1. Pathophysiology, Diagnosis and Medical Management of Myasthenia Gravis
Jung-Joon Sung MD
Department of Neurology
Seoul National University Hospital
대한흉부외과학회 제24차 춘계학술대회
May 22, 2008
It is great pleasure for me to talk here. Frankly, I am a little nervous, you know, because I am not good at speaking English, and I believe that you know more about neuroscience than anyone else around the world.
I think “from bedside to bench, bench to bedside” has two meanings. One means clinically relevant and multidisciplinary research and the other is crazy about research. From bed to bench, bench to bed. Honestly, I am not a person like that.

3. Agenda Introduction Anatomy of neuromuscular junction
Pathophysiology of exertional weakness
Etiology of myasthenia gravis
Clinical features of myasthenia gravis
Diagnosis & treatment of myasthenia gravis
Presurgical preparation
In this talk, I will touch several topics of which some ones are related with clinical situation in terms of from bedside to bench, and others are related with basic research in ALS. They are hot issues in ALS research. I will focus on my experience.
From the research about SOD1 aggregation, I will extend my idea to thinking about the selectivity of protein damage, based on the protein to protein interaction.

4. Repetitive Nerve Stimulation Test (RNST, Jolly test)
Exertional Weakness
Repetitive Nerve Stimulation Test (RNST, Jolly test)

5. Myasthenia Gravis (MG)
MG vs PBP

6. Myasthenia gravis pseudoparalytica

7. Neuromuscular Junction in MGB)
Normal vs MG

8. Myasthenia Gravis

9. Neuromuscular Junction

10. MUSK and rapsyn : clusting of AchRs
Neuregulin (or ARIA, acetylcholine receptor-inducing activity): regulate expression of AchR

11. Physiology of NMJ
Primary (immediately available) store
1,000 quanta
Beneath presynapticnerve terminal
Secondary (mobilization) store
10,000 quanta
Re-supply after a few seconds
Tertiary (reserve) store
100,000 quanta
Distant axon and cell body
Presynpatic portion 의 acetylcholine은 vesicle 에 싸여서 크게 3가지 part 로 나누어져서 저장이 되는데
Nerve terminal 에 AP 가 전달되면 바로 release 가되는 primary store . AP 전달후 수초 대개는 1-2초 정도 후에 primary store 로 이동된후 방출되는 2nd store , 만들어지는 tertiary 로 나누어집니다.

12. Action Potential in NMJ
그래서 NMJ 에서 정상적으로 action potential 이 만들어지는 과정을 보면
Resting state 에서도 spontaneous single quantal release 에 의해서 post synapse 에서 potential 이 형성이 되는데 이를 miniature end plate potential 이라하고
Presynatic terminal 에 neuronal AP 가 전달되면 VGCC 가 열리면서 immediate availabe store 에 있던 ach 가 release 되고 이는 post synapse 에서 end plate potential 을 형성합니다. 이렇게 형성된 EPP 의 크기는 release 된 Ach 의 양에 비례하여서 커지거나 작아지게 되고 all or none 의 pattern 으로 일정한 threshold 이상이 되면 single muscle fiber 에 AP 를 만들어 줍니다. 정상적으로는 EPP 는 항상 threshold 이상이므로 neuronal impulse 가 전해지면 SFAP 를 만들게 되고 threshold 이상의 potential 을 safety factor 라고 합니다. SFAP 가 모이면 하나의 muscle 에서 형성되는 CMAP 으로 측정되는 거죠.

13. Physiologic modeling of RNS
Number of quanta released during each stimulation (m) =
Probability of release (p) X
Number of quanta in the immediately available store (n)
*p; effectively proportional to the concentration of calcium, 0.2 in normal
n; 1000 in baseline
“Mobilization store” replenishes after 1-2 sec
100ms is required to pump Ca out of presynaptic terminal.
*If stimulation rate >10 Hz , Ca conc increases
이렇게 safety factor 가 감소되어있다. 특히 LEMS 에서 더많이 감소되어 있다라는 점을 기억하고 계시구요
이 RNS test 라는 것은 이 safety factor 의 차이를 이용한 것인데요
기본이 되는 개념은 다음과 같습니다.
Presynaptic Quantal release 의 수는 presynapse 에서 방출될 확률에 immediate available store 에 있는 qunata 의 수를 곱한 값으로 이 확률을 presynapse 에 축적된 ca 의 양에 비례하며 보통은 0.2 로 알려져 있습니다.
아까 보셨던 mobilizatoin store 에서 immediate store 로 ach 가 이동하는데 걸리는 시간은 1-2초 이며
VGCC 가 열리면서 influx 된 ca 가 다시 reflux 되는 데 걸리는 시간은 100ms 으로 따라서 10Hz 이상의 빈도로 자극이 들어오면 ca 의 농도가 증가 될것이라는 것을 알수 있습니다.

