Ptosis comes comes in many flavors. The most common is a variant called aponeurotic ptosis which is a dehiscence of the fascia form the lid margin either from trauma or gravitational degeneration (aging). Another type is mechanical typically due to something weighing the lid down like excess skin, upper lid edema, hemorrhage, tumor. Another is congential but this is note from childhood and would of been diagnosed by now. Another is neurogenic ptosis and has many causes such an cranial nerve III palsy, trauma, horner's syndrome. Lastly there is myogenic cause in which the muscle has a problem like myotonic dystrophy, chronic progressive external ophthalmoplegia, oculopharyngeal dystropy, or myasthenia gravis. A good examination of the lids will help sort out the cause fairly easily. I don't normally get a CT scan unless I not some difference in eye prominence suggestive of proptosis or enophthalmos. Anyway I try to sort this out for you.
Do you have other symptoms at the time of the lid drooping like headache, face pain, neck pain?
Do you have double vision?
What is your age?
How well do your eyes move from side to side, up and down?
it quite possible you have a dehiscence of the aponeurosis tissue which connects the lid elevating muscle to the the lid margin tarsal plate. patient with this have droopy lid, but have good motility of the lid. this is due to the tissue connecting the muscle to the lid margin being attenuated or stretched out. i'd recommend an ophthalmology examination so this can be sorted out and will make you feel much better.
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