Myasthenia gravis is a neuromuscular disorder characterized by abnormal muscle fatigability that improves with rest. It is caused by circulating antibodies that damage acetylcholine receptors at the neuromuscular junction. Symptoms vary but commonly involve the eyes, face, neck and limbs. Treatment involves anticholinesterase medications and thymectomy, as the thymus gland is involved in pathogenesis. Thymectomy guidelines depend on a patient's age, symptoms, and response to medications, with early surgery generally recommended for adult patients. Complete removal of thymic tissue through a standard transternal incision is required for optimal surgical treatment of myasthenia gravis.

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.