Sensory Processing and Musculoskeletal Shoulder Pain
Condition: Shoulder Pain; Healthy Volunteers
MeSH terms: Shoulder Pain
Study type: Interventional

Summary: 
      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.
    

Detailed description: 
      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.
    

Intervention name(s): Short-wave diathermy
Intervention type(s): Other
Intervention description(s): 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.
Allocation: Non-Randomized
Intervention model: Parallel Assignment
Intervention model description: This is a case-control study in which the control (healthy volunteers) will receive an intervention (SWD).
Masking: Single (Outcomes Assessor)
Primary purpose: Basic Science
Keywords: Shoulder pain; Infraspinatus muscle; Muscle pain; Generalized hypersensitivity; Pressure algometry; Conditioned pain modulation