0071643192.pdf

Myasthenia Gravis

Myasthenia gravis is an autoimmune disorder of the neuromuscular junction.

PATHOPHYSIOLOGY

■ Antibodies against acetylcholine receptors at the neuromuscular junction

→destruction of receptors →decrease in available receptors.

SYMPTOMS/EXAM

■ Fatigable muscle weakness

■ Repeated muscle use increases weakness.

■ Weakness is improved after rest.

■ Ocular muscle weakness is common →ptosis, diplopia, blurred vision.

■ Dysarthria and dysphagia may be present.

■ Exacerbated by heat, improved by cold

■ Acute myasthenic crisis= respiratory failure requiring mechanical

ventilation.

■ Triggers include infection, medications.

DIFFERENTIAL

■ Lambert-Eaton myasthenic syndrome

■ Botulism

DIAGNOSIS

■ Suspect based on clinical examination.

■ Edrophonium (Tensilon) test:

■ Edrophonium is a short-acting acetylcholinesterase blocking agent →

increased acetylcholine available at neuromuscular junction →

improvement in measured ptosis (and other symptoms).

■ Test dose must be administered first.

■ Have atropine ready in case of excessive cholinergic symptoms.

■ Serologic testing for acetylcholine receptor antibodies

■ Electromyographic testing.

TREATMENT

■ Cholinesterase inhibitors →increased circulating acetylcholine →symp-

tom control.

■ Pyridostigmine, neostigmine

■ Should NOT be used during myasthenic crisis!

■ Excessive administration →cholinergic crisis with weakness, tachycardia,

excessive secretions, etc.

■ Immunosuppressive agents for chronic control

■ Thymectomy

■ May induce early or late (2–5 years) remission of disease

■ Plasmapheresis (plasma exchange) and intravenous immune globulin (IVIG)

for acute exacerbations

Lambert-Eaton Myasthenic Syndrome

Lambert-Eaton myasthenic syndrome is an autoimmune disorder of the neu-

romuscular junction. About half the time it is associated with cancer (small

cell carcinoma of the lung, lymphoma).

NEUROLOGY

Myasthenia gravis:

Muscle weakness, worse with

repeated use and commonly

involving ocular muscles;

treat with cholinesterase

inhibitors and

immunosuppressive agents

Acute myasthenic crisis:

Acute ventilatory failure,

requiring mechanical

ventilation;

treat with plasma exchange

and/or IVIG