Blephaospasm is a chronic dysfunction of the nerve that controls the muscles around the eye. It manifests itself as an uncontrollable, forcible closure of the eyelids. It often affects both eyes at once, but it can also affect only one eye. In some cases, other muscles in the face can twitch as well, especially around the cheek and brow. Severe blepharospasm can cause the eyelids to be forcibly closed for a duration longer than the typical blink reflex, causing a variable interruption in the ability to see.

People with blepharospasm have normal vision. Visual disturbance is due solely to the forced closure of the eyelids.

Blepharospasm affects the eye muscles and usually begins gradually with excessive blinking and/or eye irritation. In the early stages it may only occur with specific precipitating stressors, such as bright lights, fatigue, and emotional tension. It is almost always present in both eyes.

As the condition progresses, it occurs frequently during the day. The spasms disappear in sleep, and some people find that after a good night's sleep, spasms do not appear for several hours after waking.

If blepharospasm causes any type of impairment, it is because muscle contractions interfere with normal function. Cognition, strength, sensory function, vision, and hearing are not affected.