When a droopy eyelid is not simple: third nerve palsy and other neurological causes

When someone comes to see me about a lid that has dropped, I ask one question before any other. How fast? A lid that came down over ten years, first noticed in a passport photograph, is one conversation. A lid that was normal on Tuesday and covers the pupil on Thursday is a different one, and it sometimes ends with me sending the patient to an emergency department today rather than booking anything with me at all.

Most droopy eyelids are not neurological. A small number are, and they are the reason I take the history the way I do. This article is about that minority: what they look like, what makes them urgent, and what you can check on yourself tonight.

When is a droopy eyelid an emergency?

When it arrived suddenly and did not arrive alone. A lid that drops over hours or days, with a pupil that has changed size, double vision, an eye that will not move properly, or a new headache, is an emergency until proven otherwise. A third nerve palsy that involves the pupil is described as one of the true emergencies in neuro-ophthalmology, because it may be the first sign of an aneurysm that could rupture, and an acute headache alongside it raises that risk sharply. The response is emergency imaging of the blood vessels of the brain, the same day.

The wider list of eye symptoms warranting same-day attention is worth knowing: sudden drooping of an eyelid, being unable to open the eye, eye pain, a very red eye, eye symptoms with headache or nausea, new light sensitivity, or any change in vision.

What is a third nerve palsy, and why does the eye sit down and out?

The third cranial nerve, the oculomotor nerve, supplies most of the muscles that move the eye, the levator palpebrae superioris that lifts the upper lid, and the sphincter pupillae, the ring of muscle that makes the pupil small. When that one nerve fails, all three jobs fail together, which is why a third nerve palsy produces ptosis, restricted eye movements and variable pupil involvement rather than an isolated droop.

The eye tends to sit down and outwards because the muscles that still work are unopposed. The droop comes from paralysis of the levator, and the double vision from the deviation of the affected eye. Causes include microvascular ischaemia, compressive lesions such as aneurysms and tumours, trauma, haemorrhage, congenital anomalies and cases where no cause is found. In adults microvascular ischaemia is the commonest, linked to small vessel disease, high blood pressure and the usual vascular risk factors. Aneurysm is the most concerning cause of an acute palsy, posterior communicating artery aneurysms in particular.

Two figures shape how carefully I examine. In adults a third nerve palsy is complete in 33 per cent of cases, and in 36 per cent there is no other neurological sign at all. In more than a third of patients the eye is the entire examination. If your main complaint is seeing two of everything rather than a lid in the way, my piece on double vision that is not a squint takes that symptom apart from the other direction.

Why does everyone keep shining a light into my eye?

Because the pupil is the discriminator. A fixed, dilated pupil on the affected side, from paralysis of the sphincter pupillae, points towards compression of the nerve from outside, which is the aneurysm scenario. Ischaemic processes typically spare pupillary function. Roughly 63 per cent of aneurysmal compressions involve the pupil, against about 17 per cent of microvascular cases, and a presentation that is not pupil sparing needs urgent CT with CT angiography to exclude a posterior communicating artery aneurysm.

What I will not do is tell you a normal pupil clears you. The current recommendation is contrast enhanced magnetic resonance imaging of the brain and orbits for every patient with an acute isolated third nerve palsy, regardless of age and regardless of vascular risk factors, with imaging repeated for anyone who has no traditional vascular risk factors or who has not improved within 6 to 12 weeks. A normal pupil is reassuring. It is not a discharge letter.

My lid is fine in the morning and gone by evening. What is that?

That pattern has a name, fatigable ptosis, and it moves myasthenia gravis to the top of the list. Myasthenia presents with variable, fatigable ptosis, often with double vision, and the lid position improves when ice is applied to the closed eye, a simple clinic test that supports a neuromuscular cause. Droopy eyelids and double vision sit alongside difficulty making facial expressions, problems chewing and swallowing, slurred speech, and weak arms, legs or neck, and symptoms tend to worsen with tiredness, often towards the end of the day.

