Some children can be born with a third nerve palsy but this type of congenital palsy is rare. The vast majority of third nerve palsies occur in later life.

Causes of a third nerve palsy include:

• Poor blood supply to the third nerve caused by a combination of factors such as high blood pressure, diabetes, high cholesterol and smoking. This is known as a microvascular palsy.
• Direct pressure on the third nerve caused by swelling of neighbouring blood vessels known as an aneurysm, or tumours can damage the third nerve.
• Head injuries can cause a third nerve palsy.
• Occasionally inflammation in the region of the nerve can cause a third nerve palsy which comes and goes. This is known as ophthalmoplegic migraine.

What are the features of a third palsy?

• Sudden onset of a droopy eyelid and an inability to open the eye.
• If the eyelid is not completely closed the patient will have horizontal and vertical double vision.
• Very limited movement of the eye upwards, downwards or inwards, with the eye being deviated downwards and outwards.
• The pupil on the affected side may be enlarged. This is usually a sign that the third nerve palsy has been caused by direct pressure on the nerve, rather than poor blood supply to the nerve.

A third nerve palsy should be considered to be a medical emergency as it can signal a dangerous swelling of a blood vessel in the brain, called the posterior communicating artery. All patients with a third nerve palsy should be assessed immediately by a doctor, ideally an ophthalmologist, a neurologist or an Accident and Emergency Physcian. If the patient’s pupil is enlarged and does not constrict when a light is shone in the eye urgent imaging studies to look for a swelling of the artery or other cause of a third nerve palsy will be arranged as soon as possible.
Once a serious cause for the third nerve palsy has been ruled out the patient can be seen in the eye clinic where specific tests will be carried out to measure the strabismus and assess the range of eye movements. Blood tests will also be carried out to investigate other causes of a third nerve palsy.

Most (80%) microvascular third palsies will resolve within 3-6 months. However spontaneous recovery is less likely to occur if the third nerve palsy has been caused by an aneurysm.

Unless there has been some recovery of the eye movements prisms are unlikely to restore an area of single vision.

If the patient still has a sizable strabismus and troublesome double vision after 6-12 months, surgery can be considered.

The aims of surgery are:

• To reduce the angle of divergent and vertical strabismus
• To restore an area of single vision

Weakening the lateral rectus muscle and strengthening the medial rectus muscle are the mainstays of surgery.

Sometimes the superior oblique muscle can be moved to a new position to help centralise the eye.

Following surgery the eye movements are usually still very limited, but often it is possible to restore a small area of single vision.

Republished, with permission, from www.squintclinic.com.