Movement disorders are a broad-ranging group of conditions that significantly alter the ability to move or control the body\u2019s muscles. Some of these conditions produce unusually or uncontrollably active muscles, while others produce unusually stiff or unresponsive muscles. Movement disorders overlap to a significant degree with various kinds of mental disorders. In some cases, unconnected movement disorders and mental disorders just happen to appear in the same individual. In other cases, movement disorders appear as symptoms of mental disorders. In still other cases, movement disorders appear as a side effect of mental disorder treatment. In addition, some movement disorders are also considered mental disorders because of their effects on psychological health. Movement Disorder Basics Doctors and researchers refer to movement disorders that produce unusually active muscles as hyperkinetic disorders. Common examples of these conditions include Huntington\u2019s disease, chorea, restless legs syndrome (RLS), Tourette\u2019s syndrome (also known as Tourette\u2019s disorder), Wilson disease, ataxia, various kinds of tremors, and various kinds of tics. Movement disorders that produce unusually restricted muscles are known collectively as hypokinetic disorders. Common examples of these conditions include Parkinson\u2019s disease, bradykinesia, Parkinson\u2019s syndrome, progressive supranuclear palsy, various conditions called postural disturbances, and generalized muscle rigidity. All told, close to three dozen separate conditions belong to either the hyperkinetic group of movement disorders or the hypokinetic group of movement disorders. More than one of out every seven Americans has one of these conditions. Co-Existing Conditions Anywhere from 34 percent to 40 percent of people with medically serious movement disorders also have some sort of mental disorder, according to an extensive report published in January 2013 in Psychiatric Times. In many cases, the presence of a movement disorder is not directly related to the presence of a mental disorder. However, even in these circumstances, the course of one health problem can substantially alter the course of the other. For example, the medications commonly used to treat bipolar disorder, depression, schizophrenia and psychotic disorders related to schizophrenia can potentially intensify existing cases of the movement disorder ataxia, as well as cases of another disorder called essential tremor. Movement Disorders as Mental Disorder Symptoms Doctors refer to movement disorders that are not faked and have no identifiable physical cause as psychogenic movement disorders. Roughly 2 percent to 3 percent of all movement disorders fall into this category, the authors of the report in Psychiatric Times explain. Generally speaking, mental disorders that produce psychogenic symptoms belong to a group of conditions called somatic symptom and related disorders (formerly known as somatoform disorders). Most affected individuals have a somatic condition called conversion disorder; other potential culprits include somatic symptom disorder, factitious disorder, and illness anxiety disorder. Examples of the movement-related symptoms associated with these conditions include psychogenic Parkinsonism, various postural disturbances, and other conditions that affect the ability to walk normally. Movement Disorders as a Treatment Side Effect Medications used to treat a variety of mental disorders can also trigger the onset of movement disorders in previously unaffected individuals. For example, use of antidepressants, antipsychotics or the bipolar disorder medication lithium can trigger a hyperkinetic movement disorder called akathisia. Use of antipsychotics can also trigger two other hyperkinetic disorders\u2014called tardive dyskinesia and acute dystonia\u2014as well as a severe, potentially fatal hypokinetic disorder called neuroleptic malignant syndrome. Use of antipsychotics, certain antidepressants, lithium, or a number of other medications can trigger a condition called drug-induced Parkinsonism, while use of lithium, antipsychotics, valproic acid, or antidepressants can trigger a type of tremor called postural tremor. Movement Disorders Viewed as Mental Disorders The basic definitions for movement disorders come from doctors and researchers who specialize in fields related to the function of the body\u2019s nervous system and muscular system. However, the basic definitions for mental disorders come from psychiatrists who belong to a nationwide organization called the American Psychiatric Association (APA). The APA judges a wide spectrum of conditions according to their potential impact on mental health, whether or not those conditions are traditionally viewed as mental health problems. This means that, because of their psychological impact, some movement disorders also qualify as mental disorders according to the APA\u2019s criteria. Examples of these conditions listed in the current edition of the APA\u2019s Diagnostic and Statistical Manual of Mental Disorders include restless legs syndrome, Tourette\u2019s disorder and a number of other tic-related disorders, mild or major neurocognitive disorder due to Parkinson\u2019s disease, mild or major neurocognitive disorder due to Huntington\u2019s disease, and mild or major neurocognitive disorder with Lewy bodies.