10.1 Myasthenia Gravis (Postsynaptic)

10.1 Myasthenia Gravis (Postsynaptic)

Quick Answer: Myasthenia gravis (MG) is the classic postsynaptic neuromuscular-junction disease. Acetylcholine-receptor (AChR) antibodies are the most common serology. The baseline compound muscle action potential (CMAP) is usually normal. Slow repetitive nerve stimulation (RNS) shows a decrement that may repair after brief exercise and then worsen during post-activation exhaustion. Single-fiber EMG (SFEMG) is more sensitive but is a physician-level needle skill, not a technologist NCS task. Edrophonium (Tensilon) is a historical clinical test, not the nerve-conduction diagnosis.

Chapter 9 taught how to immobilize a muscle, run a 2–3 Hz train, and time 10-second versus 1-minute exercise. This section teaches the MG disease pattern those trains are meant to catch. Independent OpenExamPrep study of outline item V.A.3 Postsynaptic Disorders—Myasthenia Gravis is pattern recognition: where the lesion sits, what the resting CMAP does, and how slow RNS and post-exercise trains behave. This chapter is independent study material for learners preparing for the National Registry Examination for Nerve Conduction Studies. It is not a publication of the American Association of Electrodiagnostic Technologists (AAET) and does not claim official approval, review, partnership, or exact equivalence with the handbook outline.

Mechanism: fewer receptors, thinner safety factor

A motor-nerve spike still opens presynaptic voltage-gated Ca2+ channels and still dumps quantal acetylcholine (ACh) into the cleft. In AChR-MG the attack is on the muscle side: autoimmune antibodies bind nicotinic ACh receptors on junctional folds, accelerate receptor degradation, and damage the postsynaptic membrane. Quantal size (the postsynaptic effect of one vesicle) falls. Quantal content (vesicles per nerve impulse) is relatively preserved compared with Lambert-Eaton myasthenic syndrome (LEMS). The endplate potential (EPP) therefore sits closer to threshold. Safety factor is reduced. Transmission that was 1:1 becomes 1:sometimes. Fibers that flicker around threshold drop out of the CMAP during a slow train, which is why amplitude and area fall together when the study is real.

Do not relocate MG to the nerve terminal. P/Q-type voltage-gated calcium-channel (VGCC) antibodies and SNARE cleavage are presynaptic stories (sections 10.2 and 10.3). MG is postsynaptic.

Antibodies: AChR versus MuSK

AChR antibodies are the most common serologic group in generalized MG in standard teaching. Many patients also have thymic hyperplasia; a subset has thymoma — that association is clinical, not an NCS finding. Muscle-specific kinase (MuSK) antibodies mark a smaller subset that is typically AChR-negative. MuSK-MG teaching emphasizes facial, bulbar, neck, and respiratory weakness more than a classic limb-predominant picture. Limb slow-RNS decrement may be modest or absent even when the face is clearly involved, which is why a taped nasalis or other cranial/proximal site is not optional decoration.

Other antibodies exist in the wider myasthenia literature. For this outline, lock AChR = most common and MuSK = facial/bulbar subset that may spare the hand decrement.

Clinical pattern the technologist should recognize

Fatigable weakness is the bedside signature. Extraocular muscles produce ptosis and diplopia. Bulbar involvement produces dysarthria, dysphagia, and nasal speech that worsen with talking. Limb fatigue appears with repeated effort. Symptoms often worsen later in the day. Pupils are typically spared — a high-yield contrast with botulism. Tendon reflexes are usually preserved — a high-yield contrast with LEMS. Prominent autonomic failure (dry mouth as a dominant complaint, impotence, ileus) is not the MG hallmark.

Ocular MG can live almost entirely in the extraocular muscles. A normal ulnar abductor digiti minimi (ADM) train does not clear those junctions. That is a disease-pattern statement built on the sensitivity teaching from section 9.2, not a new electrode recipe.

Baseline CMAP: usually normal

At rest, enough endplates still clear threshold that a single supramaximal shock yields a normal or near-normal CMAP in typical MG teaching. One impulse barely taxes the remaining safety factor. That is the key amplitude contrast with LEMS and botulism, where resting quantal content is so low that the baseline CMAP is small.

If ADM is 1.5 mV at rest in a warm, well-placed montage, do not call that "classic MG" on amplitude alone. Think presynaptic disease, axonal loss, or technical failure, then use the increment/decrement pattern. If ADM is 8 mV and then decrements at 3 Hz, you are in the MG amplitude neighborhood.

