Active and Passive Components of Pelvic Floor Muscle Tone in Women with Provoked Vestibulodynia: A Perspective Based on a Review of the Literature

Abstract

Aim Pelvic floor muscle (PFM) dysfunctions, especially elevated tone or tension, are suggested to play an important role in the pathophysiology of provoked vestibulodynia (PVD). However, the involvement of the PFMs remains misunderstood as the assessment of muscle tone is complex and requires a thorough understanding of muscle physiology in relation to the characteristics and limitations of current PFM assessment tools. The aim of this review was to describe the structures and mechanisms involved in muscle tone in normally innervated muscle, and to discuss and relate these concepts to the PFM findings in women with PVD. Methods A narrative overview of the literature retrieved from searches of electronic databases and hand searches. Results Muscle tone in a normally innervated muscle comprises both active (contractile) and passive (viscoelastic) components. Current methods for evaluating PFM tone such as digital palpation, ultrasound imaging, pressure perineometry, dynamometry, and electromyography may evaluate different components. Research findings suggestive of PFM hypertonicity in women with PVD include elevated general PFM tone, changes in viscoelastic properties, and at least in some women, abnormal increases in electrogenic activity. Conclusion There is a growing body of evidence to support the involvement of PFM hypertonicity in the pathophysiology of PVD. Limitations of the instruments as well as their properties should be considered when evaluating PFM tone in order to obtain better insight into which component of PFM tone is assessed. Future research is required for further investigating the underlying mechanisms of PFM hypertonicity, and studying the specific effects of physiotherapeutic interventions on PFM tone in women with PVD.

An automated intravaginal dynamometer: Reliability metrics and the impact of testing protocol on active and passive forces measured from the pelvic floor muscles

“…Stiffness, RFD, and relative peak force were all higher at the faster speed of opening compared to the slower speed. While, to our knowledge, this is the first study to demonstrate the velocity dependent nature of passive PFM forces in vivo, the results are in line with other research in skeletal muscle function and modeling . The stress relaxation response in tissues has been shown to be dependent on strain rate and hold time .…”

“…Specifically, we found that absolute peak force, relative peak force, and stiffness during passive tissue elongation were systematically higher in the first trial compared to subsequent trials at both velocities. Morin et al reported that passive forces were significantly higher in the first two cycles compared to the subsequent three cycles, and later attributed this finding to thixotropy . Thixotropy is a property of skeletal muscle that refers to an initially higher resistance to passive lengthening of the muscle which is caused by covalent bonds formed between actin and myosin monofilaments After the initial phases of a first stretch, thixotropy should no longer be present .…”

Recent advances in understanding provoked vestibulodynia

“…Recently, transperineal three-dimensional real-time ultrasound imaging has been used to investigate the morphology and function of the PFM in PVD patients 77, 78 . Because this method does not involve vaginal penetration (as with intravaginal palpation and EMG assessment) and is therefore pain-free, it has the advantage of limiting bias caused by the participant’s pain and anxiety during the procedure 79 . In accordance with previous studies, ultrasound imaging showed higher PFM tone and lesser contractile capacities among PVD patients 77– 79 .…”

“…Because this method does not involve vaginal penetration (as with intravaginal palpation and EMG assessment) and is therefore pain-free, it has the advantage of limiting bias caused by the participant’s pain and anxiety during the procedure 79 . In accordance with previous studies, ultrasound imaging showed higher PFM tone and lesser contractile capacities among PVD patients 77– 79 . These findings suggest that PVD patients display PFM impairments which are not limited to a defense reaction but are rather chronic 78 .…”

“…The mechanism suggested to be involved in such a pathway is an abnormal neuropathic output state due to changes in the peripheral and central nervous systems secondary to noxious stimuli over a prolonged period of time 83 . It was also hypothesized that trigger points within the PFM may refer pain to the region of the vestibule 73, 79 or that hypertonicity of the muscles that insert at the posterior vestibule (pubococcygeus, puborectalis, and superficial transverse perineum) can lead to allodynia in the posterior vestibule due to altered neurodynamics and neural and tissue hypoxia 20 .…”