1. MYASTHENIA GRAVIS
GENERAL THORACIC SURGERY
CHAPTER 168

2. Myasthenia gravis
• A neuromuscular disorder,
• Characterized—
1) abnormal fatigability of voluntary
muscle on repetitive activity, and recover
by rest.
2) electrophysiologically decremental
response to repetitive stimulation by single-
fiber electromyography.

3. Myasthenia gravis
3) improve by administration of
anticholinesterase drugs.
4) association with abnormality of thymus.
5) presence of circulating antibody to AChRs
and complement-mediated damage to
receptor.

4. Clinical picture
• Involvement of various voluntary muscle
group.
• Abnormal fatigability on repetitive activity
with improvement after rest.
• Progressive worsening symptoms through
the day from morning to evening.
• Ocular symptoms as diplopia and ptosis,
• Weakness and fatigue transiently.

5. Clinical picture
• Bulbar innervated musculature is affected —
dysphagia, dysarthria, difficult in
mastication, failure of respiratory muscle.
• Most serious symptoms are ventilatory
failure.
• In women — the symptoms may affected by
pregnancy, menses and stress.

7. Electrophysiology
• Single fiber electromyography.
• Record the jitter phenomenon.
• Jitter is variable temporal separation of the
response of individual muscle fibers of the
same motor unit during activation.
• Abnormally variable separation may found
more than 80% patients with myasthenia
gravis.

8. Pharmacologic treatment
• Loewi （ 1932 ） noted acetylcholine is
neuromuscular transmission in cardiac muscle, Sir
Henry Dale （ 1935 ） noted acetylcholine is
liberated at motor nerve ending in voluntary
striated muscle—1936 Nobel prize.
• Propagation of action potential down a motor
nerve fiber, release acetylcholine from synaptic
vesicles, depolarization muscle.

10. Pharmacologic treatment
• Only a small fraction of the 30-40 million
receptors per neuromuscular junction are
activated normally in response to a nerve
impulse.
• The receptors excess provide large safety
ensure the repetitively neuromuscular
transmission.

11. Pharmacologic treatment
• Anticholinesterase—physostigmine,
neostogmine, pyridostigmine— block the
cholinesterase inactivation of acetylcholine,
• Tensilon （ edrophonium chloride ） test
—rapid action and rapid subsidence, a basic
diagnostic test.

12. Pathogenesis and immunobiology
• Simpson (1960) — Autoimmune origin.
• Almon (1974) — Demonstrate circulation
antibodies to AChR site of neuromuscular
junction.

13. Pathogenesis and immunobiology
• Three possible mechanism—
1)Accelerating the degradation of
anticholinesterase receptor through the
cross-linking phenomenon.
2)Direct blocking receptor site.
3)Actual degradation the receptor site by
complement activation.

14. Pathogenesis and immunobiology
• Elevated antibody level are found in 90%
patient and roughly correlated with clinical
severity.
• Immunosuppressive agents as azathioprine,
corticosteroid, cyclosporine may have
benefit effect.

15. Pathology
• Thymoma is present in 10-15% patient with
myasthenia gravis.
• Normal 10-25%.
• Other is thymic hyperplasia.

16. Thymectomy
• Von Haberer 1917 — transcervical
thymectomy because of thymic hyperplasia
often found in thyrotoxicosis.
• Blablock 1936— upper sternotomy incision
and introduced neostigmine the operation is
success.

17. Thymectomy
• Carlens (1968), Crile (1965), Akakura
(1965) — re-describe the old technique of
transcervical thymectomy.
• Papatestas (1987) — perform more than
700 transcervical thymectomy.
• Incomplete thymectomy is the most
important problem.

18. Thymectomy
• Advantage of transcervical incision–
incision only involve soft tissue, rarely
enter the pleural space, well tolerated by
patients.
• Cooper (1988) —add self-retaining retractor
to aid in transcervical exposure and able to
extent transternal resection.
• Type of surgical exposure is most important
determinant of the extent of resection.

19. Thymoma
• 10-15% patient with MG has thymoma.
• 30-50% thymoma are associated with MG.

20. Classifications
• Modified Osserman and Genkin classification.
• Oosterhuis classification.
• Result classification.
• Immunobiological classification.

24. Present indications for thymectomy
• Patient with thymoma — the thymectomy is
indicated all.
• If no thymoma — the patient age, symptoms,
duration, severity, response to medication,
sex are factors in decision-making.

25. Present indications for thymectomy
• Thymectomy is not recommended for the
neonatal type of myasthenia gravis.
• In juvenile form — the reserve thymectomy
for patient with more severe symptoms and
lack of response to medical therapy.

26. Present indications for
thymectomy
• In adult — Cooper, Jaretzki and Papatestas all
believed patient with general symptoms should
receive early thymectomy as soon as the diagnosis
established.
• The ocular type — may try medical therapy for a
year and if the symptoms interfere the daily life,
the thymectomy should be considered.
• High incidence of unsuspected thymoma in patient
older than 40y/o with ocular symptom only.

27. Result
• Adult patient without thymoma undergoing
thymectomy has higher incidence of
complete remission.
• Complete remove all thymic tissue fom
mediastium and lower neck from standard
transternal incision is required in surgical
treatment of myasthenia gravis.