How do you determine which pupil is abnormal in anisocoria?

Compare the pupil sizes in bright and dim lighting. If the size difference increases in dim light (the smaller pupil fails to enlarge), the smaller pupil is abnormal -- it cannot dilate, suggesting Horner syndrome or simple anisocoria. If the difference increases in bright light (the larger pupil fails to constrict), the larger pupil is abnormal -- it cannot constrict, suggesting CN III palsy or Adie tonic pupil.

What is the classic triad of Horner syndrome?

The classic triad of Horner syndrome is: (1) miosis (smaller pupil due to loss of sympathetic dilation), (2) ptosis (mild upper eyelid droop from Muller's muscle loss of sympathetic innervation), and (3) anhidrosis (reduced sweating on the affected side of the face with first- or second-order lesions). The ptosis in Horner is usually mild (1-2 mm), unlike the severe ptosis of CN III palsy.

Why is a pupil-involving CN III palsy an emergency?

The parasympathetic fibers that constrict the pupil travel on the outside of CN III and are compressed first by expanding aneurysms. A new CN III palsy with a dilated, non-reactive pupil is highly suspicious for a posterior communicating artery aneurysm. If untreated, the aneurysm may rupture, causing a subarachnoid hemorrhage with high mortality. Urgent neuroimaging and neurosurgical evaluation are required.

What distinguishes an Adie tonic pupil from a CN III palsy?

Both can present with a unilateral dilated pupil, but they differ critically: Adie tonic pupil has a very slow (tonic) but present light response, no ptosis, and no extraocular muscle palsy. CN III palsy (if complete) has an absent light response, severe ptosis, and an eye that is displaced down and out due to extraocular muscle paresis. Adie is benign; CN III palsy may be an emergency.

What is a pharmacological pupil?

A pharmacological pupil is a dilated or constricted pupil caused by a drug or chemical (intentional or accidental) rather than by neurological disease. Common causes of dilation include cycloplegic eyedrops (atropine, cyclopentolate), antihistamines, and scopolamine patches. A pharmacologically dilated pupil will not constrict with pilocarpine 1% drops, which helps distinguish it from Adie tonic pupil (which does constrict) or CN III palsy (which partially constricts).

Anisocoria and Pupil Abnormalities for the CPOA Exam

What Is Anisocoria?

Anisocoria means unequal pupil sizes -- one pupil is larger than the other. A small amount of anisocoria (up to 0.4 mm) is physiologically normal and found in about 20% of the general population. Larger differences, or anisocoria that changes with lighting conditions, are more likely to be pathological.

Physiological (Simple) Anisocoria

Physiological anisocoria is the most common cause of unequal pupils. Key features:

Usually less than 1 mm difference between the two pupils.
The difference is the same in bright light and dim light.
Both pupils react normally and symmetrically to light.
No other neurological or ocular signs.
No treatment is needed -- simply reassure the patient and document.

💡 Clinical Tip: To determine whether anisocoria is physiological, compare the pupil size difference in bright versus dim lighting. If the difference stays the same in both conditions, it is likely physiological. If the difference increases in dim light (the smaller pupil fails to dilate), suspect Horner syndrome. If it increases in bright light (the larger pupil fails to constrict), suspect a problem with the larger pupil (CN III palsy, Adie).

Horner Syndrome

Horner syndrome is caused by disruption of the sympathetic pathway that dilates the pupil. The pupil on the affected side fails to dilate in dim light, making the anisocoria worse in dim conditions.

Classic triad:

Miosis: A smaller pupil on the affected side (failure of sympathetic-mediated dilation).
Ptosis: Slight drooping of the upper eyelid (Muller's muscle, which helps elevate the lid, is sympathetically innervated).
Anhidrosis: Reduced sweating on the affected side of the face (in first-order or second-order Horner).

Horner syndrome can arise from a lesion anywhere along the three-neuron sympathetic pathway (first order: hypothalamus to ciliospinal center; second order: ciliospinal center through chest to superior cervical ganglion; third order: superior cervical ganglion to orbit).

Causes range from benign (cluster headache, carotid dissection, prior chest surgery) to serious (Pancoast tumor at the lung apex compressing second-order fibers, carotid artery dissection). A new Horner syndrome requires urgent evaluation.

CN III (Oculomotor Nerve) Palsy

A complete CN III palsy produces:

Ptosis: Heavy drooping of the upper eyelid (levator palpebrae superioris paralysis).
Exotropia and hypotropia: The eye turns down and out ("down and out" position) due to unopposed action of the superior oblique (CN IV) and lateral rectus (CN VI).
Mydriasis: A large, non-reactive dilated pupil -- the parasympathetic constrictor fibers traveling with CN III are damaged.

A pupil-involving CN III palsy is a medical emergency -- it often signals an expanding posterior communicating artery aneurysm compressing CN III. Patients with sudden painful CN III palsy with a dilated pupil require immediate referral to an emergency department.

⚠️ Common Mistake: Dismissing a dilated, non-reactive pupil with ptosis as simple Adie pupil without considering CN III palsy. CN III palsy (especially with pain) is an emergency; Adie pupil is benign. Always document the full picture and report to the doctor immediately.

Adie Tonic Pupil

An Adie tonic pupil is a dilated pupil (usually unilateral) with a very slow, tonic (prolonged) constriction to prolonged light and to the near response. It results from damage to the ciliary ganglion (post-ganglionic parasympathetic fibers). Causes include viral illness, surgery, or trauma. Unlike CN III palsy, Adie pupil does not cause ptosis or extraocular muscle palsy and is not immediately dangerous.

Pharmacological Pupil

Many common medications and substances can dilate or constrict the pupil:

Dilation: Cycloplegic drops (cyclopentolate, tropicamide, atropine), antihistamines, some antidepressants, scopolamine, cocaine, amphetamines.
Constriction (miosis): Pilocarpine, opioids, organophosphate insecticides, certain beta-blockers.

A pharmacologically dilated pupil does not react to pilocarpine drops (2%), while an Adie or CN III pupil will constrict (to varying degrees). This pharmacological test helps distinguish causes.

Documentation

Document pupil size (in mm), shape, reactivity, and symmetry. Note conditions (dim vs. bright room). Example: "OD 5 mm sluggish, OS 3 mm brisk. Anisocoria worse in dim light. Suspect left Horner -- reported to Dr. [Name]."

Key Takeaways

Physiological anisocoria (up to 1 mm) is common, equal in bright and dim light, and requires no treatment.
Horner syndrome: small pupil (miosis), ptosis, anhidrosis; worsens in dim light. Can signal serious pathology.
CN III palsy: large, non-reactive pupil + ptosis + down-and-out eye. Pupil involvement is an emergency.
Adie tonic pupil: dilated, very slow light response, no ptosis or motility deficit, benign.
Anisocoria worse in dim light suggests the smaller pupil is abnormal (Horner); worse in bright light suggests the larger pupil is abnormal (CN III, Adie).