Myasthenia gravis is an autoimmune disorder that occurs when polyclonal antibodies bind to a significant number of postsynaptic acetylcholine receptors at the neuromuscular junction leading to inadequate neuromuscular transmission.1-4 It most commonly affects 10 to 30 year old females and 70 to 90 year old males. Multiple tests are available to diagnose myasthenia gravis.3,5-12 These include the use of muscle biopsies, curare, edrophonium chloride (Tensilon), electromyography, ice packs, neostigmine, nerve stimulation, and serologic testing. The edrophonium test is the most commonly used diagnostic test for myasthenia gravis.13-16 Many of these techniques are seldom used and not feasible to perform in the Emergency Department. For these reasons, only the edrophonium test and the ice pack test will be described in this chapter.
ANATOMY AND PATHOPHYSIOLOGY
Acetylcholine is a neurotransmitter of the neuromuscular junction that is released by the presynaptic nerve terminals when stimulated.5,17 An electrical potential is produced at the myoneural end plate when sufficient numbers of the postsynaptic receptors at the neuromuscular junction are bound by the released acetylcholine. This electric potential then propagates and ultimately leads to muscle contraction. Simultaneously, acetylcholinesterase rapidly terminates the neurotransmission by metabolizing the acetylcholine in the synaptic cleft of the neuromuscular junction.
Myasthenia gravis is an autoimmune disorder that occurs when polyclonal antibodies bind to postsynaptic acetylcholine receptors at the neuromuscular junction.2-5,17 This leads to inactivation of the receptors and inadequate neuromuscular transmission. Myasthenia gravis is characterized clinically by muscle weakness that develops after repetitive muscle contraction.
Patients are divided into two clinical groups. The first are patients who present with ocular complaints.10 Patients most commonly present with some degree of ocular muscle involvement. Diplopia is the most common patient complaint. Ptosis is the most visible sign noted on the physical examination. Diplopia with disconjugate gaze can be elicited by having the patient maintain a vertical gaze for approximately 3 minutes. Ptosis can also be made to worsen by having the patient maintain an upward gaze for the same duration of time. These patients may or may not have associated weakness of the pharyngeal and facial muscles that present with the complaints of dysarthria and dysphagia. These symptoms can often be elicited by having the patient count backward from 100.
The second group are patients with proximal muscle weakness. Weakness of the limbs usually involves the proximal muscles and may be asymmetric. Weakness of the muscles can be elicited by having the patient perform repetitive exercises involving the muscle groups in question. Involvement of respiratory and pharyngeal muscles should be taken very seriously as it may lead to respiratory failure or aspiration.2,3,18-21
Edrophonium chloride is a short-acting acetylcholinesterase inhibitor (anticholinesterase) used in the diagnosis of myasthenia gravis.3,17 Its onset of action is rapid (within 1 to 2 ...