MYDRIASIS: CAUSES, UNILATERAL AND BILATERAL - MEDICINE - 2025

The mydriasis is increased or enlarged diameter center hole of the eye called pupil. The pupil is the opening in the colored portion of the eye, known as the iris. The iris has two muscle groups that enlarge or decrease the size of the pupil in a reflected response to the amount of ambient light.

Thus, when the environment is illuminated, the pupil closes regulating the passage of light. On the contrary, if the environment is dark or with little light, the pupil dilates to allow the passage of as much light as possible and improve vision.

From Raziel - Own work, CC BY-SA 3.0, The decrease in the diameter of the pupil is called miosis while the increase in its diameter is known as mydriasis. Under normal conditions, both miosis and mydriasis occur simultaneously, but there may be variations caused by medications or pathological conditions.

In the case of the effect caused by the drugs, it is usually reversible once the treatment is stopped.

When asymmetry in the diameter of the pupils is observed, the patient must be studied thoroughly to discover the cause of this sign called anisocoria.

Anatomy

The eye is a spherical organ that has a transparent specialized tissue called the cornea, which allows light rays to enter. It has a posterior component that receives and processes light stimuli. This area is called the retina.

Through complex neurological and physiological mechanisms, the eye allows the processing of stimuli and the clear vision of objects.

By Artwork by Holly Fischer - http://open.umich.edu/education/med/resources/second-look-series/materials - Eye Slide 3, CC BY 3.0, https://commons.wikimedia.org/w/ index.php? curid = 24367145

The colored portion of the eye is called the iris. The iris is made up of two important muscle groups that vary the size of the central opening of the iris, called the pupil.

The muscle group that is responsible for reducing pupil size is called the iris sphincter muscle or the pupil sphincter muscle, and the one that is responsible for increasing it is the iris dilator muscle or the pupil dilator muscle.

The closing and opening of the pupil is a reflex mechanism that responds to light stimuli. This reflex regulates the amount of light that enters the eye.

In a well-lit environment, the sphincter muscle of the iris regulates the passage of light that the eye receives, reducing the pupillary diameter. This process is known as miosis.

By MikeRun - Own work, image frames taken from, CC BY-SA 4.0, When the individual is in a dark environment, the dilator muscle of the pupil is in charge of expanding the size of the pupil to let in more light. The opening or increase in the diameter of the pupil is known as mydriasis.

The circumference of the pupil varies from 2 to 4 mm in response to bright light and 4 to 8 mm in the dark. When there are no pathologies, this size is the same for both pupils.

Mydriasis

Under normal conditions, mydriasis occurs as a response of the pupillary reflex. Whereas with exposure to light, the pupil contracts; in the absence of this, the pupil dilates.

Mydriasis is the normal response of the pupil dilator muscle to darkness. It occurs to let the greatest amount of light pass through and to be able to reproduce the image that is perceived through the eyes.

By Bin im Garten - Self-photographed, CC BY-SA 3.0, The reflex is carried out by the neurological response of a specialized component of the common motor-ocular nerve.

The ultimate goal of these responses to light stimuli is to allow adequate vision of objects both in environments with bright or exaggerated lighting, as well as in dark or low light environments.

Both in natural conditions and in those caused by the doctor in the clinical evaluation, mydriasis is in both eyes equally. That is why normal mydriasis is said to be symmetrical and bilateral.

When there is a problem or blockage in the neurological mechanism that regulates the activation of the dilator and constrictor muscles of the pupil, differences in the diameter of the pupils can be seen, a condition known as anisocoria, and, in severe cases, complete absence of response..

Abnormal mydriasis can be unilateral or bilateral and can be caused by pathophysiological, benign, or malignant causes, as well as pharmacological causes.

Unilateral mydriasis

- Pathophysiological causes

External compression of the common motorocular nerve

The common motor-ocular cranial nerve is a neurological component that has both motor and reflex functions.

It is responsible for innervating various muscles that allow voluntary movement of the eye and through a specialized branch, it innervates the muscles that allow the variation of pupil size.

From Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below) Bartleby.com: Gray's Anatomy, Plate 776, Public Domain, https://commons.wikimedia.org/w/index. php? curid = 899226

Compression of this nerve by an external mass, whether tumorous, malignant or benign, or vascular, as in the case of aneurysms, causes variations in the normal response of the pupillary reflex.

For example, if there is a tumor compressing the nerve of the right eye, that pupil will be unable to respond adequately to light stimuli, remaining open or in mydriasis, even when there is a significant light stimulus. In this case, the left pupil will have a normal response.

Adie syndrome

Also known as the tonic pupil of Adie, it is the most common cause of unilateral mydriasis caused by neurological degeneration.

Patients with this condition can be totally asymptomatic, and mydriasis is sometimes discovered by a third party who notices the difference in the patient's pupil size.

