What causes it?
Anisocoria, or a difference in the diameter of the pupils in dim illumination, may be physiologic if the difference is less than 1 mm and both pupils react briskly and equally to light. Otherwise it may be pathologic.
There are four possible causes:
- Parasympathetic innervation failure causes a relatively dilated pupil that reacts sluggishly to direct light.
The major concern, as with ptosis, is a third cranial nerve palsy, especially one caused by an aneurysm. However, anisocoria is never caused by a third nerve palsy unless there are other signs of a third cranial nerve palsyptosis, reduced ocular movements, or ocular misalignment.
A common cause of isolated anisocoria is a viral infection of the ciliary ganglion (Adie's syndrome), an orbital structure that receives the parasympathetic component of the third cranial nerve.
- causes anisocoria in which the affected pupil is smaller, but both pupils react briskly to light. Ptosis is usually present but is always mild (2 mm or less).
For more information on Horner's syndrome see Ptosis in this section.
- Chemical blockade. If parasympatholytic (atropine-like) chemicals come in contact with the conjunctiva by accidental (or deliberate!) instillation, they will deactivate the iris sphincter muscle and cause (often very wide) pupil dilation.
This occurs most commonly among hospital personnel and those exposed to atropine-containing plants.
- Iris sphincter damage. Inflammation and trauma to the iris sphincter are other causes of anisocoria. The pupil is usually irregular in shape and magnified examination shows evidence of muscle damage.
What to do?
If you suspect third cranial nerve palsy, refer immediately for brain imaging. Other suspected diagnoses can wait, but anisocoria must always be evaluated unless you are convinced it is physiologic.