14. Physiologic modeling of RNS - Low Rate stimulation
stimulus
No. of quanta stored
No. of quanta released
EPP
SFAP
CMAP 1
1000
200 40 +
Normal 2
800
160 32
n/c 3
640
128 26 4
512
102 20 5
5Hz 이하의 자극을 주는 경우 immediate release ach의 양은 정해져 있기 때문에 정상적인 NMJ 에서도 점점 ach 의 방출되는 양이 줄어들고 따라서 EPP 도 감소합니다.
하지만 워낙의 EPP 가 threshold 보다 높은 상태이기 때문이 웬만큼 줄어도 SFAP 를 만들기에 충분하며 2초만 지나면 다시 pre synaptic ach 의 양이 다시 복구가 되기 때문에 CMAP의 크기는 변화가 없습니다.
Threshold : 15mV

15. Physiologic modeling of RNS - Low Rate stimulation
그림으로 나타낸거구요.

16. Physiologic modeling of RNS - Low Rate stimulation
stimulus
No. of quanta stored
No. of quanta released
EPP
SFAP
CMAP 1
1000
200 20 +
Normal 2
800
160 16
n/c 3
640
128 13 -
decrement 4
512
102 10 5
Decrement
(repaired)
MG 의 경우에는 워낙에 EPP 가 작아서 safety factor 가 부족하며 따라서 ach 의 양이 줄고 EPP 가 작아지면 결국은 threshold 를 넘지 못하는 순간이 생기고 SFAP 가 생기지 않는 m fiber 가 발생하여 CMAP 의 크기가 점차 줄어 들게 됩니다. 계속 줄어드는것을 아니고 역시 presynapse 에서 restore 되면서 다시 EPP 가 증가 하고 SFAP 가 생기면 CMAP 도 커지게 됩니다. 보통은 4-5 번째 쯤에 이런일이 발생하죠.
-- Myasthenia Gravis

17. Physiologic modeling of RNS - Low Rate stimulation
그림으로 정리해 보면
Normal MG LEMS

18. Etiology and pathogenesis
Destructive autoantibodies to AChR
Evidence:
Immunization with purified AChR would induce autoantibodies to AChR
Autoantibodies to AChR are found in human MG
Features of MG could be induced by passive transfer of autoantibodies to mice
Plasmapheresis improve S/S in MG

19. Etiology and pathogenesis
Antigen presenting cells-> CD4+ T cells-> B cells-> autoantibodies-> attack AChR
Hyperplastic thymus:
Myoid cells: surface AChR, muscle proteins
In patients, the thymus gland is almost always abnormal.
70% hyperplasia
10% thymoma
Genetics: HLA B8, DR3, DQB1
Leite MI, et al. Ann Neurol 2005

20. Pathogenic Mechanism in MG
Blocking antibody
Increased acetylcholine recpetor (AChR) degradation
Complement-mediated damage of post-synaptic membrane

21. Incidence Prevalence: 50/1,000,000 More prevalent in female
Bimodal peak in Incidence
female incidence peaks in the 3rd decade
male incidence peaks in the 6-7th decade
anti-AChR Ab:
85-90% in generalized MG
50-60% in ocular MG
10-20%: seronegative MG

22. Clinical Features Fluctuating nature: exertional weakness
Distribution of weakness
Ocular: pure in only 15-20%
Diplopia & ptosis: extraocular muscles (EOM) and levator palpabrae (LP)
90% generalization occurred in 13 months
Facial and oropharyngeal muscles
Respiratory
Limbs, never affected alone
Response to cholinergic drugs

23. Clinical Features Crisis: Respiratory muscles involvement
More likely in oropharyngeal or respiratory muscle involvement
Due to infection, surgery, emotional stress, systemic disease, aspiration
Some are spontaneous.
Vs. cholinergic crisis

