Neuromuscular Junction Disorders: Pathophysiology and Management

Anatomy and Physiology of the Neuromuscular Junction

The neuromuscular junction consists of a presynaptic motor nerve terminal, the synaptic cleft, and the postsynaptic muscle membrane 2
When a motor neuron is activated, calcium enters the nerve terminal, triggering the release of acetylcholine-filled vesicles into the synaptic cleft 2
Acetylcholine diffuses across the 30-nm gap to bind with nicotinic acetylcholine receptors on the muscle fiber's sarcolemma 2
The binding opens ion channels, allowing sodium influx and potassium efflux, which depolarizes the membrane and triggers muscle contraction 2
Acetylcholinesterase in the synaptic cleft hydrolyzes acetylcholine to terminate muscle contraction 3
The NMJ contains approximately one-half million acetylcholine-filled vesicles and up to 10,000 acetylcholine receptors/μm² 3

Myasthenia gravis is a B-cell mediated autoimmune disorder where antibodies attack acetylcholine receptors or proteins involved in receptor clustering, with a prevalence of approximately 0.5 to 12.5/100,000 per year 4, 1
The American Academy of Otolaryngology-Head and Neck Surgery recommends repetitive nerve stimulation showing decremental response for diagnosis of myasthenia gravis 6
Single fiber electromyography (SFEMG) is recommended if repetitive nerve stimulation is normal but suspicion of NMJ disorder remains high, and is more sensitive than RNS 6
Acetylcholine receptor antibodies are present in nearly all generalized myasthenia gravis cases and 40-77% of ocular myasthenia gravis cases 5

The American College of Chest Physicians recommends 3,4-diaminopyridine (Firdapse) for treatment of Lambert-Eaton myasthenic syndrome 7
Thymectomy is recommended for myasthenia gravis, especially if thymoma is present 5, 1
Immunosuppressive therapy is recommended for myasthenia gravis 1
Early recognition and diagnosis are crucial to prevent life-threatening complications like respiratory failure, and treatment should be tailored to the specific NMJ disorder 1
Patients with NMJ disorders may have altered sensitivity to certain medications, particularly neuromuscular blocking agents used in anesthesia 2, 3
The American Heart Association recommends monitoring for respiratory compromise in patients with generalized myasthenia gravis 1