Anisocoria and Horner's Syndrome

Medically reviewed by Alastair Lockwood on 08 March 2021

Anisocoria is a condition where one pupil is larger than the other. You can be born with anisocoria or develop it later in life.

Several factors can cause anisocoria, including Horner's syndrome. In this case, an underlying medical condition may be diagnosed by the doctor. This article looks at Horner's syndrome and its relationship to anisocoria.

What is Horner's syndrome?

Horner's syndrome, also known as Horner-Bernard syndrome or oculosympathetic palsy is a rare disorder resulting from nerve damage. The symptoms of Horner's syndrome include a drooping upper eyelid, unequal pupil sizes and a constricted pupil on the affected side. People with congenital Horner's syndrome often have a lighter coloured iris of the eye on the affected side compared to the iris of the eye on the unaffected side.

Horner's syndrome is the result of a disrupted nerve pathway from the brain to the face and the eye on one side of the body. The pathway is triggered as part of the “fright / fight /flight” reflex. The underlying causes for this vary and, in some cases, an underlying cause cannot be found.

Congenital Horner's syndrome is when the condition is passed down through families and is present from birth. Acquired Horner's syndrome appears after the newborn period. Both are the result of damage to the sympathetic nervous system.

Can Horner’s syndrome affect both eyes?

Horner's syndrome typically affects one side of the face, affecting only one eye. It does not usually cause problems with a person's vision.

How is Horner’s syndrome related to anisocoria?

The sympathetic nerves are responsible for dilating the pupil and raising the eyelid. Horner's syndrome is caused when these nerves are damaged.

Damaged sympathetic nerves cause the pupil in the eye on the affected side to become smaller compared to the eye on the unaffected side. The affected eye also has problems dilating. When the pupils in both eyes are different sizes at the same time, this is known as anisocoria.

Horner’s syndrome symptoms and causes

Signs and symptoms associated with Horner's syndrome usually only affect one side of the face.

What are the three classic signs of Horner's syndrome?

The three classic signs of Horner's syndrome are:

• A constricted pupil (miosis)
• Drooping of the upper eyelid (ptosis)
• Reduced sweating or no sweating on the whole side of the face or a patch on the affected side of the face (anhidrosis)

Other symptoms of Horner's syndrome include:

• A sunken appearance to the eye (enophthalmos)
• A difference in pupil size (anisocoria)
• Delayed pupil dilation or little pupil dilation in dim light
• A slight elevation of the lower lid

Children with Horner’s syndrome might have additional signs and symptoms, including:

• A lighter iris colour in the affected eye
• Lack of flushing (on the affected side) as a result of heat, emotional reactions or physical exertion

Causes of Horner’s syndrome

Horner's syndrome is a result of nerve damage from injury, compression or disease. Causes include:

• Neck or shoulder injury during birth
• A stroke in the brainstem
• Trauma to the neck, chest or upper spinal cord (or surgery involving them)
• Migraines
• Tumours
• Lung cancer
• Damage to the aorta
• Injury

The cause of Horner’s syndrome is not always identifiable. These cases are known as idiopathic Horner syndrome.

Does Horner's syndrome cause pain?

Horner's syndrome can have associated sudden neck pain and headaches, depending on the cause.

What nerve is damaged in Horner's syndrome?

The nerves that are damaged in Horner's syndrome are part of the sympathetic nervous system. These in part run independently and in part are carried by other nerves, eg: occulomotor nerve. The sympathetic nervous system regulates the functions that enable you to respond to changes in your environment. It is sometimes referred to as your body's 'fight or flight' response.

The oculosympathetic pathway is responsible for sending messages from the brain to the eye. The signals in this pathway send messages to your body so that it can respond to stress.

Does Horner’s syndrome go away?

Horner's syndrome will often go away if the underlying cause is reversible. If the nerves have been physically interrupted, the effects are frequently permanent.

What can be done for Horner's syndrome?

The management of Horner's syndrome entails understanding the underlying cause and excluding the serious ones. Treatment then depends on this underlying cause and might include:

• Surgery, chemotherapy or radiation (for cancer)
• Antibiotics for infectious diseases
• Anti-inflammatory drugs
• Clot-dissolving agents (for certain strokes)
• Minimally invasive treatments (for carotid artery dissection)

Diagnosis of Horner's syndrome

Diagnosing Horner's syndrome can be challenging as there are many other medical conditions which have the same symptoms, such as a dilated pupil or a drooping eyelid. Other conditions with these symptoms include optic neuritis and third nerve palsy.

Those with anisocoria should consider getting a diagnosis for Horner's syndrome. A dilation lag of the smaller pupil will help determine if one has Horner's syndrome. It also helps to distinguish the symptoms from physiologic anisocoria.

Sometimes the underlying cause of Horner's syndrome is harmless; however, other times, it can be life-threatening, for example, lung cancer, carotid artery dissection or neuroblastoma.

A detailed account of medical history and a physical exam are essential for the diagnosis of Horner's syndrome. A doctor will ask questions about the symptoms to try and decipher the underlying cause. They will also ask questions to see if Horner’s syndrome is the result of medication or injury from a medical procedure.

Patients with Horner's syndrome will also undergo an eye exam. During the exam, the doctor will look at their pupillary dilator muscle by seeing how their pupils respond to light and dark. The eye exam will also check eye movement.

Pharmacologic tests

Pharmacologic tests are sometimes used to help diagnose where the interruption of the nerve supply is in Horner's syndrome. This includes using apraclonidine eye drops or cocaine to see how your eye responds.

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