Global collaboration produces largest analysis of brains of patients with trichotillomania
Trichotillomania is a psychological condition where a person compulsively pulls out their own hair. The condition is in the same diagnostic category as obsessive compulsive disorder (OCD), but typically doesn’t involve the intrusive obsessive thoughts that occur in OCD. Despite affecting 0.5-1% of the population, trichotillomania has not received a lot of research attention or funding.
The lack of funding for trichotillomania research has meant that studies of the neurobiology of condition have only included small numbers of participants. Small studies can lead to unreliable and contradictory results. To solve the problem of small samples, researchers at the University of Cambridge, the University of Chicago, Harvard Medical School, the University of Cape Town and the University of Stellenbosch pooled together existing sets of MRI brain scans of trichotillomania patients. The resulting pooled dataset included brain scans 76 patients with trichotillomania and 41 typical controls.
Lead author Dr Samuel Chamberlain
Hair-pulling in trichotillomania can be thought of as a behavioural habit, which the individual is unable to suppress. At a neurobiological level, this could be due to differences in brain areas, such as the dorsal striatum, that are involved in habit formation, and/or brain areas in the frontal cortex that are involved in habit suppression. In their analysis of the pooled MRI dataset, the research team looked for differences in the thickness of the cortex (the outer layer of the brain) and differences in the volume of subcortical structures (underneath the cortex) including parts of the striatum, thalamus and hippocampus.
The results of the analysis, published in Brain Imaging and Behaviour in June, show that patients with trichotillomania have increased thickness in regions of the frontal cortex involved in suppression of motor responses: the right inferior frontal gyrus (rIFG) and other nearby brain regions. This large area of cortex was on average 0.14mm thicker in the trichotillomania patients than in controls. Brain damage to the rIFG leads to response inhibition deficits, suggesting that the area plays a critical role in regulating habitual behaviour. Trichotillomania patients and their first-degree relatives also show response inhibition deficits on neuropsychological tests.
“Trichotillomania is a common mental health condition but we know very little about its basis in the brain and how to treat it. This study suggests that the right inferior frontal lobe, which regulates our habits, develops differently in people who have trichotillomania. In future work we plan to explore whether treatments capable of enhancing function in this brain region may be useful for patients with trichotillomania.”
Differences in the rIFG thickness have also been found in studies of the brains of patients with OCD. However, OCD patients show reduced thickness in this area in comparison to typical controls, rather than the increased thickness shown in trichotillomania. It seems that, although both conditions are characterised by difficulties controlling habitual behaviours, trichotillomania may have a distinct neurobiological basis.
The authors believe that an increased thickness of the rIFG is related to trait tendency to develop trichotillomania, rather than the state of having the condition. There are two reasons for suggesting this. First, there was no relationship between symptom severity and rIFG thickness in the patients with trichotillomania in the current study. Second, another study found that healthy relatives of people with trichotillomania also have increased rIFG thickness. This suggests that relatives share this genetically determined neurological trait, which makes them susceptible to developing trichotillomania.
The study did not identify significant differences in volumes of subcortical regions involved in habit formation in trichotillomania. This may be because the team’s analysis was optimised to look at the thickness of the cortex rather than volumes of subcortical brain areas. The researchers plan to carry out additional analysis using more sensitive techniques. In doing so, they hope to further characterise the neurological basis of this understudied condition.