Hence, other medical conditions that include tics or tic-like movements—for example, autism or other causes of tics—must be ruled out.Patients referred for a tic disorder are assessed based on their family history of tics, vulnerability to ADHD, obsessive–compulsive symptoms, and a number of other chronic medical, psychiatric and neurological conditions. In individuals with a typical onset and a family history of tics or OCD, a basic physical and neurological examination may be sufficient. There are no specific medical or screening tests that can be used to diagnose Tourettes; the diagnosis is usually made based on observation of the individuals symptoms and family history, and after ruling out secondary causes of tic disorders (tourettism).Delayed diagnosis often occurs because professionals mistakenly believe that TS is rare, always involves coprolalia, or must be severely impairing. The DSM has recognized since 2000 that many individuals with Tourettes do not have significant impairment; diagnosis does not require the presence of coprolalia or a comorbid condition, such as ADHD or OCD. Tourettes may be misdiagnosed because of the wide expression of severity, ranging from mild (in the majority of cases) or moderate, to severe (the rare but more widely recognized and publicized cases). About 20% of people with Tourette syndrome do not realize that they have tics.Tics that appear early in the course of TS are often confused with allergies, asthma, vision problems, and other conditions.
The genetic relationship of ADHD to Tourette syndrome, however, has not been fully established. A genetic link between autism and Tourettes has not been established as of 2017. Mechanism
The exact mechanism affecting the inherited vulnerability to Tourettes is not well established. Tics are believed to result from dysfunction in cortical and subcortical brain regions: the thalamus, basal ganglia and frontal cortex. Neuroanatomic models suggest failures in circuits connecting the brains cortex and subcortex; imaging techniques implicate the frontal cortex and basal ganglia. In the 2010s, neuroimaging and postmortem brain studies, as well as animal and genetic studies, made progress towards better understanding the neurobiological mechanisms leading to Tourettes. These studies support the basal ganglia model, in which neurons in the striatum are activated and inhibit outputs from the basal ganglia.Cortico-striato-thalamo-cortical (CSTC) circuits, or neural pathways, provide inputs to the basal ganglia from the cortex. These circuits connect the basal ganglia with other areas of the brain to transfer information that regulates planning and control of movements, behavior, decision-making, and learning. Behavior is regulated by cross-connections that "allow the integration of information" from these circuits. Involuntary movements may result from impairments in these CSTC circuits, including the sensorimotor, limbic, language and decision making pathways. Abnormalities in these circuits may be responsible for tics and premonitory urges.The caudate nuclei may be smaller in subjects with tics compared to those without tics, supporting the hypothesis of pathology in CSTC circuits in Tourettes.