Trichotillomania (TTM) is characterized by repetitive stereotypical hair-pulling from different sites resulting in noticeable hair loss 1. Phenomenological observations have suggested that symptoms of repetitive hair-pulling are reminiscent of the compulsions seen in obsessive-compulsive disorder (OCD) 23. For example, both TTM and OCD patients describe compulsive urges and ritualistic behaviours 24. Comorbidity data also suggest some overlap between TTM and OCD 2. Thus, a number of authors have suggested that TTM might be classified with OCD in a spectrum of disorders having similar phenomenology 45678.

However, in addition to overlapping phenomenology between OCD and TTM, there are also significant differences. For example, in contrast to compulsions in OCD, hair-pulling in TTM is not in response to obsessive thoughts (such as worry about harm to self or others) but rather because of an irresistible urge and the promise of gratification when pulling out hair 26. Also, unlike patients with OCD whose symptoms change over time in terms of focus and severity (e.g. from washing of hands to checking locks, stoves, appliances, etc) 9, TTM patients usually only present with hair-pulling without evolution to non-self-injurious compulsive rituals.

Examination of demographic variables in OCD and TTM supports the argument that these are two distinctive disorders. TTM is much more prevalent in females (10:1 female to male ratio) 10 whereas OCD is equally common in males and females 11. Age of onset also differs somewhat: TTM typically presents in early adolescence, with the mean age of onset of hair-pulling in males later than that in females 101213 whereas OCD has its onset from childhood through to early adulthood 14, but with males reporting an earlier onset compared to females 15.

Additional clinical observations further support a distinction between OCD and TTM. Patients with TTM tend to have fewer comorbid obsessive-compulsive symptoms, as well as less depression and anxiety compared to OCD patients 16. Response prevention in OCD patients eventually leads to anxiety reduction, whereas in people with TTM it may lead to an increase in anxiety 17. Although a selective response to serotonergic reuptake inhibitors (SRI's) has been suggested to characterize both OCD and TTM, there is good evidence that response to SRI's is sustained in OCD, whereas the evidence-base for the efficacy of these agents in TTM is much more mixed.

Relatively few empirical studies have, however, documented the phenomenological similarities and differences between OCD and TTM 31819. A large clinical database comprised of patients with OCD and TTM provided us an opportunity to investigate the relationship between these conditions in terms of demographic and clinical variables.

A comparison of women with TTM and with OCD found significant differences in clinical variables; OCD patients had more comorbidity, greater disability, increased childhood interpersonal trauma (specifically sexual abuse) and more maladaptive schemas. Fewer TTM patients, however, reported having responded to treatment.

The gender ratio findings in both OCD and TTM groups were similar to other surveys where a mean female:male ratio of 1.5:1.0 in OCD 113233 and approaching 10:1 in TTM 34 were documented. OCD patients' mean total score on the YBOCS (i.e. 20.1 ± 8.0) puts them in the "moderate" severity category 22. The mean hair-pulling severity score on the MGHHPS (16.1 ± 6.5) was similar to that reported in other studies 3536. Taken together, these data suggest that our patients are not dissimilar from those assessed at other sites.

Our comorbidity findings are consistent with existing data suggesting that depressive and anxiety disorders are highly prevalent in both OCD and TTM, and significantly more prevalent in OCD 31618. Indeed, compared to TTM, comorbidity in OCD is greater across a range of different diagnostic categories including mood (MDD, dysthymia), anxiety (panic disorder), OCD-related (hypochondriasis) and personality disorders (OCPD). Such comorbidity appears to extend also to impulse control disorders (intermittent explosive disorder), a finding which argues against the current classification of trichotillomania as a member of this spectrum of conditions.

Our findings of increased disability in OCD is consistent with studies on OCD suggesting it is one of the most impairing of all medical disorders 37. A number of clinical studies have emphasized the burden of OCD across different domains, including higher rates of divorce and separation than in subjects without OCD 13 and significantly impaired instrumental functioning (work, school, home making and family life) 38394041. However, the impairment and distress due to TTM should not be underestimated: TTM can be associated with serious sociological and psychological effects (e.g. strong feelings of shame and embarrassment 42, as well as avoidance behaviour including potentially dangerous avoidance of medical care 43) resulting in a significant decline in quality of life (QOL) for patients, their family members and significant others 4445.

