**5.2 Direct observation**

One of the hallmarks of behavior analysis is the use of direct observation to quantify behavior. This preference is based on the premise that direct observation is more objective than indirect methods such as self-report or clinician ratings of tic severity. Several studies, published in behavior-analytic outlets, demonstrate the value of using direct observation to quantify changes in tic frequency when evaluating behavioral treatments for tics59. Still, many researchers outside behavior analysis have largely preferred indirect measures over direct observation60. Among the foremost concerns raised by these researchers is that observations conducted within a clinical or research context may not generalize to other settings such as school or home. Generalization between settings is an important issue in both research and clinical practice. Indeed, it is not uncommon for parents to report that a child's tics are more or less severe while at the clinic compared to when the child is at home61 Such reactivity to setting has been attributed to several factors including natural fluctuation, reinforcement contingencies, children's ability to volitionally suppress or temporarily withhold tics, reactivity to observation and internal states such as anxiety62.

Regardless of the reason for contextual variation in tics, such fluctuations have important implications for the measurement of tics. If the scientific and clinical community is to have confidence in the results of behavior-analytic work utilizing direct observation methodology, observations conducted within a research setting must be generalizable to other settings. Clinic- and homebased observations are highly related, suggesting that, in general, clinic observations correspond well with home observations63. However, examination of individual data shows that generalization should not necessarily be assumed; many children exhibited differential tic frequencies across the two settings, suggesting that, whenever possible, observations should be conducted in multiple settings. Lack of consensus regarding the most reliable, valid and feasible methods for collecting and coding direct observation data has also been cited as a reason for the preference of indirect measures over direct observation64. Practitioners and researchers in disciplines outside behavior analysis may be more likely to use direct observation methods if the effort associated with their use can be reduced, without any sacrifice of their validity and capacity to generate representative samples of target behaviors.

Direct observation to longer samples and event-frequency coding to a less arduous timesampling method (*i.e.* partial-interval coding) have been used to evaluate outcomes in tic research65, although partial-interval coding is more user-friendly because it does not require the observer to record each occurrence of the tic; thus, it might be preferred over the event frequency method. However, partial-interval coding cannot be recommended as an alternative if it does not yield a reliable measure of the behavior. Because simulation studies have suggested that partial-interval coding may underestimate the frequency of high-rate short duration responses, especially if they occur in rapid succession or as bouts, as is the case with many tics66.

Epidemiology of Tics 173

between the primary investigators and coders and recoding of videotapes until agreement

A key factor in understanding these divergent results of epidemiological studies concerns the sample size, randomization, stratification, steps in epidemiological assessment and clinical aspects to warrant quality of databases67. Most relevant studies about prevalence are

**Author Year Population Methodology Prevalence Strenghts Limitations** 

6.63% were found to have tics, 4.24% were male and 2.43% female. Further, 4.8% of the children had transient tics. All together, 6.6% of 7 to 15 year old children currently had or had experienced some kind of tic disorder during the last year.

21.2% had tics. Community sample

should minimize problems with ascertainment bias, controlled study.

Investigators used the DSM-IV instead DSM-III-R criteria. The first one requires that the tics cause a marked disturbance or significant functional impairment. Inclusion criteria were that tics had occurred sometime during the last 12 months. Same physician performed both the telephone interview and the clinical assessment.

Data obtained from the Child Behavior Checklist (CBCL) can be influenced by which parent or teacher completed the scale, tic severity

and psychotropic medication, factors that were not included in analyses.

As a result of a decision from the ethics committee, investigators were not allowed to ask the teachers about their pupils´ tics in the main study and some cases may have been missed. Some parents were no Swedish and the screening questionnaire could be not easy to complete in its language. However, in these cases a professional translator helped the parents. If the child refused further participation, the telephone

criterion is reached.

Kurlan R, Como PG, Miller B, Palumbo D, Deeley C, Andresen EM, et al5.

Khalifa N, Knorring ALV68.

**5.5 Not clinical studies of tics** 

2002 Communitybased study of school children 12.5 to 15.7 years old.

2003 4479 Swedish school children aged 7 to 15 years

showed in Table 2, including their methodological aspect.

1596 children assessed using interviews to determine the prevalence of tics

psychopathological disorders.

