Anisocoria – Miosis and Mydriasis – Pupils of different sizes
Anisocoria refers to the condition where the pupils of your eyes are not the same size. The pupil is the black ring in the center of the iris; its size is regulated by a circular muscle (the pupillary sphincter) and involuntary nerves that respond to stress hormones. It's worth noting that one in five people naturally has pupils of different sizes. However, a sudden change in the size of one pupil can be a symptom of a serious eye or neurological problem.
What is anisocoria?
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Anisocoria refers to two asymmetrical pupils. Either one is dilated (mydriasis), or the other is constricted (miosis).
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This asymmetry can be benign and present from birth; this is called physiological anisocoria.
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It can also reveal neurological damage or be the result of eye trauma.
Symptoms
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Anisocoria is most often asymptomatic. The patient may complain of photophobia (sensitivity to light) in cases of mydriasis or night blindness (dark vision) in cases of miosis (constricted pupil).
The associated emergency symptoms
Warning! Anisocoria is sometimes caused by a neurological problem. If you experience any of these symptoms along with anisocoria, call 15 immediately or seek emergency medical attention:
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Drooping eyelid – Ptosis.
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Diplopia – Double vision due to paralysis of an eye nerve or muscle
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Eye pain
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Fever
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Severe headaches
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Reduced perspiration on half of the face.
Who is affected?
Anisocoria, also known as anisocoria, is a rare condition. People at risk of developing anisocoria include those who have:
A known neurological condition.
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A history of trauma, inflammation, or eye surgery.
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A high risk of stroke.
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An active viral infection.
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Adie's tonic pupil is known to be a disease related to insufficiency of the pupillary sphincter (muscle).
Causes
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In general, anisocoria is caused by an abnormal dilation (mydriasis) or an abnormal constriction (miosis) of the pupils.
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An injury or damage to the pupillary muscles or the nerves controlling them can lead to changes in pupil size.
Other common causes include:
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Physiological anisocoria – also called simple or essential anisocoria – affects 1 in 5 people from birth. It is a stable and benign anatomical anomaly present at birth. The difference in pupil size is usually less than or equal to 1 mm between the two eyes. Its exact cause is unknown. Light and near vision reflexes are intact, and the degree of anisocoria is generally the same in light and darkness.
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Eye trauma – Anisocoria usually results from known eye trauma or follows surgery, an inflammatory condition (uveitis) or acute glaucoma.
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Eye drops or toxic substances – Pharmacological anisocoria can present as mydriasis or miosis after the administration of an active ingredient that acts on the dilating muscles or sphincters of the pupils. Most often, this involves the administration of Atropine or Mydriaticum, a product intended to dilate the pupil for an ophthalmological examination.
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Pupillary malformations – Congenital abnormalities in the structure of the iris can contribute to abnormal pupil sizes and shapes present from childhood. These include rare conditions such as aniridia, coloboma, and ectopia pupillae.
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Horner's syndrome – In this syndrome, the patient presents to varying degrees with the triad of miosis, ptosis, and enophthalmos (retracted eye). Anisocoria is more pronounced in the dark. Diagnosis is most often made by CT scan, looking for nerve compression in the upper chest (tumor or thrombosis).
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Adie's tonic pupil – This condition is caused by damage to the parasympathetic ciliary ganglion, which innervates the pupillary sphincter and ciliary muscle. Aberrant reinnervation results in pupil dilation, with minimal constriction in response to light. The condition most often affects women between the ages of 20 and 40, typically on one eye only. Seventy percent of cases are associated with diminished deep tendon reflexes (Adie's syndrome).
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Third cranial nerve palsy (3) – When the oculomotor nerve (third cranial nerve) is affected, the pupil dilates (mydriasis). The condition is frequently accompanied by ptosis, strabismus with downward and outward gaze, and loss of accommodation (focusing ability). A compressive lesion of the nerve due to head trauma, aneurysms, or, rarely, a tumor, should be investigated.
Diagnosis of anisocoria
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To confirm anisocoria, your ophthalmologist will examine your pupils in a well-lit room and in a dark room. This will allow them to see how your pupils react to light and may help determine which pupil is abnormal.
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Your eye movements will also be assessed to look for associated muscle paralysis or diplopia.
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The initial assessment concludes with an examination using a tabletop microscope (slit lamp). This helps to identify potential causes.
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If there is doubt about which side is affected, a pharmacological test (instillation of Neosynephrine drops) can help with the diagnosis.
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In cases of sudden onset of anisocoria, an urgent brain MRI will likely be performed to look for a serious cause threatening visual and vital prognosis.
Treatment of anisocoria
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The treatment for anisocoria is to treat its cause. Eye drops to constrict the pupil in cases of mydriasis or to dilate it in cases of miosis are ineffective.
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In cases of traumatic mydriasis, iris plasty may be considered in order to reconstruct the iris.
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In the case of Adie's tonic pupil, it will spontaneously return to its normal morphology.
Support
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The management of anisocoria varies depending on the cause (etiology).
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Physiological anisocoria is often asymptomatic and does not require intervention.
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Mechanical anisocoria secondary to trauma may require surgical intervention to compensate for the structural defect. The operation is called pupilloplasty.
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Mechanical anisocoria secondary to other ophthalmic conditions such as uveitis or acute angle glaucoma can be managed medically if necessary.
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Drug-induced anisocoria usually disappears when the causative agent is stopped. For example, the pupil constricts 3 to 5 days after stopping atropine.
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Adie's tonic pupil can be compensated for with glasses to improve vision and pilocarpine to constrict the pupil.
Expert opinion
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Consultation with a neurologist or neuro-ophthalmologist is recommended for atypical cases.
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A brain MRI is most often performed as an emergency in order to rule out life-threatening causes.