10.2 Lambert-Eaton Myasthenic Syndrome

10.2 Lambert-Eaton Myasthenic Syndrome

Quick Answer: Lambert-Eaton myasthenic syndrome (LEMS) is a presynaptic disorder. Antibodies against P/Q-type voltage-gated calcium channels (VGCC) reduce acetylcholine release. The baseline CMAP is low — the key contrast with typical myasthenia gravis (MG). About 10 seconds of isometric exercise produces a large CMAP facilitation/increment. High-frequency RNS (20–50 Hz) can do the same but is painful. Autonomic symptoms and hyporeflexia that may improve after exercise complete the clinical picture.

Independent OpenExamPrep study of outline item V.A.2.a is this disease pattern, not a second copy of the RNS setup chapter. Recall from chapter 9: immobilize, warm, stay supramaximal, and know that brief exercise is the usual substitute for a long 20–50 Hz train. Then interpret what the numbers mean when the lesion is presynaptic. This material is independent study content for the National Registry Examination for Nerve Conduction Studies. It is not an AAET laboratory manual and does not invent an official AAET increment cutoff.

Mechanism: fewer calcium channels, fewer vesicles at rest

In LEMS, IgG antibodies attack P/Q-type (Cav2.1) VGCCs clustered at active zones of the motor-nerve terminal. Less Ca2+ enters when the action potential arrives. Fewer synaptic vesicles fuse. Quantal content falls. The resting endplate potential in many fibers sits below threshold, so those fibers never join the surface CMAP. That is why the baseline CMAP is small even though the motor axons in the nerve trunk may still conduct and sensory axons are not the primary target.

High-rate activity changes the arithmetic. During tetanic firing or brief strong isometric exercise, residual Ca2+ accumulates in the terminal. Quantal content rises. Fibers that were silent at rest now fire. The CMAP grows in amplitude and area. That growth is true facilitation (more acetylcholine release), not pseudofacilitation (tighter synchrony with a shorter duration and relatively unchanged area).

Lock the compartment: LEMS is presynaptic calcium-channel disease. It is not fewer nicotinic receptors (MG) and not SNARE cleavage (botulism).

Clinical associations: SCLC versus autoimmune

Two teaching streams share the same synapse.

Paraneoplastic LEMS is classically tied to small-cell lung cancer (SCLC). The tumor expresses VGCC-like antigens; the immune response hits the NMJ. New LEMS in a smoker or older adult is a cancer-hunt problem for the clinician. Nerve conduction does not image the lung. It shows the presynaptic pattern that should trigger that hunt.

Autoimmune (non-paraneoplastic) LEMS occurs without demonstrable SCLC, often in a younger or otherwise autoimmune patient. The NCS pattern can be the same. Do not wait for a pathology report to recognize a low CMAP plus large increment.

Bedside clues that belong on the same flash card:

• Weakness often proximal and lower-limb (rising from a chair) more than fluctuating extraocular MG
• Dry mouth and other autonomic features (constipation, erectile dysfunction, reduced sweating)
• Hyporeflexia or areflexia that may improve after brief exercise as more junctions fire
• Strength that may briefly improve after a few seconds of effort (facilitation of strength), the clinical cousin of CMAP increment
• Ocular symptoms less dominant than in MG, though they can occur

Pupils are not the botulism story here, but autonomic involvement is still a LEMS hallmark. Reflexes that "come back" after the patient squeezes or walks a few steps are a teaching pearl; they are not an NCS cursor.

Baseline CMAP: the key contrast with MG

Typical MG: normal resting CMAP, then a slow-RNS decrement. Typical LEMS: low resting CMAP, then a large post-exercise increment. If you only look for decrement, you will mislabel LEMS as MG, because a low-quantal-content synapse can also decrement on a 2–3 Hz train. Decrement answers "is safety factor thin?" Increment plus a small baseline answers "is the terminal failing to release enough ACh at rest?"

