J Manipulative Physiol Ther. 2016 (Jun); 39 (5): 339347 ~ FULL TEXT
Heidi Haavik, BSc (Chiro), PhDip (Science), PhD,
Bernadette A. Murphy, DC, MSc, PhD,
Jennifer Kruger, BSc (Nursing), MSc, PhD
Director of Research,
Centre for Chiropractic Research,
New Zealand College of Chiropractic
OBJECTIVE: The aim of this study was to investigate whether a single session of spinal manipulation of pregnant women can alter pelvic floor muscle function as measured using ultrasonographic imaging.
METHODS: In this preliminary, prospective, comparative study, transperineal ultrasonographic imaging was used to assess pelvic floor anatomy and function in 11 primigravid women in their second trimester recruited via notice boards at obstetric caregivers, pregnancy keep-fit classes, and word of mouth and 15 nulliparous women recruited from a convenience sample of female students at the New Zealand College of Chiropractic. Following bladder voiding, 3-/4-dimensional transperineal ultrasonography was performed on all participants in the supine position. Levator hiatal area measurements at rest, on maximal pelvic floor contraction, and during maximum Valsalva maneuver were collected before and after either spinal manipulation or a control intervention.
RESULTS: Levator hiatal area at rest increased significantly (P < .05) after spinal manipulation in the pregnant women, with no change postmanipulation in the nonpregnant women at rest or in any of the other measured parameters.
CONCLUSION: Spinal manipulation of pregnant women in their second trimester increased the levator hiatal area at rest and thus appears to relax the pelvic floor muscles. This did not occur in the nonpregnant control participants, suggesting that it may be pregnancy related.
KEYWORDS: Chiropractic; Manipulation; Pelvic Floor Disorders; Pregnancy; Spinal Manipulation; Ultrasonography
From the Full-Text Article:
The role of the pelvic floor muscles (PFMs) in spinal stabilization has been well documented. [1, 2] The PFMs are coactivated with the abdominal muscles particularly transversus abdominis during exercise and increases in intraabdominal pressure.  The PMFs, also known as the levator ani muscle complex. are intimately involved in the birth process, mainly during the second stage of labor. The consequences of a difficult vaginal delivery, particularly when intervention is required, are strongly correlated to the development of PFM dysfunction. This often manifests as stress urinary incontinence, pelvic organ prolapse, and/or fecal incontinence.  The social and economic cost of pelvic floor dysfunction is enormous. 
It has previously been demonstrated that sacroiliac manipulation significantly improves the feed-forward activation of the transversus abdominus.  Lumbar spine mobilization has been shown to change the activation of the abdominal oblique muscles.  Recently, real-time ultrasonographic imaging was used to demonstrate improved contraction of the transversus abdominus muscle following sacroiliac joint manipulation.  As the PFMs are known to be coactivated with transversus abdominis,  we hypothesize that sacroiliac and/or lumbar spine manipulation can affect PFM function.
Women who have increased bladder neck descent and a concomitant increase in levator hiatal area are more likely to have an uncomplicated vaginal delivery.  If lumbopelvic manipulations are able to alter PFM function, then this could be beneficial during the second stage of labor. The ability of the PFM to stretch during vaginal delivery is highly likely to be related to the risk of PFM damage. Reduction in the incidence of PFM damage and consequent sequelae is a research priority.  As the use of chiropractic care during pregnancy is becoming more popular, this technique could be of benefit in the future.
Chiropractic care is often used in the care of pregnant women, particularly for low back pain. A survey of 1,531 women in South Australia found that 35.5% of women experienced moderately severe low back pain during pregnancy and that two-thirds of this group had persistent back pain following pregnancy.  A study of obstetric caregivers and pregnant women in New Haven, CT, found that 61.7% of the pregnant women and 36.6% of the obstetric caregivers would consider chiropractic care for low back pain during pregnancy.  A retrospective study of 400 pregnancies and deliveries was undertaken by interview of 170 consecutive female patients presenting to 5 chiropractic offices in the Niagara Peninsula in Canada. Back pain was reported during 42.5% of the pregnancies and 44.7% of the deliveries. Those that had received manual manipulation reported significantly less pain during labor.  A retrospective case series studies found that chiropractic care, averaging only 1.8 visits, led to clinically important improvement in 16 of 17 cases of low back pain in pregnancy with no adverse effects. 
Quantitatively assessing the effect of spinal adjustment on PFM function has not previously been done. However, the use of 3-/4-dimensional (3D/4D) transperineal ultrasonography has been shown to be a reliable and effective method to assess PFM function.  A number of biometric parameters have been identified to quantify the function and morphology of the PFMs using this technique with good reproducibility.  The goal of this study was to use 3D/4D ultrasonography to determine if spinal manipulation alters pelvic floor function in pregnant women.
This study is the first to show that spinal manipulation of pregnant women in their second trimester appears to relax the PFMs at rest as reflected by an increase in levator hiatus area measured with translabial 3D ultrasonography. No changes occurred postmanipulation in the nonpregnant control group; thus, the changes seen in the pregnant group may be unique to pregnancy. A second novel finding is that the nonpregnant control group that consisted of a convenience sample of local chiropractic students appears to be able to elicit an effective voluntary Valsalva maneuver to a similar degree only previously seen in elite nulliparous women  or in pregnant women. 
