MYASTHENIA GRAVIS

GENERAL THORACIC SURGERY

CHAPTER 168

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.

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.

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.

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.

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.

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.

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.

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.

Pathogenesis and immunobiology

• Simpson (1960) — Autoimmune origin.

• Almon (1974) — Demonstrate circulation antibodies to AChR site of neuromuscular junction.

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.

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.

Pathology

• Thymoma is present in 10-15% patient with myasthenia gravis.

• Normal 10-25%.

• Other is thymic hyperplasia.

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.

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.

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.

Thymoma

• 10-15% patient with MG has thymoma.

• 30-50% thymoma are associated with MG.

Classifications

• Modified Osserman and Genkin classification.

• Oosterhuis classification.

• Result classification.

• Immunobiological classification.

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.

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.

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.

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.