Many different pupil responses can be abnormal and can indicate a problem in the eye. The pupil is controlled by two major cranial nerves: the oculomotor nerve and the sympathetic nervous system. Pupillary response to light will change depending on how much light is shining into the pupil, sending signals back to your brain for processing.
This article discusses what causes an abnormal pupillary response, pupils’ examination, pupillary reflexes, and some common abnormalities associated with pupil reactivity.
What is an abnormal pupillary response?
An abnormal pupillary response is a pupil not responding normally to light, resulting in abnormal pupil size. It can either be a small pupil or a large pupil. There are many different reasons why the pupil may appear this way, and it will affect how your brain processes visual information.
Pathways of the pupillary reflex
- The afferent limb starts at the retina – which sends signals through cranial nerve II (optic) to reach both sides of the midbrain superior colliculus. The efferent limb then takes these messages from the midbrain superior colliculus, where they cross over before traveling down cranial nerve III (oculomotor). In addition to controlling pupil size with light stimuli, signals also travel down pathways to control eye movements.
- The pupil will dilate when the pupil on one side of the body is stimulated, which constricts when light stimuli are applied to both pupils.
Examination of pupil reactivity
- General pupil evaluation: The pupillary reflex can be tested by shining a bright flashlight into each pupil in turn while observing how quickly it responds (light reflex). If this doesn’t work, you can cover up one eye at a time and see if there is any difference between them.
This would indicate that more than likely, your brain has a normal vision for both eyes; however, something could still be wrong with either the afferent or efferent limb.
- Pupillary observation: Next, observe whether they usually respond together (tonic pupil) or not (narrow pupil, dilated pupil).
You can further examine the pupil by testing whether it usually responds to accommodation and convergence. There are three reflex tests:
- Light reflex: Shine a bright light into one pupil and observe the other pupil’s response.
- Swinging flashlight test: Hold a finger or pen at arm’s length in front of each eye in turn, covering first the left eye and then the right. First, note which pupil is dilated (usually if you cover it for more than three seconds). Then watch to see whether it constricts when uncovered; this indicates that both eyes generally work with normal vision.
- Near reflex test: This is a form of pupil diameter measurement. You can measure pupil sizeby having your patient look at any small object, such as their wristwatch held about 12 inches from their nose, while you cover either pupil alternately. The pupil will appear to jump back again after several seconds.
Abnormal pupillary responses
We will now take a look at various pupil responses which are considered abnormal.
One pupil is larger than the other; one pupil does not dilate (constrict) in response to light or near pupil measurement, and there is a difference between pupil size when looking straight ahead compared to looking up at the ceiling. The greater degree of anisocoria suggests that pupillary constriction requires more effort because it occurs less readily.
- Caused by: This can be caused by damage to the iris sphincter muscle on one side resulting from disease, trauma, or surgery affecting either eye independently, but it may also involve both eyes simultaneously as part of Horner syndrome.
Unilateral large pupil
The large unilateral pupil is a pupil that is larger in a dark room. A similar size to the other pupil when light shines into either eye.
- Caused by: The pupil is larger because the pathway for pupil constriction has been interrupted, blocking parasympathetic input to one pupil.
This can be caused by damage to the iris sphincter muscle on one side resulting from disease. Trauma, or surgery affecting either eye independently. Still, it may also involve both eyes simultaneously as part of Horner syndrome.
Unilateral small pupil
The small unilateral pupil occurs when a person’s pupil will not dilate in response. Light exposure regardless of which eye stimulus is exposed to light at any given time.
This condition produces asymmetric photophobia with decreased visual acuity. Impaired color vision compared with normal individuals under similar laboratory conditions. This disorder can occur during infancy, where there are developmental anomalies, or later in life due to brain injury.
- Caused by: Injury to the nerve that carries information from the pupil position in the ipsilateral eye. This nerve comprises two nerves, each carrying fibers for pupil constriction and pupil dilation, respectively.
Impaired pupillary light reflex
The impaired pupillary light reflex is characterized by a pupil that is non-reactive to light or a pupil that reacts only in the presence of a strong light stimulus.
- Caused by: An abnormality of the pupil light reflex pathway, such as damage to the ipsilateral optic nerve.
Relative afferent pupillary defect (RAPD)
The pupil may appear smaller than expected concerning the size of a normally reactive pupil.
The cause is an abnormal function or absence of one eye. Causing loss of consensual response and resulting in relative dilation on that side. It can be unilateral or bilateral.
- Caused by: Damage to either optic nerve from glaucoma, retinitis pigmentosa, trauma, etc., which results in a lesion affecting both eyes but only affects vision contralaterally so that there is no perception of light entering the damaged eye
This is divided into two.
Unilateral non-reactive pupil
The unilateral non-reactive pupil is the result of a lesion affecting one optic nerve. These pupil findings are seen in
- Optic neuritis
- Compressive lesions, such as aneurysm or tumor
- Papilloedema is associated with increased IOP, mainly when there is no other cause for raised pressure.
Caused by: Lesions involving third cranial nerve (locomotor) pathway at any site.
Bilateral non-reactive pupil
This condition is evident by bilateral non-reactive pupil findings.
- Caused by: It is caused by a lesion of the third cranial nerve, which may be above or below the nucleus in the midbrain. although a nuclear lesion will affect pupil size and reactivity more severely than an intranuclear one.