Nociceptive reflex receptive fields are enlarged in patients with knee osteoarthritis : preliminary results

J. A. BIURRUN MANRESA; M. B. JENSEN; K. PETERSEN; C. LAURSEN; O. SIMONSEN; L. ARENDT-NIELSEN; O. K. ANDERSEN

Yokohama

Congreso; 16th World Congress on Pain; 2016

International Association for the Study of Pain IASP

Aim of Investigation Chronic pain conditions are often associated with pain hypersensitivity, but the underlying mechanisms are not fully understood. A number of recent studies have identified enlarged reflex receptive fields (RRF) in patients with endometriosis, chronic low back pain and chronic neck pain patients, most likely associated with widespread central sensitization. The assessment of the RRF thus provides a new tool in the identification of possible mechanisms behind hypersensitivity states in pain patients. In line with this, the aim of the present study was to determine if patients suffering from severe knee osteoarthritis (OA) present enlarged RRF.MethodsThirty patients diagnosed with severe painful knee OA (pain lasting more than 6 months at an intensity of at least 3/10 on a visual analogue scale) were recruited. Additionally, twenty-one age- and gender-matched healthy controls participated in the study. Patients were requested not to take any analgesic medication for 24 h before the experiment. Ten surface stimulation electrodes were mounted in a non-uniform grid on the plantar side of the foot, and one large common anode was placed on the dorsum of the foot. Each stimulus consisted of a constant current pulse train of five individual 1 ms pulses delivered at 200 Hz (felt as a single stimulus) by a computer controlled electrical stimulator. Initially, the volunteers were familiarized with electrical stimulation, and then pain thresholds were determined individually for each of the ten stimulation sites using a standardized staircase procedure. The final stimulation intensity for each site was adjusted to 1.3 times the pain threshold. Afterwards, each site was stimulated five times in random order (fifty stimulations in total), with an inter-stimulus interval ranging from 10 to 15 s. Electromyographic (EMG) activity was recorded from the ipsilateral tibialis anterior (TA) muscle. Root-mean-square (RMS) amplitudes of the EMG were calculated in the 60 - 180 ms post stimulation interval, as well as the probability of occurrence of a reflex, defined as the number of trials in which a reflex was detected divided by the total number of trials. RRF sensitivity and probability maps were derived using two-dimensional interpolation, and RRF sensitivity area and RRF average probability were calculated as outcome variables. RRF sensitivity area is defined as the fraction of the sole of the foot encompassing RMS amplitudes larger than a threshold defined as the peak RMS amplitude minus two times the standard deviation of the remaining RMS amplitudes. RRF average probability is the mean probability of occurrence of a reflex across all sites. Differences in RRF area between groups were assessed using a non-parametric Mann-Whitney rank sum test, whereas differences in RRF average probability were assessed using an independent samples t-test.ResultsData from 6 patients and 3 healthy controls were discarded because reflexes could not be detected following any of the stimulations. RRF sensitivity areas were significantly enlarged in knee OA patients (median = 0.42, IQR = 0.43) compared to healthy controls (median = 0.16, IQR = 0.28; U = 107, p = 0.006). Additionally, RRF average probability was significantly higher in knee OA (mean = 0.51, SD = 0.25) compared to healthy controls (mean = 0.32, SD = 0.26; t40 = 2.310, p = 0.026).ConclusionThese results provide evidence for expansion of RRF in knee OA patients compared to age- and gender-matched healthy controls, most likely as a result of widespread central sensitization.