Total population and their parents were asked to fill in a questionnaire covering both motor and vocal tics. A three- stage procedure was used: screening, interview and clinical

investigation. SGT were diagnosed according to DSM-IV criteria.

and

### **5.3 Yale global tic severity scale**

Yale global tic severity scale (YGTSS) is a clinician-completed rating scale used to rate tic severity along several dimensions based on parent and child reports and clinician observations during the interview64. Each dimension is represented by a subscale designed to quantify the number, frequency, duration, intensity and complexity of both motor and vocal tics. Each subscale includes several descriptions to help the clinician make his or her ratings. Guided by these descriptions, each subscale is issued a rating between 0 and 5, with higher scores indicating greater severity. Examples of descriptions included on the number subscale are single tic, multiple discrete tics and multiple discrete tics plus several orchestrated paroxysms of multiple simultaneous or sequential tics where it is difficult to distinguish discrete tics. Examples of items on the frequency subscale are ''rarely—specific tic behaviors have been present during the previous week; these behaviors occur infrequently, often not on a daily basis; if bouts of tics occur, they are brief and uncommon and ''always—specific tic behaviors are present virtually all the time". Examples of items on the intensity subscale include ''minimal intensity—tics not visible or audible (based solely on patient's private experience) or tics are less forceful than comparable voluntary actions and are typically not noticed because of their intensity'' and ''severe intensity— tics are extremely forceful and exaggerated in expression; these tics call attention to the individual and may result in risk of physical injury because of their forceful expression''. Examples on the complexity subscale include ''borderline—some tics are not clearly 'simple in character'' and ''severe—some tics involve lengthy bouts of orchestrated behavior or speech that would be impossible to camouflage or successfully rationalize as normal because of their duration or extremely unusual, inappropriate, bizarre or obscene character''. Examples of interference items include ''minimal—when tics are present, they do not interrupt the flow of behavior of speech'' and ''severe—when tics are present, they frequently disrupt intended action or communication''. Finally, examples of items on the impairment subscale include ''minimal—tics associated with subtle difficulties in self-esteem, family life, social acceptance or school or job functioning'' and ''severe—tics associated with extreme difficulties in self-esteem, family life, social acceptance or school or job functioning''. The five subscales are rated separately for motor and vocal tics. The motor subscales are then summed to produce an overall motor tic severity rating and the vocal tic subscales are summed to provide an overall vocal tic severity rating; each ranges from 0 to 25. The motor and vocal tic severity ratings are then summed to produce an overall tic severity score that ranges from 0 to 50. Studies have shown the YGTSS total tic score to have acceptable internal consistency, good inter-rater reliability and acceptable convergent and divergent validity in samples of adults and children.

#### **5.4 Gross-site procedural training**

Prior to the beginning of the study, a face-to-face meeting between personnel for tic assessment must held to review the standardized observation protocol and to conduct training on YGTSS administration and scoring. Sample tapes of children with tics may be used to conduct cross site YGTSS training and direct observation coding. Tapes included an interview and YGTSS administration conducted by the primary investigators with a child and his or her parents, along with a while direct observation segment of the child (at least 10 minutes). YGTSS training has to continue until the clinicians obtained agreement of at least 90% on the training. Disagreements during training have to be resolved by discussion

Yale global tic severity scale (YGTSS) is a clinician-completed rating scale used to rate tic severity along several dimensions based on parent and child reports and clinician observations during the interview64. Each dimension is represented by a subscale designed to quantify the number, frequency, duration, intensity and complexity of both motor and vocal tics. Each subscale includes several descriptions to help the clinician make his or her ratings. Guided by these descriptions, each subscale is issued a rating between 0 and 5, with higher scores indicating greater severity. Examples of descriptions included on the number subscale are single tic, multiple discrete tics and multiple discrete tics plus several orchestrated paroxysms of multiple simultaneous or sequential tics where it is difficult to distinguish discrete tics. Examples of items on the frequency subscale are ''rarely—specific tic behaviors have been present during the previous week; these behaviors occur infrequently, often not on a daily basis; if bouts of tics occur, they are brief and uncommon and ''always—specific tic behaviors are present virtually all the time". Examples of items on the intensity subscale include ''minimal intensity—tics not visible or audible (based solely on patient's private experience) or tics are less forceful than comparable voluntary actions and are typically not noticed because of their intensity'' and ''severe intensity— tics are extremely forceful and exaggerated in expression; these tics call attention to the individual and may result in risk of physical injury because of their forceful expression''. Examples on the complexity subscale include ''borderline—some tics are not clearly 'simple in character'' and ''severe—some tics involve lengthy bouts of orchestrated behavior or speech that would be impossible to camouflage or successfully rationalize as normal because of their duration or extremely unusual, inappropriate, bizarre or obscene character''. Examples of interference items include ''minimal—when tics are present, they do not interrupt the flow of behavior of speech'' and ''severe—when tics are present, they frequently disrupt intended action or communication''. Finally, examples of items on the impairment subscale include ''minimal—tics associated with subtle difficulties in self-esteem, family life, social acceptance or school or job functioning'' and ''severe—tics associated with extreme difficulties in self-esteem, family life, social acceptance or school or job functioning''. The five subscales are rated separately for motor and vocal tics. The motor subscales are then summed to produce an overall motor tic severity rating and the vocal tic subscales are summed to provide an overall vocal tic severity rating; each ranges from 0 to 25. The motor and vocal tic severity ratings are then summed to produce an overall tic severity score that ranges from 0 to 50. Studies have shown the YGTSS total tic score to have acceptable internal consistency, good inter-rater reliability and acceptable convergent and divergent validity in