One clean rule separates it from the emergency above: myasthenia may mimic a third cranial nerve palsy, but the pupil will never be involved. It is also why I ask about swallowing and breathing rather than only about the lid. If myasthenic symptoms suddenly worsen, for example severe difficulty breathing or swallowing, urgent hospital treatment may be needed. Diagnosis involves a blood test, an assessment of how well the nerves are working, and scans such as CT or MRI, and treatment involves avoiding triggers, medicine to improve muscle weakness, and in some cases surgery to remove the thymus gland. Rarer still are antibody mediated syndromes that disturb the nerves of eye movement, a group my colleagues and I reported as a case series, listed under my research.

What does a small droop with a small pupil mean?

That combination points to Horner syndrome, a different mechanism altogether: partial ptosis, a small pupil on the same side, and reduced sweating on that side of the face. The droop is mild because the muscle that has lost its nerve supply is the superior tarsal muscle, a small sympathetic helper behind the levator, not the levator itself. A small droop with a small pupil and a large droop with a large pupil are two different diagnoses, and the pupil tells you which you have.

Horner syndrome is here because of what can cause it. Dissection of the carotid artery in the neck is the critical concern, especially with one-sided headache and facial or neck pain, and suspected dissection needs urgent assessment and treatment. New onset matters: a Horner syndrome that appeared recently is far more worrying than one visible in photographs from years ago. Weight loss, coughing up blood or enlarged lymph nodes alongside it point towards malignancy instead, and are equally a reason to be seen quickly.

Most droopy eyelids are not neurological, though?

Correct, and I want that said plainly so nobody books a brain scan for a lid slipping since 2015. The most common form in adults is aponeurotic ptosis, also called involutional ptosis, where the levator muscle still works but its tendon has stretched or pulled away from the tarsal plate it is meant to lift. Its signature is preserved levator movement, a skin crease sitting higher than it should, and a thinned upper lid with redundant skin above it. Contact lens wear, both hard and soft, chronic eye rubbing and age are named risk factors.

There is also a group where the lid margin has not moved and only looks as though it has: heavy skin resting on the lashes, a descended brow, or an eye sitting lower in the socket. That is why the assessment covers brow position, eye movements and globe position, not the lid alone. The full breakdown sits in ptosis that is not simply ageing, and if you are wondering why your eyes look heavier than they used to, what makes eyes look tired is the better starting point. The rest sit under eyelid conditions. The line I hold is this: acute drooping with a headache, a pupil abnormality or an eye movement problem earns urgent neurovascular imaging. A gradual droop in an otherwise well eye does not.

Once the cause is known, can the lid be lifted?

Often, yes, but the order is fixed: cause first, lid last. Most ischaemic third nerve palsies improve within a month, with complete recovery by three months, so an operation in week two would aim at a moving target. Conservative management suits pupil sparing cases in patients over 50 with vascular risk factors, meaning observation with a plan, not instead of one. Where an aneurysm is found it is treated as a surgical emergency, and treating it may result in complete or partial recovery of the nerve in over 80 per cent of patients.

One further reason to wait is aberrant regeneration. After a compressive or traumatic injury, though not after vascular causes such as diabetes, healing nerve fibres can reconnect to the wrong muscle, so the lid or the pupil moves when the eye moves. It changes what an operation can sensibly aim for, which is another argument for letting things settle before anyone marks a skin crease. Where myasthenia is the cause, treatment is directed at the disease, not the eyelid. When the time does come to lift a lid surgically, the measurements and the choices are set out in my piece on ptosis surgery in adults.

If you take one thing from this page, take the pairing rule. A slow droop on its own, in a comfortable eye, is an eyelid problem and can wait for a clinic appointment. A droop with a changed pupil, a new headache, double vision or an eye that will not move is a today problem. Nobody has been embarrassed in front of me for going to hospital and turning out to be fine.