OCD patients reported significantly more sexual abuse than TTM patients (p = .04). This finding differs from our previous data suggesting similar rates of childhood interpersonal trauma (CIT) in OCD and TTM 46. However, the current sample size is much increased, resulting in more power to detect smaller differences. Indeed, increased rates of OCD (and other anxiety disorders) have previously been linked with a history of physical and sexual abuse during childhood 4748. Nevertheless, in both OCD and TTM, dissociative symptoms – which are present in a minority of patients in both conditions – are positively correlated with a history of childhood interpersonal trauma 49, so that a potential role for CIT in some TTM patients should not be ignored 50.

TTM patients had significantly more novelty seeking (NS) than OCD patients, whereas OCD patients scored significantly higher on harm avoidance (HA) compared to TTM. Our findings are consistent with previous work on temperament / character in OCD, showing increased HA and decreased NS 515253. Of note, compared with mean temperament scores obtained in a normal community sample 30, both TTM and OCD scored high on HA. NS scores in the TTM sample were higher than in the OCD sample, but compared to normal controls, these fell in the "medium" range. The higher NS in TTM may however point to greater dopaminergic involvement in this disorder, and might also be used to argue that TTM lies closer to the more impulsive risk-/novelty-seeking pole of an impulsive-compulsive (IC) spectrum of disorders 54.

OCD patients had more maladaptive cognitive schemas than TTM, i.e. mistrust / abuse, social isolation, shame / defectiveness, subjugation and emotional inhibition. The schemas that OCD and TTM patients differed on are included in 2 of the 4 higher order factors (i.e. "impaired autonomy" and "disconnection") described by Lee and colleagues' YSQ factor model 55. While schemas are thought to represent responses to life experience, including the experience of a disorder, they may also reflect underlying symptoms. Given that maladaptive schemas in OCD were not reminiscent of its characteristic symptoms, it is likely that they at least partly reflect life experience. An increased number of maladaptive schemas in OCD is consistent with higher rates of comorbidity, disability, and functional impairment. Nevertheless, further empirical investigation is needed to assess the relationship between schemas and illness course.

Hormonal influences have previously been investigated in OCD 56 and TTM 57. For example, it has been noted that menarche, premenstruum, pregnancy 58, and menopause 59 may be related to onset or relapse in OCD. Similarly, in a study that investigated the relationship of the menstrual cycle and pregnancy to compulsive hair-pulling, premenstrual symptom exacerbation was reported for actual hair-pulling, urge intensity and frequency, and ability to control pulling 57. In that study the impact of pregnancy on TTM was less clear. Our findings suggest that significantly more OCD patients than TTM patients report an association between pregnancy/puerperium and the onset of illness. This finding is in part consistent with previous work suggesting that the postpartum may constitute a risk for the onset of OCD in women 60. Taken together our data suggest both similarities and differences in the role of sexual hormones in the mediation of OCD and TTM.

Although rare, brain injury may play a role in some cases of OCD 6162; in only one OCD patient (and none of the TTM patients), head injury was associated with onset of obsessive-compulsive symptoms. No data could be found on the potential role of brain injury in the etiology of hair-pulling.

A number of patients associated the onset of their OCD onset with an infection, possibly bacterial pharyngitis. This finding is consistent with a body of data suggesting post-streptococcal disease is a cause of OCD in children and adolescents 63, and perhaps also adults 6465. There is less work demonstrating a role for autoimmune factors in TTM 66. Notably, the data on bacterial pharyngitis were based on retrospective assessment and could have been contaminated by memory bias.

