Changes in the Pupil Due to Traumatic Brain Injury

Pupil changes due to traumatic brain injury are one of the most visible signs. In addition to a basic pupil exam, the presence of normal or reactive pupils does not rule out a traumatic brain injury. 

Certain brain injuries can cause impaired cerebral perfusion, which leads to a significant decrease in the amount of blood perfusing the brain and a consequent decrease in the amount of blood reaching the pupils. In this blog post, we will look at the changes in pupil response that manifest in several forms of brain injury and how these changes affect the patient.

What is a traumatic brain injury?

A traumatic brain injury is a condition in which a significant impact on the head causes damage to the brain. The damage to the brain can be caused by sudden movement, rotational acceleration, compression, or deceleration.

Traumatic brain injury is usually classified as mild, moderate, or severe based on the Glasgow Coma Scale (GCS).

What is a pupillary response?

A pupillary response refers to the changes in the percent change in pupil size that occur in response to various stimuli in normal individuals. Many stimuli can lead to pupillary constriction, such as light, near vision, and emotional states such as fear or anger.

Other stimuli can dilate the pupil, such as darkness, far vision, and head rotation. Normal pupillary responses are very complex and adaptive. The responses are divided into direct and consensual responses.

• Pupil dilation: This is usually due to the eye being directly exposed to an intense light source. Dilation is caused by the contraction of the dilator muscle. The dilator muscle contraction is controlled by parasympathetic signals from the Edinger-Westphal nucleus in the brain stem. This signals the release of norepinephrine (noradrenaline) from the nerve endings.
• Pupil constriction: This is usually a result of the eye being in a dark environment. Constriction is caused by the contraction of the sphincter muscle. The sphincter muscle contraction is controlled by sympathetic signals from the superior cervical ganglion. This signals the release of acetylcholine from the nerve endings. Pupil constriction or dilation can be observed as a result of accommodation.

Types of abnormal pupillary responses

Just like there are normal pupillary responses, there are also abnormal responses. Abnormal pupillary responses are usually caused by parasympathetic and sympathetic nervous systems problems. They include

1. Anisocoria

Anisocoria is a pupillary response in which both pupils have different responses. In other words, one pupil dilates, and the other constricts or vice versa.

Anisocoria is usually caused by the mechanical compression of one eye’s iris due to a foreign object. If the object is small, one pupil will constrict, and the other will dilate.

2. Relative Afferent Pupillary Defect (RAPD)

The pupillary defect associated with the relative afferent pupillary defect is caused by dysfunction in the parasympathetic nervous system instead of the sympathetic nervous system.

The term “relative” is used because the pupil’s response depends on the response of the fellow pupil. For example, if the right pupil dilates, the left pupil will be smaller than usual. If both pupils dilate, the right pupil becomes larger than usual.

3. Adie’s (Tonic) Pupil

Adie’s pupil is a form of relative afferent pupillary defect. The main difference is that this pupil has a slow onset and a long-lasting effect. This pupil usually occurs at rest, and in the dark, the pupil dilates.

Adie’s pupil is often caused by damage or dysfunction to the bilateral Edinger-Westphal nucleus or fibers that control the pupillary response.

4. Argyll Robertson Pupil

The Argyll Robertson pupil is associated with hepatic encephalopathy. It causes a small pupil and constriction of the iris sphincter, the constriction occurs in dim light and at rest, but it relaxes with bright light.

Argyll Robertson pupils are caused by bilateral damage to the parasympathetic fibers of the oculomotor nerve between the third cranial nerve and the Edinger-Westphal nucleus in the midbrain.

The use of the pupilometer in diagnosing traumatic head injuries

The pupilometer is a wonderful invention that has been able to help diagnose traumatic head injuries. It is a device that shines a bright light into the eye and measures the size of the pupil. The larger the pupil, the more serious the head injury. The pupilometer has been used to help diagnose traumatic brain injury and concussion and monitor pupil changes as the injury heals.

Conclusion

The pupilometer is a helpful tool for doctors to check pupil dilation velocity when dealing with patients suffering from traumatic brain injuries. The pupilometer provides a standardized and objective measurement of the diameter and reactivity of pupils as well as other ocular function tests.