Persistent pain, in conditions such as complex regional pain syndrome and chronic low back pain, is associated with cortical changes and altered tactile acuity.[2-4, 6, 7] Tactile acuity is thus considered a clinical signature of primary somatosensory representation in these conditions and is increasingly being assessed in clinical practice to evaluate the extent of cortical reorganisation in chronic pain patients and to monitor change as they recover.
So how good are these tests? The most common test of tactile acuity is two-point discrimination (TPD). However, we searched the literature and were surprised to find that tests of TPD in regions other than the fingertip had not been properly assessed. For a test to be clinically useful, it must be able to be performed quickly and provide reliable results – that is, if you measure a person twice consecutively it should render the same measurement.
We assembled a group of 28 physiotherapists and trained them to assess TPD with cheap hardware-style mechanical callipers (approx. AU$15).. We asked them to assess the tactile acuity of 28 healthy young university students in four locations: the back, the neck, the palm and the sole of the feet. The clinicians were asked to conduct a series of five assessments; at first increasing the distance between the two points on the callipers and then decreasing the distance, in order to hone in on the subject’s TPD threshold.
We assessed whether an individual clinician could efficiently and reliably reassess TPD and whether measurements taken by two separate clinicians were comparable. We also assessed whether clinician experience, in years, affected reliability.
Each assessment took around 3 minutes to complete, suggesting our protocol was clinically viable, and clinical experience did not influence capability. We found individual clinicians could reliably assess TPD in all four of the assessed areas but measurements taken by different clinicians were only comparable for the neck and the feet.
Importantly however, large variability was observed in all assessments and it is quite likely that this variability would be greater in patients with altered tactile acuity. Many factors affect tactile acuity including skin temperature, body hair, patient cooperation and fatigue, clinician concentration and application pressure to name a just a few. It is likely that these factors contributed to the variability we found and would likely contribute to variability in the clinic. As such, caution is needed when interpreting changes in tactile acuity in individual patients. The change in TPD threshold would need to be substantial before they can be confidently attributed to a true change rather than chance.
Mark Catley is a PhD candidate in the Body in Mind Research Group (at University of South Australia) in Adelaide. When he is not busy researching, Mark works as a physiotherapist in a rehabilitation hospital. He is interested in the brain’s involvement in the transition from acute pain to chronic pain, and is currently investigating the relationship between cognitive variables, mood and sensory function in people with back pain.
He also has a very particular approach to cooking rice. For perfectly cooked rice: 2/3 cup rice, double that in COLD water, and then 8mins in microwave uncovered. Actually, he has a particular approach to many things – including windows. He is the only BiM team member you should ever get to clean a window.
1. Catley MJ, Tabor A, Wand BM, & Moseley GL (2013). Assessing tactile acuity in rheumatology and musculoskeletal medicine–how reliable are two-point discrimination tests at the neck, hand, back and foot? Rheumatology (Oxford, England) PMID: 23611918
2. Flor H, Elbert T, Braun C, Birbaumer N: Extensive reorganization of primary somatosensory cortex in chronic back pain patients. Neurosci Lett 224:5-8, 1997.
3. Maihofner C, Handwerker HO, Neundorfer B, Birklein F: Patterns of cortical reorganization in complex regional pain syndrome. Neurology 61:1707-15, 2003.
4. Maihöfner C, Handwerker HO, Neundörfer B, Birklein F: Cortical reorganization during recovery from complex regional pain syndrome. Neurology 63:693-701, 2004.
5. Pleger B, Foerster A-F, Ragert P, Dinse HR, Schwenkreis P, Malin J-P, Nicolas V, Tegenthoff M: Functional Imaging of Perceptual Learning in Human Primary and Secondary Somatosensory Cortex. Neuron 40:643-53, 2003.
6. Pleger B, Tegenthoff M, Ragert P, Forster AF, Dinse HR, Schwenkreis P, Nicolas V, Maier C: Sensorimotor returning in complex regional pain syndrome parallels pain reduction. Ann Neurol 57:425-9, 2005.
7. Pleger B, Ragert P, Schwenkreis P, Förster AF, Wilimzig C, Dinse HR, Nicolas V, Maier C, Tegenthoff M: Patterns of cortical reorganization parallel impaired tactile discrimination and pain intensity in complex regional pain syndrome. Neuroimage 32:503-10, 2006.