Introduction to the Pupillary Reflexes
The pupil, the small black circle in the middle of our eye, changes size to control how much light gets to the back of the eye. This is known as the pupillary reflex and they should be tested during every eye examination that you perform1,2. In this article, we’ll look at what the pupil is, what it does, and how it works. We’ll also learn how to check pupil reactions, including the direct, consensual, near, and RAPD reflexes. Whilst there are numerous abnormal pupil responses and findings, these will remain outside the scope of the article and feature in an upcoming “Skill Centre” article in the future.
Understanding the pupil and its reflexes can seem complex (I personally remember being confused by the neurophysiology of it all and what each of the abnormal responses were) but it’s important for optometry students. By the end of this article, you’ll have a good understanding of the pupil and how to check its reactions. This will help you in your studies and future career as optometrists.
Please note: this article is to aid education only and not to be used to diagnose or manage conditions – if you are concerned about the appearance of your pupils, please seek advice from an optometrist or other eye care professional. Additionally, clinicians should use their own professional judgement and manage their patients accordingly. The Eye Care Advocate and its author(s) will accept no responsibility for the management of your patients.
What is the Pupil?
The pupil is the black circular opening located in the center of the iris of the eye. It functions much like the aperture of a camera, controlling the amount of light that enters the eye, as well as changing the depth of focus3,4,5. The pupil appears black because the light entering through the pupil is almost entirely absorbed by the the tissues inside the eye5.
The size of the pupil changes in response to the brightness of light around you. In bright light, the pupil constricts (becomes smaller) to allow less light in4. In dim light, the pupil dilates (becomes larger) to allow more light in4. This automatic adjustment is known as the pupillary light reflex. Additionally, the pupil constricts to near stimuli in what is called the pupillary near response3.
The size of the pupil is controlled by two muscles within the iris, the sphincter pupillae (controlled by the parasympathetic nervous system) and the dilator pupillae (controlled by the sympathetic nervous system)6. The balance between the two muscles, and their relevant nervous system controls, allows for fine control on pupil size at any given moment. The diagram below shows how these muscles are configured.
The sphincter pupillae is a ring of smooth muscle that contracts to constrict the pupil (operating along the blue arrows1).
The dilator pupillae expands radially, like the spokes on a wheel. When this muscle contracts (along the yellow arrows1), the pupil dilates.
Innervation of the Pupil
So far, the idea of the pupil has been relatively straightforward, but the innervation and the neurophysiology of the pupillary response is where it can become complex. This section will try and simplify the pupillary light reflex as much as possible.
To start, it is worth familiarising yourself with the following terms:
The Afferent Pathway: The afferent pathway, also known as the sensory or ascending pathway, carries sensory information from the peripheral tissues and organs towards the central nervous system (the brain and spinal cord)7. In the case of pupillary light reflexes, think of it as the signals leaving the pupil and heading towards the brain7.
The Efferent Pathway: The efferent pathway, also known as the motor or descending pathway, is a part of the nervous system that carries signals away from the central nervous system (the brain and spinal cord) towards the peripheral tissues and organs7. These signals, or impulses, lead to various physiological responses such as muscle contraction or gland secretion. In the case of the pupillary light reflex, it starts at the Edinger-Westphal nucleus (see below) and takes the signal back to the eye (the iris and subsequently controls the pupil)1,7.
A way that I like to remember the direction of the signals in these pathways is:
AFFERENT is APPROACHING the central nervous system and the
EFFERENT is EXITING the central nervous system.
Parasympathetic Innervation of the Pupil
The first explanation will focus on the parasympathetic pathway that controls the sphincter pupillae. You may find that the following image looks rather complicated, but please do not let it alarm you – I will attempt to break this down into easier steps and to help clarify the pathway.
The image represents the parasympathetic visual pathway. The AFFERENT pathway (APPROACHING the brain) is in red and blue and the EFFERENT pathway (EXITING the brain) is seen in purple. Image adapted from Orr (2014)1.
Step 1 – Light falls on the retina
As light is shone into an eye, the right eye in this example, the photoreceptors of the retina generate a signal and the process begins.
