Some people have pupils that are slightly different sizes or shapes, so variations can be normal and are usually harmless. However, if there is a significant difference or one of the pupils has only recently changed there could be an underlying issue.

The pupil is the hole in the centre of the iris (the coloured part of the eye) and its primary function is to control the amount of light that enters the eye. Pupils do this by contracting and dilating, thus making them appear smaller and larger respectively. When your pupils are different sizes, it is called anisocoria and there are several different causes. Sometimes your pupils will return back to their normal size without need for treatment, but you should seek medical attention if the condition persists or has followed a head trauma or injury. Here are some of the reasons why pupils may be different sizes or shapes.

Migraines

Dilated pupils can be a symptom of certain migraines. This specific type of severe headache is thought to cause abnormal paralysis of the motor nerves of the eye. This not only causes persistent dilated pupils but headache pain will be focused around the eyes. Sufferers also often complain of blurred vision.

Holmes-Adie Syndrome

Holmes-Adie Syndrome (HAS) is a neurological disorder where one pupil is significantly larger than the other and constricts slowly in bright lights. It is thought to be caused by a viral infection that leads to inflammation and damage in the part of the brain that controls eye movement. The condition usually progresses slowly in one eye before developing in the other. People with HAS may also suffer profuse sweating and it is more common in young women.

Horner's Syndrome

Horner's Syndrome is another neurological disease that can arise due to various reasons, including tumours and medical treatments. The condition affects the sympathetic nervous system, which is responsible for the body's fight or flight mechanism. Horner's is characterised by abnormally small pupils and may be accompanied by a drooping eyelid and bloodshot eyes. Decreased sweating on the affected side of the body is also common.

Colobomas

A coloboma is a gap in one of the eyes structures, and when it affects the iris it can make the pupil appear misshapen. Colobomas are most common in the bottom half of the iris, giving the pupil a characteristic keyhole shape. Colobomas are congenital, meaning they are present from birth, and are due to the baby's eyes not developing properly during pregnancy. The impact on your eyesight will depend on where the coloboma is located, although those in the iris usually have a limited effect on vision.

Iritis

When the iris becomes inflamed it is called iritis. The condition is categorised as either traumatic - from an injury to the eye - or non-traumatic, which is normally due to an infection or disease. The most common symptoms include bloodshot eyes, pain in the eye, headaches, blurred vision and small or misshapen pupils. You are also likely to experience pain when a bright light is shined on the affected eye.

Other underlying causes

Without visiting an optometrist, it is difficult to diagnose why you may have different sized or odd-shaped pupils. However, there are a number of more serious reasons for suffering anisocoria.These include:

• Brain abscess
• Brain tumour
• Meningitis
• Seizures
• Aneurysms
• Bleeding into the brain

These are rare conditions but it is important to seek medical help if you have anisocoria and want to rule them out.