Bulbar Weakness, a Negative Acetylcholine Receptor Antibody, and an Intolerant Pill
Ms. L is a 38-year-old teacher with 3 months of fluctuating ptosis, evening diplopia, nasal dysarthria, and head drop. She has no limb weakness. Acetylcholine receptor binding antibody is negative; MuSK antibody is positive. Repetitive stimulation at 3 Hz shows a 22% decrement in frontalis and 16% in the spinal accessory, but no decrement in abductor digiti minimi. Single-fiber EMG of frontalis shows increased jitter with blocking. Chest CT shows a 1.3 cm thymic remnant. Her neurologist started pyridostigmine 60 mg four times daily; she reports severe cramping, diarrhea, and new fasciculations with no improvement in ptosis. Seated FVC is 68% predicted and falls to 52% supine; single-breath count is 22.
1. Explain the pattern of repetitive nerve stimulation abnormalities and the serology. Why was the hand muscle normal while the face and neck were not?
Read the worked answer
The distribution is the diagnosis.
In myasthenia gravis, the decrement reflects loss of the safety factor at the postsynaptic membrane. It appears first in proximal, facial, bulbar, and axial muscles, which have lower safety margins and are more fatigable than distal limb muscles. Testing only the abductor digiti minimi misses a large fraction of generalized and nearly all pure bulbar disease; the correct screen includes facial, spinal accessory, and proximal muscles, as was done here.
Why MuSK rather than AChR:
- MuSK antibodies impair clustering of acetylcholine receptors through the agrin-LRP4-MuSK pathway. The receptor loss is often more diffuse and the thymus is usually normal, without the follicular hyperplasia typical of AChR-positive disease.
- MuSK disease characteristically produces prominent bulbar, facial, neck extensor, and respiratory weakness with relatively spared limbs, which is exactly her phenotype.
- AChR binding antibodies are negative in most MuSK-positive patients, so a negative AChR titer must never be treated as excluding myasthenia when the clinical picture fits.
- Single-fiber jitter with blocking in frontalis confirms a neuromuscular junction defect and is the most sensitive test when repetitive stimulation is equivocal.
The 1.3 cm thymic remnant is normal thymic tissue for her age.
Key clinical inference: the normal hand study does not lower her risk. Her supine FVC of 52% predicted predicts ventilatory failure far better than limb strength does.
2. How do you manage her medications, immunotherapy, thymectomy question, and respiratory risk? Justify each choice against her specifics.
Read the worked answer
1. Stop the pyridostigmine. MuSK-positive patients frequently tolerate acetylcholinesterase inhibitors poorly - cramping, diarrhea, and fasciculations without benefit are the classic pattern, and continued escalation can precipitate a cholinergic crisis that mimics worsening myasthenia. Taper rather than stop abruptly so any true worsening is interpretable.
2. Start immunosuppression promptly. Prednisone at moderate dose with a steroid-sparing agent, or rituximab as first-line in MuSK disease given its strong record here. B-cell depletion is particularly attractive in MuSK because the pathogenic antibody is directly pathogenic and titers track disease. Monitor for infection and infusion reactions.
3. No thymectomy. Thymectomy has an established role in AChR-positive generalized disease with thymoma or hyperplasia; in MuSK-positive disease the thymus is typically normal and benefit is not established. Recommend against it.
4. Respiratory and bulbar monitoring is the immediate priority. Supine FVC 52% predicted with a 16-point drop from seated, plus single-breath count of 22, signals restrictive bulbar and axial weakness. Admit for monitoring if there is any trend downward, check negative inspiratory force, and set clear thresholds for elective intubation. Treat any precipitant (infection) aggressively.
5. Medication avoidance list: aminoglycosides, magnesium, fluoroquinolones, beta-blockers, and checkpoint inhibitors. Coordinate with anesthesia if surgery is ever needed.
3. How would Lambert-Eaton syndrome change the electrodiagnostic picture and the workup, and what would you do if the MuSK antibody had been negative?
Read the worked answer
Lambert-Eaton differs at every step.
- Baseline CMAPs are low, not normal, because the presynaptic calcium channel antibody reduces quantal release.
- Brief maximal exercise or 10 seconds of 50 Hz stimulation produces facilitation - an increment of well over 100% (commonly 200-400%), the opposite of the decrement seen here.
- Autonomic features (dry mouth, constipation, erectile dysfunction) and proximal leg weakness with areflexia that returns after exercise are typical; ptosis and diplopia are milder and often absent.
- Single-fiber EMG shows jitter, but jitter alone cannot distinguish presynaptic from postsynaptic disease; the stimulation pattern does.
- Any confirmed LEMS mandates chest imaging and smoking-related cancer screening for small cell lung cancer, with repeat surveillance for at least 2 years if the first screen is negative.
If MuSK had been negative, the next steps are:
- Test LRP4 and agrin antibodies; repeat AChR testing with cell-based assays, which are more sensitive than radioimmunoassay.
- Consider seronegative myasthenia if the clinical and electrophysiologic picture fits; it is a diagnosis of exclusion and still responds to immunosuppression.
- Reconsider mimics of bulbar weakness: brainstem lesion, motor neuron disease (fasciculations, wasting, hyperreflexia - she has none), oculopharyngeal muscular dystrophy (family history, cricopharyngeal involvement), thyroid eye disease, and botulism if acute.
- Do not start pyridostigmine empirically as a diagnostic trial; it delays the real answer and can confound the examination.