In our study, more OCD patients reported a positive response to treatment (with CBT or SRI's) than TTM patients. These data should be interpreted cautiously given the retrospective assessments. Nevertheless, there is evidence that SRI's in TTM may not be as effective over the long-term as in OCD 2. About 40–60% of OCD patients respond to the first trial of an SRI 67, with a proportion of non-responders to a single SRI responding to administration of a second SRI 68. In comparison, it has been suggested that TTM patients judge their treatment (including pharmacotherapy, psychotherapy, and behaviour modification) to be relatively ineffective 69. The usefulness of SRI's in TTM has been investigated in a number of studies with results so far being equivocal. For example, Christenson et al 70 were unable to document efficacy for fluoxetine in a placebo-controlled trial in which patients received 6 weeks of the active agent in doses of up to 80 mg/day. Anecdotal evidence also suggests that the effectiveness of SRI's in TTM may wane with time 71.

Although there is evidence for the usefulness of behaviour therapy in both OCD 72 and TTM 73, the focus of the treatment differs in the two disorders (exposure and response prevention in OCD versus habit reversal in TTM). Keuthen et al 74 have suggested that "state-of-the-art" behavioral and pharmacological treatments offer substantial clinical benefit to patients with TTM, but in general clinics there may be relatively little experience with highly specialized interventions.

Several limitations of the current study should be acknowledged. First, interviewers were not blind to the patients' psychiatric diagnosis, so potentially biasing clinician's assessments. Nevertheless, a structured diagnostic instrument ensures a reasonable degree of reliability. Second, instruments employed in the current study are intended for use in adults rather than younger subjects. However, in the case of children and adolescents, the SCID-I was supplemented with a clinical interview of parents or guardians, and self-report data was included only when it was clear that questionnaires had been completed meaningfully. Third, source of referral, and the duration of OCD/TTM, were not controlled for in the analysis, so potentially biasing the analyses. However, given the chronicity of both conditions, this is unlikely to have materially affected the findings. Fourth, males, as well as patients with comorbid OCD and TTM were excluded from the investigation; so that the results here may not be generalizable to all OCD or TTM subjects. Given evidence that the phenotype of OCD 75 and of TTM 10 varies with gender, additional work on male subjects should be undertaken in the future.

In conclusion, our data suggest that despite some overlap, TTM differs from OCD in terms of demographics (gender distribution), associated clinical variables (e.g. comorbidity, cognitive schemas, temperament/character profiles and disability), precipitating factors (trauma history) and treatment response. It has been suggested that although TTM is not the same as OCD, it lies on a compulsive-impulsive spectrum of disorders 54. However, it is notable that impulsivity may be an important component of OCD 76, and rather than viewing OCD and TTM on a single dimension, compulsivity and impulsivity should arguably therefore be seen as lying on orthogonal dimensions. Although TTM patients had more novelty seeking, OCD patients were more likely to have intermittent explosive disorder; such data support a view that TTM should not be classified as an impulse control disorder. Indeed, TTM may have more in common with conditions characterized by stereotypical self-injurious symptoms, such as skin-picking 77. Differences between OCD and TTM may reflect contrasts in underlying psychobiology, and may necessitate contrasting treatment approaches.

The author(s) declare that they have no competing interests.

CL:

• participated in the design of the study,

• was responsible for patient recruitment,

• did most of the clinical assessments,

• performed the statistical analyses,

• helped to obtain funds for the research, and

• was responsible for the final writing up of data.

SS:

• participated in the design of the study,

• assisted with recruitment of patients, and

• supervised writing and statistical analyses.

PLduT:

• participated in the design of the study,

• assisted with recruitment of patients, and

• did some of the clinical assessments.

DGN:

• was the primary statistical consultant for analyses.

DJHN:

• participated in the design and coordination of the study,

• was responsible for patient recruitment, and

• assisted with clinical assessments.

RS:

• assisted with literature review of cognitive schema data, and

• asisted statistical analysis of schema data.

DJS:

• conceived of the study

• supervised coordination, statistical analysis, and writing,

• did the final revision of paper before submission, and

• assisted with obtaining of funds.

All authors read and approved the final manuscript.