Pupil Inequality (Anisocoria)

What Is Anisocoria?

Pupil inequality (anisocoria) represents a frequently encountered situation. Causes can range from completely benign (physiological anisocoria) to very serious (carotid dissection, cerebral aneurysm). Approximately 20% of the population has physiological anisocoria (the difference between the 2 pupils is less than 1 mm). Sometimes, anisocoria is accidentally discovered by an observer. Other times, it is accompanied by other ocular signs (diplopia, pain, ptosis, motility abnormalities).

Is It a Permanent Condition?

When discovered, the presence of anisocoria mandatorily implies ophthalmologic examination. The size of pupils in light and darkness is examined, and their shape is assessed. Anisocoria greater in light suggests that the abnormal pupil is the one with the larger diameter. Anisocoria more pronounced in darkness suggests that the abnormal pupil is the one with the smaller diameter.

What Are the Causes of Anisocoria?

If the Abnormal Pupil Is Small (Miotic)

Causes can be multiple:

• It can be the result of instilling drops with miotic effect (pilocarpine).
• Eye inflammations can be accompanied by miosis. Iridocyclitis produces miosis, accompanied by eye pain, redness, precipitates in the anterior chamber.
• Horner's syndrome, characterized by miosis, mild ptosis, iris heterochromia, anhidrosis on the affected side, is produced by sympathetic paresis (in stroke), cervical spine involvement (tumor, inflammation), lung apex lesions (tumors, metastases), thyroid adenoma, internal carotid dissection.
• Argyll Robertson pupil (of syphilitic cause) – is an asymptomatic condition in which both pupils are miotic, irregular, with slight anisocoria. Specific is the weak or absent reactivity to light, but with preservation of constriction in convergence.

If the Abnormal Pupil Is Larger (Mydriatic)

Several causes may be involved:

• Instillation of mydriatic drops (e.g., atropine)
• An eye trauma with damage to the iris sphincter
• Adie's tonic pupil is characterized by irregular mydriasis, photophobia, minimal reaction to light and tonic to convergence. Specific is hypersensitivity to diluted pilocarpine. Often, the condition is idiopathic, but can also appear after trauma, orbital surgical interventions, in herpes zoster infection.
• Oculomotor nerve (III) palsy is associated with ptosis, extraocular muscle palsies. When accompanied by pupil involvement (fixed mydriasis), the causes of palsy can be: aneurysms of the posterior communicating artery, tumors, brain trauma, cavernous sinus lesions, orbital diseases. N. III palsy without pupil involvement is found in microvascular ischemic diseases (diabetes, hypertension).

Can It Be Treated?

As can be observed, in few situations is anisocoria the result of a strictly ocular condition. Ophthalmologic examination must be completed with neurological examination, accompanied by imaging (brain MRI, cervical spine, orbit, neck, upper chest – for tumors, metastases). Cerebral angiography can diagnose a cerebral aneurysm, a carotid dissection. Complete clinical and paraclinical examination can detect arterial hypertension, diabetes, signs of infection (syphilis).

Treatment of anisocoria is not always easy to achieve, as it addresses the cause that produced the condition. The other symptoms accompanying anisocoria (ptosis, strabismus) are also treated. The role of the ophthalmologist in detecting and treating anisocoria is important, but not singular. Collaboration with other medical and surgical specialties is mandatory to correctly diagnose and treat this condition.