Hemifacial spasm occurs when the facial nerve (cranial nerve VII) becomes irritated or compressed, usually by a small blood vessel near its starting point at the brainstem. This irritation triggers irregular signaling to the facial muscles, causing unexpected and uncontrollable twitching or contractions.
Although the condition can develop gradually, it tends to worsen over time if left untreated. In some cases, hemifacial spasm may happen after a nerve injury or inflammation. Though not life-threatening, hemifacial spasm can be physically and emotionally distressing, interfering with speech, blinking, and daily interactions.