Prior to the beginning of the study, a face-to-face meeting between personnel for tic assessment must held to review the standardized observation protocol and to conduct training on YGTSS administration and scoring. Sample tapes of children with tics may be used to conduct cross site YGTSS training and direct observation coding. Tapes included an interview and YGTSS administration conducted by the primary investigators with a child and his or her parents, along with a while direct observation segment of the child (at least 10 minutes). YGTSS training has to continue until the clinicians obtained agreement of at least 90% on the training. Disagreements during training have to be resolved by discussion

**5.3 Yale global tic severity scale** 

samples of adults and children.

**5.4 Gross-site procedural training** 

between the primary investigators and coders and recoding of videotapes until agreement criterion is reached.

#### **5.5 Not clinical studies of tics**

A key factor in understanding these divergent results of epidemiological studies concerns the sample size, randomization, stratification, steps in epidemiological assessment and clinical aspects to warrant quality of databases67. Most relevant studies about prevalence are showed in Table 2, including their methodological aspect.


Epidemiology of Tics 175

57 cases were identified after direct observation in the classroom, thereby prevalence was 6.5%. The vast majority of tics were mild in severity and duration. Most of identified cases were quite mild, not leading to major functional disability.

The study was conducted in three successive steps: information to parents and teacher by way of speeches and projection of videotapes; anonymous fulfilling of specified questionnaire by teachers and parents and identification of children as possible tic disorder according to questionnaire; and confirmation of presence of tics by 20 minutes direct observation of children at school.

Children were screened by inquiring their parents and teachers. Children indicated as ticpositive by the screening procedure were investigated using semi-structured questionnaires and the Polish version of YGTSS scale. A validity study involved random selection and investigation of 130 non indicated subjects. Screening procedure had

Lifetime prevalence of 9.9% (95%CI 7.1–12.6%) and a point prevalence of 6.7% (4.3– 9.1%). Lifetime prevalence of ICD-10 tic disorders was 2.6% (95% CI 1.2–4.1%) for transient tic disorder (TTD).

Linazasoro G, Blercom NV, Ortiz C58.

Stefanoff P, Wolanczyk T, Gawrys A, Swirszcz K, Stefanoff E, Kaminska A, et al70.

2006 867 Children of two schools, 4 to 16 years.

2008 12-15 year old Warsaw schoolchildren attending 24 randomly selected schools.

spent four morning hours observing the students and noted any symptoms resembling

High participation rate and collaboration offered by parents and teachers data highly reliable. Investigators chose one private and one public school to exclude possible selection bias related to the socioeconomic status. Blind observation phase of

Investigators decided not to interview directly parents and children, but some children could be identified during

direct observation phase. Diagnosis of tics was not confirmed. Investigators emphasize the convenience of including a later step in the methodology to confirm the diagnosis. Length of direct observation in the classroom was 20 min, which could be quite limited. To observe children during their routine school activity requires a considerable degree of mental concentration.

Reference group differed significantly from the total study population in terms of gender distribution. The number of subjects with tics found in this group could be even higher, leading to lower positive predictive value. Teachers interviewed in the study protocol knew their students for

tics.

the study.

The schools were chosen randomly and should not differ systematically from other mainstream schools.


