Anisocoria (Unequal pupils)

The pupil is the hole in the center of the iris, that allows light to enter the retina. The iris is a thin strip of smooth muscle which regulates the amount of light entering the eye by controlling the size of the pupil.
pupil
The iris actually consists of two smooth muscles:  There is a circular group called the sphincter pupillae, which is innervated by the parasympathetic fibers that travel with the oculomotor nerve, and when activated constricts the pupil. There is also a radial muscle group called the dilator pupillae, innervated by sympathetic nerves from the superior cervical ganglion, which when activated causes the pupil to dilate.

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Anisocoria (unequal pupils) can be physiologic, or can result from a variety of disorders of the nervous system.

anisocoria

Anisocoria – the pupils are unequal, is the the pupil too large, or is the left pupil too small?

The first step in evaluating anisocoria is to determine which is the abnormal side – this can be accomplished by comparing the pupil sizes in response to bright light and dull ambient light:

horners, dark

If the smaller of the pupils is abnormal, a miosis, then the anisocoria will be more apparent (the affected pupil will be smaller) in dull ambient light conditions (a dark room), and less apparent (the pupils will be more equal) in bright light.

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mydriosis

Alternatively, if the large pupil is the abnormal side, mydriasis, it will fail to contract in response to intense light.

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Miosis (small pupil)

A unilateral miosis can be physiologic or can indicates a lesion affecting the sympathetic nerve fibers to the eye known as Horner’s syndrome.

Physiologic anisocoria is mild (usually less than 0.5 mm difference in size between the pupils, and the amplitude of the difference does not vary greatly under dim or bright light conditions.

Horner’s syndrome causes unilateral miosis, and the affected abnormal pupil either does not dilate or has a delayed dilation in the dark. Furthermore, there is usually an associated lid ptosis and facial hypohidrosis. Horner’s syndrome is important to diagnosis, because it can indicate a lesion in the brain stem (stroke), neck (carotid artery dissection) or lung apex (Pancoast’s tumor).

Horner pancoast

Right Horner’s syndrome (ptosis and miosis), from R apical lung tumor (Pancoast tumor).

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Mydriasis (Dilated Pupil)

This is most commonly caused by an anticholinergic drug, such as atropine or hyoscyamine, hyoscyamine, either from a medication, eye drop or misplaced nebulizer:

nebulizer, anisocoria

Unilateral mydriasis, caused by direct anticholinergic drug effect on iris because of a badly fitting face mask.

Damage or compression of the oculomotor nerve buy an aneurysm or brain herniation can also cause mydriasis, typically associated with ptosis and ophthalmoparesis:

R ptosis, occulomotor palsy (eye is deviated down and outwards), with a dilated pupil, caused by nerve compression from aneurysm (red arrow)

R ptosis, occulomotor palsy (eye is deviated down and outwards), with a dilated pupil, caused by nerve compression from aneurysm (red arrow)

Mydriasis from Adie’s Syndrome

Adie’s syndrome presents with abrupt onset mydriasis often associated with loss of deep tendon reflexes, thought to be the result of a viral infection that causes damage to neurons in the ciliary ganglion (the area of the brain that provides parasympathetic control of eye constriction). Clinical exam may reveal sectoral paresis of the iris sphincter or vermiform iris movements:

tonic pupil