Discussion
The primary focus of this study was to examine whether lesions in certain elements of the CSTC circuit were associated with the development of OCD symptoms or tics in patients with TBI. Overall, we found that patients with TBI did not differ in the presence or severity of obsession, compulsions or tics compared with controls. Also, there were no significant differences in obsessions, compulsions or tics before and after brain injury in patients with TBI. Therefore, our findings suggest that general TBI is not specifically associated with the development of obsessions, compulsions or tics. However, when we examined whether damage to particular elements of the CSTC circuit were associated with obsessions, compulsions or tics, we did find that damage to specific brain areas was associated with the development of compulsions and tics. Specifically, our results suggest that damage to the left dlPFC may be associated with the development of compulsive actions, behaviours, and thought patterns. Studies in the past have implicated the left and right dlPFC in OCD symptoms24; however, in this study, only left-sided lesions in dlPFC were associated with increased compulsions. While we do not have a simple explanation for this finding, a recent study suggested that the left dlPFC is important in context-dependent shifting of on-task and off-task thought.25 Therefore, damage to the left dlPFC might be expected to decrease this shifting ability and may leave patients predisposed to enacting repetitive compulsive activities. Prior studies have shown that dysfunctional connectivity between dlPFC and putamen is associated with symptom severity in OCD.26 27 The same study demonstrated that altered connectivity between these regions was associated with deficits in goal-directed learning. While the authors did not comment on the relationship between dlPFC-putamen disconnection and compulsivity specifically, a subsequent study demonstrated that deficits in goal-directed learning are associated with trait compulsivity.28 Our study suggests that damage to the left dlPFC results in increased compulsivity in a TBI population. Interestingly, two recent meta-analyses found evidence that transcranial magnetic stimulation targeting the dlPFC can alleviate some symptoms in OCD.29 30 However, a direct connection between altered dlPFC activity and compulsivity in OCD remains to be established.
In our cohort of patients with TBI, damage to the BG was associated with increased tics, similar to prior studies in animals and individual case studies.31 The overall output of the BG is thought to inhibit motor activation21 32 and prior studies in animals suggest that damage to or inactivation of the globus pallidus, one of the output nuclei of the BG, can result in a release of motor inhibition and repetitive stereotyped behaviours.33 We speculate that this mechanism may also play a role in tic generation in patients with BG damage. One notable limitation of the current study is that the definition of tics we used does not allow tics to be well-distinguished from stereotypies. Whereas tics are associated with features such as premonitory urges and may be stress related or sometimes suppressible, stereotypies tend to cluster in longer periods and have a tendency to involve similar muscle groups in consistent patterns that may look rhythmic.34 The YGTSS does not specifically distinguish between these aspects of the movements it identifies and quantifies. Notably, a recent study looking at the co-occurrence of stereotypies and tics in patients with Tourette Syndrome found that tics and stereotypies could be identified by the YGTSS and the Stereotypy Severity Scale (SSS), respectively. Future studies may benefit from including the YGTSS and SSS when there is concern for tics and/or stereotypy.35
While psychiatric illnesses are conceptualised as circuit-level disorders,24 the results from this study highlight the possible involvement of individual CSTC circuit elements in the development of compulsions and tics in particular. Indeed, recent studies have suggested that compulsive behaviour may be an important transdiagnostic symptom that has the potential to be amenable to specific targeted brain interventions.36–39 Our study adds to this literature by suggesting that dlPFC damage and dysfunction may play a role in compulsive behaviour and that BG damage and dysfunction may be involved in tics.
Our study has a number of important limitations. First, there were unequal numbers of patients in the different ROI groups, which could affect the power to detect associations. Second, the imaging and behavioural evaluations were performed at different phases of the study, and so separated by several years (although the lesions were stable between phases 3 and 4). Third, CT imaging of the BG and thalamus did not have the anatomical resolution of MRIs and our characterisation of damage to these regions was, by necessity, rudimentary. Fourth, all the patients in the current study are male. Fifth, the time elapsed between the brain injury, imaging, and assessment of symptoms may have confounded our results. Sixth, we employed modified versions of the Y-BOCS and YGTSS that were completed by an informant and assessed symptom presence and severity before and after brain injury (in the case of TBI subjects). Because the subjects sustained their brain injuries many years prior to their assessment, there was a chance for recall bias of the informant. All of the informants were family members and did know the patient before the brain injury. Finally, there were also some differences in the baseline characteristics of the subjects with TBI in the study and the control subjects. Despite these limitations, the present study is the first, to our knowledge, to examine the contributions of different CSTC circuit elements to obsessions, compulsions and tics in patients with focal penetrating TBI. A next step will be to test whether damage or dysfunction in the dlPFC or BG in other disorders, such as neurodegenerative illnesses, is associated with the development of compulsive behaviours or tics, respectively.
In summary, our results suggest that damage to the left dlPFC is associated with an increase in compulsions and damage to the BG is associated with the development of tics. Our findings contribute to understanding the role of particular CSTC nodes in compulsions and tics. Further work should explore whether there is circuit-level dysfunction occurring in these lesioned patients.
Research ethics Statement: This study obtained ethics approval by the IRB as detailed in the methods section of the paper.