Background
Pelvic floor disorders (PFDs), caused by weakened pelvic muscle, connective tissue, and fascia, are associated with decreased quality of life and increased economic burden. Impairment of defecation is a public health issue with a prevalence of approximately 37% in Iran [
1‐
3]. Since fecal incontinence (FI) results from reduced mechanical pressure of anal sphincter tissue and the inability to obstruct the anal canal, anal sphincter injury or defect, as one of the PFDs, is considered among the leading causes of FI [
4,
5]. Epidemiologic studies demonstrate that 2–15% of the population suffer from FI [
6‐
8], and the prevalence increases to 13–23% with older age and in the female gender (possibly due to labor injuries) [
9]. Although FI has enormous negative impacts on a patient’s daily life, social activities, quality of life, and mental health, it lacks a complete and standard treatment [
6,
10,
11]. Although there are controversies about the effectiveness of surgery in the long term for the treatment of traumatic injuries of the anal sphincter in some cases [
12], but still anal sphincter repair surgery is the primary treatment approach for anal sphincter injury [
13]. Other treatment methods such as artificial anal sphincter and using mesh, are not ideal due to their high morbidity rates and the possibility of device failure [
14,
15]. Efficacy of bulking agents is limited by various factors, such as absorbance of the injected agent, their migration, and fat emboli and granuloma formation [
16,
17]. Therefore, there is increasing gravitation towards supplementary treatment approaches that restore the lost tissue to maintain efficacy in long-term follow-up.
Antioxidants, including Vitamin E, are among the agents that can repair the muscle tissue of the sphincter in anal sphincter injury, leading to the control of FI. Vitamin E is effective in repairing striated muscle (voluntary) through its antioxidant effect, inhibiting inflammatory factors, such as interleukin-6 (anti-inflammatory effect) [
18,
19], its angiogenic effect [
20], and protecting myocytes from membrane damage and loss [
21]. Another property of vitamin E is its anti-fibrotic effect by decreasing TGF-β expression, which is why vitamin E could be helpful in anal sphincter muscle repair and controlling FI [
22,
23].
Trolox (6-hydroxy-2 5 7 8-tetramethyl chroman-2-carboxylic acid), a water-soluble vitamin E, is commonly used in experimental studies since it is easily administered orally in laboratory animals due to its powder form and water-solubility [
24]. Accordingly, vitamin E could be an efficient strategy for repairing muscles due to its antioxidant, anti-inflammatory, angiogenic, and anti-fibrotic properties and ability to prevent muscle cell loss. Thus, we aimed to evaluate the effect of vitamin E in repairing external anal sphincter (EAS) muscle defects in rabbits.
Discussion
The sphincteroplasty, as the primary treatment option for FI, offers satisfactory results in the short term; however, the FI symptoms could reappear after two years [
28,
29] due to fibrous tissue formation in the sphincter defect site, which is not as functional as muscle tissue. Therefore, restoring the muscle tissue in place of fibrous tissue could potentially lead to improved results in the long term. Vitamin E may be an effective strategy for repairing the muscle due to its antioxidant, anti-inflammatory, angiogenic, anti-fibrotic, and protective properties against myocyte loss. Thus, we aimed to evaluate the efficacy of vitamin E in anal sphincter muscle defect of rabbits to establish a treatment approach with long-term and permanent effects.
The anal canal resting pressure is a result of IAS (85%) and EAS (15%) tone, and maximum squeeze pressure is caused by EAS tone [
30]. Therefore, a significant reduction in the abovementioned indexes occurs in case of defects in these muscles. In the present study, due to the fourth-degree rupture sphincterotomy (EAS, IAS, and mucosa tear) induction [
25] in rabbits, we observed a substantial decline in the resting and maximum squeeze pressures in the sphincterotomy (control) group compared to the intact group. After three months of Trolox administration, there was a significant improvement in the resting pressure of the Trolox group compared to the control group.
