Nizar Souayah, MD, FAAN, FAANEM, FANA, explains why many cases labeled seronegative MG may result from incomplete antibody testing. He outlines a three-step diagnostic approach to confirm true disease and rule out alternative conditions.
What is your systematic approach to confirming a seronegative MG diagnosis?
Transcript
The majority of patients labeled seronegative where most of the time not true seronegative. They were incompletely tested. My approach has three steps.
First, confirm the phenotype. I need to see true fatigability weakness that fluctuates — not fixed weakness. Ptosis that worsens with sustained upward gaze. Voice that change over time. Chewing that deteriorates by the end of the meal. If the weakness is fixed, reconsider the diagnosis entirely.
Second, repeat and expand the antibody panel. Many community labs only run acetylcholine receptor binding antibodies. That misses the blocking antibodies, the modulating antibodies, and the muscle-specific kinase antibodies. If those were never sent, the workup is incomplete.
In addition, cell-based assay are also increasingly detecting antibodies that conventional testing misses.
Third, confirm muscular junction dysfunction with electrophysiology. Single-fiber EMG is the most sensitive test that we have — above 90% sensitivity, even in ocular disease. And seronegative patients, I go directly there rather than relying on repetitive nerve stimulation alone. If a complete antibody panel is negative and single-fiber EMG is normal, I stop and reassess.
Consider other differential diagnosis, including thyroid eye disease, mitochondrial disease, or functional neurologic disorder.
The take home message with the seronegative myasthenia that seronegative myasthenia should only be diagnosed after complete antibody testing and objective visual confirmation. Do not treat ambiguity as a disease.
