Patients with POP showed no decrease in muscle mass, but a significant decrease in muscle quality compared to patients without POP. However, decrease in muscle quality was not an independent risk factor.
Age-related declines in muscle mass and quality are among the most widespread changes associated with aging. The term sarcopenia was coined by Rosenberg to describe pathological declines in skeletal muscle mass, strength and function associated with aging [11]. After 35 years old, healthy individuals lose muscle mass at a rate of 1–2% per year and muscle strength at a rate of 1.5% per year, accelerating to about 3% per year after 60 years old [12]. As a result, muscle cross-sectional area of the thigh decreases by about 40% between 20 and 60 years old. The magnitude of the decrease in fat-free mass is amplified in sedentary individuals compared to physically active individuals [12, 13]. The average adult can expect to gain about 0.45 kg [14] of fat per year between 30 and 60 years old [15], in addition to losing muscle mass. Such changes in body composition are often masked because of the relative stability of body weight, in a condition referred to as sarcopenic obesity, which further increases the risks of disability and morbidity. Although losses of muscle mass and strength with aging have long been known, sarcopenia has recently become a hot topic in gerontology. Several processes and mechanisms have been proposed for the multifaceted etiology of sarcopenia, including lifestyle, systemic factors (such as inflammatory cytokines), changes in the local environment (e.g., vascular disorders), changes in the neuromuscular system, and changes in specific processes within muscles [16]. All these factors ultimately disrupt the balance between anabolic and contractile effects, leading to muscle protein degradation, muscle cell loss, and changes in specific processes within the muscle.
The three most common risk factors for POP are the number of vaginal deliveries, postpartum weight gain, and aging. Awwad et al. found that the prevalence of POP in nulliparous women was 3.6%, compared to 6.5% in women with one delivery, 22.7% in women with two deliveries, 32.9% in women with three deliveries, and 46.8% in women with four deliveries. The higher the number of deliveries, the higher the prevalence [17]. The effects of vaginal delivery on the pelvic floor musculature include direct injury to soft tissues and nerves [18]. The results of direct injury to soft tissues is thought to be temporary and the ptosis is likely to improve. In the case of muscle tears, irreversible changes such as vascular damage are thought to occur after birth. In addition, although the details are unknown, the nervous system may be physically damaged by the pressure of the infant's head during delivery, and this damage may accumulate with repeated deliveries, resulting in secondary damage to the pelvic floor muscle groups.
Obesity is a risk factor for POP. BMI at 1 year postpartum is reportedly associated with the occurrence of POP [18]. Weight control after the postpartum period is also an important factor in the development of POP [1–4, 17].
The most characteristic changes in body composition with aging are an increase in adipose tissue mass and a decrease in bone and muscle mass [1–4, 17]. Losses in muscle mass, in particular, are thought to lead to muscle weakness and POP. However, Awwad et al. reported in 2012 that the incidence of POP is particularly high among women who rarely develop new POP after menopause [17]. Nutritional and weight management guidance for these women might thus help prevent the onset of premenopausal POP and reduce the number of POP patients [17]. In addition, observation and close follow-up are appropriate for mild, asymptomatic women, and most women do not experience symptoms until the bulge protrudes beyond the vulva. Pelvic floor muscle training (e.g., Kegel exercises) allows systematic contraction of the anorectal muscles to strengthen the pelvic floor and has been proven to improve symptoms of stress urge incontinence and mixed urinary incontinence, and has also been reported as effective in women with mild POP [19].
Our study showed that poor muscle quality was an important factor in the development of POP, but not an independent risk factor. However, patients with POP occasionally have no history of vaginal delivery; the incidence of POP among patients without a history of delivery has been reported as 5% [17, 19]. In women, muscle mass has been reported to not change markedly throughout life [20], and muscle mass was maintained even in POP in the present study. The most important thing to be aware of is the loss of muscle quality, which may be a very important factor in the development of POP, as hidden sarcopenia associated with increased BMI may actually be a very important factor.
Muscle mass is an essential part of the criteria for the diagnosis of sarcopenia, and whole-body measurements with dual-energy X-ray absorptiometry have been cited as a method for this purpose [2]. To popularize the assessment of body composition, greater reliance on measures already included in the diagnosis or treatment of patients may be important. Psutka et al. [7] calculated a skeletal muscle index from images obtained at the level of the third lumbar vertebra during abdominal CT. Such legitimate, scientifically valid approaches have increasingly been used in the last few years to introduce discussions and interventions on body composition [7–11]. This is one reason why we measured muscle mass on CT in the present study.
However, some limitations to the present study must be considered. First, the number of patients was too small to obtain conclusive results. Second, strict criteria were not set for the selection of tomographic images to be measured. Third, this retrospective study was limited to patients from a single center.
The present findings suggest that rather than generalized muscle loss, muscle quality is associated with the pathophysiology of POP. Methods of preventing and addressing POP starting during pregnancy and postpartum should be implemented. Guidance for proper weight control during pregnancy and weight regain after delivery appears extremely important [18, 19].
From this point of view, the next step is pelvic floor muscle exercises. This is a non-invasive preventive method that has been shown to significantly improve symptoms after 6 months of practice and a consensus has been reached on its utility. In addition, avoiding undue pressure on the abdomen is important [18, 19]. Preventing the loss of muscle quality is also important. To maintain muscle mass, consuming at least 1 g of protein per kilogram of body weight per day is important, along with good oral health care to maintain the ability to eat well, and continued daily exercise habits from the time of sexual maturity [21].