SEP 23, 2016
Myasthenia gravis, Neuro-Ophthalmology/Orbit
- 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.