Ocular Myasthenia Gravis: Optometry Case #33

Three pairs of glasses in five months is the clue that something is being measured that will not hold still. Refraction assumes a stable eye; hers is not stable, and the pattern of instability is the diagnosis. She is symptom-free first thing in the morning, worst by about nine at night, and partially recovers after an afternoon rest — fluctuating weakness with diurnal variation, better with rest. That single pattern separates this from every structural cause on the differential, because a compressed nerve, a fibrotic muscle or a stroke does not get better after a nap. The examination then demonstrates fatigability directly rather than taking her word for it. Sixty seconds of sustained upgaze makes the ptosis progressively worse, which is the sign in real time. The Cogan lid twitch — the lid overshooting upward and drifting down after a saccade back from prolonged downgaze — points the same way, though it is not specific to this condition. And the ice pack test does the reverse experiment: two minutes of cooling the closed lid lifts it almost completely, a test with a reported sensitivity of 76.9% and a specificity of 98.3%, with resolution of ptosis in over 90% of patients. Note its limitation — it tests the lid, not the eye muscles. Two negatives carry as much weight as the positives. The misalignment corresponds to no single cranial nerve and differs from the pattern recorded at her last visit, which is not how nerve palsies behave. And both pupils are equal and briskly reactive: in most patients with this condition the pupil examination is normal, and that is exactly what distinguishes it from a pupil-involving third nerve palsy, from Horner syndrome and from botulism. A ptosis with diplopia and a normal pupil is a very different conversation from a ptosis with a dilated pupil, which is a compressive aneurysm until proven otherwise. The laboratory result is the trap. The acetylcholine receptor antibody is negative, and in generalised disease that would count heavily against the diagnosis, where 80% to 99% of patients are positive. In purely ocular disease only about half are — reported between 30% and 77% — so a negative result changes very little. This is why single-fibre electromyography, the most sensitive test available, is the one that confirms it. Two things follow. Chest imaging is done to exclude a thymoma, and here a normal plain film is reassuring but not conclusive, so cross-sectional imaging follows. And she needs to know what to watch for: around 85% of patients have eye involvement at presentation, and 50% progress to generalised disease affecting bulbar, axial and limb muscles within two years, with 90% of that progression happening inside that same window. Her speech, swallowing, neck flexion, limb strength and breathing are all normal today. The next two years are when that matters most.