Brief recall of slow RNS — then the MG envelope

Do not rebuild chapter 9. Recall only what the MG tracing needs:

• Immobilize (tape, board, isometric). Movement artifact can raise or lower CMAP amplitude and mimic decrement.
• Warm the muscle. A cool limb can mask decrement by raising safety factor.
• Stimulus supramaximal before the train.
• Slow RNS typically 2–3 Hz (sometimes 3–5 Hz), about 8–10 stimuli.
• Common teaching measures percent decrement from the first CMAP to the fourth CMAP.
• A decrement of 10 percent or more is a frequently cited laboratory teaching threshold, with lab variation. It is not an official unpublished AAET cut score.

The MG envelope is often U-shaped: decrement usually maximal by about the fourth response, then a slight repair later in the train as transmitter dynamics shift. Read area with amplitude. A peak that falls while duration stretches and area holds is not the same as fiber dropout.

Proximal and facial muscles (nasalis, trapezius, anconeus) are often more sensitive than ADM or abductor pollicis brevis (APB) because clinically involved junctions carry a thinner safety factor. Study a technically clean distal muscle and a weak proximal/facial muscle. In MuSK disease, that facial emphasis is even more important.

Post-exercise: repair, then exhaustion

Chapter 9 defined the clock. MG uses that clock as follows.

Brief isometric exercise (about 10 seconds) is the facilitation / repair window. Immediately afterward, a myasthenic decrement may lessen or disappear. The resting CMAP itself usually does not leap to double or triple its baseline the way a LEMS CMAP can. Repair of decrement is not a 167 percent increment.

Prolonged isometric exercise (about 1 minute), then slow trains at about 1, 2, and 3 minutes, is the post-activation exhaustion window. A myasthenic decrement may worsen as vesicles and receptor availability are taxed. Exhaustion is delayed; it is not the immediate 10-second tracing.

SFEMG and Tensilon: know their lane

SFEMG can show increased jitter and blocking and is more sensitive than RNS for a neuromuscular-transmission defect. It is a physician-level needle examination. It is not a technologist NCS skill on this outline. Mention sensitivity; do not write an SFEMG protocol and do not claim to perform it as part of routine nerve conduction studies.

Edrophonium (Tensilon) is a short-acting acetylcholinesterase inhibitor used historically at the bedside to look for brief improvement of ptosis. It is a clinical/historical test. It is not the nerve-conduction diagnosis. It does not replace a valid, warm, immobilized slow-RNS study. Safety (including bradycardia) belongs to the clinician performing that test, not to a CMAP cursor. An ice-pack test at the bedside is likewise clinical, not an NCS result.

Worked MG numbers

ADM is taped, warm, and 8.0 mV at rest. A 3 Hz train of 10 stimuli: first CMAP 8.0 mV, fourth 6.4 mV. Percent decrement = (8.0 − 6.4) / 8.0 × 100 = 20 percent, with a matching area drop and a U-shape. After 10 seconds of isometric fifth-digit abduction, the first CMAP is still about 8.1 mV and the fourth is 7.6 mV (about 6 percent) — repair, not a LEMS jump. After 1 minute of exercise, a 3 Hz train at 2 minutes shows 7.8 mV then 5.5 mV (about 29 percent) — exhaustion. None of these percentages is an official AAET published cutoff; they illustrate the pattern.

If the same 20 percent "decrement" appeared only while the wrist flexed and vanished after retaping, it was movement, not MG. If the muscle was iced "for comfort" and the train was flat, you may have masked a real decrement.

Exam traps

• Decrement from movement artifact (G1 slide, cathode shift, wrist roll) labeled as MG
• Cool limb masking decrement, then reporting no neuromuscular-junction defect
• Studying only ADM in ocular, bulbar, or MuSK disease
• Calling a large post-exercise CMAP increment MG (think LEMS)
• Treating Tensilon as the NCS gold standard
• Treating SFEMG as a technologist needle task
• Inventing an official AAET decrement cut score instead of teaching ≥10% as common laboratory language with variation

The MG pattern in one line: postsynaptic AChR (or MuSK subset), normal resting CMAP, slow-RNS decrement that may repair after brief exercise and worsen in exhaustion, pupils and reflexes usually spared.