The clinician should observe the direct pupillary response in the illuminated eye and the consensual response in the fellow eye. The response to testing each eye separately with a bright light source is then assessed. 6 The presence of ptosis and/or extraocular muscle involvement suggests a pathological process regardless of the magnitude of anisocoria. If anisocoria remains the same in both lighting conditions, and the difference between pupil sizes is no more than 2 mm, the aetiology is likely to be physiological.
Pupil size should be assessed in both the dark (sympathetic-dominated dilation) and the light (parasympathetic-dominated constriction). If the patient’s irides are dark and the pupils are difficult to visualise, the clinician can use an ophthalmoscope to assess differences in the red reflex. A baseline measurement of pupil size should be made under ambient lighting conditions with both pupils equally illuminated. Any asymmetry of the colour of the two irides (heterochromia) should be noted. The examiner should observe the shape, size and location of the patient’s pupil. Testing this forms the basis of the swinging-torch test: if the response to shining a light in one eye differs from shining it in the other, this is referred to as a relative afferent pupillary defect (RAPD). 2,5ĭirect and consensual responses should be identical, whichever eye is illuminated. 2,5 Third-order sympathetic neurons then travel to the orbit with the internal carotid artery and its branches, ultimately innervating the dilator pupillae muscle in the iris. 5 From here, second-order sympathetic neurons travel back up the sympathetic chain to synapse in the superior cervical (stellate) ganglion. 2 Sympathetic pupillary fibres originate in the hypothalamus, travel down the brainstem and cervical spinal cord to exit at the first thoracic level. Parasympathetic pupilloconstrictor fibres travel in the oculomotor nerve, synapsing in the ciliary ganglion before reaching the sphincter pupillae muscle in the iris. 5 It is this bilateral innervation of the Edinger-Westphal nucleus that results in both direct and consensual responses to light shone in one eye. 2 Information from the optic nerve passes to the ipsilateral pretectal nucleus and then on to the Edinger-Westphal nuclei on both sides. The pupillary light response involves both afferent (optic nerve) and efferent (oculomotor nerve and sympathetic) pathways. Examination should include assessment of visual acuity, visual fields to confrontation, pupil testing, extraocular motility and whether or not ptosis is present. 5 It is important to ask about previous or current malignancies and neck trauma. Associated visual and/or neurological symptoms should be sought, including visual blurring, visual loss, disturbance of visual fields, or diplopia. 2,5Ī thorough history should include asking about the use of new medications or inadvertent ocular contact with foreign substances by rubbing the eyes. 2–4 Non-physiological anisocoria indicates disease of the sympathetic or parasympathetic pathways supplying the pupil, or a problem with the iris itself.
Physiological anisocoria is common: approximately 20% of normal people have different-sized pupils. 1 This article aims to guide management in both of these situations. Indeed, new onset anisocoria may be an early sign of a life-threatening emergency. The general practitioner (GP) may discover anisocoria during examination for a seemingly unrelated problem. The aetiology may be physiological, pathological or pharmacological. A difference in pupil size between the eyes is known as anisocoria.
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