Gastrointestinal
Electromyographic Activity of the Anterolateral Abdominal Wall Muscles During Rectal Filling and Evacuation

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Background

The role of the anterolateral abdominal wall muscles (AAWMs) at defecation has not received sufficient attention in the literature. We investigated the hypothesis that the AAWMs exhibit increased electromyographic (EMG) activity on rectal distension, which presumably assists in rectal evacuation.

Materials and methods

The effect of rectal balloon distension on the AAWMs EMG and on anal and rectal pressure was examined in 23 healthy volunteers (37.2 ± 9.4 SD years, 14 men, 9 women); this effect was tested before and after rectal and AAWMs anesthetization.

Results

The rectal and anal pressures increased gradually upon incremental rectal balloon distension starting at 70 mL balloon distension until, at a mean of 113.6 ± 5.6 mL, the balloon was expelled to the exterior. The AAWMs showed no EMG activity at rest or on rectal distension up to the time of balloon expulsion when they exhibited significant increase of EMG. This effect was abolished on individual rectal or AAWMs anesthetization but not with saline administration.

Conclusions

AAWMs appear to contract simultaneously with rectal contraction; this action seems to increase the intra-abdominal pressure and assist rectal evacuation. The AAWMs contraction upon rectal contraction appears to be mediated through a reflex, which we call the “recto-abdominal wall reflex”. Further studies are required to investigate the role of this reflex in defecation disorders.

Introduction

The anterolateral abdominal wall muscles (AAWMs) consist of the external and internal oblique (EOM, IOM), transverse abdominis (TAM), and rectus abdominis (RAM) muscles [1]. They consist of striated muscle bundles which contract voluntarily [1]. They act together to perform multiple functions some of which involve the generation of a positive pressure within the abdominal cavity [2]. Activities such as expiration, defecation and micturition may be assisted by the generation of a positive intra-abdominal pressure (IAP) [3]. Also parturition, coughing, and vomiting are usually aided by such a positive pressure.

Under normal resting conditions, the AAWMs provide support to the abdominal viscera and retain the normal abdominal contour [1, 2]. Congenital absence of these muscles as in “prune belly syndrome [4]” would lead to lack of support of the abdominal viscera. When the IAP is increased, AAWMs’ contraction plays an important role in the maintenance of the abdominal wall tone [1, 2].

Defecation is initiated when rectal contents distend the rectum until, at a certain volume, the stretch receptors in the rectal wall are stimulated and evoke the recto-anal inhibitory reflex [5]. The latter results in rectal contraction and internal anal sphincter relaxation. While the role of the pelvic floor muscles, including the levator ani and puborectalis muscles, at defecation is well established [6, 7, 8, 9, 10], the function of the AAWMs at defecation is as yet poorly addressed in the literature. We hypothesized that the AAWMs exhibit increased electromyographic (EMG) activity on rectal distension, which presumably assists in rectal evacuation. This hypothesis was investigated in the current study.

Section snippets

Subjects

The study comprised of 23 healthy volunteers (14 men, nine women, mean age 37.2 ± 9.4 SD years, range 27 to 47 y). After receiving full information about the nature of and their role in the study, they gave an informed consent to participate in the tests to be done. The results of the physical examinations including neurological assessment were normal. Laboratory work-up composed of blood count, renal and hepatic function tests, as well as electrocardiography had unremarkable results. The Cairo

Results

No adverse side effects were encountered during or after performing the tests and all of the subjects were evaluated.

The resting rectal pressure recorded a mean of 7.6 ± 1.2 cm H2O, and the anal pressure a mean of 72.4 ± 8.4 cm H2O (Table 1). The EMGs of the EOM, IOM, TAM, and RAM recorded no basal activity as these muscles are striated: rectal balloon distension in increments of 10 mL of normal saline and up to 60 mL did not effect significant rectal or anal pressure changes or AAWMs EMG

Discussion

The AAWMs partially cover the abdominal cavity and share in maintaining the intra-abdominal pressure (IAP) constant under normal physiological conditions [1, 2]. They respond to variations of the IAP. They reflexively contract upon increase of the IAP, a response mediated through the straining-abdominal wall reflex (SAWR) [6]. Coughing or straining trigger the SAWR with a resulting contraction of AAWMs.

The AAWMs, consisting of striated muscle fibers, contract voluntarily [1, 2]. Like the other

Acknowledgments

The authors acknowledge Margot Yehia, who assisted in preparing the manuscript.

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