An Orthoptist's guide to: Pupils
Assessing pupils is an important part of your Orthoptic examination. The pupils offer a great insight into what is happening along the neural pathway, and are affected in a number of ocular conditions. This post will go through the key features of the pupils as well as the important reflexes they are a part of.
How do pupils constrict?
Pupil constriction, known as miosis, occurs due to constriction of the sphincter pupillae. The sphincter pupillae is innervated by the parasympathetic fibres in the 3rd cranial nerve. It uses the neurotransmitter Acetylcholine to bind to muscarinic receptors.
How do pupils dilate?
Pupil dilation, known as mydriasis, occurs due to constriction of dilator muscle. The dilator muscle is innervated by the sympathetic fibres carried via blood vessels running within the 5th cranial nerve. It uses the neurotransmitter Noradrenaline which binds to Alpha receptors.
What is the light reflex?
The light reflex refers to the reflex responsible for pupillary constriction in response to direct/consensual innervation. Its main role is to regulate the intensity of light entering the eye.
How does the light reflex work?
The pupillary light reflex begins when light enters the eye and stimulates the retina. Sensory impulses travel via the optic nerve (CN II), chiasm, and optic tracts to the pretectal nucleus, forming the afferent limb of the reflex. From here, fibres project bilaterally (crossed and uncrossed) to both Edinger-Westphal nuclei, meaning each afferent input gives rise to two efferent pathways: one producing a direct response in the same eye and the other producing a consensual response in the opposite eye. The efferent limb originates in the Edinger-Westphal nucleus, where preganglionic parasympathetic fibres exit with the oculomotor nerve (CN III), synapse in the ciliary ganglion, and send postganglionic fibres to the sphincter pupillae muscle. This results in constriction of both pupils when light stimulates either eye.
What is the dark reflex?
The dark reflex is the opposite of the pupillary light reflex. Instead of constriction, the pupils dilate to let more light enter the eye when in a dim environment.
How does the dark reflex work?
The dark reflex works by the retina sensing a lower intensity of light, resulting in a reduction of activity in the parasympathetic pathway (responsible for the light reflex, Edinger-Westphal nucleus → CN III → sphincter pupillae). This activates the sympathetic pathway, where the hypothalamus sends descending signals through the spinal cord to the ciliospinal centre of Budge (C8–T2). From there, preganglionic sympathetic fibres exit, synapse in the superior cervical ganglion, and then postganglionic fibres travel along the internal carotid artery into the orbit. These sympathetic fibres innervate the dilator pupillae muscle, causing the pupil to dilate (mydriasis).
In summary:
Light reflex = parasympathetic → constriction of pupil via the sphincter pupillae
Dark reflex = sympathetic → dilation of pupil via the dilator pupillae
What is the near reflex?
The near reflex refers to the response of the eye when shifting focus from a distant object to a near object. It involves a triad of actions—1. convergence of the eyes, 2. accommodation of the lens, and 3. pupillary constriction (miosis), all controlled by the parasympathetic nervous system.
The pathway begins with visual input from the retina through the optic nerve, chiasm, and tract to the lateral geniculate body, then via the optic radiation to the visual and prefrontal cortices. From there, signals descend to the oculomotor nucleus. Convergence is mediated by activation of the medial rectus muscles via CN III, while accommodation and miosis occur through the Edinger–Westphal nucleus, with fibres travelling in CN III to the ciliary ganglion and then through short ciliary nerves to the ciliary muscle and sphincter pupillae. What you will see on examination as the patient changes focus from the distance to near, is convergence of the eyes, accommodation of the lens and constriction of the pupils.
In summary:
Visual input travels → Retinal ganglion cells – optic nerve – optic chiasm – optic tract
LGB – optic radiation – visual cortex – prefrontal cortex – CN III nucleus – CN III – MR – Convergence of eyes
Superior branchium – pretectal area – EWN – CN III nerve – ciliary ganglion – short ciliary nerves – ciliary muscles innervated – sphincter contract – miosis (constriction of pupil)
LGB – optic radiation – visual cortex – prefrontal cortex – CN III nucleus – CN III – ciliary ganglion – short ciliary nerves – ciliary muscles contract – accommodation induced
How to test the near reflex?
Ask the patient to fixate at a target in the distance
Hold up a detailed target (for example a Snellen stick) at 25cm
Ask the patient to look at the target
Watch to see constriction of the pupil - assess the speed of pupillary constriction and the roundness of each pupil
What is anisocoria?
Anisocoria is a difference in pupil size greater than 0.4mm. It can be physiological or the sign of an underlying condition.
Physiological anisocoria affects 15-20% of the population. The pupil difference is present and similar in light and dark conditions.
When assessing anisocoria, the main question is to figure out whether the problem pupil is the bigger or smaller one.
If the larger pupil is the abnormal one - the anisocoria is larger in bright light, as the normal pupil will constrict in this situation. This indicates a problem along the efferent (parasympathetic) pathway as the abnormal pupil failed to constrict. The causes of this could be:
Iris damage - noted with abnormal iris on slit lamp examination
Instil pilocarpine 0.1% - if the larger pupil constricts, this indicates Adie’s tonic pupil
If there is no reaction to pilocarpine - this could indicate mydriasis secondary to pharmacological agents (eg dilating eye drops, drug use)
If the smaller pupil is the abnormal one - the anisocoria is larger in darkness, as the normal pupil will dilate in this situation. This indicates a problem along the afferent (sympathetic) pathway as the abnormal pupil failed to dilate.
Instil Iopidine 1% - if the smaller pupil dilates, this indicates Horner’s syndrome
If there is no reaction (neither pupil dilates), this points to physiological anisocoria
What is a relative afferent pupillary defect (RAPD)?
A relative afferent pupillary defect (RAPD) occurs when there is a difference in pupillary constriction in response to light stimulus in one eye compared to the other. It indicates a problem in the afferent limb of the pupillary light reflex pathway. It is typically caused by significant retinal disease or optic nerve pathology, which prevents adequate sensory input from reaching the pretectal nucleus. Clinically, it is detected using the swinging light test. For example, in a right RAPD:
Pupils equal in size before illumination
Light shone directly into left eye, both pupils respond to stimulation of left eye and constrict
Light shone directly into right eye, neither pupil responds to stimulation of right eye, you do not see any movement and may even see some dilation (as the left eye dilates in the darkness)
Both pupils react normally near stimulation
Common causes include large retinal lesions and optic neuropathies.
This is a brief overview of pupillary functions and how you test them.
