Introduction

Dyspareunia is defined as genital pain that occurs just before, during or after sexual intercourse. This term is used for both sexes, but for obvious reasons it is more frequently discussed in relation to women [1]. It was first described in ancient Egypt in the Ramesseum Papyrus, the oldest surviving illustrated Papyrus roll [2]. Its frequency often depends on the period in a woman’s lifetime. The frequency of dyspareunia ranges from 8–22%, according to different authors [3]. However, it is thought to be more frequent in women about 40 years of age [4]. Sexual intercourse concomitant with a pelvic organic lesion or wound is a probable cause of pain in most cases.

Another reason could be non-physical aspects of dyspareunia or sexual trauma. Moreover, sexual dysfunctions can cause dyspareunia. Pelvic pain could be present at the site of the pelvic inflammation, vaginal mucous inflammation, discharge, endometriosis, mucous atrophy, pelvic surgery or after other affections [5, 6]. However, in these cases the pain depends not only on penile-vaginal intercourse but has an organic origin [7–9]. In most cases it disappears after regular treatment of its origin. It could be better described as vulvodynia. In its essential definition dyspareunia in women is considered an idiopathic condition without a typical organic constitution, i.e. it is only present with penile-vaginal penetration and not followed by any other condition [1, 10]. This dysfunction is very often associated with painful spasms of the pelvic muscles [11, 12].

For this reason, the term “vaginism” was probably included in this group. This dysfunction is defined as a painful spasm of the pelvic muscles that completely anatomically disables vaginal penetration [8]. The complex was named GPPPD (genito-pelvic pain or penetration disorder) [13].

The intensity (level of dyspareunia) was first defined by Marinoff et al. [14]. The authors proposed a four-point scale ranging from 0–3 (0 = no pain during intercourse, 1 = pain during intercourse that does not prevent intercourse, 2 = pain during intercourse that interrupts intercourse and 3 = pain that prevents intercourse). The scale describes pain limitations during sexual intercourse. Despite proper diagnostics, determining a cause and treatment for dyspareunia remains problematic.

Pain associated with muscle, surroundings and interpenetrating fascial tissue seems to be one of the most common diagnoses reported in general practice. Studies have focused on functional principles, including an altered expression of muscle metabolism and enhancing the role of peripheral factors in persistent chronic pain [15–17]. Some studies have reported an increased concentration of end products of anaerobic glycolysis and impairing oxidative metabolism by local hypoxia, presented as an insufficient capacity for muscle recovery because of inflammation and metabolic pattern changes [18]. Long-term hypoxia in perineal muscles could cause muscle and perimuscular changes, resulting in chronic pain not adequately responding to standard therapy. During the entrance examination to our previous study on dyspareunia, we noted significantly lower pulse oximetry levels in the perineal area of affected women [15, 19]. We focused on this event to explain the reason for the muscle and perimuscular changes. Specifically, we aimed to compare pulse oximetry oxygen saturation (SpO2) of dyspareunia-affected women (treatment group) to healthy, pain-free women (controls).

Participants and method

A retrospective study was performed to compare the treatment group with the controls. The study participants were women who took part in our previous study on dyspareunia conducted between 2017 and 2019, following a former feasibility study (previously published) [20].

All study procedures were conducted in accordance with the ethical standards of the institutional and research committees and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. The study protocol was approved by the Ethical Committee of the Teaching Hospital at Charles University, Prague, and all patients gave their informed consent and confirmed their participation by signing a consent form. This retrospective study protocol was also approved by the Ethical Committee of the Teaching Hospital at Charles University, Prague.

Method

During their regular examinations, women in the treatment and control groups were measured for SpO2. The oximetry measurements were performed in the posterior commissure of the vagina between the mucous part of the vagina and the dermal part of the perineum, involving the bulbospongiosus muscle (Fig 1). The measurements were done in supine position with the biggest ear-clip sensor pulse oximeter and a plain vital functions measuring gadget. Measurement of SpO2 was recorded as a percentage. SpO2 is determined by measuring the absorption of two emitted light wavelengths, red at 660 nm and near-IR at 940 nm. The light transmitted through the tissue is detected by a photodiode on the opposite side. The pulse oximeter uses the relative amount of red and IR light absorbed to define the proportion of oxyhaemoglobin in deoxyhaemoglobin [19, 21, 22]. The ability of the pulse oximetry to detect SpO2 of only arterial blood is based on the principle that the amount of red and IR light absorbed fluctuates with the cardiac cycle as the arterial blood volume increases during systole and decreases during diastole. The blood volume in the capillaries, veins, fat, skin and bone remains relatively constant [19].

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Fig 1. SpO₂ measurement area in perineum region.

https://doi.org/10.1371/journal.pone.0281268.g001

Results

There were 62 participants with 31 in each group (Table 1). The Shapiro-Wilk test confirmed that a Gaussian distribution of the variables was not met. The Mann-Whitney nonparametric U test was run to determine differences in perineum oximetry between the treatment and control groups. The distribution of SpO2 differed statistically between the two groups (p = 0.002). The median SpO2 was 91 in the treatment group and 92 in the control group. We computed the sample size to ensure that a two-sided test with α = 0.05 has 80% power to detect a true effect. With our data, 50 participants (25 per group) were needed, lower than the actual number of study participants (n = 62).

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Table 1. Distribution of oximetry data in treatment and control groups.

https://doi.org/10.1371/journal.pone.0281268.t001

Discussion

The present study confirms the presence of depressed oxygenation in the examined areas of the female perineum in dyspareunia-affected women. Due to local ischemia, abnormal tissue oxygenation is most likely present in the focused areas. The hypothesis was that a microcirculation change occurs in regions producing muscle pain [15]. Some authors have studied blood flow and reported a notable change in local microcirculation. They assumed that regulation of the microcirculation is disturbed in fibromyalgia, which could lead to sensitisation of the intramuscular nociceptors [18]. The lower density of capillaries in the focus area was also discussed. Histological tissue findings detected so called moth-eaten and ragged red fibres that may indicate mitochondria’s abnormal proliferation. This pathological distribution of mitochondria may be caused by local hypoxia from low blood flow in tender areas. A feasible alternative cause of hypoxia could be physical inactivity, which produces mitochondrial insufficiency and abnormal levels of critical metabolites for contractile muscle force [23]. Some authors refer to this phenomenon as deconditioning. Reduced daily activity causes deconditioning, affecting the overall picture of notable anatomical and biochemical defects in muscle [15, 24].

Yet, it should be noted that this study type of pain is only connected with cohabitation. If the pain is present at different time points or permanently, we have to describe the condition differently, probably as vulvodynia. We believe this is the first study expressly focusing on oximetry in the context of dyspareunia.

The strength of our study is that the technique used is painless for the patient, straightforward, low-cost and time efficient. Moreover, the procedure could be easily repeated without problems. It demands only standard vital function measuring unit with an adult ear probe to gather the data. The major disadvantage is the lack of standardisation for conducting this procedure. Another weakness of this study could be that it could overlook some diseases, which may bias the findings.

Conclusions

Idiopathic dyspareunia is inherent in cohabitation muscle pain that standard therapy could neither explain nor treat. During the examination, we detected statistically significant decreased levels of SpO2 in our patient cohort. We compared pelvic oximetry between dyspareunia-affected and treated women and healthy controls (no dyspareunia). This comparison showed significant hypoxia in the perineal muscle area (p = 0.002). Our results may help understand the source of this pain and more accurately aim the treatment directly in the defective area.

Acknowledgments

We would like to thank our midwives, whose help was crucial for this work.