Miosis is the constriction or narrowing of the pupil, the dark center of the eye. This adjustment is an involuntary reflex that regulates the amount of light entering the eye. The pupil’s ability to change size is a natural protective mechanism, ensuring the light-sensitive retina receives an appropriate level of stimulation. The size of the pupil normally ranges between 2 millimeters in bright light and 8 millimeters in darkness for a healthy adult. Miosis is one half of the eye’s delicate balance to control visual input, automatically responding to environmental changes to optimize vision.
The Physiological Mechanism of Pupillary Constriction
The control of pupil size is managed by the autonomic nervous system, specifically through a balance between the sympathetic and parasympathetic branches. Pupillary constriction is primarily mediated by the parasympathetic nervous system, which acts as the brake on the pupil’s size. The process begins when light hits the retina, sending a signal through the optic nerve to the brainstem.
This signal ultimately reaches the Edinger-Westphal nucleus, a structure near the oculomotor nerve nucleus. Nerve fibers then travel with the oculomotor nerve (Cranial Nerve III) and synapse at the ciliary ganglion inside the orbit. From this ganglion, postganglionic fibers travel to the sphincter pupillae muscle, a circular band of muscle fibers within the iris.
When the sphincter pupillae muscle contracts, it acts like a drawstring, physically reducing the pupil’s diameter. This action is unopposed by the sympathetic nervous system, which normally works to dilate the pupil via the radial dilator pupillae muscle. Miosis also occurs as part of the accommodation reflex, which happens when the eye focuses on a near object, ensuring a clear image.
Common Medical Causes of Abnormal Miosis
When miosis occurs abnormally, it is often a sign of an underlying medical issue or external influence, rather than a normal response to light. Pharmacological agents are one of the most frequent causes of pathological miosis. Opioids, such as morphine, heroin, and fentanyl, are well-known to cause miosis by activating parasympathetic receptors in the eye.
Other medications can also induce this effect, including certain drugs used to treat glaucoma, like pilocarpine, which directly stimulate the pupillary sphincter muscle. Exposure to toxic substances, such as organophosphate insecticides, can lead to severe miosis by inhibiting an enzyme called acetylcholinesterase. This causes an overstimulation of the parasympathetic system. This drug-induced constriction is often bilateral.
Neurological conditions represent another category of abnormal miosis, typically resulting from a disruption of the sympathetic pathway that normally promotes dilation. Horner’s syndrome is the most common example, caused by damage to the sympathetic nerve chain anywhere from the brainstem down to the eye. The loss of sympathetic input leaves the parasympathetic system unopposed, causing the pupil to constrict and often resulting in a slight drooping of the eyelid.
Miosis can also be caused by local eye inflammation, such as in cases of uveitis or iritis, where the iris itself is swollen. The inflammation can cause the iris muscles to spasm or become sticky, forcing the pupil into a constricted state. Brainstem strokes, particularly those affecting the pons, can also result in severely constricted pupils due to damage to central control centers.
Distinguishing Miosis from Other Eye Changes
Miosis must be distinguished from other changes in pupil size and symmetry. Mydriasis is the opposite condition, referring to pupil dilation, which is driven by the sympathetic nervous system. Anisocoria is a condition where the two pupils are of unequal size, which can be caused by either abnormal miosis in one eye or abnormal mydriasis in the other.
The clinical context is important for diagnosis. If anisocoria is present and the pupil difference is greater in dim light, the smaller, miotic pupil is usually the abnormal one, suggesting a problem with sympathetic dilation, such as Horner’s syndrome. Conversely, if the size difference is greater in bright light, the larger, non-constricting pupil is the one with the problem, suggesting a failure of the parasympathetic system.
Sudden, newly acquired miosis, especially when it affects only one eye, requires immediate medical evaluation. When unilateral miosis is accompanied by symptoms like pain, double vision, or a drooping eyelid, it may signal a serious underlying neurological issue.