A total of 68 children (56 boys, 12 girls) aged 6–11 years were identified with tic disorders. The period prevalence was 2.9% (95% CI 2.3 to 3.7). The prevalence was 4.4% in boys and 1.1% in girls, with no detectable trends at age 6–11. Situation related tics were noted in 37 cases. A significant correlation was found between the presence of tic disorders and impaired school performance.

The purposes of the study, the definition and characteristics of the tic disorders were illustrated by authors to all the school teachers, with the support of videotaped interviews. Investigators used stringent and different diagnostic criteria; data sources and movement disorders were excluded. Different sources have been used to identify subjects with tic disorders, including direct observations at school, parents´ interview/questionnair

e, teachers´

interview/quiestionnair e and clinical examination. A pilot study was conducted on 232 children from one school, evenly distributed across school years (two classes for each school year). This sample served to test the validity and reliability of the school teachers as a source for the ascertainment of patients with tics. In each class, one investigator with experience in the field of movement disorders

Lanzi G, Zambrino CA, Termine C, Palestra M, Ferrari O, Orcesi S, et al69.

2004 The study population comprised 2347 primary school children from the city of Pavia, Northern Italy, 5-12 years from 15 primary schools.

Using trained school teachers as the source of cases, all children with motor or vocal tics occurring intermittently and unpredictably out of a background of normal motor activity were accepted. The type, frequency, and circumstances of tic disorders were noted. School performance was correlated to the presence of tics. Diagnostic criteria for a tic disorder were those of the Tourette Syndrome Classification Study Group.

interview was expanded to make a correct diagnosis. Thus it is plausible that misdiagnoses were minimized. The study has not focused on the severity of the disorders, functional impairment as well as comorbid disorders, school problems and learning disabilities.

Case ascertainment was no attempt to verify the teachers´ observations through direct examination of all the affected children.


Epidemiology of Tics 177

Prevalence of any tic disorder was 0,8%. Both tics and ADHD were diagnosed most often in the age group 7-12 years. Tic disorders were observed in 2,3% of patients with ADHD.

Prevalence rates within this large sample established the remitting course of tics. The study confirms the co-ocurrence of tics and ADHD in children and adolescents, presenting both perspectives: the rate of ADHD in patients with tics as well the rate of tics in patients with ADHD.

These outpatients have no detailed information about the exact diagnostic procedures. Short time of follow-up. Diagnoses in children made more often by a medical specialist and by a primary care physician in adults. The study is difficult to compare with previous reports, which is related to different study design. The absolute administrative prevalence rates should be interpreted cautiously. Some people could not be assisted to SHI.

12-months administrative prevalence rates as well as rates of cooccurrence of tics and ADHD based upon the number of diagnosed cases of tics disorders.

Schlander M, Schwarz O, Rothenberger A, Roessner V71.

Ortiz B, David M, Sánchez Y, Mira J, Sierra JM, Cornejo JW72.

2009 2.2 million live records during 2003 covered by Statutory Health Insurace (SHI) in Norbaden, Germany, 0 to 50 years old.

2011 346 students of public basic school.

Table 2. Not clinical studies of tics.

Students were assessed by structured questionnaire, interview and 20 minutes of clinical examination. Comorbidity with ADHD was detected by DSM-IV criteria. Severity and interference produced by tics was determined by apply **YGTSS**.

Tics were present in 17.97% and GTS in 3.4% of scholars. According to time onset, 27.6% had transient tics and 72.4% had chronic tic disorder. 53.4% of patients with tics agree with DSM-IV ADHD criteria. Mean age to tics presentation was 9 years old. There was no difference in tics frequency between children studying in public and private schools.

First stud

y in South America establishing prevalence of tics in children. Wide clinical sample that includes training to teachers and parents, structured questionnaire, clinical assessment and direct observation in the classroom by experts in tics detection. Percentage of children with GTS was higher than other studies probably because evaluators considered milder cases and depicting that the disorder was not as rare as previously was believed.

