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    I See Double When I Get Tired

    AAO 2015

    • Myasthenia can mimic any eye movement pattern.
    • Fatigable ptosis on sustained upgaze suggests ocular myasthenia, but can also be present in up to 38% of patients who do not have the disease.
    • Onset of a new fatigable ptosis over a background of fatigable diplopia is highly suggestive of myasthenia gravis, making decompensation of a congenital strabismus as the cause of the diplopia much less likely. However, some patients presenting with a decompensation of a congenital strabismus can also have ptosis from other causes, most often mechanical.
    • Lack of systemic fatigable weakness is consistent with a diagnosis of ocular myasthenia; in a recent study, 20.9% of patients converted to generalized myasthenia over 2 years.
    • The Tensilon test has a high specificity for myasthenia, but can cause syncope.
    • The rest test is a sensitive and specific assessment for ocular myasthenia, but requires rigorous documentation before and after a 20-minute session of eye closure.
    • A decrement on repetitive nerve stimulation (RNS) testing is highly specific for myasthenia, but has a sensitivity as low as 24%, so a lack of decrement does not rule out myasthenia.
    • Other entities that can mimic ocular myasthenia because of a diurnal variability include Miller Fisher syndrome and silent sinus syndrome.