Horner syndrome: symptoms, causes, tests and what happens next

The sympathetic pathway to the eye is long. Patient.info describes first-order fibres starting in the hypothalamus and running down the brainstem to the spinal cord, second-order fibres leaving the cord at the T1 level and rising over the apex of the lung to the neck, and third-order fibres passing alongside the internal carotid artery into the eye. Damage anywhere along this route can cause Horner syndrome.

The reference lists causes by where the damage occurs, and notes that preganglionic lesions are less common but more likely to be malignant. A rare congenital form also exists, and in some people the cause is not found.

Patient.info lists symptoms of inability to open the eye fully on the affected side and loss of sweating on that side, with facial flushing in preganglionic lesions and orbital pain or headache in postganglionic ones. There may be other symptoms depending on the cause, such as head, neck or facial pain on the affected side with a carotid artery dissection.

Signs include a constricted pupil that is more apparent in a darkened room, in which the affected pupil lags behind the other when the light is removed, dry skin on that side of the face, and a partly drooping eyelid, sometimes with mild retraction of the opposite lid. In congenital Horner syndrome the iris on the affected side may stay blue while the other turns brown.

Moorfields Eye Hospital notes that droopy eyelids can also come from nerve or muscle conditions such as myasthenia gravis or third nerve palsy, which is why a doctor looks at the pupil as well as the lid.

Patient.info says investigations are guided by the suspected cause. A chest X-ray may show an apical lung tumour, CT or MRI may identify a stroke, and CT angiography or carotid ultrasound may demonstrate carotid artery dissection.

Pharmacological testing can help confirm the diagnosis and show whether the damage is before or after the neck ganglion. Apraclonidine drops have little effect on a normal pupil but dilate a pupil affected by Horner syndrome, and are an alternative to cocaine drops, which were used universally in the past. Hydroxyamphetamine drops can then help localise the lesion.

A doctor will also consider look-alikes such as other causes of ptosis and pupil abnormalities, and the use of eye drops that constrict the pupil in one eye.

There is no treatment for Horner syndrome itself. Patient.info states that it is a physical sign, so management involves diagnosing the underlying condition and treating it as appropriate.

Moorfields describes ways of treating a droopy lid generally, such as surgery on the lid if it affects vision. Ptosis from Horner syndrome is usually partial, and any lid surgery is considered separately after the cause has been dealt with.

A new drooping eyelid with a small pupil, especially with pain in the neck, face, shoulder or arm, a sudden severe headache, or transient loss of vision, needs urgent assessment because it can signal a carotid artery dissection or stroke. Patient.info notes that pain and transient visual loss may be due to carotid dissection.

The NHS says to call 999 if you think you are having a stroke, which includes a drooping face, arm weakness or numbness, slurred or confused speech, blurred vision or loss of sight, dizziness and a severe headache. Do not drive yourself to A&E.

Patient.info says congenital Horner syndrome can appear after birth trauma or as a rare inherited form. An iris that is lighter on one side (heterochromia) may be seen because iris pigmentation is under sympathetic control and is usually complete by about age 2.

In children, the reference says acquired Horner syndrome with no known cause such as birth trauma requires thorough investigation, because there is frequently a serious underlying cause, including neuroblastoma, trauma, rhabdomyosarcoma and brainstem vascular malformation. A child with a new droopy lid and unequal pupils should be seen urgently.

On the NHS, a GP, optometrist or emergency department assesses new lid droop and unequal pupils. Depending on pain and other signs, imaging and referral to ophthalmology, neurology or a stroke service follow, often the same day if there is neck pain or neurological symptoms.

Private ophthalmology and neurology appointments are available, as is private MRI and angiography. The urgency does not change by route: new Horner syndrome with pain or neurological symptoms should go to emergency care rather than waiting for a routine appointment.