Authors detected that hyperactivity was confused with tics by parents and teachers on parents and teachers questionnaires


3–4 years and were coping with them on a daily basis as their formal tutors. A relatively high proportion of false negative subjects in the reference group is the result of poor knowledge of involuntary movements in the polish population and indicates a possible underestimation of prevalence estimates. classification of tic disorders based on the ICD-10 criteria could yield misdiagnosing of tic disorders. The ICD classification reflects the current concepts of tic disorders as a behavioral continuum, and two most severe syndromes chronic tic disorders and GTS—differ only in terms of duration of tic symptoms. The use of ICD criteria should not constitute a problem in terms of comparability with previous epidemiological studies, especially that most of them that utilized DSM-III-R criteria, very similar toICD-10.

high sensitivity (92%) and low positive predictive value (18%). International Classification of Diseases-10 (ICD-10) criteria were used for tic disorders.


Table 2. Not clinical studies of tics.

Eapen V, Lees A, Lakke JPWF, Trimble MR; Robertson MM76.

Prior AC, Tavares S, Figueiroa S, Temudo T77.

2006 78 Children and teenagers with tics diagnosed based by DSM-IV criterions.

Table 3. Clinical warding studies of tics.

2002 Study about report of eight adult patients with adultonset motor tics and vocalizations.

All the cases were evaluated using the National Hospital Interview Schedule for the assessment of GTS and related behaviors; this is a standardized, semistructured instrument that includes systematic assessment of personal and family history of GTS, ADHD and

OCB.

Retrospective analyses from clinical archives of child neurology outpatients of Hospital General de Santo Antonio,

Spain.

Epidemiology of Tics 179

onset tics were more likely to have a symptomatic or secondary tic disorder, likely caused by infection, trauma, cocaine use, and neuroleptic exposure.

All patients there were a potential trigger event, such as drug exposure, viral or bacterial infection, physical trauma, cerebrovascular disease or psychiatric illness prior to the onset of tic symptoms.

The interview was conducted with the patient and a family member (usually a parent) who knows the patient well enough to give relevant details about childhood.

Wide clinical sample.

Further study of

phenomenology and natural history of these adult-onset cases is needed.

Boys were derived to the clinic because symptoms of ADHD instead of tics. Descriptions were extracted of patients untreated. Sex selection bias may take place (male:female ratio was 5.5:1) because ADHD was more frequent in boys. Neurobehavioral and family aspects

may be overestimated by the type retrospective of study.

the

These may have acted as a trigger to unmask the symptoms in a constitutionally predisposed individual. 50% of cases, there was either a personal or family history of GTS-related behavior. The symptoms were severe in 75% and 50% suffered extreme occupational or social disadvantage as a direct result of tics.

84.6% were boys. Family history of tics, depression and OCD occurred in 30%. ADHD was the most

neuropsychologycal disorder (67.9%). In more than two thirds of the patients, tics were simple. Mean age for tics was 7 years old. 59.7% of tics were chronic and 45.7% of those were

frequent

GTS.

### **5.6 Clinical studies of tics**

Clinic-based studies are believed to underestimate the frequency of tics, as only a small fraction of children and adults with tics are brought to a health care provider for evaluation73. There is evidence of a reporting bias in community studies with 50% of the children with observed tics reported to have tics by their parents. Results of investigations also support the previously reported findings that tics wax and wane in severity and frequency over time, as individual shad fluctuating symptoms over the observation period74. Most relevant clinical studies about prevalence are showed in Table 3.


Clinic-based studies are believed to underestimate the frequency of tics, as only a small fraction of children and adults with tics are brought to a health care provider for evaluation73. There is evidence of a reporting bias in community studies with 50% of the children with observed tics reported to have tics by their parents. Results of investigations also support the previously reported findings that tics wax and wane in severity and frequency over time, as individual shad fluctuating symptoms over the observation

**Authors Year Patients Methodology Results Strenghts Limitations** 

5.35% presented for the first time with tic disorders after the age of 21. 2.18% of patients had a history of previous childhood transient tic disorder, but in 3.16% of patients, the adult onset tic disorder was new. Among the new onset cases, 1.45% of patients developed tics in relation to an external trigger, secondary tic disorders. The remaining patients had idiopathic tic disorders. The categorical

The study shows that adult onset tic disorders represent an underrecognised condition that is more common than generally appreciated or reported and clinical evidence suggesting that adult tic disorders are part of a range of illness that includes childhood onset tics and GTS. Authors propose a new classification of tic disorders in adult age category that is subdivided by disease course into tic disorders that persist from childhood, tic disorders that represent a recurrence of transient childhood tics and genuine new onset adult tic disorders.

