**2.1 Definition of tics and their practical ways to characterize them since clinical view**

As we described elsewhere, tics are defined as "sudden, rapid, recurrent, nonrhythmic, stereotyped motor movements or vocalizations"and have clinical concerns1.Tics are likely to differ in terms of the body location, number, frequency, complexity, intensity or forcefulness, noticeable and resulting social consequences2. Intervals between tics can range from seconds, hours or even days. The forcefulness with which a tic is performed can range from slight and barely noticeable to intense and obvious. Over time, the frequency and intensity of tics may wax and wane and maybe influenced by a variety of internal and external stimuli including private events, contextual variables and social reinforcement contingencies3.

Tics can also vary considerably in their complexity. Tics that involve the contraction of a single muscle group are typically referred as simple tics and those that involve the contraction of multiple muscle groups are typically considered complex. Simple tics are typically of very short duration (*i.e.* 1 second) and include such behaviors as eye blinking; jerking of the face, head, torso, or limbs; coughing; sniffing; throat clearing and making singles syllable sounds. Complex tics are often sustained for longer durations or occur in paroxysms and can include virtually any orchestrated pattern of behavior otherwise

Epidemiology of Tics 165

**3. Biological frame to understand the base of tics and related comorbidities** 

Figure 1 is a practical schema explaining the mechanism and pathways of tics and related disorders based in experimental models resulting of neuroanatomy and neurobiology

Fig. 1. Cerebral cortex giving glutamatergic excitatory projections to striatum.

CM-PF: centromedian -parafascicular nuclear complex.

Five parallel circuits connecting the cortex to the striatum27 28:

associative and a centromedial / limbic.

field with the caudate nucleus.

caudate head.

Abbreviations: Glu: glutamate, DR: dopamine receptors, DA: dopamine, GABA: gamma aminobutiric acid, enk: encephaline K, SNpc: substantia nigra pars compacta, SNpr: substantia nigra pars reticulata, Sust P: susbstantia P, GPe: external globus pallidus, GPi: internal globus pallidus, NST: nucleus subthalamic, VA: ventral anterior, VT: ventral nuclei,

The cerebral cortex provides excitatory glutamatergic projections to striatum. The striatum has a topographic distribution as follows: somatosensorial dorsolateral, intermediate /

1. Motor circuit: gives origin to motor tics and arises in the supplementary motor area. 2. Oculomotor circuits: is the potential source for ocular tics and connects the frontal eye

3. Dorsolateral prefrontal circuit: related to difficulties in executive functions, motor planning, cognition and attention. It is connected to Brodmann areas 9 and 10 and

4. Lateral orbitofrontal circuit: It is associated with OCD, irritability and mania. It originates from the inferolateral prefrontal cortex and projects to ventromedial caudate.

**3.1 Mechanism and pathways of tics and comorbidities** 

grounds.

meeting the definition of a tic. Common examples include picking, tapping, gesturing, mimicking the gestures of others (echopraxia), repeating one's own speech (palilalia), mimicking the speech of others (echolalia) and the production of inappropriate words or sentences.

Tics comprise a group of movement disorders. Thus, transient tics, the most common form of the disorder, consists of single or multiple motor and/or vocal tics that occur for at last four weeks but no longer than 12 months. GTS has onset before 18 years of age, characterized by motor and vocal tics over more than a year, there is never a tic-free period of more than 3 consecutive months, not produced by Huntington disease neither viral encephalitis and produces a negative personal impact. The disorder is called chronic motor tics if the criteria of GTS are present but vocal tics are absent. By contrast, if there are vocal tics but no motor tics the disorder is called chronic vocal tics4.
