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Jonathan A. Barnett


  • Anisocoria = unequal pupil size

  • Constriction of the pupil is driven by parasympathetic innervation; Dilation by sympathetic innervation

  • Physiological anisocoria = pupil sizes differ by \<1mm. Normal.

Conditions that can cause anisocoria:

  • Benign episodic mydriasis

    • Intermittent episodes of pupillary dilation and possible association with migraine. No other significant neurologic or ocular symptoms present. Important to exclude other causes of mydriasis prior to this diagnosis.
  • Horner’s syndrome

    • Pupillary miosis + ipsilateral upper lid ptosis

    • May be congenital or acquired

    • Acquired causes include: trauma involving sympathetic pathway, carotid dissection, cavernous sinus pathology, stroke, neck or thoracic surgery, or Pancoast / mediastinal tumor (rare).

    • If suspect new onset Horner’s, consider Neurology and Ophthalmology consult.

  • CN3 Palsy

    • Pupillary mydriasis (“blown pupil”) + adduction, supraduction, infraduction deficit, “down and out position”, upper lid ptosis. Must rule out a compressive lesion on CN3.

    • If new onset, consider head imaging and Neurology and Ophthalmology consult

  • Medications (especially anticholinergics)

    • Commonly seen in young patients with scopolamine patches who forget to wash hands after handling the patch; dilation can last one day to a couple weeks.
  • Trauma

    • Damage to the iris sphincter after eye trauma can cause a dilated pupil that is poorly reactive to light.
  • Eye surgery

    • Patients who have undergone intraocular surgery (like cataract surgery) may have an irregularly shaped, less reactive, and slightly dilated pupil as compared to the other eye due to damage or stretching of the iris during the operation.
  • Infections

    • Viral infections and syphilis can cause parasympathetic denervation, resulting in a relatively mydriatic pupil that is poorly reactive to light. However, the accommodative pupillary response (i.e., constriction at focusing on an object at near) may be intact.

    • “Adie’s Tonic Pupil” = Idiopathic cause of parasympathetic denervation presenting similarly as above. However, viruses (HSV) are sometimes thought to be behind idiopathic cases.

  • Acute angle closure glaucoma

    • During an acute attack, the pupil may be mid-dilated and sluggish to light. However, patients will also have symptoms of acute angle closure (e.g., headaches, nausea/vomiting, eye pain, etc. See Glaucoma section for more info).