The short answer
Most ptosis is age-related tendon stretching with normal pupils. A droop with a smaller pupil is Horner syndrome, a sign that needs imaging of the whole nerve pathway including the lung apex.
The commonest cause of a drooping eyelid is age-related stretching of the eyelid tendon: slow, painless, pupils normal.
Clinicians sort a new droop by the pupil. A larger pupil with double vision or pain is an emergency; a smaller pupil suggests Horner syndrome.
Horner syndrome is a sign, not a disease: ptosis, a constricted pupil more obvious in dim light, and reduced sweating on that side of the face.
Because the nerve pathway dips into the top of the chest, an apical lung (Pancoast) tumour can cause it; these are 3-5% of lung cancers.
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The full explanation.
What a drooping eyelid usually turns out to be
Most ptosis is mechanical and benign. The commonest cause by a wide margin is age-related stretching of the tendon that lifts the eyelid, which comes on slowly over years, often affects both eyes to different degrees, causes no pain, and leaves the pupils completely normal. Long-term contact lens wear, previous eye surgery, eyelid swelling from a stye or chalazion, and lifelong congenital droop account for most of the rest.
What makes a drooping eyelid worth a careful look is not the eyelid. It is the pupil underneath it, and what else is happening on the same side of the body.
The three patterns that change the assessment
Clinicians sort a new droop largely by pupil size.
Droop with a larger pupil on the same side, often with double vision, a painful eye, or severe headache, suggests a third cranial nerve palsy. This can indicate an aneurysm and is treated as an emergency.
Droop that comes and goes, worse late in the day or after using the eyes, with double vision and normal pupils, suggests myasthenia gravis, a neuromuscular condition.
Droop with a smaller pupil on the same side suggests Horner syndrome. The classic triad is a mildly drooping lid, a constricted pupil, and reduced sweating on that side of the face. The difference between the pupils is more obvious in a dim room than a bright one, and the affected pupil is slow to widen when the lights go down. Horner syndrome itself is not a disease; it is a sign that something has interrupted a nerve pathway that runs from the brain, down into the chest, up the neck alongside the carotid artery, and into the eye.
Why Horner syndrome points at the chest
Because that pathway dips into the top of the chest, a tumour at the apex of the lung can compress it. These are called Pancoast or superior sulcus tumours. They make up roughly 3% to 5% of lung cancers and behave unlike other lung cancers: cough and coughing blood are often absent, and the first symptom is usually pain in the shoulder, present in up to 96% of cases.
The full pattern to recognise is a quiet, slowly developing droop with a small pupil, accompanied by any of: shoulder or shoulder-blade pain that no injury explains, pain running down the inner arm, numbness or tingling in the ring and little fingers, weakness or wasting of the small muscles of the hand, hoarseness, or a smoking history. Documented diagnostic delays of five to ten months are common with these tumours, precisely because the parts get assessed separately.
Other causes along the same pathway include carotid artery dissection, which typically causes a painful Horner syndrome with neck or face pain and carries a stroke risk, and strokes, spinal lesions or neck masses higher up.
What a workup involves
Assessment begins with photographs or observation of both pupils in bright and dim light, checking for the dilation lag that suggests Horner syndrome. Pharmacological testing with apraclonidine eye drops can confirm it, though the test can be falsely negative in the first five to eight days after onset.
Confirmation is followed by imaging of the whole pathway, not just the head, because the lesion can be anywhere along it. That usually means MRI of the brain, neck and spinal cord, MR or CT angiography of the carotid arteries, and CT of the chest with attention to the lung apices. A plain chest X-ray is not sufficient to exclude an apical tumour, because overlying bone and soft tissue shadows make that region hard to read. Nerve conduction studies and a chest MRI may be added if hand weakness is present.
How long is too long to wait
Go to an emergency department the same day for a droop that appears suddenly, or one accompanied by double vision, a large pupil, severe headache, neck or facial pain, facial weakness, slurred speech or loss of balance.
A gradual droop with an unchanged pupil and no other symptoms can be reviewed at a routine appointment. A new droop with a smaller pupil on the same side should be assessed within days, and it is fair to ask one specific question: does the imaging plan include the top of my lungs?
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Common questions
How can I tell whether my pupil is smaller on the drooping side?
Look in a mirror in a dim room, then in bright light. In Horner syndrome the difference between the pupils is more obvious in the dark, and the affected pupil is slow to widen. Photographs in both lighting conditions are genuinely useful to bring to an appointment.
My eyelid has drooped slowly over years. Do I need imaging?
Usually not. A gradual droop with normal, equal pupils and no other symptoms is typically age-related aponeurotic ptosis and is assessed routinely by an eye clinician, often with no scan at all.
What is the apraclonidine test?
Eye drops that reverse the pupil difference in Horner syndrome because of nerve supersensitivity. It confirms the diagnosis but can be falsely negative in the first five to eight days after symptoms start, so a negative early test may need repeating.
Why would an eye problem need a chest scan?
The sympathetic nerve supply to the eye travels from the brain down into the upper chest before returning up the neck. A lesion anywhere along that loop produces the same eye sign, so imaging covers brain, neck and chest apex rather than the eye alone.
Which version of this is a true emergency?
A sudden droop, or one with double vision, a larger pupil, severe headache, or neck or facial pain. A painful Horner syndrome can indicate a torn carotid artery, which carries stroke risk and needs same-day imaging.
Questions to ask your doctor
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Written by: Cancer Explained Editorial TeamSources last checked: 2026-07-30Last updated: 2026-07-30Next planned review: 2028-07-29
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source verified — This page was created with AI assistance and checked against the sources listed on it. Source checking is not a medical review.
Human medical review: not completed. Cancer Explained is not clinician-reviewed, and that is a deliberate design choice rather than a gap we are waiting to close. We restate published federal guidance and cite it; the authority belongs to the source, not to us. That is why every page names where its claims come from — so you can verify us instead of trusting us. Use it to understand your situation and to ask better questions of the people treating you.
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