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The Pupil of Our Eye and Problems Related to Its Size

Written by
Dr. Manjunath Natarajan
and medically reviewed by iCliniq medical review team.

Published on Aug 21, 2018 and last reviewed on Sep 11, 2018   -  3 min read



The pupil is a circular opening in the iris of our eye with variable size. The main function of the pupil is to constrict in the presence of light and hence regulate the amount of light entering the eye. This mechanism of light regulation is quite important in determining the clarity of vision. Excessive light entering into the eye can bleach the photopigments and cause blinding. This article gives in-depth knowledge about the size of the pupil, its normal behavior and the main symptoms in cases of pupillary disturbances and simple measures to cope with it.

The Pupil of Our Eye and Problems Related to Its Size

The pupil is an opening in the center of a muscular diaphragm, the iris. This unique property of iris musculature to contract and hence shrink their size gives the pupil the characteristic property to widen or constrict in size. This plays a very important role in improving the quality of the image we see by reducing the scattering of excess light and by preferentially letting light oriented in the same angle to the orientation of the light receptors in our eye called rods and cones.

The normal size of the pupil is 3 to 4 millimeters in both eyes. A variation of 2 mm between the 2 eyes is considered normal. That means, even if one eye of a patient has a bigger or smaller pupil by 2 mm, it does not necessarily mean that s/he has some ocular problem. The same applies to both eyes.

The process of control of the size of the pupil is done by nervous control. The arm of the nervous system responsible for this is the autonomous nervous system. This essentially means that the size of our pupils is not in our hands but under autonomic nervous feedback control. The center of this reflex is placed in the Edinger-Westphal nucleus in the midbrain and is relayed through the third cranial nerve and its branches. It is thus implied that any injury or lesions involving the cranial cavity, midbrain or third nerve will produce pupillary abnormalities.

The procedure of testing the pupil is fairly simple and is done by using a torchlight, preferably a pen torch. The light causes a constriction of both pupils in a normal patient when shone on one eye. This is because of the shared nervous supply, also called in medical terms as crossing over of internuncial fibers. The attribute of a brisk and comprehensive contract encompassing the whole of the pupil is considered normal.

External Factors Apart from Light Which Play a Role in Constricting the Pupil

These include -

External Factors Which Play a Role in the Dilatation of the Pupil

These include -

Problems Seen in Patients with Dilated Pupils

Although people having constricted pupils have almost no problems with daily activities, the same cannot be said of patients with dilated pupils. As mentioned earlier, pupil plays an important role in vision by regulating the amount of light entering the eye, thus preventing scattering of light and a drop in vision. Thus dilated pupils can cause problems like glare and an aversion to bright light (called photophobia).

Treatment Measures for Dilated Pupils

  1. The main treatment mode is the treatment of the underlying causative disease. A visit to the ophthalmologist will facilitate this.
  2. Good quality sunglasses, preferably with UV/ polaroid filters.
  3. Brimonidine - this is a prescription medication originally intended for lowering intraocular pressure inside the eye. It has been found to reduce pupil size in several studies.
  4. Using indoor car lighting to reduce the amount of glare while driving at night.

For more information consult an eye care ophthalmologist online -->


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Last reviewed at:
11 Sep 2018  -  3 min read


Dr. Manjunath Natarajan

Dr. Manjunath Natarajan

M.B.B.S, M.S OPHTHALMOLOGY, Fellow of International Council of Ophthalmology (U.K), M.R.C.S Edinburgh

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