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why is one pupil dilated and not the other

There isn't enough information to answer with any specificity as more details are required. In the case where one pupil does not respond to dilating drops there are several possibilities. The iris could be partially stuck to the lens because of prior inflammation or trauma or prior surgery. The condition of one pupil being larger than the other without having received dilating drops is called. The causes of this are too numerous to discuss in detail. Most common is simple anisocoria, which is usually a small difference between the pupil sizes and is a normal variant. If this is a new observation then the ophthalmologist would have to determine if the abnormal pupil is the smaller one or the larger one.

A unilaterally constricted pupil can be caused by local ocular factors such as inflammation or trauma but also may be related to a failure of the dilator muscle due to a neurological condition such as Horner's syndrome. Horner's syndrome can be a benign condition or be caused by a vast array of things such as a tumor in the chest, carotid artery dissection, migraines, trauma and some medications to name just a few. It can also be congenital. Some drops and some systemic medications and drugs may cause a constricted pupil.

A unilaterally dilated pupil can be a benign finding such as a tonic pupil. It can be related to local factors such as a history of eye trauma or surgery. It can also be a result of paralysis of the iris constrictor muscle which may result from many neurological conditions but especially from involvement of the third nerve. If that is the case this can be an emergency resulting from a brain aneurysm. There are also many drugs and systemic medications that can cause a dilated pupil. The main point here is that differences in pupil size, reactivity, or reaction, while often benign, may be a harbinger of a very serious condition or even an emergent life threatening condition.

Examination by an ophthalmologist is required.
Nervous system disorders that cause unequal pupils are those that affect the 3rd cranial nerve or certain parts of the sympathetic or parasympathetic nervous system (the ). These pathways carry nerve impulses to the pupil and to the muscles that control the eye and eyelid. Thus, people with nervous system disorders that affect the pupil often also have a drooping eyelid, double vision, and/or visibly misaligned eyes.

Brain disorders that can affect these pathways include strokes, brain hemorrhage (spontaneous or due to head injury), and, less commonly, certain tumors or infections. Disorders outside the brain that affect the sympathetic nervous system include tumors and injuries that involve the neck or upper part of the chest. Horner syndrome refers to the combination of a constricted pupil, drooping eyelid, and loss of sweating around the affected eye. Horner syndrome is caused by interruption of the sympathetic nerves to an eye due from any cause.

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