Hemifacial spasm causes muscles on only one side of the face to contract. It affects both men and women and usually develops in middle age. More than 4000 people in the UK are thought to be affected. Hemifacial spasm develops gradually. Initially the muscles surrounding the eye may be affected by muscle spasms, which continue to spread and affect other muscles on the same side of the face, especially the jaw and mouth. Some patients may experience a clicking sound in the ear on the affected side each time a muscle contracts.