Miosis is a condition in which the pupils of the eyes are always too small, whether in light or dark. Small pupils let in less light to the eye, making it more difficult for someone to see clearly.
Age, injuries and inflammation to the eyes, some medications and a family history cause most of the cases of miosis.
Any consistent presenting of miosis is cause to see a doctor because it could be a symptom of another underlying condition that is more serious.
What Is Miosis?
Miosis is an eye condition in which the pupils are routinely too small for the lighting conditions.
Under normal circumstances, the pupil changes size between 2 to 8 mm throughout the day, depending on lighting conditions. Normal pupil size in adults ranges from 2 to 4 mm in bright light and from 4 to 8 mm in dark conditions. In most cases, the pupils of both eyes are the same size, regardless of the amount of light.
The pupil shrinks in bright light to protect the eye from excess light from entering the eye. When a pupil stays shrunken even when not in bright light, a condition of miosis, that indicates an underlying problem.
Doctors consider pupils abnormal if they do not dilate in the dark and if they do not constrict in brightly lit conditions.
The leading causes of miosis include age, inflammation, medication, cluster headaches, head injuries, neurosyphilis and a family history of the condition.
Age-related miosis is common in newborns and in older adults. A newborn’s pupils stay small for up to two weeks after birth. The older a person gets, the smaller their pupils become because of weakening dilator muscles in the iris.
Injuries to the eye or other maladies that cause swelling will stop the pupil from widening when it needs to. Uveitis is a condition that causes swelling in the iris and the tissues surrounding it.
Healthy people can develop eye inflammation, but when it presents it can indicate a larger issue. Research has linked eye inflammation with other conditions such as HIV, tuberculosis, rheumatic disease, lung disease, shingles and gastrointestinal disease.
Inflammation mostly happens in younger and middle-aged people. It can happen in one or both eyes, with accompanying symptoms such as:
- Blurred vision
- Pain in the eye
- Eye redness
- Light sensitivity
Certain types of medications, like those that treat muscle spasms, anxiety and seizures, cause the shrinking of the pupils. Examples of these medicines include diazepam and antihistamines. Similarly, narcotics can also cause miosis.
Medicated eye drops are not usually in this group. In fact, eye doctors use drops for the opposite effect, to widen pupils, so that they may get a better look inside the eye to detect any abnormalities.
These kinds of headaches cause severe pain above or around the eye, forehead or temple. Cluster headaches only occur on one side of the head and can be chronic or episodic.
One common accompanying symptom of a cluster headache is miosis. Some cluster headache symptoms include:
- Eye redness
- Drooping eyelid
- Runny nose
- Mood change
- Sensitivity to light and sound
Neurosyphilis takes place when an untreated syphilis infection reaches the brain. The nervous system invasion can happen at any stage of the infection. Once inside the midbrain, the infection causes a type of miosis known as the Argyll Robertson pupil.
When you contract this type of miosis, your pupils do not shrink when exposed to light. However, they do contract when focusing on a nearby object.
You can be born without the muscles that control the pupil. Muscles could be missing entirely or have a defect, a condition known as microcoria, or congenital miosis.
One or both parents can pass on the defect to a child. It can also happen in either one or both eyes. Most nearsighted people have this condition.
People who have congenital miosis may also experience abnormal development of the whole anterior eye segment and are likely to have a higher risk of getting glaucoma.
After a head injury, one of the first things a doctor checks is pupil dilation. How the pupils respond will point to a severe head injury or not.
Pinpoint pupils are an indication of hemorrhaging or an eye injury. Note that opiates given to manage the pain can also cause shrinking pupils.
This rare condition affects how the brain controls one side of the brain, including the eyes. It can make one pupil smaller than the other. Although it is generally genetic, it also presents because of a head or neck injury.
A person can also develop Horner’s syndrome if their neck, brain or chest does not develop properly. Kids with a rare kind of cancer called neuroblastoma can also develop this condition.
In addition to shrunken pupils, patients with Horner’s syndrome may show symptoms such as:
- Raised lower eyelid
- Droopy upper eyelid
- Lighter eye color in the eye with a shrunken pupil
- Little to no sweat on the side with a miotic eye
Exposure to Pesticides
Another cause of miosis is exposure to pesticides. In addition to shrunken pupils, patients may exhibit other symptoms of pesticide exposure, including:
- Breathing difficulties
- Retching and vomiting
- Rapid pulse
- Muscle weakness
When to See a Doctor
Any case of miosis is reason enough to see an eye doctor. Constricted pupils are not a disease, but they can indicate an underlying medical condition that could be more threatening to eyesight or overall good health.
Anyone experiencing Miosis for no apparent reason should also seek medical attention.
Miosis could indicate severe medical conditions like pesticide poisoning or opioid dependency. Left untreated, they can lead to life-threatening situations. If someone is suspected of being poisoned by a pesticide, they need to seek emergency medical attention immediately.
Similarly, you should see a doctor when you contract a disease that causes miosis, such as anterior uveitis. Some of these diseases can cause blindness and permanent eye damage if left untreated.
Argyll Robertson Pupils. (March 2022). American Academy of Ophthalmology.
In Case of Pesticide Poisoning. (November 2021). U.S. Environmental Protection Agency.
Horner Syndrome. (May 2021). StatPearls.
Successful long-term management of iris floccule and miosis in a patient with a strong family history of thoracic aortic aneurysms and dissections associated with an MYH11mutation. (June 2014). JAMA Ophthalmology.
Small Pupils Importance: How Pupil Dilation Affects Iris Biometrics. (October 2010). The University of Notre Dame.
Autosomal dominant congenital miosis with megalocorna. (June 1992). Ophthalmic Pediatrics and Genetics.
Chapter 58: The Pupils. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. (1990). Spector, Robert. Butterworth Publishers.
Last Updated May 3, 2022
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