Causes of Anisocoria

Anisocoria is recognised when pupils of the eye are not equivalent in size. Pupils control the amount of light that gains entry into the eye. In bright light, it narrows. In dimmer light, it widens to allow more in. The muscles in the iris, known as the iris dilator, and constrictor experience problems resulting in anisocoria. Effects of anisocoria on the eyes range from just a small difference in the pupils to a noticeable difference. Several factors determine whether it is a simple difference in size, or if there is an underlying neurological problem. According to, 20 per cent of people have simple anisocoria and it isn't an issue. The difference in size and whether it is consistent helps determine if there is an issue.

Horner's Syndrome

Horner's syndrome is caused by damage to the oculosympathetic nerve. This nerve keeps the eyelid open. The pupil is unusually small. It does not respond immediately in areas that suddenly darken. This can be a signal of damage to the sympathetic nerves that run down to the carotid artery and back to the brain. Sympathetic nerves control bodily functions such as sweating, heart rate and blood pressure. The causes of Horner's syndrome can be as serious as a stroke or tumour, or there may be no known cause. An MRI is an effective way to determine the cause of Horner's syndrome.

Third Cranial Nerve Damage

A pupil that is abnormally large may mean there is damage to the oculomotor nerve, which controls movement of the eye. The oculomotor nerve also controls the muscle responsible for opening the eyelid, and the iris constrictor. If there is a problem with this nerve, also known as the third cranial nerve, the patient may experience double vision, an expanded pupil or an eyelid that droops. These signs are also a key indicator of an aneurysm. This requires immediate attention. The patient must receive testing involving neuro-imaging, such as magnetic resonance imaging (MRI), to detect the aneurysm. Aneurysms that rupture can cause internal bleeding and death. Having third cranial nerve damage can be caused by garden chemicals, certain medicines, caffeine, alcohol, a virus or diabetes.

Adie's Pupil

Another type of anisocoria is known as Adie's pupil. The causes are unknown. It has similar symptoms to third cranial nerve damage. It is a degeneration of the nerve fibres controlling the eye muscles due to a viral infection of unknown origin. These muscles aid in controlling eye movement. The symptoms are usually seen in young women. It can occur in either one pupil or both. Patients with Adie's pupil have difficulty focusing on closely held objects. The pupils usually return to normal by themselves without treatment.

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About the Author

Since 2009, Crystal Widger has contributed to a variety of online publications. Many of these topics covered health, fitness, woodworking, gardening, sociology, psychology, economics, and pets. Widger holds a Bachelor of Arts in psychology, with plans to pursue her MBA in the near future.