercise
Stiffness often painful especially around chest.
Marked variation in intensity of myotonia
Myotonia aggravated by oral potassium or carbonic anhydrase inhibitors.

32. Paramyotonia Congenita
Eulenburg, 1886.
Stiffness usually affecting bulbofacial, neck & hand muscles.
Stiffness is brought on by repeated contraction or exercise.
Stiffness is triggered by exposure to cold.
Infant noted to have prolonged eye closure after crying or face washed with cool water.

33. Paramyotonia Congenita
Electrophysiology
Myotonia maybe more prominent in distal muscles.
Muscle cooling may have profound effect which is pathognomic: transient dense fibrillations appear with cooling which eventually disappear below 28°C; as further cooling occurs, all myotonic discharges completely disappear below 20°C giving way to paralysis of muscle. This may last up to an hour after muscle is warmed to room temperature.
Short exercise test may produce drop in CMAP which may show marked delay in recovery to baseline CMAP up to an hour (unlike myotonic dystrophy and Myotonia congenita); muscle cooling maybe necessary in some patients to bring out the decremental response.
RNS no decrement unless muscles cooled or after exercise.

34. Hyperkalemic periodic paralysis
Attacks of periodic weakness provoked by fasting, rest after exercise or cold.
Attacks are brief, lasting minutes to hours, and accompanied by hyporeflexia.
K is usually elevated during attacks.
Attack relieved by ingesting carbohydrates.
Develop progressive weakness during adulthood.

35. Hyperkalemic periodic paralysis
Electrophysiology
May not have myotonia between attacks.
Rarely myopathic units.
Myotonia seen early in the attack but disappear as weakness progresses.
Muscle cooling ahs no effect.
Short exercise test produces no decrement.
Long exercise test produces immediate increase in CMAP, especially if initial CMAP is low. Followed by progressive decline in CMAP by 50% over 20-40nminutes with most of decline in the first 20 minutes.
RNS causes no decrement unless after prolonged exercise.

36. Hypokalemic Periodic Paralysis
Usually present in teenage yeas
Attacks can be prolonged, usually occurring on awakening and accompanied by hyporeflexia
K is low during attacks
Myotonia is never present.
Eventually, proximal myopathy.
Rarely associated mild sensory axonal polyneuropathy.

37. Hypokalemic Periodic Paralysis
Electrophysiology
Rarely associated mild sensory axonal polyneuropathy.
Never myotonia
Effects of cooling unknown.
Short exercise test causes no decrement.
Long exercise test produces immediate increase in CMAP, especially if initial CMAP is low. Followed by progressive decline in CMAP by 50% over 20-40nminutes with most of decline in the first 20 minutes.
RNS causes no decrement unless after prolonged exercise.

38. Other Muscle disorders associated with Myotonia
Acid maltase deficiency
inflammatory: polymyositis
congenital: myotubular myopathy
drugs (clofibrate, statins, propranolol, fenoterol, terbutaline, colchicine, penicillamine, cyclopsorin)

39. Exceptions to the Rule
Channelopathies always thought of as disorders of fast inactivation, but newly discovered diseases due to slow inactivation.
Hayward LJ, Sandoval GM, Cannon SC. Defective slow inactivation of sodium channels contributes to familial periodic paralysis Neurology, Apr 1999; 52: 1447.

40. Slow Inactivation of Na Channel (Heat Sensitive)
Sodium currents by perforated patch clamp from wild-type (WT) and P1158S Na+ channels in transfected tsA201 cells at 22 and 32 °C. Test 10-millisecond pulses were given every 5 seconds to a potential between -80 and 35 mV from the holding potential of -100 mV. WT Na+ currents at 22 °C (a) and 32 °C (b) are shown. P1158S Na+ currents showed a slower inactivation time course and slight late currents at both 22 °C (c) and 32 °C (c). I to V curves of Na+ current recorded by perforated patch clamp at 22 °C (e) and 32 °C (f) are also shown. Test 10-millisecond pulses were given every 5 seconds to a potential between -80 and 35 mV. Filled squares = WT, 22 °C; filled triangles = P1158S, 22 °C; open squares = WT, 32 °C; open triangles = P1158S, 32 °C.

