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What is Miosis Eye (Pupillary Constriction)? Is miosis a serious medical condition?

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Author : Dr Deepak Garg
Pupil Miosis

The term “Miosis” is used to describe the constriction or narrowing of the eye’s pupil, the central hole in the iris (the colored part of the eye). In non-medical terms, this is referred to as ‘pinpoint pupils.’ This phenomenon can be simply described as the opposite of dilated pupils or mydriasis. The autonomic nervous system regulates this natural mechanism, controlling how much light enters the eye and, consequently, how the lens of the eye focuses. The signal for regulating pupil size is initiated only when the optic nerve of the eye is healthy. Thus, pupil reactions are significant in medicine.

The pupil contracts in bright light to block an excessive amount of light, while it dilates in dim light to let more light in for clearer vision. In ambient light conditions, the normal pupil size is 2-3mm. Miosis or pinpoint pupils are a natural and necessary function of the eye, but they can also be a sign of several different illnesses. For instance, they might indicate an underlying condition such as inflammation, nerve injury, or the use of specific treatments or medications. Persistent pinpoint pupils or miosis may present a dangerous medical problem, depending on the underlying cause, and necessitate urgent treatment and evaluation by an eye care specialist to address any potential underlying issues.

Causes of Miosis or Constricted pupils:

Exposure to intense light is the most common cause of this miosis, although other factors can also contribute to it. The pupils automatically contract in response to bright light to reduce the amount of light entering the eyes and minimize the risk of harm to the sensitive internal structures. Sometimes, infants are born with constricted pupils, a condition known as congenital miosis. Congenital miosis, or microcoria, is a developmental ocular anomaly where the pupil size is smaller than 2mm.

  • Miosis due to Horner Syndrome:

Horner Syndrome is a condition that affects the nerve innervation of the face and eye on the affected side. The cranial nerve responsible for eyelid movements and pupillary dilation is the oculomotor nerve. In Horner syndrome, it’s the sympathetic nerve that travels through the oculomotor nerve that is specifically affected. Consequently, the pupils in Horner Syndrome are constricted, a phenomenon known as miosis. Along with a constricted pupil, the affected eye appears smaller due to drooping of the upper eyelid.

Horner Syndrome can be diagnosed by your eye doctor using a swinging flashlight test. Abnormal miosis can occur due to weakness in the iris dilator muscles, which are responsible for pupil dilation, and this may cause miosis. Patients commonly complain that the affected eye appears smaller than the other. If this happens suddenly and does not resolve, it is important to seek medical attention as soon as possible. Prior knowledge of the sympathetic pathway is important for localizing the lesion and understanding the rationale behind the clinical presentation of Horner syndrome.

  • Miosis Caused by drugs:

Miosis can be caused by recreational drugs such as organophosphates, benzodiazepines, and clonidine. Pilocarpine is another in-office eyedrop used to purposefully constrict the pupils in certain eye conditions. Certain prescription drugs like morphine, heroin, and hydrocodone can also cause miosis. Hypertensive patients who are on blood pressure medication, such as clonidine, may experience pupillary miosis. Some studies have revealed that chronic cigarette smoking can lead to miotic pupils due to the influence of nicotine over time. Therefore, avoiding cigarette smoking can reduce the likelihood of developing miotic pupils.

  • Miosis Caused due to uveitis:

Uveitis is a group of diseases caused by inflammation of the middle layer of the eye. The most common cause of miosis is anterior uveitis, which involves the inflammation of the iris. This is an example of abnormal miosis. If there is active inflammation, your eye doctor might prescribe appropriate anti-inflammatory drugs to prevent this abnormal miosis. Occasionally, the affected person may experience blurred vision, which typically improves as the drops are administered. Increased intraocular pressure may also occur, leading to pain in the affected eye.

  • Miosis Caused due to brain injury:

The control centers of all the nervous innervation to the eye are situated in the brain, thus any head injury involving the control centers in the brain can cause miosis. Especially the sympathetic nervous system. Additionally, miosis can also be caused due to a stroke, internal bleeding in the skull or brain. Thus, the first thing any healthcare professional checks in head injury cases is pupils, because that can tell how bad the impact has been even, furthermore, can allow the doctor to give you the prognosis of the individual. Any severe head injury, especially if there was loss of consciousness, an immediate medical attention must be seeked.

