Clinical case review
Review the complete Optometry case, its five progressive clues, the diagnosis, and the clinical reasoning behind the answer.
Want to test yourself first? Play the original case before reading the clues and explanation below.
WhatsTheDx publishes a new Optometry case every day. Five clues, one diagnosis, and no account needed to play.
Play today's Optometry caseThese are the case findings in their original reveal order, moving from the broader presentation to the most discriminating evidence.
Clue 1
A 58-year-old woman books an appointment convinced her new glasses are wrong. She has started closing one eye to read in the evening, and has been doing it for weeks without quite noticing.
Clue 2
This is her third pair in five months. A second optician added prism to one of them, and that did not help either. The double vision disappears completely when either eye is covered. And there is a pattern to it: she is fine first thing in the morning and worst by about nine at night, and a nap in the afternoon buys her a couple of good hours.
Clue 3
By the end of the day her right upper lid sits distinctly lower than the left, and photographs on her phone show it changing from week to week. Asked to hold her gaze up at a target for sixty seconds, the lid drops progressively further as she does it. The pattern of misalignment does not correspond to any single cranial nerve, and it is not the same pattern it was at her last appointment.
Clue 4
Both pupils are equal and briskly reactive, with no relative afferent pupillary defect. There is no pain, no proptosis, no lid retraction and no lid lag. One more sign: after she looks down at the floor for fifteen seconds and then flicks her eyes back to straight ahead, the upper lid shoots up too far and then drifts back down.
Clue 5
An ice pack held on the closed lid for two minutes lifts it almost completely. The acetylcholine receptor antibody comes back negative, as does the muscle-specific tyrosine kinase antibody — which does not settle the question the way it would in a patient with more widespread weakness. Single-fibre electromyography of the orbicularis oculi is abnormal. Chest imaging shows no anterior mediastinal mass, and her speech, swallowing, neck flexion and limb strength are all normal.
Diagnosis
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.
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Open the clinical referenceFor education and entertainment only. This fictional case is not medical advice and does not replace supervised clinical training, diagnosis, or treatment.