41. Exceptions to the Rule
Hypokalemic paralysis always thought of as Ca channel disease, new families discovered with mutation in SCN4A.
Japanese family with AD heat induced myotonia and cold induced paralysis with hypokalemia.
Bulman DE. Scoggan KA. van Oene MD. Nicolle MW. Hahn AF. Tollar LL. Ebers GC. A novel sodium channel mutation in a family with hypokalemic periodic paralysis. Neurology. 53(9):1932-6, 1999 Dec 10.
Sugiura Y. Aoki T. Sugiyama Y. Hida C. Ogata M. Yamamoto T. Temperature-sensitive sodium channelopathy with heat-induced myotonia and cold-induced paralysis. Neurology. 54(11): , 2000 Jun 13

42. SCN4A Mutations Multiple Phenotypes
KPP: Kalemic Periodic Paralysis PC: Paramyotonia Congenita
PAM: Potassium Aggravated Myotonia

43. Exceptions to the Rule
Multiorgan involvement : paramyotonia congenita with SCN4A mutation affecting cardiac repolarisation.
Andersen-Tawil syndrome: mutation in K channel causes dysmorphic features, long QT, periodic paralysis. Exercise test is similar to periodic paralysis. New channel mutations.
Pereon Y. Lande G. Demolombe S. Nguyen The Tich S. Sternberg D. Le Marec H. David A. Paramyotonia congenita with an SCN4A mutation affecting cardiac repolarization. Neurology. 60(2):340-2, 2003 Jan 28.
M. R. Donaldson, J. L. Jensen, M. Tristani–Firouzi, R. Tawil, S. Bendahhou, W. A. Suarez, A. M. Cobo, J. J. Poza, E. Behr, J. Wagstaff, P. Szepetowski, S. Pereira, T. Mozaffar, D. M. Escolar, Y.-H. Fu, and L. J. Ptácek PIP2 binding residues of Kir2.1 are common targets of mutations causing Andersen syndrome Neurology :
Katz JS. Wolfe GI. Iannaccone S. Bryan WW. Barohn RJ. The exercise test in Andersen syndrome. Archives of Neurology. 56(3):352-6, 1999 Mar
S. Sampaolo, A. A. Puca, V. Nigro, V. Cappa, V. Sannino, G. Sanges, V. Bonavita, and G. Di Iorio Lack of sodium channel mutation in an Italian family with paramyotonia congenita Neurology : 1549

44. Andersen-Tawil Syndrome
Syndactyly of the second and third toes on the left foot is evident, and toes on the right foot are dramatically shortened. The patient possesses a small, reduced chin and broad, flat nose—typical physical features associated with Andersen-Tawil syndrome. In the accompanying EKG, asterisks denote sinus beats, and overhead bar designates bidirectional ventricular tachycardia.

45. Exceptions to the Rule
Compound heterozygotes confounding the diagnosis!!!
S. Nagamitsu, T. Matsuura, M. Khajavi, R. Armstrong, C. Gooch, Y. Harati, and T. Ashizawa A "dystrophic" variant of autosomal recessive myotonia congenita caused by novel mutations in the CLCN1 gene Neurology :

46. S. Nagamitsu, T. Matsuura, M. Khajavi, R. Armstrong, C. Gooch, Y
S. Nagamitsu, T. Matsuura, M. Khajavi, R. Armstrong, C. Gooch, Y. Harati, and T. Ashizawa A "dystrophic" variant of autosomal recessive myotonia congenita caused by novel mutations in the CLCN1 gene Neurology :
The proband shows muscular hypertrophy of the quadriceps and hamstring and atrophy of the distal forearm and distal lower leg muscles. He has no "hatchet" facies or frontal baldness.