The syndrome is triggered by a viral or bacterial infection that causes neurological damage to the communication pathways of the iris muscles.

The pupil of these patients may have a slow response or be completely paralyzed to light stimuli.

Benign episodic mydriasis

Isolated and transient episodes of mydriasis are caused by overactivity of the neurological fibers that regulate the muscles of the iris.

The pathophysiological mechanism by which this response occurs is not fully understood. However, it is closely associated with conditions such as migraine headaches, diabetes mellitus, and high blood pressure.

In these cases the patient also has blurred vision, pain around the eye and sensitivity to light.

- Pharmacological causes

Atropine eye drops are used to cause the pupil to dilate during the physical examination in the ophthalmology office.

By NIH - http://www.nei.nih.gov/rop/photos.asp (follow page link to relevant subject; page may have changed since upload), Public Domain, https://commons.wikimedia.org/w /index.php?curid=4169250

The direct effect of this type of topical medication is mydriasis. When the pupil is dilated, a proper evaluation of the retina can be made through an examination known as the fundus.

Atropine drops are also used to treat some eye conditions.

Bilateral mydriasis

- Pathophysiological causes

Miller Fisher syndrome

Miller Fisher syndrome is a condition in which the body creates antibodies against its own nerves. This means that the body's protective system does not recognize nerve fibers and attacks them through special cells, causing them to be destroyed as if they were a foreign organism.

The patient with this condition presents three typical symptoms, which are uncoordinated movements, decreased or absent reflex response and pupillary mydriasis with paralysis to stimuli.

From NIH - http://openi.nlm.nih.gov/imgs/rescaled512/2939409_JOP2010-291840.001.png, Public Domain, Although its cause is not exactly known, it is associated with viral infections such as chicken pox, rubella, cytomegalovirus and HIV, among others.

Silvio aqueduct syndrome

The aqueduct of Silvio is a brain structure that serves as a passage for the flow of cerebrospinal fluid.

When this area of ​​the brain becomes blocked, a process of increasing the amount of intracranial fluid called hydrocephalus begins.

The increase in the amount of fluid inside the skull increases the intracranial pressure causing compression of the nucleus in which the nerves that regulate the movement of the pupil originate.

In these cases, dilated pupils are observed with little reaction to light stimuli.

Brain damage

When a patient with significant head trauma is physically examined, and his pupils are found to be bilaterally dilated and there is no evidence of response to stimuli, this is considered a sign of irreversible brain damage.

The dilation response occurs by an increase in normal pressure within the skull due to inflammation of the brain or blood pooling caused by trauma.

When mydriasis lasts more than 6 hours in this type of patient, it is an indicative factor of poor prognosis and most likely death of the injured person.

- Pharmacological causes

Some of the treatments for the control of psychiatric pathologies have an effect on the neurological signals that control pupillary movement.

Most of the time, the effect caused by these treatments is temporary and the response normalizes when they are stopped.

Tricyclic antidepressants, antipsychotics, and serotonin reuptake inhibitor drugs, as well as some migraine treatments, are drugs that are associated with bilateral mydriasis.

Illicit stimulant drugs such as amphetamines, cocaine and MDMA (ecstasy) cause significant dilation of the pupils, reaching a size that covers almost the entire iris.

Other drugs with hallucinogenic effect such as LSD (acid), hallucinogenic mushrooms and mescaline, are also causes of bilateral mydriasis.

In all these cases the pupils react to the light stimulus in a normal way and mydriasis improves when the trigger factor is removed. However, other symptoms, such as eye movements and trouble focusing, can persist and be irreversible.

References

1. Purves, D; Augustine, GJ; Fitzpatrick D. (2001). Neuroscience: Anatomy of the eye. Taken from: ncbi.nlm.nih.gov
2. Ludwig, PE; Motlagh, M; Czyz, CN. (2019). Physiology, Eye. StatPearls. Treasure Island (FL). Taken from: ncbi.nlm.nih.gov
3. Bloom, J; Motlagh, M; Czyz, CN. (2019). Anatomy, Head and Neck, Eye Iris Sphincter Muscle. StatPearls. Treasure Island (FL). Taken from: ncbi.nlm.nih.gov
4. Belliveau, AP; Somani, AN; Dossani, RH. (2019). Pupillary Light Reflex. StatPearls. Treasure Island (FL). Taken from: ncbi.nlm.nih.gov
5. Payne, WN; Barrett, MJ. (2019). Anisocoria. StatPearls. Treasure Island (FL). Taken from: ncbi.nlm.nih.gov
6. Skeik, N; Jabr, FI. (2011). Migraine with benign episodic unilateral mydriasis. International journal of general medicine. Taken from: ncbi.nlm.nih.gov
7. Athanasiou, A; Balogiannis, I; Magras, I. (2017). Lasting bilateral mydriasis after traumatic brain injury may not always be a lost case. Surgical neurology international. Taken from: ncbi.nlm.nih.gov