Looking into a mirror, you will easily be able to see your pupils. They are the round dark centers of the eye or the gap within the iris, the coloured part of the eye. Health care professionals assessing a patient’s pupils can provide insight into the health of not only the eye but also the rest of the body.
The size and shape of the pupil is controlled by nerve impulses from the brain. There are actually two different classes of nerves. The first class is responsible for making the pupil larger (dilated), while the other makes the pupil smaller (constricted).The dilating group of nerve fibres originates from the lower portion of the brain. They travel down to the thoracic region of the spinal cord then back up to the head along the internal carotid artery and eventually through the back of the eye. The constricting group of nerve fibres originates from the lower portion of the brain, but travels into the head and enters through the back of the eye. The balance between these two types of nerve fibres determines the size of your pupil.
The amount of light entering the eye can change the size of the pupil. This is called the light reflex. Increasing the amount of light will cause the pupil to constrict. In dim light conditions, the pupil will dilate. When focusing on an object that is close to your face, the pupils will constrict. This is called the near or accommodative reflex.
When assessing a patient’s pupils, we look at the pupil size, shape, reaction to light, and any asymmetry between the pupils.
The normal size of pupils ranges from 3mm to 8mm in diameter. Normally pupils are round however they can take an oval or distorted shape due to previous eye inflammation, trauma, intraocular eye surgery or congenital defects.
Abnormalities in pupils to the reaction of light can be caused by optic nerve disease or significant arterial occlusions in the retina.
Usually the pupils have the same diameter between left and right. On occasion there can be a difference in the size between the eyes. This is referred to anisocoria. For some people this can be normal. In other instances this can be caused by certain eye drops, trauma, intraocular surgery, tumours or blood vessel damage near the pathways of the nerve fibres responsible for controlling the size of the pupil.
The next time you look in the mirror, pay particular attention to both your pupils. If anything should appear out of the ordinary regarding your pupil size or shape, do not hesitate to consult the care of your local optometrist or ophthalmologist.