In this month’s Research Refresh on the Small Animal platform, we dive into an article on pressure algometry, specifically looking at a modified technique and its validity, repeatability and reliability (Lane & Hill, 2015). But one article gives us only a snapshot, so let’s look at what else we know about this tool and its use specifically for us as Vetrehabbers.
Pressure algometry is a tool used to measure a mechanical nociceptive threshold, or the point at which an animal first demonstrates an adverse reaction to increasing focal pressure. Think of it as measuring the amount of force needed to elicit a pain response.
Therefore, we can theoretically use this tool to measure:
- the presence or absence of pain,
- an abnormal pain threshold, for example hyperalgesia,
- the response to treatment interventions.
This allows us to more objectively report back to owners and veterinarians regarding the status of a patient and their progress.
Is this a clinically applicable tool?
Let’s start with this question, because at the end of the day, if we are not able to use a measuring tool easily, consistently and quickly in practice, it’s not even worth speaking about!
The pressure algometer is a small hand-held device, portable and easily stored. Mechanical and electrical variants of these devices are available.
The tip of the algometer is applied directly to the area being tested, perpendicular to the skin. A steady increase in pressure is applied to the skin, until either a behavioural or a physical response is seen, such as a local twitch response, the patient turning and looking at the site, withdrawal of the limb, vocalisation, etc.
As soon as one of these signs is seen, the algometer is removed and the reading is noted, which will reflect the highest amount of pressure applied.
Personally, I have not used an algometer before, but this method sounds simple and easily integrated into my initial palpation assessment and follow-up assessments, and would not add time to the overall evaluation. We have spoken to a few therapists, including Solange Mikail, who regularly use pressure algometry in their practices.
Is pressure algometry reliable?
There is conflicting data in answer to this question, but overall the results look promising.
Briley et al (2014) looked at the feasibility and repeatability of mechanical quantitative sensory testing in normal, healthy and pain-free dogs with a high quality of life. They found that repeatability was good for an individual dog, but not as a representative of a population. This is perfect in our clinical application, where we really want a tool that will help us monitor the progress of each patient individually.
Coleman et al (2014) assessed the use of algometry at common surgical sites at different time points, on healthy and pain-free dogs. Dogs were tested twice in one day, and again 10-14 days later. Their findings suggested that learning and anticipation occurred over time, and that this significantly affected the results. Variables such as the order of testing site, time and day all affected the measurements.
This is not great news, as variations in readings over time would completely nullify the tool’s usefulness in measuring a response to treatment and progress in a patient.
Making behavioural adaptations
As pressure algometry readings are primarily dependent on a behavioural response from the patient, it makes sense that we take some behavioural factors into consideration when applying this tool, especially in light of Coleman et al.’s findings.
For example, when dogs are presented with a choice of position, they tend to be more comfortable and responsive. Allowing dogs to remain sitting or standing as opposed to placing them in lateral recumbency can improve the quality of the measurements taken (Harris, et al, 2015).
In addition, if we apply pressure to a healthy, pain-free dog until they respond, then release that pressure every time we get a response, we have in effect a powerful training technique for a certain response. It makes sense that the patient would quickly learn that when they respond, the pressure goes away. How do we counteract that trained response?
I love the adaptations Lane and Hill (2015) made to their method, as discussed in the Research Refresh. By setting a maximum threshold of pressure, every application of the algometer doesn’t elicit a pain response, and a learned reaction is not achieved by the patient. To me, this is the most powerful adaptation made to the method of applying this tool. Additional adaptations included keeping the owner present, having a silent algometer, and using testing sites in random order. They also used an algometer with a 1cm tip.
A useful tool across species
If you treat multiple species, it helps to know that pressure algometry can be applied to equine patients as well. In the members’ portal, we recently looked at a paper by Long, McGowan and Hyytiäinen (2020), where pressure algometry was successfully used to measure the response to tail pull as a treatment intervention in horses with back pain.
Objective outcome measures
As we increasingly focus on using our clinical work for research prospects, or adding data to a centralised, global research database, it becomes more and more important that we use objective outcome measures wherever we can.
What I would love to see more of, as we evaluate this tool, is its use on patients that are compromised, its use on our patients. Studies like the one above by Long, McGowan and Hyytiäinen (2020) show us multiple things:
- Their research was conducted in clinical setting, meaning results can be accurately taken back to clinic.
- Their research shows us whether or not the measuring tool is repeatable and valuable.
- It also show us whether or not our interventions have the desired effect.
As we gear up for Vet Rehab Summit 2021, we are continually wanting to encourage you, our Vetrehabbers around the world, to get involved in clinical research. Let’s increase our database, prove our efficacy, and start validating our modalities and techniques. In the end, it will only benefit our profession and by extension, every one of us.
- Briley JD, Williams MD, Freire M, Griffith EH & Lascelles BD. 2014. Feasibility and repeatability of cold and mechanical quantitative sensory testing in normal dogs. Vet J., 199(2): 245-250
- Coleman KD, Schmiedt CW, Kirkby KA, et al. 2014. Learning confounds algometric assessment of mechanical thresholds in normal dogs. Vet Surg, 43(3): 361-367.
- Harris LK, Murrell JC, Van Klink EG & Whay HR. 2015. Influence of experimental protocol on response rate and repeatability of mechanical threshold testing in dogs. Vet J., 204(1): 82-87.
- Hughes, LE. 2020. Outcome Measures: Pressure Algometry. Four Leg News, Sept- Oct. [Online]. https://www.fourleg.com/media/9(5)FourLegNews.pdf
- Lane, DM & Hill, SA. 2016. Pressure algometry measurement of canine muscular pain near the thoracolumbar junction: Evaluation of a modified technique. Vet Anaesth Analg, 43(2):227-34. Epublished 2015 Sep 11.
- Long K, McGowan CM & Hyytiäinen HK. 2020. Effect of caudal traction on mechanicalnociceptive thresholds of epaxial and pelvic musculature on a group of horses with signs of back pain. Journal of Equine Veterinary Science. [Online]. doi: https://doi.org/10.1016/j.jevs.2020.103197.
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