Horner syndrome: a slightly droopy eyelid and a small pupil on the same side

Horner syndrome is a slightly droopy upper eyelid and a smaller pupil on the same side of the face, sometimes with less sweating on that side. It is not a disease in itself but a sign that the sympathetic nerve supply to the eye has been interrupted somewhere between the brain and the eye. Finding where, and why, is the real task.

What is Horner syndrome, and what does it look like?

Horner syndrome is a combination of three findings on one side: ptosis, miosis and anhidrosis. Ptosis is the medical name for a droopy upper eyelid. Miosis is a pupil that is smaller than normal. Anhidrosis is reduced sweating, which is often not noticed by the person. AAO's EyeWiki describes the droop as mild, under 2 mm, which is a small difference and easy to put down to tiredness or to the way a face sits.

The pupil is usually the more telling sign. A small pupil on one side is most obvious in dim light, because the healthy pupil widens in the dark and the affected one lags behind, so the difference between the two grows. That difference between the two pupils is called anisocoria. In children, EyeWiki notes that a Horner syndrome present from birth may come with a difference in iris colour (heterochromia) and a face that flushes unevenly.

Many things droop an eyelid, and most of them are not neurological. My article on ptosis that is not ageing sets out the whole list, and the one on neurological causes covers third nerve palsy, in which the pupil is usually large, not small. That contrast is the quickest way to tell the two apart.

What is the sympathetic pathway, in plain words?

It is a three-part relay of nerves that starts in the brain and ends at the eye, and Horner syndrome means the relay has been broken at one of its links. The sympathetic system is the part of the nervous system that controls things we do not decide, such as how wide the pupil is, how much we sweat and how the small muscles in the face behave.

EyeWiki describes the relay in three links. The first-order neuron starts in the hypothalamus, deep in the brain, and runs down through the brainstem to the upper chest part of the spinal cord (levels C8 to T2). The second-order neuron leaves the cord, passes over the top of the lung, and travels up the neck in the cervical sympathetic chain to a relay station called the superior cervical ganglion. The third-order neuron leaves that ganglion and climbs along the internal carotid artery, the main artery to the brain, through the skull and past the cavernous sinus to the eye.

At the end of the line the nerve supplies the muscle that widens the pupil and a small lid-lifting muscle, Müller's muscle, which sits beneath the main lifting muscle, the levator. Lose the signal and the pupil narrows and the lid sits a little lower. That is why one small cluster of signs can come from lesions in the brain, the chest or the neck, and why the path of the nerve is the map for the search.

What causes Horner syndrome?

The cause depends on which link of the relay is damaged, and the doctor names it first, second or third order. EyeWiki gives examples at each level.

• First order (central): stroke, demyelination (loss of the insulating coat of nerves, as in multiple sclerosis) and tumours in the brainstem or spinal cord.
• Second order (pre-ganglionic): lesions at the top of the chest or the lower neck, including mediastinal tumours and Pancoast's tumour, a cancer at the top of the lung.
• Third order (post-ganglionic): carotid artery dissection, aneurysm and disease at the cavernous sinus.

The two to keep in mind are carotid dissection and a tumour at the top of the lung. A dissection is a tear in the wall of the carotid artery, and a lung-apex tumour presses on or invades the nerve as it passes over the lung. Neither is common, and neither can be ruled in or out by looking at the eye. That is why the examination looks for the level of the lesion and for any neurological or chest findings that go with it.

In babies and children, Horner syndrome can be present from birth, and a child's pattern is a paediatric matter. Little Eyes 101 explains the urgent signs in a child on its page about an eyelid suddenly drooping in a child, written by a paediatric ophthalmologist.

When is Horner syndrome an emergency?

A new Horner syndrome with neck pain, head or face pain, or other neurological symptoms is an emergency until proved otherwise. EyeWiki states that an acute, painful Horner syndrome should be considered a neurological emergency and evaluated for dissection of the internal carotid artery, because of the risk of stroke.

The reason is the third-order link described above. The sympathetic fibres run along the wall of the carotid artery, so a tear in that wall can damage them and cause Horner syndrome at the same time as pain in the neck or head. The danger is a clot forming where the artery is torn, which can travel to the brain. So the eyelid is the visible warning of a problem in a large artery. Weakness, numbness, trouble speaking, or loss of vision in addition to a new droop, are other reasons to go to an emergency department at once, and the NHS lists a suddenly drooping eyelid as a reason for an urgent appointment in its own right.

Timing matters in the other direction too. A Horner syndrome that has been there for years and is visible in old photographs is far less worrying than one that began recently, and sorting out which applies is itself part of the assessment. Do not wait to see if a new droopy lid with a smaller pupil, and pain in the neck or head, settles on its own.

How is Horner syndrome confirmed?

By looking at both pupils in the dark and the light, often followed by eye drop tests, and then by scans that follow the pathway. The examination comes first. The drops help to confirm that the sympathetic supply really is affected, and imaging looks for the cause.

EyeWiki describes two drops. Apraclonidine, an alpha-adrenergic agonist, produces a reversal of the anisocoria in Horner syndrome: the small pupil enlarges and the difference can swap sides, although a false negative can occur soon after the onset. Cocaine drops block the re-uptake of the nerve transmitter norepinephrine (noradrenaline), so a normal pupil widens and a Horner pupil does not, which makes the difference between the two bigger. Both are tests done by a doctor in a clinic, not something to try at home.

Imaging is then fitted to the suspected level, and EyeWiki's approach is: scans of the brain and spinal cord for a central cause, the neck and chest for a pre-ganglionic cause, and the cavernous sinus when there is also a problem with eye movements. For a painful acute case the neck arteries are imaged for dissection. What the doctor does not do is order the same test for everyone. The history and the examination decide where to look.

Does the droopy eyelid need surgery?

Usually not at first, and never before the cause is known. The droop in Horner syndrome is typically small, under 2 mm, so it often matters less to vision than to appearance. Treatment is directed at the cause, and the eyelid is a separate question for later.

Once the cause has been dealt with, if the lid still bothers, there are options, and they are not the same as for age-related ptosis. EyeWiki's page on blepharoptosis notes that apraclonidine and phenylephrine drops give a short, temporary lift, which is why these drops are sometimes discussed, though not as a cure. Where a drop gives a good response in mild to moderate ptosis, the same page describes a Müller muscle-conjunctival resection as one of the operations. Which operation suits a given lid is a decision for an oculoplastic surgeon after an examination, and the general guide is my article on ptosis surgery in adults.

One firm point: do not treat a new droop as a cosmetic problem. The cause is looked for first, and any plan for the lid comes after. More about the area is in the eyelid conditions section, and the hub article on why eyes look tired sets out how a lid that looks tired is separated from one that is not.