These photoreceptors on the retina transmit their signal through the visual pathway, with the nasal fibres decussating (crossing over) at the optic chiasm to head to the contralateral (opposite) side of the brain and the temporal fibres remaining on the ipsilateral (same) side of the brain.
These fibres transmit the signals along the optic tract on both sides of the brain to reach the pre-tectal nuclei on both sides of the midbrain.
It is this part of the pathway that makes up the AFFERENT PATHWAY of the reflex1,8.
Step 2 – Pre-tectal nucleus projects bilaterally to both Edinger-Westphal nuclei
Once this signal has reached the pre-tectal nucleus it is processed and projected bilaterally to both Edinger-Westphal nuclei1.
It is the Edinger-Westphal nuclei that then pass this signal on. Think as this stage as a mini-relay station for the signal as it goes through the brain.
Step 3– CNIII, the ciliary ganglion and short ciliary nerve
The Edinger-Westphal nuclei then send the signal away from the brain down the oculomotor nerve (CNIII) to the ciliary ganglion1.
At the ciliary ganglion, the signal is transmitted through the short ciliary nerve to the sphincter pupillae on the iris.
When the signal reaches this muscle, the pupil constricts. Given the bilateral nature of the pathway, this is why both pupils constrict simultaneously1.
The Overall Process
The animations above try to break down the topic into smaller chunks. I have created a full animation and enlarged it for ease of access. The resulting overall animation can be seen below:
The Near Response
The near response is brought through signals arriving at the Edinger-Westphal from the visual cortex of a perceived near target. The signal follows through to the ciliary nerve to the ciliary body, causing accommodation, and to the subnucleus of the medial rectus muscle, causing convergence. This highly linked triad of responses allow the eye to adjust to a variety of lighting conditions and viewing distances whilst maintaining optimal vision.
Sympathetic Innervation of the Pupil
The sympathetic innervation of the pupil is responsible for the innervation of the dilator pupillae muscle, with the innervation allowing enlargement of the pupil (or mydriasis). The pathway is relatively simple:
There are three neurons that make up the sympathetic innervation of the pupil.
The first neuron sends the signal from the posterior hypothalamus, down the brain stem and to the cilio-spinal centre of Budge1.
The second neuron then takes this signal from the cilio-spinal centre to the superior cervical ganglion, which is found in the neck.
The final neuron follows the path of the internal carotid artery before joining the ophthalmic division of the trigeminal nerve1.
The sympathetic fibers then journey via the nasociliary nerve and along the long ciliary nerves until they reach their destination; the dilator pupillae of the iris. Note: This pathway is an EFFERENT pathway1 as it is EXITING the brain.
Assessing the Pupils and Pupillary Reflexes
The pupil assessment is a key component to every eye examination as it can indicate if there are any abnormalities of the visual pathway or nervous systems. In addition to pupillary reflex assessment, you should also assess the shape and size of each pupil and compare the right to the left.
Measurement of Pupil Size and Shape
Measurement should be taken under normal room illumination, with the patient’s glasses off. Ask them to look at a distance target – such as the spotlight on the testing chart and avoid encroaching their line of sight. Use a ruler or a hemisphere scale (such as that on the side of a pen torch) and note the findings.
Pupils are normally round and equal in size between the eyes, although approximately 20% of the population will have some degree of asymmetry in size8. This is known as anisocoria. The size of the pupil varies in light levels, with the typical diameters being between 3 to 6 mm in brighter conditions and 4-8 mm in dimmer conditions8.
There are four key tests to assessing the pupil reactions. These are; the direct pupillary response, the consensual pupillary response, the near reflex and the swinging flashlight test. We shall now look at these tests.
Assessing the Direct and Consensual Pupillary Reflexes
The Direct Pupillary Reflex: This is the immediate response of the pupil of the eye constricting when a light is shone directly into it1,8. For example, shining a light into the right eye and watching the right pupil constrict would be assessing the direct pupillary reflex of the right eye (see the animation below the “consensual pupillary reflex section”).
