The Pupil Entrance to the Eye

The pupil is the hole that allows images to enter the eye and determines the level of brightness inside the eye. The pupil can sharpen vision by constricting and blocking out peripheral image rays that cause a larger blur circle. This can be demonstrated by looking through a pinhole (1 to 1.5mm hole).

During the day or in a bright environment the pupil constricts (miosis). At night, the pupil dilates to let more light into the eye. A larger pupil is the cause for most patients night driving problems. A larger pupil allows more peripheral image rays into the eye. A larger surface area of the cornea is required to focus images which exaggerates nearsightedness or astigmatism present. If cataracts are present, a larger pupil leads to more image ray scattering and bluriness.

Pupil size can be affected by factors other than light. When reading up close the pupil constricts. The pupil is closely tied to the focusing system. This link is located anatomically at the ciliary ganglion. The ciliary ganglion controls innervation of the ciliary muscle (focusing system) and the iris sphincter (pupil constriction).

Parasympathetic and sympathetic nervous systems alter the size of the pupil as well. The parasympathetic system (rest and digest) causes the pupil to constrict (miosis) and the sympathetic system (fight or flight) stimulates pupil dilation (mydriasis). Parasympathetic and sympathetic input are the reason pharmaceutical and recreational drugs influence pupil size.

When pupils are unequal in size (anisocoria) evaluation is needed. Unequal pupil size could a brain aneurysm (third nerve palsy), lung cancer (Horner's syndrome), syphilis (Argyll-Robertson), inflammation inside the eye (iritis), trauma to the eye, neurological damage, pharmacological agent in the eye or be something as simple as fatigue (physiological anisocoria). There are several pupil reflexes the Eye Doctor will check for to narrow down the cause of the problem.

A direct response to light is when light is shined into the right eye and the right pupil constricts. The consensual response is when light is shined into the right eye and the left pupil constricts. Both eyes contract simultaneously without delay. The signal follows a complex route to allow this to occur.

When light is shined into one eye the retinal cells send a signal to the optic nerve (cranial nerve 2 right eye), which sends part of the signal to the pretectal nucleus (optic chiasm), onward to the Edinger-Westphal nucleus, which sends the signal to the ciliary ganglion (cranial nerve 3 left eye) and finally the signal reaches the iris sphincter contracting the pupil of the opposite eye.

The swinging flashlight test is done to check the afferent pupillary pathway (described above). During the swinging flashlight test the light is moved as quickly as possible between the eyes. Asymmetric pupil responses usually indicate an optic nerve damage in the eye with the weaker response. The most common cause of optic nerve damage is glaucoma.

The most common cause of unequal pupils is physiological (benign, unknown). It is common with fatigue to have unequal nervous sytem (parasympathetic, sympathetic) stimulation to the Edinger-Westphal nucleus resulting in one eye constricting or dilating more than the other. In physiological anisocoria pupil responses to light and near should help equilibriate the pupils. Migraines can also cause benign, abnormal pupil responses. If you're unsure it's safest to have your pupils examined by an Eye Doctor or physician.

Summary
The pupil significantly impacts the brightness and clarity of images.
Pupils constrict in response to light, near and parasympathetic stimulation.
Both pupils should constrict equally and simultaneously.
Unequal pupils are usually due to a benign cause.
Unequal pupils should be examined by an Eye Doctor or Physician as they could indicate a serious health problem.