Botulinum toxin for endometriosis pelvic pain


“Studies like ours may open a new venue of treatment for women with chronic pelvic pain, especially when associated with pelvic floor muscle spasm,” said principal investigator Barbara Karp, MD.

Women with endometriosis-related chronic pelvic pain (endo-CPP) were more likely to report benefit with botulinum toxin (BoNT) compared to placebo, according to new research in Fertility and Sterility.

The randomized trial found that at 1 month after masked injection, benefit was reported by 73% who received BoNT versus only 29% who received placebo.

BoNT also led to longer-lasting benefit (P = 0.023) and a greater percentage improvement (P = 0.034) than placebo up to 1 year post-injection.

Furthermore, only the BoNT cohort had a decrease in the number of pelvic floor muscles in spasm (P = 0.019).

The principal investigator of the study is Barbara Karp, MD, Office of the Clinical Director, National Institute of Neurological Disorders and Stroke, National Institutes of Health (NIH), in Bethesda, Maryland.

In the 1990s, Karp was chief of the NIH Neurology Consultation Service, during which time she started a collaboration with first author Pamela Stratton, MD, chief of the NIH Gynecology Consultation Service, focused initially on the occurrence of headache in patients with endometriosis.

“As we discussed endometriosis and the prominent symptom of persistent pelvic pain, we appreciated that spasm of pelvic floor muscles might be a contributing factor,” Karp told Contemporary OB/GYN®. “We proposed treating the pelvic floor spasm with toxin in this population.”

The study, which recruited participants from across the United States, was conducted at the Warren Grant Magnusen Clinical Center at the NIH in Bethesda from 2014 to 2019.

All 29 study patients, aged 18 to 55, were identified as having pelvic floor spasm associated with their endo-CPP.

Patients received 1 injection of 100U BoNT (n = 15) or placebo (n = 14) into pelvic floor muscles with spasm. An optional second injection was offered at the subject’s request, any time 1 to 12 months after the first injection.

Assessments included pelvic exam for pelvic floor muscle spasm, pain rating, medication tracking, and the Oswestry Disability Index (ODI).

“We hypothesized that BoNT would diminish spasm and pelvic pain,” Karp said. “So we were gratified, but not surprised, by those findings.”

Adverse events were mild, with no difference in incidence between the two groups (P = 0.11).

“Studies like ours may open a new venue of treatment for women with chronic pelvic pain, especially when associated with pelvic floor muscle spasm,” Karp said. “One advantage of this approach is that BoNT injections can be safely combined with other therapies, including other medications, pelvic floor physical therapy and complementary approaches.”

However, much more research is needed to optimize the use of BoNT for pelvic pain, according to Karp. “We chose a single dose of a single toxin type, based on our prior experience in other conditions, and developed a strategy to injection that could be implemented in an office setting,” she said. “Further studies will help determine if factors like a different dose or customized doses might be better.”

Technical issues, such as whether multiple injection sites in the pelvic floor muscles are needed or if a single injection site is sufficient, also need to be clarified.

“For most other conditions, toxin needs to be reinjected roughly every 12 weeks to maintain benefit,” Karp said.

To enhance the safety of BoNT in the study population, the authors advocate the use of a needle guidance technique like electromyography (EMG), which was used in the study, or imaging like ultrasound, to ensure that the needle is accurately placed in the muscle.

“Higher doses should be used with caution, as they could cause side effects like incontinence,” Karp said.


  1. Stratton P, Tandon K, Phan V, et al. Randomized, placebo-controlled trial of botulinum toxin for endometriosis-related chronic pelvic pain. Fert. Steril. Abstract only. Volume 116, Issue 3, Supplement, E52. September 1, 2021. doi:org/10.1016/j.fertnstert.2021.07.148
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