A technically valid low CMAP still requires the usual checks: correct G1 on the motor point, supramaximal stimulus, warm muscle, no co-stimulation errors. A 1.5 mV ADM with a positive takeoff because G1 is off the belly is not LEMS. A 1.5 mV ADM with a crisp negative onset that then triples after 10 seconds of isometric abduction is the teaching pattern.

Brief 10-second exercise and high-frequency RNS

Brief isometric exercise (about 10 seconds) is the facilitation maneuver. The patient pushes against a fixed resistance without peeling tape. Immediately record a single CMAP (and often a slow train). In LEMS teaching the CMAP jumps. Percent increment in common laboratory language is:

Percent increment = (Amplitude_after − Amplitude_before) / Amplitude_before × 100

Worked numbers: ADM 1.5 mV at rest becomes 4.0 mV immediately after 10 seconds of isometric exercise.

(4.0 − 1.5) / 1.5 × 100 = 166.7 percent, typically rounded in teaching talk to about 167 percent.

That is a large true increment. Typical electrodiagnostic teaching often cites facilitation greater than about 60 percent, and many sources treat about 100 percent as a more specific ballpark for classic LEMS. Labs vary. These figures are typical electrodiagnostic teaching, not official unpublished AAET cut scores. Do not memorize a fake registry number such as "exactly 250 percent required by AAET."

High-frequency RNS at 20–50 Hz produces the same calcium-accumulation physiology and can show a similar increment. It is painful. Chapter 9 already taught that many laboratories substitute brief exercise in an awake adult. Fast RNS still exists as a teaching and occasional laboratory tool, especially if the patient cannot cooperate with a clean 10-second isometric contraction. It is not the first button on a facial muscle, and a writhing, screaming tetanus is not an interpretable increment.

After the immediate facilitation window, the CMAP may sag again. LEMS is not diagnosed from a single noisy peak on a moving hand. Immobilize, repeat, and confirm that area rose with amplitude.

Slow RNS still matters — do not use it alone

Because resting quantal content is low, slow RNS (2–3 Hz) in LEMS may show a decrement. That decrement does not convert the study into MG. The discriminating cluster is low baseline CMAP + large brief-exercise (or fast-RNS) increment + autonomic/reflex bedside features. A normal 8 mV ADM that decrements 18 percent and then repairs without ever doubling is still the MG neighborhood.

Rest the muscle before the baseline CMAP. A patient who climbed onto the table and then received an "at rest" shock may already be facilitated, hiding the low baseline that makes LEMS obvious.

MG versus LEMS: exam-ready comparison

Worked laboratory scenario

A 62-year-old with proximal leg weakness, dry mouth, and reduced ankle jerks is referred for ulnar motor NCS. ADM is taped to a board, warm, belly-tendon montage, supramaximal. Resting CMAP is 1.5 mV with a negative onset. A 3 Hz train decrements modestly. After 10 seconds of isometric abduction, the immediate CMAP is 4.0 mV (about 167 percent increment) and area rose with the peak. Ankle jerks that were absent are now obtainable. High-frequency RNS is offered but declined because of pain; the exercise increment already shows the presynaptic pattern. The technologist reports the numbers and the quality of immobilization, not a homemade cancer diagnosis and not an invented official cutoff.

If instead the 1.5 mV peak became 1.7 mV with a shorter duration and unchanged area after a sloppy finger wiggle, that is not 167 percent LEMS facilitation. Retape and repeat.

Exam traps

• Missing the low baseline CMAP and calling every decrement MG
• Treating pseudofacilitation (duration down, area flat) as LEMS increment
• Running 50 Hz on a moving, painful limb and calling the envelope facilitation
• Inventing an official AAET increment cut score
• Forgetting autonomic features and reflex facilitation as clinical partners of the tracing
• Declaring SCLC from one ulnar CMAP (NCS shows the synapse, not the chest CT)
• Confusing MuSK MG (facial, often normal-ish CMAP, less limb decrement) with LEMS (low CMAP, large increment)

LEMS in one line: presynaptic P/Q-type VGCC antibodies, low resting CMAP, large 10-second-exercise increment, autonomic symptoms, hyporeflexia that may improve after effort.