context: Many people at some point experience spasm-like movements of particular muscles. These movements, known as tics and twitches, often affect the eyelids or face. They can, though, occur anywhere in the body. In most instances, tics and twitches are harmless and temporary. In some cases, though, they may be caused by a tic disorder. Tic disorders generally can be managed with treatment and lifestyle changes. While many people use the terms tic and twitch interchangeably, there are differences between these two forms of movements. Tics. There are two types of tics -- motor tics and vocal tics. These short-lasting sudden movements (motor tics) or uttered sounds (vocal tics) occur suddenly during what is otherwise normal behavior. Tics are often repetitive, with numerous successive occurrences of the same action. For instance, someone with a tic might blink his eyes multiple times or twitch her nose repeatedly. Motor tics can be classified as either simple or complex. Simple motor tics may include movements such as eye-blinking, nose-twitching, head-jerking, or shoulder-shrugging. Complex motor tics consist of a series of movements performed in the same order. For instance a person might reach out and touch something repeatedly or kick out with one leg and then the other. Tics are often classified not as involuntary movements but as unvoluntary movements. This means that people are able to suppress the actions for a time. The suppression, though, results in discomfort that grows until it is relieved by performing the tic. While people of all ages can experience tics, they are most prevalent in children. Experts say that around 25% of children experience tics. And tics are far more likely to affect boys than girls. No one knows exactly what causes tics to occur. Stress and sleep deprivation seem to play a role in both the occurrence and severity of motor tics. Doctors once believed that certain medications, including some used to treat attention deficit hyperactivity disorder, induced tics in children that were prone to them. Newer studies, though, suggest this is not the case. Twitches. Unlike tics, the majority of muscle twitches are isolated occurrences, not repeated actions. Muscle twitches are also known as myoclonic jerks. They are entirely involuntary and cannot be controlled or suppressed. One type of muscle twitch is benign essential blepharospasm. Blepharospasm refers to the muscles of one or both eyelids twitching uncontrollably. This often occurs repeatedly over a sustained period of time. In extreme cases, which are rare, benign essential blepharospasm may also involve the eyebrows, mouth, and neck. While an eyelid twitch may mimic an eye-blinking tic, it is different because it cannot be controlled. It also occurs most often in adults. Your doctor may be able to determine whether you or your child is experiencing tics or an eyelid twitch based upon the symptoms. Experts believe that the eyelid twitching of blepharospasm is caused by the misfiring of certain cells in one area of the brain. Eyelid twitches may be aggravated by having dry eyes. They may also be worsened by stress, lack of sleep, caffeine, and harsh light conditions. The majority of tics are not severe. So they have very little effect on a person's quality of life. In some instances, though, tics may occur often enough to be disruptive and troubling. When they do, they can affect many areas of a person's life, including school, work, and social life. Doctors use four characteristics to identify and diagnose tic disorders: the age when tics began duration of the tics severity of the tics whether tics are motor or vocal or both Transient tic disorder. This disorder most commonly appears in youth. It affects between 5% and 25% of school-age children. Transient tic disorder is characterized by the presence of one or more tics for at least one month but less than one year. The majority of tics seen in this disorder are motor tics, though vocal tics may also be present. Many children with the disorder experience multiple episodes of the transient tics, which may vary in how they manifest over time. Chronic motor or vocal tic disorder. While transient tics disappear within a year, chronic tics can last for a year or more. Chronic tic disorder is characterized by the presence of one or more long-lasting tics. They may be either motor or vocal, but not both. For a diagnosis of chronic tic disorder, symptoms must begin before age 18. Chronic tics occur in less than one in 100 children. Tourette's syndrome. In some instances, what appears to be a chronic tic may be a sign of Tourette's syndrome. This syndrome is the most severe tic disorder. It is characterized by the presence of both motor tics and vocal tics. Since many people with the disorder have not been diagnosed, it is unknown exactly how many people in the U. S. are living with Tourette's syndrome. Experts estimate that around 200,000 people in the U. S. have the condition. Symptoms typically begin when children are between ages 5 and 18 years. The severity of Tourette's syndrome often changes over time. There may be periods of reduced tic frequency followed by heightened tic activity. Fortunately, many people with Tourette's syndrome find that their condition improves as they get older. The treatment for tic disorders depends on the severity of the condition. In many instances, no treatment is needed and the tics will resolve on their own. In other cases, doctors may prescribe behavioral therapy, medication, or a combination of the two. Behavioral therapy helps people learn to manage their tic symptoms and reduce tic frequency. Medications are typically used to reduce tic frequency and enhance a person's daily life. This usually does not result in the complete remission of tic symptoms.