The Consensual Pupillary Reflex: This is the immediate and simultaneous constriction of the pupil of the eye when a light is shone in the opposite eye1,8. For example, shining a light in the right eye and watching the pupil constrict in the left eye would be assessing the consensual reflex of the left eye (see the animation below).
An animation demonstrating the direct and consensual pupillary responses of the eyes to a bright light.
The animation above also demonstrates how the assessment is conducted – ensuring that the pen torch is shone from beneath the eyes to avoid encroaching the patient’s line of sight thus potentially inducing the near reflex. Ideally, you would like to dim the room lights slightly but not too dim as this will make it difficult to see the pupil on darker irides.
Assessing the Near Pupillary Reflex
The Near Reflex: This is the constriction of the pupil when the focus of the eye is changed from distance viewing to near viewing. An eye that is shifting from a distance to a near target would show a constriction of the pupil and when it shifts from a near target to a distance target, it would show dilation of the pupil. This needs to be performed if there is an abnormal finding on the direct or consensual pupil reflex assessment – as far as current understanding, there is no condition or disease where the near pupillary reflex is lost when there are normal reflexes to light1,8.
In order to assess this reflex, you ask the patient to look at a distant target then shift their view to a near target (such as an image on the budgie stick/near fixation target). Observe the extent and the speed of the constriction as your patient does this, before asking them to look at a distance target and observing the extent and the speed of the dilation8.
Assessing for an RAPD – the Swinging Flashlight Test
Swinging Flashlight / Assessing for a Relative Afferent Pupillary Defect (RAPD): A relative afferent pupillary defect is an abnormal finding when assessing pupils. This test requires “swinging” a light between the pupils and watching their responses8. If a pupil dilates when the light is shone into it (instead of the expected constriction), then there is an RAPD present indicating damage to the afferent pathway and further investigation is required. An RAPD is also referred to as a Marcus Gunn Pupil9.
In order to perform this test, have your patient looking ahead at a distance target. Hold a BRIGHT flashlight below the patient’s right eye from a distance of approximately 10 cm away and observe the direct response8. Pause over this eye for 3 seconds8 before BRISKLY moving it to the the left eye and watch the responses from the pupil. Repeatedly alternate this between the two eyes, being sure to pause for the 3 seconds over each eye.
A normal response is one where both the pupils constrict when a light is shone directly onto them, followed by a very brief dilation as the light is snapped between the two eyes and then a rapid constriction when the light is shone directly on the other eye8. This can be seen in the animation below:
An animation of the swinging flashlight test, with a normal response demonstrating an absence of a relative afferent pupillary defect.
It is important to note that the pupils will likely dilate slightly once the eye has been completely illuminated. This is due to an overestimation of the eye constricting to the initial presentation of light, but the eye should still remain constricted overall8. It is therefore vital that you pay attention to the initial constriction as the light is shone into the eye.
An abnormal response, that is to say the presence of a relative afferent pupillary defect is one where the affected eye will dilate its pupil as soon as the light is shone upon it. This is because the consensual dilation response occurs as the light moves off of the fellow eye overpowers the the constriction response (if any) of the affected eye8. Below is an animation of a right RAPD.
An animation of the swinging flashlight test, with a normal response demonstrating the presence of a right relative afferent pupillary defect.
In some cases, the afferent pathway may be only mildly damaged and not completely disconnected and as such, the extent of the RAPD can be varied. It can therefore be difficult to detect subtle RAPDs. It is therefore recommended to use a BRIGHT pen torch as this will cause significant constriction of the contralateral (other eye) without the defect, allowing improved ability to observe the afferent pupil defect on the affected eye10.
Additionally, those that rush this test may merely miss these subtle signs. Make sure you are brisk in the movement between the two eyes – a common problem I see in optometry clinics and assessments with student optometrists is a slow loop under the nose when alternating between the eyes. This increased time between light presentations will make it more difficult as both eyes will dilate during this alternation, reducing the ability to directly compare between the pupils, again, meaning a subtle RAPD may be missed.
Recording Pupillary Assessments
If you have performed an assessment on your patient’s pupils, be sure to record your findings. These findings can help detect pathology later on in a patient’s lifetime and having these records accurately documented may help aid, or prevent, a referral being made.
