Fatigable ptosis, a 3 Hz decrement and a drug interaction
A 58-year-old teacher with fatigable weakness. For six weeks she has had double vision appearing in the late afternoon, drooping eyelids and difficulty chewing meat; rest improves her strength. Examination shows ptosis that worsens within 40 s of sustained upward gaze. Repetitive 3 Hz stimulation of the facial nerve shows a 22 % decrement in compound muscle action potential amplitude between the first and fourth responses. A 2-minute ice pack on the eyelid temporarily reduces the ptosis. Serum anti-acetylcholine receptor antibodies are 2.4 nmol/L (reference < 0.2). Chest CT shows a 3 cm anterior mediastinal mass.
She was prescribed ciprofloxacin for a urinary tract infection and reported worsening weakness within 24 h. A colleague proposes giving suxamethonium for a minor procedure without neuromuscular monitoring, arguing that “the block is already there”.
1. Explain the 22 % decrement at 3 Hz in terms of quantal release and the safety factor. Why is the first response normal, and why is low-frequency stimulation the appropriate test?
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At the normal neuromuscular junction the endplate potential (EPP) produced by one impulse is several times larger than the threshold needed to trigger a muscle action potential - the safety factor is roughly 3–5. That surplus is what allows transmission to succeed every time even though quantal release fluctuates.
During a 3 Hz train (impulses 333 ms apart):
- Each impulse releases a similar number of vesicles, but at this frequency the terminal cannot fully replenish the readily releasable pool, so quantal content falls slightly with successive impulses.
- In a healthy junction the EPP remains far above threshold despite that fall, so the compound muscle action potential amplitude does not change.
- Here roughly 70 % of the postsynaptic acetylcholine receptors are lost or blocked by antibodies. The EPP for a given amount of acetylcholine is proportionally smaller, so the safety factor is eroded.
- The first impulse still crosses threshold - hence a normal first response - but by the fourth the combination of slightly reduced release and a reduced postsynaptic response drops the EPP below threshold in some fibres. Those fibres drop out, and the summed potential falls by 22 %. A decrement > 10 % is abnormal.
Low frequency is the right test because it stresses the release mechanism minimally. A presynaptic disorder (Lambert–Eaton) shows the opposite pattern - low-frequency decrement with a marked increment after high-frequency stimulation or brief exercise - so the choice of 3 Hz versus 30–50 Hz distinguishes the two.
2. Explain the improvement with the ice pack and the deterioration after ciprofloxacin, using the same physiological framework. What should be done about her medication?
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Both observations are explained by the size of the endplate potential relative to threshold.
- Ice pack: cooling the muscle reduces acetylcholinesterase activity in the synaptic cleft, so acetylcholine persists longer and can bind the surviving receptors. With more receptor occupancy per impulse, the EPP rises closer to its normal size and the safety factor improves - ptosis lessens within a couple of minutes. A positive ice-pack test is a bedside demonstration that the deficit is postsynaptic and transmitter-related, not a structural muscle problem.
- Ciprofloxacin: fluoroquinolones reduce acetylcholine release from the presynaptic terminal (and have GABA-A-related effects). In a junction already operating with a small safety factor, even a modest fall in quantal content is enough to push more fibres below threshold, which is why weakness worsened within 24 h.
The two effects are additive because they act on different sides of the cleft and both narrow the same margin.
Management: the drug should be stopped or switched to an agent with less neuromuscular effect, and the change documented in her record; if an antibiotic is essential, the dose and renal function should be checked and her strength monitored. She should be warned to report new weakness after any new prescription, and she needs a written list of agents to avoid. The deterioration is also a prompt to review her disease control and to expedite the thymic assessment, since she is currently not on adequate treatment.
3. Assess the colleague's proposal to use suxamethonium without monitoring. Include the receptor biology, the anaesthetic risk and what the mediastinal mass implies.
Read the worked answer
The proposal is unsafe and should be rejected.
Receptor biology. Suxamethonium is a depolarising blocker that acts as an agonist at the nicotinic receptor. In chronic denervation or prolonged receptor loss, muscle fibres upregulate extrajunctional acetylcholine receptors across the whole sarcolemma, and the junction itself has fewer functional receptors. Two consequences follow:
- Depolarisation of a much larger membrane area releases a large potassium efflux; in this setting suxamethonium can cause severe hyperkalaemia and cardiac arrest.
- With few receptors available, the block may be prolonged and unpredictable, and the patient cannot be assumed to behave like a normal patient.
A non-depolarising agent, if needed at all, should be given at a reduced dose with train-of-four monitoring and a nerve stimulator, and reversal planned. Avoiding neuromuscular blockade entirely, or using a regional technique, is preferable.
The mediastinal mass. A 3 cm anterior mediastinal mass in a patient with acetylcholine receptor antibodies raises thymoma, which occurs in about 10 % of patients with myasthenia gravis (thymic hyperplasia in about 70 %). That mandates contrast CT or MRI, a thymectomy opinion, and a preoperative anaesthetic assessment that includes respiratory function - the mass itself and bulbar weakness both increase anaesthetic risk. Note also that the antibody titre (2.4 nmol/L) does not correlate with disease severity, so it cannot be used to justify a lighter anaesthetic plan.