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Anisocoria is actually not such an uncommon condition. The causes of this issue can vary from completely harmless to life-threatening so there are various factors that one has to consider in determining what issues cause problems for affected individuals.

The various possible causes of unequally dilated pupils will be discussed here further.

Physiological anisocoria

This means that the unequally dilated pupils are completely normal and there is no other condition associated with this occurrence. Individuals with asymptomatic anisocoria live completely normal lives and there is no abnormality in their visual acuity.

Horner syndrome

Horner syndrome is the medical name given to a condition where there is a combination of signs and symptoms due to a disruption in the nerve pathway from the brain to the eyes and face of one side of the body. The nerve disruption can be caused by conditions such as strokes, a spinal cord injury, or a tumour.

Together with anisocoria, the signs and symptoms of Horner syndrome include a decreased pupil size (miosis), a drooping eyelid (ptosis), and decreased sweating (anhidrosis) all involving the affected side.

Oculomotor nerve palsy

Oculomotor (third) nerve palsy that affects the dilation of the pupil is often caused by compressive issues in the area of the brain where the nerve is located. The side of the brain where the oculomotor nerve palsy occurs results in a pupil that remains dilated (mydriatic) and is poorly reactive to light. Therefore, the affected side is the one where the pupil remains open despite stimulation to light.

Pharmacological pupil

Out of all the causes of anisocoria, a pharmacologically induced dilatation of the pupil is the largest in this scenario. The accidental or intentional administration or instillation of atropine, or atropine-like medications, can cause dilatation of one or both pupils. If one pupil is involved then the condition is referred to as pharmacological anisocoria.

The pupil here will fail to respond to light stimuli and the rest of the examination won't yield any other abnormalities involving the anatomy and function of the affected eye.

Mechanical damage

Damage to the muscle of the iris itself due to trauma, infection, or even surgery may result in anisocoria. Acute angle closure causes a sudden rise in intraocular pressure and is a very important cause of mechanical dysfunction of the iris that leads to poor reactivity of the pupil. 

The clinical presentation of acute angle closure includes pain, edema of the cornea, increased intra-ocular pressure, and a light-fixed mid-position pupil.

Tonic pupil

A tonic pupil is one that responds poorly to light stimuli but briskly when focusing on nearby objects and the position of the pupil is maintained for a prolonged period, thus the description of the pupil being tonic. 

Conditions associated with this situation include:

  • Severe afferent nervous system damage.
  • Autonomic neuropathies such as those as a result of diabetes or a vitamin B12 deficiency.
  • Parinaud dorsal midbrain syndrome.
  • Aberrant regeneration of the oculomotor cranial nerve.
  • Argyll Robertson pupils.

Transient anisocoria

This term describes a temporary anisocoria that resolves on its own and may be caused by benign conditions such as a migraine headache, especially when there are no other associated symptoms present.

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