A hemifacial spasm is a nervous system disorder in which the muscles on one side of the face start to twitch involuntarily. Hemifacial spasm may be caused by a blood vessel touching a facial nerve, a facial nerve injury or a tumor, or it may not have a cause.In a majority of the cases, the spasm starts near the eye and progresses down the face over time. Sometimes, it starts near the chin and progresses upward. The twitching is usually not painful, but it can be embarrassing and interfere with normal expression and vision.Hemifacial spasm can be caused by injury to the facial nerve, a tumor or blood vessel compressing the nerve, or Bell’s palsy. The most common cause is compression of the facial nerve by the anterior inferior cerebella artery where the nerve begins at your brainstem. The compression causes the nerve to misfire making your facial muscles contract. The most important symptom is the twitching that usually begins around the eye. At first the twitching spasms may come and go. Gradually the spasms may become worse and the spasm may become permanent. The left side is more often affected than the right. The twitches may spread to involve other muscles on the same side of the face. The mouth and jaw are often involved. The corner of the mouth can become pulled up by permanent spasm. Some people also hear a clicking sound on the side that is affected when the spasm comes. In some cases the spasms may affect vision as well.