In our cohort, approximately 8 out of 10 women with VVS had chronic orofacial pain. The most common and widely studied form of chronic orofacial pain -temporomandibular disorder- affects 7−15% of the adult population; 80% of the treated cases are women in their early to mid-adulthood.21
At the present, we can not irrefutably make a statement about the prevalence of a specific category of OFP. However, on formal evaluation by an orofacial pain specialist (as part of an ongoing follow up study) the majority of our participants have signs and symptoms of temporomandibular disorder.
It is intuitively perplexing as to why an idiopathic orofacial pain disorder (affecting 10% of the general population)22
is highly prevalent among women with idiopathic genital pain. Another related question that comes to mind is the mechanism by which the distribution of signs and symptoms of OFP mirror the spectrum of psychological characteristics among women with VVS. Women with VVS and symptoms of OFP had higher levels of anxiety, somatization, and psychological distress; duration and severity of self-reported pain with intercourse did not differ among categories of OFP.
The association between psychological traits and OFP among women with VVS may, in part, be explained by specific genetic variants which mediate the activities of central pain regulatory pathways. For example, polymorphism of the gene encoding catechol-o-methyltransferase (COMT) is a potent and independent risk factor for the development of chronic pain conditions, such as temporomandibular disorder.23
Furthermore, the associations between the haplotype variations for the β2 adrenergic receptor and psychological traits, such as anxiety and somatization, have been identified.24
We hypothesize that subgroups of women with VVS may share the same genetic vulnerability as women with temporomandibular disorder.7
Thus, the associations between certain psychological traits and signs/symptoms of OFP among women with VVS suggest that an inherent susceptibility may permit or even precede the development of VVS in certain women. We therefore speculate that women with VVS are a heterogeneous population and that the observed clinical phenotype is composed of several interactive biological and psychological factors.
Based on our findings and an emerging body of evidence, we hypothesize that the experience of pain with attempted intercourse may be governed by two interdependent processes: 1) a biologic impairment in pain regulatory mechanisms (similar to what is seen among women with temporomandibular disorder and other idiopathic pain conditions), and 2) a genetic predisposition to a heightened inflammatory response in vulvar mucosa. Mucosal sensitivity, though a necessary component of the clinical manifestation of VVS, may not be sufficient for the development of this disorder in all women. Experience of pain, though clinically similar, could in fact be primarily driven by different pathophysiologic processes, some that are centrally mediated (i.e., biochemical abnormality in pain processing) and some that are peripherally mediated (i.e., biochemical abnormality in the inflammatory cascade in the mucosa).
While these results are intriguing, it is important to note that our population reflects the severe end of the spectrum of patients with VVS seen in a referral-based university clinic. The findings from our patient population may not be generalizable to VVS patients seen in primary care. Furthermore, the 137 women who completed the questionnaires that allowed classification of OFP status may differ in important ways from those not reached or not willing to participate in research. It is difficult to hypothesize the direction of bias due to non-response. However, it is unlikely that women who opted against participation or could not be contacted would change our findings because the classification of these women would likely fall equally among the three subgroups. Lastly, we did not use the ‘gold standard’22
clinical exam for diagnosis of OFP but instead used a validated questionnaire used to identify temporomandibular disorder, the most common form of OFP. This may have resulted in a higher prevalence of OFP. However, the differences across groups with respect to psychosocial variables are in agreement with previous research in temporomandibular disorder 25
and support the likelihood that our categories based on questionnaire responses adequately classified OFP groups.
This study is cross-sectional and therefore temporal sequence cannot be established. Specific psychological traits may precede or be modified by chronic pain disorders.6
Psychological traits may be consequences of living with chronic pain; more importantly, however, such traits may precede the development of chronic pain and be amplified with disease progression.
Research in chronic pain disciplines (e.g., orofacial pain, fibromyalgia) requires conceptual models that examine the interplay between psychological and biological factors and their ultimate effect on pain pathophysiology. This model is lacking in VVS. Our findings provide additional evidence in support of VVS being an idiopathic pain disorder akin to fibromyalgia and TMD. To that end, we must: 1) objectively measure the clinical presentation of VVS as it relates to the vulvar mucosa, pelvic floor muscles, psychological traits, and central pain processing mechanisms; 2) investigate the potential for distinctive subtypes of VVS; and 3) develop a conceptual framework, incorporating these measures, in order to guide research in the pathophysiology and treatment of VVS.