The Levator Hiatus and Pregnancy
The other levator hiatal dimensions in the pregnant group are similar to what has been shown previously for pregnant women in their third  and second trimester.  It has been postulated that the mechanical and hormonal effects of pregnancy can lead to biomechanical, neurological, or neuromuscular changes to the pelvic floor and pelvic organ supports [38, 39] that may contribute to pelvic floor dysfunction, independent of delivery mode. A previous study found that both hiatal dimensions and urethral mobility were markedly higher in women in late pregnancy and at 4 months after giving birth,  suggesting a very substantial mechanical and/or hormonal effect of pregnancy on the pelvic floor.
Our study did not find any differences between the hiatal dimensions at baseline between the group of women in their second trimester and our nonpregnant control group. The hiatal areas for both groups at rest are similar to what has been shown previously for nonpregnant women. [36, 40] After spinal manipulation, the hiatal area of the pregnant women at rest was on average 14.0 ± 2.0 cm2. This is similar to what has been previously shown for women in late pregnancy (average of 15.1 ± 3.2 cm2). This increase in hiatal area at rest for the pregnant group following spinal manipulation may be a result of the manipulation itself, as this was not present following a sham (control) maneuver.
This relaxation of the levator ani muscles is likely to be beneficial for a vaginal delivery, suggesting that spinal manipulation may be of benefit to pregnant women to relax their PFMs if this does not occur naturally for them. However, this would need to be explored further to see if chiropractic care improves labor outcomes. In this regard, it should be highlighted that the pregnant women were manipulated where clinically indicated as assessed by the chiropractor, and this varied from woman to woman. It is unknown whether specific segments need to be manipulated to induce the observed effects in this study or whether it is improving the function of dysfunctional segments that produces the relaxation of levator hiatus as observed in this study.
Nonpregnant Changes With Spinal Manipulation
Our study also found that both groups were able to produce hiatal areas of at least 20 cm2 on the voluntary Valsalva maneuver. This is similar to what is seen in women in late pregnancy.  However, it was an interesting and unexpected finding to discover the large levator hiatus areas that the nonpregnant control group was able to produce during the voluntary Valsalva maneuver. This may be due to the fact the control group consisted of chiropractic students who may have received chiropractic care more regularly because a previous study has shown that sacrum manipulation increased phasic perineal contraction and basal perineal tonus in young healthy nulliparous women. 
Previous studies have also shown that spinal manipulation can alter motor control in a variety of ways in asymptomatic persons. [10, 4244] The timing of core muscle contractions,  muscle-specific changes in intracortical facilitatory and inhibitory processing, and control has been observed, [43, 44] as have changes in cortical drive,  lowered recruitment threshold of motor neurons to Ia afferent input,  prevention of fatigue,  and an increase in maximal voluntary muscle contractions of a lower limb muscle. 
It has been hypothesized that spinal manipulation of dysfunctional spinal and/or pelvic segments improves somatosensory processing and sensorimotor and mulitimodal integration, [34, 4547] thus producing improved motor control. As the control participants had received more frequent chiropractic care, they may be more kinesthetically aware  and did not have the same degree of co-contraction during a voluntary Valsalva as is often seen in nulliparous women. This was also observed previously in a cohort of elite athletes.  There was no difference between the chiropractic students levator hiatal area values at rest and during contractions as compared with other normal control participants in previously published studies. [35, 36]
Because of the small sample size, the results in this study need to be interpreted with caution. Future research should follow up these findings both to investigate the effect of spinal manipulation during pregnancy and the potential effects on birth outcomes, and to explore why chiropractic students can contract their PFMs to the degree that they could in this study. Limitations of this study include that the time frame for the interventions in the pregnant cohort was not exactly the same as in the nonpregnant cohort, which may have influenced the comparisons between the groups.
Another consideration to be noted is that some of the pregnant women were naive to chiropractic care, whereas those in the nonpregnant group were all familiar with chiropractic. It is therefore possible that the effect seen in the pregnant group may be a consequence of first exposure to chiropractic, although for those pregnant women familiar with chiropractic care, the effect was still present, suggesting that the postmanipulation changes in pelvic floor function were a genuine clinical outcome. Future work could include larger cohorts of pregnant women who have or have not had chiropractic care in the past. It should also be noted, in light of the current findings, that pregnant women who have perineal hypotonia may not be suitable to receive spinal manipulation.
This study showed that spinal manipulation of pregnant women in their second trimester appears to relax the pelvic floor muscles (PFMs) at rest, as reflected by an increase in levator hiatus area measured with translabial 3D ultrasonography. No changes occurred postmanipulation in the nonpregnant control group; thus, the changes seen in the pregnant group may be due to the hormonal changes of pregnancy. This relaxation of the levator ani muscles seen with spinal manipulation may mean that spinal manipulation could be of benefit to pregnant womens vaginal delivery by aiding the relaxation of their PFMs if this does not occur naturally for them.
A second novel finding is that the nonpregnant control group, which consisted of a convenience sample of local chiropractic students, appears to be able to perform a voluntary Valsalva maneuver to a similar degree only previously seen in elite athletic  or pregnant women. 
This study shows that spinal manipulation of pregnant women in their second
trimester appears to relax the PFMs at rest, as reflected by an increase
in levator hiatus area measured with transperineal 3D ultrasonography.
No changes occurred postmanipulation in the nonpregnant control group;
thus, the changes seen in the pregnant group may be unique to pregnancy.
This relaxation of the levator ani muscles seen with spinal manipulation
suggests that spinal manipulation could be of benefit to pregnant
womens vaginal delivery by aiding the relaxation of their
PFMs if this does not occur naturally for them.
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