Hair-pulling disorder (trichotillomania, HPD) is a disabling condition that is characterized by repetitive hair-pulling resulting in hair loss. Although there is evidence of structural grey matter abnormalities in HPD, there is a paucity of data on white matter integrity. The aim of this study was to explore white matter integrity using diffusion tensor imaging (DTI) in subjects with HPD and healthy controls. Sixteen adult female subjects with HPD and 13 healthy female controls underwent DTI. Hair-pulling symptom severity, anxiety and depressive symptoms were also assessed. Tract-based spatial statistics were used to analyze data on fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD) and radial diffusivity (RD). There were no differences in DTI measures between HPD subjects and healthy controls. However, there were significant associations of increased MD in white matter tracts of the fronto-striatal-thalamic pathway with longer HPD duration and increased HPD severity. Our findings suggest that white matter integrity in fronto-striatal-thalamic pathways in HPD is related to symptom duration and severity. The molecular basis of measures of white matter integrity in HPD deserves further exploration.
Hair-pulling disorder (trichotillomania, HPD) is a disabling condition that is characterized by repetitive hair-pulling resulting in hair loss. HPD may interfere with social relationships, family life and work, and has been associated with significant functional impairment (Woods et al., 2006). Currently defined as an impulsive control disorder not otherwise specified (DSM-IV), HPD has also been conceptualized as an obsessive–compulsive (OC) spectrum disorder due to partial overlaps with obsessive–compulsive disorder (OCD), Tourette's syndrome and stereotypic behaviors such as skin-picking (Lochner et al., 2005, Chamberlain et al., 2009 and Ferrao et al., 2009).
A number of the putative OC spectrum disorders may be mediated by fronto-striatal circuitry (Boulougouris et al., 2009 and Fineberg et al., 2010). Imaging data on HPD are somewhat inconsistent. A number of structural and imaging studies have found evidence of fronto-striatal abnormalities (Grachev, 1997, O'Sullivan et al., 1997 and Keuthen et al., 2007). On the other hand, there is also data on alterations in a broad range of other areas (Chamberlain et al., 2009) implicating fronto-striatal-thalamic pathways. For example, in an investigation of structural abnormalities in HPD using computational morphometry (Chamberlain et al., 2008), there were increased grey matter densities in widespread areas including the prefrontal lobe, anterior cingulate, supplementary motor areas, striatum, amygdala and hippocampus.
To date, structural imaging studies in HPD have focused on structural grey matter abnormalities, rather than on white matter integrity. The introduction of diffusion tensor imaging (DTI) techniques allows specific assessment of white matter integrity. Chamberlain et al. (2010) undertook DTI in 18 subjects with HPD and found decreased fractional anisotropy (FA) in the anterior cingulate, orbitofrontal cortex, presupplementary motor areas and temporal lobe in HPD compared to healthy controls, indicating disruption in white matter integrity. These findings are partly consistent with DTI findings in OCD, where white matter abnormalities have been found in the fronto-striatal-thalamic pathways (Menzies et al., 2008).
In addition to FA, established DTI parameters include mean diffusivity (MD), axial diffusivity (AD) and radial diffusivity (RD) (Alexander et al., 2007). MD represents a global average of all diffusion directions (Assaf and Pasternak, 2008), with increased MD suggesting damaged and/or disorganized white matter tracts. AD is a specific measure of diffusion along axons, which decreases when there is axonal damage (Harsan et al., 2006 and Sun et al., 2006). RD measures perpendicular diffusion towards membranes, which is increased when myelin is damaged (Song et al., 2002 and Song et al., 2005). MD, AD, and RD have not yet been reported in HPD. We therefore undertook DTI of individuals with HPD and healthy controls to assess FA, MD, AD, and RD. The vast majority of individuals with HPD are female (Christenson et al., 1994 and Chamberlain et al., 2007), and we therefore included only women. We hypothesized that there would be disruption of white matter integrity in the fronto-striatal-thalamic pathways in HPD, and that some of these disruptions would be associated with clinical variables such as HPD duration and HPD severity.
