A recent study in Sweden compared sterile water injections to acupuncture for relief of labor pain. It found that sterile water produced significantly greater pain relief and relaxation. It concluded, “Women given sterile water injection experience less labor pain compared to women given acupuncture.”
I’m puzzled, because the study also says “there were no significant differences regarding requirements for additional pain relief after treatment between the 2 groups.” 85% and 90% got nitrous oxide, 40% and 47% got epidurals, and other conventional interventions were also used. It seems to me the conclusion could just as well have been “Women given sterile water injections report less labor pain than women given acupuncture, but require just as much additional pain relief.”
Could this be a case of biased researchers selectively interpreting data in the most favorable light? I read the abstract to my husband (who is not a doctor) and he immediately noticed the apparent contradiction. But Dr. George Lundberg didn’t mention it when he reported on the study for the Medscape Medical Minute. In fact, he inexplicably said “Pain relief was sufficient in both groups to prevent the need for other pain therapy.” That’s not what the article says.
I won’t attempt to dissect the study. I’ll assume that it was properly carried out and that its main conclusion (water injections better than acupuncture) is correct. But I do have questions about what it really tells us.
This experiment was not blinded: patients can tell whether they are getting an injection or acupuncture. I wondered why they didn’t include any placebo controls like sham acupuncture or dry needling. It seems they already assumed that both treatments were more effective than placebo; but I don’t think that has been established.
Perhaps all this study shows is that a placebo injection is more effective than a placebo needling procedure. We already knew that placebos are “effective” in reducing pain, and that there is a hierarchy of placebo effects with injections being higher on the list than practically anything besides sham surgery. If we assume that sterile water injections are a placebo, this study just provides one more confirmation that acupuncture is no more effective than placebo.
Do sterile water injections make sense?
Is the method plausible? Anywhere from 0.1 ml (that’s 1/50 of a teaspoonful) to 0.5 ml of sterile water is injected into or just under the skin, forming a small bleb. How could a small superficial bleb affect deep nerves and affect pain transmission up the spinal cord?
Several hypothetical mechanisms have been proposed:
1. A counterirritant effect. Sterile water injections hurt. This hypothesis fits with the fact that sterile saline is less painful and doesn’t work as well as sterile water (sterile water has an osmotic effect in addition to the distention of tissues by the fluid). If it’s only a counterirritant, there might be better, less invasive ways of providing it.
2. Endorphin production. But we know placebos can produce endorphins too.
3. The gate control theory of pain. But other methods that are supposed to work by the gate control theory, like acupuncture or TENS (transcutaneous electrical nerve stimulation), don’t work as well as sterile water.
4. “It may work like acupuncture.” But there is no good evidence that acupuncture is anything more than an elaborate placebo system. And if it works like acupuncture there would be no need for the water; needling could be used alone.
5. “The needles may be hitting acupuncture points.” But no one has convincingly shown that acupuncture points even exist, and acupuncture needles are inserted deeper below the skin.
Sterile water works fast: it sometimes relieves pain for the very next contraction. My colleague Dr. Atwood is an anesthesiologist who has done a lot of epidurals. Before inserting the spinal needle, he numbs the skin by injecting lidocaine intradermally and subcutaneously. It stings. He tells me he has never noticed any effect on the patient’s perception of pain over the next few contractions. Then when the epidural kicks in, there is a dramatic effect.
So sterile water injections don’t look very plausible. Admittedly, implausible things do sometimes turn out to be true. So what does the evidence really show about the effectiveness of this treatment?
I found quite a few studies, but they are problematical. Most are small studies. Some are from non-English-speaking countries that are statistically less likely to publish negative studies (Iran, Thailand, etc.). Some used intradermal injections, others used subcutaneous injections. One study showed that intradermal injections were more painful than subcutaneous ones. Some injected one site, others injected four. Some repeated the injections when the effect wore off after 2-3 hours. Several studies were from Scandinavia, where sterile water injections are popular among midwives. I couldn’t find any US studies.
Most studies used normal saline as a control, and it may not be an adequate control. Several studies reported severe pain with water injections but little or no pain with saline injections. Subjects might be able to guess which group they were in by the amount of pain. If their guesses were better than chance, you wouldn’t have an adequate placebo control. I couldn’t find any evidence of “exit polls” asking patients which group they thought they were in.
A systematic review concluded that it was effective but it was based only on 4 studies they found suitable for analysis:
• Ader et al. compared sterile water to saline. Sterile water worked better but there was no difference in the requirement for pethidine (Demerol).
• Trolle et al. compared sterile water to saline and found it twice as effective (89% vs. 45%).
• Martensson et al. compared 0.1cc of intradermal water, 0.5 cc of subcutaneous water, and 0.1 cc of subcutaneous saline. The two water groups were equally effective and superior to the saline.
• Labreque et al. compared sterile water injections to TENS and to standard care (massage, etc.) Water worked better than the other two, but there was no difference in epidural requests, and fewer women said they would choose it again.
Systematic reviews are limited by the quality of the studies they review. Since the controls in these studies are questionable, I don’t know how much we can trust the conclusions.
What about Non-Obstetric Pain?
What really bothers me is that if these injections really work so well, why aren’t they being used for other types of pain like post-op pain and trauma? I found only 3 studies of other types of pain. A study from Iran showed sterile water was more effective than saline for renal colic. A small study in Norway showed no benefit from either saline or sterile water injections for cervicogenic headache. And there was a whiplash study, but it was not really comparable because it targeted trigger points and the pain relief lasted much longer.
Sterile water injections seem to appeal mainly to midwives and natural childbirth advocates. They argue that it’s inexpensive, readily available, does not require a prescription, has no risks, is easily administered, and “won’t make the drug companies rich.” And it doesn’t require an obstetrician or an anesthesiologist. One commenter on Medscape said, “Sad that the technique isn’t being given a chance here in the U.S. Probably because it doesn’t fit in with the paternalistic, manipulative, technologically dominated, controlled style of obstetric medicine in this country.”
A bias and an agenda shine through such comments. It’s strange that natural childbirth advocates support sterile water injections, because there’s nothing “natural” about them. Some people seem to think there is merit in women suffering pain in labor rather than accepting interventions. I don’t. I think it’s cruel to persuade women to suffer pain unnecessarily when modern obstetrics can relieve pain safely and effectively.
Some support using any placebo that helps the patients get through the ordeal. Placebos relieve pain and are unlikely to harm women or their babies, but they’re UNETHICAL! We need to look for ways to harness the power of the placebo without lying to patients. One way to do that might be to tell them it’s a placebo and get their informed consent. Studies have shown that knowing you’re getting a placebo doesn’t necessarily destroy the placebo effect. I will always remember a patient in my internship who was given a placebo injection instead of the Demerol she usually got in the ER for her severe headaches. It worked just as well as the Demerol! When she was confronted with the fact that a placebo had relieved her pain, she asked if she could get it the next time she had a headache because it really worked!
I wonder about the wisdom of using counterirritants. Hitting your thumb with a hammer would probably work great as a counterirritant, but I couldn’t recommend it. I suspect simple distraction measures and suggestion might be nearly as effective, and they are not invasive and don’t produce pain or stinging.
Mind you, I’m not claiming that sterile water is not better than placebo; I’m just saying it’s premature to conclude that that it is better, based on currently available evidence that is questionable and has not been shown to be properly blinded.
This article was originally published in the Science-Based Medicine Blog.