The study cannot estimate the prevalence of adult onset tic disorders in general population. Establishing that an adult patient with apparent new onset tics did not have tics during childhood can rarely be done with certainty because some patients are unaware of their tics and reliable observers who knew the patient as a child may not be available. All patients were self referred for tics, a referral bias that usually selects for more severely affected people.

breakdown among 22 patients was: idiopathic new onset tics in seven (32%), new onset secondary

disorder in six (27%) and recurrent childhood tic disorder in nine (41%).The appearance of the tic disorder, the course and prognosis, the family history of tic disorder and the prevalence of obsessive-compulsive disorder were found to be similar in adult patients with recurrent childhood tics and those with new onset adult tics. Adults with new

tic

period74. Most relevant clinical studies about prevalence are showed in Table 3.

Patients' charts were retrospectively reviewed for demographic information, age of onset of tics, tic phenomenology, distribution, the presence of premonitory sensory symptoms

and tic suppressibility, family history and associated psychiatric features. These patients' videotapes were reviewed for diagnostic confirmation and information was obtained about disability, course, and response to treatment in a structured follow up interview.

**5.6 Clinical studies of tics** 

2000 411 adults with tic disorders who presented between 1988 and 1998 to the movement disorders clinic at Columbia-Presbyterian Medical Center after the age of 21.

Chouinard S, Ford B75.


Table 3. Clinical warding studies of tics.

Epidemiology of Tics 181

**Author Year Methodology Results Strenghts Limitations** 

1596 children interviewed, 21.2% had tics, 38.4% with ADHD, 10.9% with OCD, 29.2% with social phobia, 9.4% with agoraphobia, 14.8% with separation anxiety, 21.2% with anxiety disorder, 1.2% with mania and 17.4% oppositional defiant disorder (ODD).

One quarter of all children exhibited problem behaviors. The monthly point prevalence was significantly higher during winter months compared with the spring months. Behavior comorbidity is associated with the more persistent tic symptoms versus all tic symptoms, as children with isolated tics lasting only 1 to 2 months did not have increased rates of problem behaviors, whereas those with a more persistent

course did.

ADHD was present in 67.9%, learning difficulties in 59%, sleep disorders in 23.1%, developmental delay in 21.8%, unspecified mental retardation in 16.7%, ODD in 10.3%, obsessive compulsive symptoms (OCS) in 7.7%, epilepsy in 6.4%, autism in 3.8%, migraine/headache in 3.8% and depression in 2.6%

Community sample is intended to minimize problems with ascertainment bias, controlled study.

Children were followed by three independent raters and problem behaviors were classified by their impact.

Wide clinical sample. DSM-IV criteria were used for tics, which allow comparison with other studies.

Data obtained from the CBCL can be influenced by which parent or teacher completed the scale, tic severity and psychotropic medication, factors that were not included in analyses.

Conclusions about seasonal prevalence are limited because the children were not observed from July through October. It's possible that clinic-based studies overestimate the frequency of comorbid behavior problems, in part because the behavior problems can be more troublesome than the tic symptoms and become the motivating factor for seeking treatment. It is also possible that clinicbased studies estimate accurately the prevalence of comorbid conditions and that the discrepancy came from the inappropriate generalization of clinic based data to community

populations.

Data was extracted from clinic archives and patients were selected by an unknown and notvalidated questionnaire.

**6.1 Studies of comorbidity in tics** 

of tics and psychopathological disorders. A standard psychiatric interview and standardized rating scales were utilized to diagnose childhood behavioral disorders.

2002 553 children of

2006 78 Children and teenagers with tics diagnosed based by DSM-IV criterions. Retrospective analyses from clinical archives of child neurology outpatients of Hospital General de Santo Antonio, Spain.

Table 5. Related disorders in tics.

kindergarten through sixth grade, observed monthly from November 1999 to June 2000 by 3 raters. Problem behaviors were rated as absent, subclinical or clinical in following categories: disruptive, hyperactive, impulsive, aggressive, anxious and distracted.

2002 Community-based study of school children since 12,5 to 15,7 years old using interviews to determine the prevalence

Kurlan R, Como PG, Miller B, Palumbo D, Deeley C, Andresen EM, Eapen S, *et al*5.

Snider LA, Seligman LD, Ketchen BR, Levitt SJ, Bates LR, Garvey et al79.

Prior AC, Tavares S, Figueiroa S, Temudo T77.
