Miosis is the medical term for the constriction of the pupil, the black circle in the center of the eye. This size change is controlled by muscles within the iris, the colored part of your eye. Two specific muscles, the iris sphincter and the iris dilator, work in opposition to adjust the amount of light that passes through the pupil to the retina. Miosis is defined as the pupil shrinking to a diameter of two millimeters or less.
Physiological Causes of Miosis
Miosis is frequently a normal and automatic response to environmental changes, with the pupillary light reflex being a well-known example. When a bright light enters the eye, photosensitive ganglion cells detect the intensity and signal the brain. The brain then activates the iris sphincter muscle to constrict the pupil, limiting light exposure and protecting the sensitive retinal tissue.
Another normal cause is the accommodation reflex, which happens when you shift your focus from a distant object to one that is nearby. To see a near object clearly, the eyes converge, the lens changes shape, and the pupils constrict. This constriction helps prevent diverging light rays from the close object from scattering across the retina, which would result in a blurred image.
Finally, pupil size naturally changes with age. As individuals get older, the muscles that control the pupil can lose some of their elasticity and strength. This often leads to age-related miosis, where the pupils become smaller and less responsive to changes in light. This is a reason why older adults may find it more difficult to see in low-light conditions.
Pathological Causes of Miosis
While often normal, persistent miosis can also be a sign of an underlying health issue. These causes are distinct from standard physiological responses, and distinguishing between them is an important part of a medical evaluation.
One category is medications and substances. Opioid drugs, including morphine, fentanyl, and heroin, are well-known for causing bilateral, or “pinpoint,” pupils. Certain prescription medications can also induce miosis, including miotic eye drops like pilocarpine used for glaucoma, as well as some antipsychotic and high blood pressure medications.
Various medical conditions can lead to abnormal pupillary constriction. Neurological issues are a concern, with Horner’s syndrome being a notable example. This condition results from damage to the sympathetic nerves connecting the brainstem to the face and eyes, causing a triad of symptoms on one side of the face: a constricted pupil, a drooping eyelid, and decreased sweating. Other causes can include inflammation of the iris (uveitis or iritis), brainstem strokes, and infections like neurosyphilis.
Exposure to certain toxins is another cause. Organophosphate insecticides and some types of nerve agents are chemicals that can disrupt the nervous system’s control over the iris muscles, leading to pronounced miosis. This type of exposure is considered a medical emergency.
Associated Symptoms and Diagnosis
Pathological miosis is often accompanied by other signs that point toward the underlying cause. Associated symptoms may include a drooping eyelid (ptosis), eye pain, sensitivity to light, blurred vision, redness of the eye, or headaches. In the case of Horner’s syndrome, a sign is that the difference in pupil size between the two eyes becomes more pronounced in the dark.
Diagnosing the cause of miosis begins with a physical examination, where a doctor will assess pupillary reactions to light and near objects, as well as eye movements. The patient’s medical history, including any medications or substance use, is reviewed. To confirm a diagnosis of Horner’s syndrome, a pharmacological test may be used. The apraclonidine eye drop test is a diagnostic tool; in an eye affected by Horner’s syndrome, the pupil will dilate in response to the drops. If a neurological cause like a stroke or tumor is suspected, imaging tests such as an MRI or CT scan may be ordered.
Management of Miosis
The approach to managing miosis is not to treat the small pupil itself but to address the root cause. Because miosis is a symptom, its resolution depends on successfully managing the underlying condition.
For instance, if miosis is determined to be a side effect of a particular medication, a doctor may adjust the dosage or switch to an alternative drug. In cases of uveitis, or inflammation of the iris, treatment involves anti-inflammatory eye drops, often containing steroids, to reduce the inflammation. Should the cause be an infection such as neurosyphilis, a course of antibiotics would be prescribed.
When a more serious condition like a brainstem stroke or a tumor causing Horner’s syndrome is identified, management becomes more involved, potentially requiring surgery or other specialized neurological interventions. A prompt medical evaluation is important whenever unexplained miosis is observed, as a proper diagnosis is the first step toward the correct course of action.