I personally record that the patient has/has no direct, consensual or near response, alongside whether there was an RAPD or not. If there are any other abnormal findings (such as anisocoria or a misshapen pupil, then note it down!) It is generally good practice to record pupils sizes – especially when considering contact lens fitting as larger pupils may experience glare from a poorly considered back optic zone parameter.
A Caution About PERRLA
A common acronym within optometry and ophthalmology is the one of PERRLA. This acronym is Pupils Equal Round and Responsive to Light and Accommodation. Whilst this may be a good note for younger patients with normal pupils, I encounter many practitioners use this as a way of saying pupils are “normal” without thinking about what they have written.
However, this does miss out the fact that approximately 20% of the population have essential anisocoria, so this acronym doesn’t apply here. Additionally, as patients get older they experience presbyopia and lose their accommodation (or even more notably have intraocular lenses following cataract surgery). Therefore, can you truthfully put that the pupils respond to accommodation when the eye cannot actually accommodate?
The General Optical Council and College of Optometrists both make it clear that you have to make full, clear and accurate records as part of your role as an optometrist – so is PERRLA really an accurate recording of what you have observed? Whilst in the majority of cases, other eye care professionals will know what you mean when you have put this, just be wary that in some uses that this acronym could be used against you if inappropriately used and a patient complaint arose.
Summary
Whilst the pupil doesn’t appear to many as much more than a black spot at the front of the eye, this article hopefully has shed some light on its purpose, function and complex nature that enables optimal vision.
I strongly recommend that you read some of the sources below, as their wording and explanations helped shift my understanding to the next level, whilst unearthing some facts that I did not even think to look up.
As always, there is a skill activity included at the end of this article that can help test your knowledge. Let me know how you get on in the comments. If you think this article is useful and you think a fellow student or a pre-registration optometrist would appreciate a read, be sure to pass it on to them! All views to the site are greatly appreciated and encourage the production of further content!
Skill Activity
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Further Reading
- Orr J (2014). Characteristics of normal and abnormal pupils – part 1. Optometry Today 14/11/2014: 52-55.
- College of Optometrists. Conducting the eye examination or sight test. [Online]. Available at: https://www.college-optometrists.org/clinical-guidance/guidance/knowledge,-skills-and-performance/the-routine-eye-examination/conducting-the-routine-eye-examination [Accessed: 9th April 2024].
- McDougal D, and Gamlin PDR (2008) Pupillary control pathways. In The Senses: A Comprehensive Reference; Masland RH, Albright T, Eds; Academic Press: Cambridge; Volume 1, pp. 521–536.
- Yoo H, and Mihaila DM (2023). Neuroanatomy, Pupillary Light Reflexes and Pathway. StatPearls [Online.]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK553169/ [Accessed: 9th April 2024].
- Courn S (2022). Pupil of the eye and its function, size and testing. Journal of Contemporary Medical Education 12(9): 1-2.
- Bloom J, Motlagh M, and Czyz CN (2023). Anatomy, head and neck: eye iris sphincter muscle. StatPearls [Online]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK532252/#:~:text=The%20sphincter%20muscle%20fibers%20are,light%20reflex%20or%20during%20accommodation. [Accessed: 9th April 2024].
- Remington LA (2012). Chapter 12 cranial nerve innervation of ocular structures. In: Clinical Anatomy and Physiology of the Visual System Third Edition; Remington LA Eds; Butterworth-Heinemann; pp. 219-232.
- Prokopich CL, Hrynchak P, and Elliott DB (2007). Ocular health Assessment. In: Clinical Procedures in Primary Eye Care Third Edition; Elliott DB, Eds; Butterworth-Heinemann: Philadelphia; pp. 221-318.
- Simakurthy S, and Tripathy K (2023). Marcus Gunn pupil. StatPearls [Online]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK557675/#:~:text=Marcus%20Gunn%20pupil%20is%20indicative,the%20cause%20should%20be%20treated. [Accessed: 12th April 2024].
- Browning DJ, and Buckley EG (1988). Reliability of brightness comparison testing in predicting relative afferent pupillary defects. Archives of Ophthalmology 106(3): 341-343.
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