  • Miosis Caused due to Neurosyphilis (Argyll Robertson Pupil):

Syphilis is a sexually transmitted disease commonly treated with antibiotics in its early stages. In the later stages of the disease, it can reach the nervous system through the cerebrospinal fluid, which is a clear and transparent fluid that nourishes the brain. The bacteria eventually affect the sympathetic nervous system, leading to miosis of the pupil. This results from the iris sphincter muscle constriction due to the inability of the iris dilator muscles to function.

Diagnosis of Miosis:

Similar to mydriasis, the diagnosis of miosis can be made by a healthcare professional through a simple torchlight examination that assesses the pupils’ response to light.

Therapeutic Miosis:

  • Getting Rid of Reading Glasses:

Recently, a pupil constricting eye drops containing Pilocarpine Hydrochloride has been approved by the US FDA for treatment of reading number which patients get after the age of 40 years. It is a prescription drug effective only in early stages of reading glasses where your power is around +1.50. These eye drops are yet to be launched in India and are currently prescribed only in the US. These eye drops are not a replacement to reading glasses, rather a temporary solution to it only in earlier years of presbyopia.

  • Getting rid of Glare:

In certain individuals, the size of the pupil is larger than normal, causing significant glare due to optical discrepancies in the eye known as aberrations. To mitigate these aberrations, pupil-constricting eye drops are prescribed, effectively reducing glare to a significant extent.

  • Glaucoma Management:

In cases of angle-closure type glaucoma, where the drainage area for the clear fluid inside the eye is too narrow, pupil constriction can help open up this area, lowering the pressure inside the eye to a certain extent.

Treatment of Miosis:

The treatment for miosis depends on its underlying cause. Addressing the root problem is often necessary to treat constricted pupils. For instance, discontinuing the use of drugs that cause pupil constriction is typically recommended. In the case of eye disorders like uveitis or iritis, your eye doctor may prescribe medications, often in the form of eye drops, which may include anti-inflammatory drugs like steroids or anti-infectives. Sometimes, migraines can cause constricted pupils, so treating the migraine can resolve the miosis. If you have Horner’s syndrome related to a tumor, your doctor might recommend surgery. Additionally, in the case of strokes, your doctor may use drugs or techniques to dissolve or remove blood clots for treatment.

Frequently Asked Questions

1. What is the difference between mydriasis and miosis?

Mydriasis is when the clear hole in the iris, called the pupil, is enlarged or dilated, whereas Miosis is when the pupils are constricted.
Learn more about Mydriasis here “

2. What should I do if I notice persistent or unexplained miosis in my eyes?

Persistent, unexplained miosis of the eye requires medical attention; therefore, report to your eye doctor as soon as possible.

3. Can stress or anxiety cause temporary miosis?

Stress and anxiety can cause our body to release norepinephrine, which induces a fight-or-flight response. This makes our nervous system alert and causes the pupils to constrict. This type of pupil constriction is natural and temporary, and once the stressor is gone, pupils return to their normal size.

4. Are there specific eye drops or medications used to dilate pupils during surgery?

Yes, just before your surgery, depending on the type of procedure being performed, the surgeon can pharmacologically dilate your pupils.

5. Is miosis reversible or permanent?

Miosis is a natural response of the iris muscles, just like mydriasis, and in the absence of any comorbidities, miosis is a reversible phenomenon.

6. Can miosis affect both eyes at the same time?

Miosis is a phenomenon that occurs in both eyes simultaneously. It is a binocular phenomenon.

7. What is the size of a miotic or constricted pupil?

A pupil size of 3mm and less is known as a miotic pupil.

8. How does miosis affect vision?

A miotic pupil behaves like a pinhole, which is known to reduce the extra rays of light entering the eye, thereby reducing the aberrations of the eye. This is known to improve the sharpness of vision. However, it’s worth noting that prolonged constricted pupils may not be healthy in the long term.

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