α-Tocopherol, the most common vitamin E form in human tissues, protects cells against inflammatory and degenerative processes [
31,
32]. Vitamin E has an influential role in muscle repair and protection. In a study by Mancio et al. in 2017, vitamin E supplement administration for 14 days had beneficial effects against myonecrosis, inflammatory response, and oxidative stress [
18]. Vitamin E could enhance sarcolemma (muscle cell membrane) integrity, decrease serum creatine kinase level, and inhibit IgG binding to skeletal muscles. Cell membrane repair is the primary mechanism by which vitamin E aids muscle repair [
21]. Since vitamin E is fat-soluble, it can penetrate the phospholipid bilayer structure of sarcolemma and influence the physical properties of the cell membrane, such as fluidity, and thus contribute to membrane repair [
33].
Moreover, the potent antioxidant effect of vitamin E is another aspect of its role in cell membrane repair, mainly inhibiting lipid peroxidation [
34]. In addition to the antioxidant property, the findings of several studies have pointed to the impact of vitamin E on inflammatory factors regulation in different pathological states [
35‐
37]. Vitamin E administration in MDX mice has shown favorable effects on the inflammatory process of muscle and decreases the histological and molecular inflammatory factors that damage muscles [
18]. Vitamin E regulates the signal transduction and gene expressions involved in the inflammatory processes [
38]. Moreover, α-Tocopherol inhibits NF-κB activity [
39], among the main anti-inflammatory properties of vitamin E in injured muscles. Moreover, vitamin E directly decreases the interleukin-6 caused by muscle damage [
18]. Therefore, vitamin E has a robust anti-inflammatory effect via sarcolemma protection, decreasing lipid peroxidation, inhibiting NF-κB, and reducing interleukin-6 levels.
Anti-fibrosis is another critical function of vitamin E, achieved through TGF-β expression regulation [
22]. TGF- β is a profibrogenic cytokine that is increased in fibrotic lesions and scar formation up to four times. Accordingly, the present study’s findings demonstrate the significant collagen decline in the sphincterotomy site in the Trolox group relative to the control group. In general, the healing of sphincter tissue occurs through several main mechanisms that vitamin E provides. These mechanisms include anti-fibrotic, anti-inflammatory, antioxidant properties and protection of myocytes against inflammatory and apoptotic processes of damaged sphincter tissue. Reducing fibrosis tissue on one hand and preventing the loss of sphincter tissue myocytes along with muscle regeneration on the other hand increases the ratio of muscle tissue to fibrosis tissue in the lesion site. In other words, maintaining the myocytes of the sphincter tissue is a far more important factor than the production of new myocytes in achieving the desired results regarding sphincter muscle tissue function.
Increases the ratio of muscle tissue to fibrosis tissue in the lesion site is proved to be a crucial factor in improving anal sphincter injuries and FI, according to animal studies and clinical trials. In 2020, Sarveazad et al. revealed that the significant decrease in collagen content secondary to human adipose-derived stem cells and low-level laser therapy in the rabbit’s anal sphincterotomy site resulted in muscle content increase compared to the control group and a rise in the resting and maximum squeeze pressures of anal sphincter tone and motor units’ number in electromyography. Our findings were similar regarding the collagen content reduction, increase in the anal sphincter tone in resting and maximum squeeze, and electromyography data [
26]. The results of a clinical trial by Sarveazad et al., published in 2017, showed that the increases the ratio of muscle tissue to fibrosis tissue at the sphincterotomy site of patients who had FI due to EAS injury improved the Wexner score (FI score) and significantly increased the number of motor units [
27].
The rise in the resting and maximum squeeze pressures of anal sphincter tone is caused by the increases the ratio of muscle tissue to fibrosis tissue of the sphincterotomy site. In 2014, Baldelli et al. showed that Trolox enhances myogenesis by regulating proliferative-activated receptor gamma coactivator-1 alpha (PGC-1α) [
40].
The strength of the present study includes long follow-up time (90 days) and performing functional tests. Nevertheless, our study has some limitations. We did not evaluate the molecular pathways involved in myogenesis, and the study design was not dose-dependent. Thus, it is recommended to design further studies evaluating different doses of Trolox and investigate the involved molecular pathways in addition to the functional tests and histological evaluation.
Because no patient with sphincter lesions knows before being injured, in order to mimic the most similarity with clinical injuries, it is suggested that future studies be designed in such a way that Trolox is administered after sphincterotomy (not at the time of sphincterotomy or before). Among other suggestions, in future studies, Trolox should be administered in different degrees of sphincter damage (with different lengths and extension) so that the real effectiveness of Trolox in repairing the sphincter muscle can be judged.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.