25. Diagnostic Tests Repetitive nerve stimulation(Jolly test)
3-5 HZ, more than 10 % decremental response
positive in 90% generalized type
Single-fiber EMG:
increased jitter
presence of block
Antibodies to AChR:
no false-positive except Lambert-Eaton syndrome or thymoma without MG
titer is not related to severity
Response to cholinergic drugs: Tensilon (Edrophonium) test, Neostigmine test

26. Discrepancy btw severity and AChR Ab titer
Vincent A. Semin Neurol 2004

27. Laboratory data, others
Antibodies to myofibrillar proteins
Titin, Ryanodine receptor
85-95% in MG with thymoma
Anti-MuSK (muscle specific tyrosine kinase)
0-70% in patients without anti-AChR Ab
Other autoimmune disease: hyperthyroidism (5%)
Chest CT: thymic hyperplasia, 15% thymoma

29. Pathogenic mechanisms in SNMG
Vincent A, et al. Lancet Neurol 2003

30. SNMG in Korea
Sung JJ, et al. Journal of Clinical Neuroscience 2005

31. MuSK-antibody-positive SNMG
IgG MuSK antibodies assayed by immunoprecipitation
Vincent A, et al. Lancet Neurol 2003
Sung JJ, et al. Journal of Clinical Neuroscience 2005

32. Clinical features of MuSK-antibody-negative and –positive SNMG patients in Korea
No. of patients
MuSK positive
MuSK negative
(n=4)
(n=19)
MuSK antibodies (cpm precipitated/5 ul serum)
>17,000 cpm
< 400 cpm
Most disabling symptoms
ocular 9
pharyngeal 2 4
respiratory
limb weakness
axial weakness
Other autoimmune disease 3
Other autoantibodies 1

33. Immunosuppresive treatment 4(100%) 4(21%) Thymectomy 1
Clinical features
No. of patients
MuSK positive
MuSK negative
(n=4)
(n=19)
Immunosuppresive treatment
4(100%)
4(21%)
Thymectomy 1
Change in status
improved 4 13
unchanged 6
Treatment response (PIS)
CSR 2 PR
MM-0
MM-1
MM-2
MM-3 3 7
PIS= postintervention status; CSR = complete stable remission;
PR = pharmacological remission; MM = minimal manifestations

34. Suggested Four Types in MG
SPMG
AChR antibody only
AChR antibody + titin, ryanodine receptor antibody
SNMG
(Ocular SNMG)
MuSK antibody
Undefined SNMG

35. Evoli A. Acta Neurol Scand 2006

36. Treatment(1) Symptomatic treatments
Anticholinergics (anti-AChE drugs): pyridostigmine (Mestinon)
Plasmapheresis
Intravenous immunoglobulin (IVIg)
Alter clinical course
Thymectomy
Steroids
other immunosupressive drugs
Azathioprine(Imuran), cyclophosphamide, cyclosporin, Mycophenolate mofetil

37. Treatment(2) Cholinergics: anti-AChE drugs
Side effects: due to muscarinic effects
Tearing, salivatoin, secretion, diarrhea, GI cramps, meiosis, bradykardia, hypotension
Cholinergic crisis:
Overdosage of anti-AChE drugs
Desensitization of AChR result in weakness
Much salivation, meiosis
Tx: decrease dosage, atropine

38. Immunosuppression

39. Presurgical Preparation (I)
Risk factors in difficult ventilator weaning
MGFA classification II or higher
Duration for more than 6 years
History of steroid requirement
Prior history of respiratory insufficiency
Vital capacity less than 2.9 L
Pyridostigmine dose greater than 750 mg/d 48 hours before surgery
Maximal expiratory force less than 40 to 50 cm H2O
Kernstine KH. Thorac Surg Clin 2005

40. Presurgical Preparation (II)
After appropriate management, unless emergent surgery
If unable to decrease AChEIs in order to reduce respiratory complication, change to IV form of AChEIs (1/30 of oral dose in pyridostigmine)
Plasmapheresis, reduce requisite dose of AChEIs

41. Presurgical Preparation (III)
Steroid, increase post-op complication
Oral steroid, change to IV steroids, and consider stress dosage (eg. Methylprednisolone 500 mg IV on OP day)

42. Drugs with Adverse Effects on MG
depolarizing muscle relaxant: succinylcholine
aminoglycoside:
block presynaptic calcium currents
decreased release of Ach
ampicillin
erythromycin
chlorpromazine
quinidine
procainamide
beta blockers
calcium channel blockers
Tetracycline
Magnesium
penicillamine
interferon