This article was published in the October 2016 edition of Medical Observer under the title, “Treating acute low back pain” (pp. 62-63). (PDF)
Ahmed, a 25-year-old builder recently presented with acute low back pain. I recalled a discussion on the GPs Down Under forum that lamented the evidence that paracetamol is not effective . Several GPs were enthusiastic about sterile water injections. What is the evidence for this treatment modality?
What is the effect of sterile water injections (intracutaneous or subcutaneous) on acute low back pain?
What does the research evidence say?
Step 1: The Cochrane Library
There is a review on the use of sterile water injections for (low back) pain management in labour only, which interestingly concluded that there was a lack of robust evidence .
Step 2: TripDatabase
I conducted a search using the TripDatabase PICO search tool (Participant: “acute low back pain”, Intervention: “sterile water injection”, Comparator: “placebo”, Outcomes: “pain”). The first and only relevant result was a randomised trial conducted by Cui and colleagues, published in the Brazilian Journal of Medical and Biological Research in 2016 . Let’s look at this study in more detail.
I will use the randomised controlled trial appraisal sheet from the Centre for Evidence Based Medicine .
Participants: who was studied?
68 participants (41 women, 27 men) with first episode of acute low back pain (no radiation to limbs, < 2 weeks duration, pain severity ≥ 5/10 on visual analogue scale [VAS]) were recruited from the First People’s Hospital of Lianyungang City, Jiangsu Province, China.
Important exclusions: suspected serious pathology, presumptive lumbar nerve root compression, previous spinal surgery, pregnancy, received any analgesia in previous 12 hours.
Mean age (32 years) and BMI (24 kg/m2). Mean duration of low back pain (5.5 days). Mean pain severity (VAS = 6.7).
Intervention: what was the exposure?
Intracutaneous injections of sterile water at all tender and trigger (tender and radiating sensation on palpation) points, using a 2 mL plastic syringe with an 0.40 mm diameter (27 gauge) needle.
0.5 mL of sterile water to create a bleb at each injection site. 3-5 injections in rapid succession, with a break of 1-2 min before further injections.
Comparator: what was the control/alternative?
Isotonic saline was used rather than water. The same pain physician administered all injections.
Outcomes: what was measured?
Primary outcome: pain intensity (VAS), recorded at baseline, 10, 45, and 90 min, and 1-day post-treatment.
Internal validity: are the trial results valid?
Randomised patient assignment?
Yes. The randomisation process was computer generated.
Groups similar at the start?
Yes. The groups were mostly similar (see Table 1) .
Groups treated equally apart from assigned treatment?
All patients accounted for?
Yes. There were a small number (< 10%) of participants whose 1-day results were not available (see Figure 1) .
Measures objective? Or patients and clinicians kept blinded?
Unclear, possibly no. Self-reported pain measures are subjective. The study is described as “double-blinded” but it does not appear that the clinician who administered the injections was blind to the treatment allocation. Participant blinding might not have been effective.
What were the results?
Primary outcome: the difference in pain between the control (saline) and intervention (sterile water) groups as measured by the VAS (0 to 10) favoured the intervention at all time periods:
- 10 min: 2.2 (95% CI, 1.4 to 3.0)
- 45 min: 2.5 (1.8 to 3.3)
- 90 min: 2.5 (1.8 to 3.3)
- 1 day: 2.9 (1.9 to 3.6)
Discussion and conclusion
Taking the results at face value, the effects on pain appear clinically significant. The intervention group’s pain was described to have improved from an average of 7/10 to 3/10 within 10 minutes, which was sustained the next day . If true, this is a dramatic effect.
However, I have concerns regarding the study internal validity. No method of blinding the procedural clinician is described. Sterile water also causes immediate injection site pain. It seems possible that neither the clinician nor participant during the actual administration of the intervention were actually blind. This introduces an important bias (see StatFacts).
Results from a single, small trial need to be interpreted cautiously. Small trials have low positive predictive values (that is, false positives are common) and tend to have an exaggerated estimate of the effect size . There is also the oddity that this study from China was published in a regional medical journal from the opposite side of the globe.
So, how do we interpret the evidence? The use of sterile water injections for acute low back pain cannot be justified routinely on the basis of a single study. However, given the (limited but supportive) evidence for other types of pain , apparent dramatic benefit, low likelihood of harm, and the absence of immediate effective treatments, it may have a role in patients who consent after being informed to its uncertain value.
Experimenter- and subject-expectancy effects
Experimenter-expectancy bias results in the researcher subconsciously influencing the study towards the desired outcome (e.g., through their facial expressions, or tone of voice). Subject-expectancy are similarly from the participant. Subjective, and self-reported outcomes are especially susceptible to this form of bias. Double-blind design is used to control this threat to study validity.
- Machado GC, Maher CG, Ferreira PH, et al. Efficacy and safety of paracetamol for spinal pain and osteoarthritis: systematic review and meta-analysis of randomised placebo controlled trials. BMJ 2015;350:h1225.
- Derry S, Straube S, Moore RA, Hancock H, Collins SL. Intracutaneous or subcutaneous sterile water injection compared with blinded controls for pain management in labour. Cochrane Database of Systematic Reviews 2012;1:CD009107.
- Cui JZ, Geng ZS, Zhang YH, Feng JY, Zhu P, Zhang XB. Effects of intracutaneous injections of sterile water in patients with acute low back pain: a randomized, controlled, clinical trial. Braz J Med Biol Res 2016 Mar;49(3).
- Centre for Evidence-Based Medicine. Critical Appraisal tools. 2014 [cited 2015 2015 Dec 1]; Available from: http://www.cebm.net/critical-appraisal/
- Button KS, Ioannidis JP, Mokrysz C, et al. Power failure: why small sample size undermines the reliability of neuroscience. Nat Rev Neurosci 2013 May;14(5):365-76.