The development of chronic pain is a common complication in shoulder pain conditions. Half of these type of patients exhibit persistent pain even after a period of six to twelve months from the start of their pain treatment. Persistent pain is most likely a result of different biological alterations, including but not limited to tissue damage. Another potential cause is neurogenic inflammation, which can be triggered by applying excessive mechanical stress to a structure or region. Neurogenic inflammation can lead to peripheral sensitization and sensitizing the musculoskeletal tissues in the process, and this sensitization will last until the input is removed. Nevertheless, even though peripheral factors are sufficient to perpetuate pain, the role of central mechanisms cannot be excluded as a potential cause of sensitization. Moreover, the evidence is not conclusive about deficiencies in pain modulation processes and presence of central mechanisms in chronic pain conditions. It is well known that chronic shoulder pain presents a multi-factorial nature. Hypersensitivity in this condition has been linked to persistent activation of peripheral nerves, which can result in an increased excitability of the sensory input and reduced inhibitory effect of pain modulatory mechanisms. On the other hand, the role the central sensitization plays is not completely clear in this type of patients, but could also contribute to the hypersensitivity in some patients.However, it has been shown that there is a remarkable variability in the presence of central effects among chronic patients. The aim of this study is to evaluate changes in sensory processing and in pain modulatory mechanisms in individuals with and without shoulder pain.
This quasi-experimental study comprise a sample of 64 volunteers with and without unilateral shoulder pain (32 volunteers for each group). Patients of both genders, aged between 18 and 35 years old will be potential candidates for participating in the study. To be included, they must meet inclusion and exclusion criteria. Inclusion criteria for the patient group requires volunteers to report unilateral shoulder pain for more than 6 weeks and to be symptomatic at the time of evaluation. The study will be conducted in two experimental sessions on different days, at least 48 to 72 hours apart each other in order to avoid carryover effects. In each experimental session, several measurements will be performed to assess different physiological parameters, including: visual analog scale (VAS), pressure pain thresholds (PPT), temperature thresholds, specific and generic self-report questionnaire, shoulder joint position sense, and muscle activity. In order to test change induced by transient muscle pain, healthy participants will receive shortwave diathermy (SWD) applications. Volunteers will be invited to participate in the study through social networks and e-mail. In the first session, participants will be interviewed to collect anthropometric data and health history. Basic upper quadrant physical examination will be performed to check inclusion or exclusion criteria. Then, VAS, shoulder joint position sensation, American Shoulder and Elbow Surgeons (ASES) score, and Örebro Musculoskeletal Pain Questionnaire (ÖMPQ) will be assessed. Heat and cold perception threshold as well as heat pain threshold will be assessed using a controlled thermode. Finally, to conclude the first session, conditioned pain modulation (CPM) will be performed to asses inhibitory descendent mechanisms. The CPM protocol will use a cold water bath as the conditioned stimulus, and pressure pain thresholds will be quantify in the belly of the infraspinatus muscle of both shoulders before and after immersion of the hand in cold bath. In the second session, shortwave diathermy (SWD) will be apply to the healthy participants in order to induce a transient muscle pain. The stimulation will be performed using a CEC M-8 shortwave thermotherapy unit. Before and after the SWD, infraspinatus muscle and deltoid muscle myoelectric activity will be recorded. A multichannel array (6 channels) will be distributed over the infraspinatus muscle to evaluate changes in the distribution of the muscle activity due to pain, whereas monopolar recording will be performed in the medial and posterior deltoid muscle to assess potential crosstalk between recordings. The volunteers will position the arm in 90 degree elevation in the scapular plane (arm flexion), maintaining this position for 8 seconds. This block will be completed in 5 repetitions with 15 seconds of rest between for each trial. Finally, a fatigue task will be executed. For this, flexion and extension of the arm will be executed dynamically in a range of motion between 30 and 90 degree in the scapular plane using a additional weight calculated according to the volunteer body mass (1.25 kg in participants under 68 kg and 2.5 kg in participants over 68 kg). The flexion and extension speed will be guided using a 60 beats/s metronome. In this session, unilateral shoulder pain volunteers will only performed the motor tasks, i.e., they will not received any type of intervention. The healthy and unilateral shoulder pain volunteers will draw their pain distribution on a digital body chart (following the physiotherapist's instructions). Pain drawings will be performed using a PC screen. A modified self-report Likert scale and will be used to follow the temporal progression of muscle pain/soreness, and the McGill questionnaires will be used to describe the sensation. Sample size considerations: PPT in healthy participants usually increases 50 kPa after cold water immersion. Sample sized was calculated considering a probability of committing a type I error (α) of 5%, a statistical power (1 - β) of 80%, with a mean standard deviation of the outcome of 48 kpa and a non-inferiority limit of 30 kpa. A sample of 64 volunteers (32 for each group) are required to be 80% sure that the lower limit of a one-sided 95% confidence interval (or equivalently a 90% two-sided confidence interval) will be above the non-inferiority limit of -30. Using this sample the results will show that difference between the healthy and unilateral shoulder pain volunteers (if there is any) cannot be considered clinically relevant.
The SWD will be applied using a CEC M-8 shortwave thermotherapy unit (CEC Electrónica S.R.L., Argentina) that emits RF at a frequency of 27.12 MHz. The device has two rectangular capacitive applicators (18 × 12 cm), which will be placed below and above the dominant shoulder. The coplanar application will be done using the continuous wave mode. The intensity of SWD will be gradually increased until the volunteer perceives a sensation of heat, and once familiar with this perception, the intensity will be increased until a sensation of constant but tolerable pain is evoked. This sensation will be maintained throughout the SWD application, and three repetitions will be performed. Healthy volunteers will be in a lateral supine position, and a cotton towel will be used to absorb perspiration and avoid unwanted heat effects.
Rosario, Santa Fe Province, Argentina