May 31, 2023


Health Lasts Longer

Causes of Abnormal Pupil Reactions

Dilated Pupils: Causes and What to Do

People might experience a wide range of unusual pupil responses that could signal an eye disease. Two primary cranial nerves govern the pupil: the oculomotor nerve and the sympathetic nerves. Depending on how much light enters the pupil, the pupillary light reflex to light changes, sending information back to the brain for processing.

Pupil evaluation, pupillary reflexes, and some frequent anomalies of pupil responsiveness are all covered in this article.

What does an abnormal pupillary response mean?

Pupils that do not react appropriately to light have an abnormal pupillary response, manifesting as an enlarged pupil. Pupils may range in size from tiny to gigantic. Your pupil may seem this way for a variety of reasons, and your brain’s processing of visual information will be impacted as a result.

Pathways of the pupillary reflex

  • In order to reach both sides of the midbrain’s superior colliculus, afferent impulses go down cranial nerve II (optic) from the retina. In the midbrain’s superior colliculus, efferent limbs receive these instructions, which cross over before travelling down cranial nerve III (oculomotor). Additionally, signals are sent along these pathways to govern eye movements in addition to regulating pupil size.
  • An eye’s pupils widen when just one side of the body is targeted by light stimulation, and they close when both eyes get it.

Examination of pupil reactivity

1. General pupil evaluation

To check for the pupillary reflex, shine a strong flashlight into each eye and time how fast the pupil dilates in response to the light (light reflex). Try covering one eye at a time to check if there is any change in vision while the other eye remains open.

This would suggest that your brain has proper vision in both eyes but that the afferent or efferent limb may still be malfunctioning.

2. Pupillary observation

Next, keep an eye out to see whether they always react in unison (tonic pupil) or not (narrow pupil, dilated pupil).

Pupillary reflexes

You may further conduct pupil measurement by determining whether it reacts to accommodation and convergence normally. Three reflex tests are available:

  • Light reflex: Look at how the other pupil reacts when you shine a strong light into one of their eyes.
  • Swinging flashlight test: Each eye should be covered by an object held at arm’s length, first with the left eye and then the right one. Begin by identifying which pupil is dilated (usually, if you cover it for more than three seconds). To determine whether both eyes function normally, look to see if the pupil constricts while exposed.
  • Near reflex test: Pupil diameter may be determined using this method. Your patient may stare at any little item, such as a wristwatch, while you cover each pupil alternatively while measuring their pupil size. After a few seconds, the pupil will seem to jerk forward and then back.

Abnormal pupillary responses

We are now going to examine some of the abnormal pupil responses that have been documented.

1. Anisocoria

There is a difference in pupil size between looking straight ahead and looking up at the ceiling; one pupil does not dilate (constrict) in response to light or near pupil measurement, and one pupil is bigger than the other. Because pupillary constriction happens less often with a higher degree of anisocoria, it takes more effort.

Illness, trauma, or surgery on one side of the face may damage the iris sphincter muscle on the other side of the face, but Horner syndrome can also cause this.

2. Unilateral large pupil

Unilaterally big pupils are those that are larger in the dark and smaller when light is shining into either eye but only in one eye.

Since parasympathetic signals cannot reach one of the eyes, the pupil has become bigger as a result.

The iris sphincter muscle on one side of the eye may be damaged by illness, trauma, or surgery on the other eye. Horner syndrome, on the other hand, may affect both eyes at the same time.

3. Unilateral small pupil

In those with tiny unilateral pupils, no matter which eye stimulation is exposed to light, the pupil will not enlarge in response to light.

This syndrome causes asymmetric photophobia, with reduced visual acuity and poor colour vision when compared to normal people under the same laboratory circumstances. As early as infancy, this condition may manifest itself as

In the ipsilateral eye, this is caused by damage to the nerve that provides information about the pupil position. Two nerves make up this nerve, and each one carries fibres that control the dilatation and constriction of the pupil.

4. Impaired pupillary light reflex

Having a pupil that doesn’t respond to light or a pupil that only responds in the presence of a strong light stimulus is an indicator of the defective pupillary light response.

Optic nerve injury in the ipsilateral eye might induce this anomaly in the pupil light reflex circuit.

5. Relative afferent pupillary defect (RAPD)

When compared to the typical pupil size, the pupil may look smaller than anticipated. Because of the lack of consensual response and accompanying relative dilatation on one side, the reason might be either unilateral or bilateral: an aberrant function or absence in one eye.

As a consequence of an optic nerve lesion affecting both eyes but just the vision on the other side of it, there is no experience of light entering the affected eye when this happens.

Non-reactive pupil

There are two parts to this.

1. Unilateral non-reactive pupil

Having a non-responsive pupil on one side of the face is caused by an optic nerve injury. Findings like these may be seen in pupils.

Compressive lesions, such as an aneurysm or a tumour, as well as papilloedema, are all linked to elevated intraocular pressure (IOP). However, this is more often the case when there is no other explanation for the elevated pressure.

Third cranial nerve (locomotor) lesions at any location are the cause.

2. Bilateral non-reactive pupil

Bilateral non-reactive pupillary results reveal this syndrome. An intranuclear or nuclear third cranial nerve lesion is to blame for this condition, which affects pupil size and reactivity more than an intranuclear lesion.


There are many causes of abnormally fast reactions and pupil reactions. Fortunately, most people know of their causes in advance. This article has explored what causes abnormal pupil reactions and how experts use pupillary size measurement to treat them.