Sensory manifestations in experimental and work-related chronic neck-shoulder pain

Little is known about the mechanisms leading to chronic neck-shoulder musculo-skeletal disorders. The aim of the present study was to investigate and compare the sensory manifestations of experimental or chronic neck-shoulder pain under controlled, low load, repetitive work. Experimental and clinical experiments were carried out. In Experiment 1, experimental muscle pain was induced in healthy subjects by intramuscular injection of hypertonic saline in the trapezius (n = 10) and infraspinatus (n = 10) muscles. Experiment 2 was performed on 18 workers with (n = 12) and without (n = 6) chronic neck-shoulder pain. Sensory assessments were performed before and/or after a session of controlled, low load, repetitive work. The pain intensity was assessed by a visual analogue scale. The pain quality and location(s) were monitored together with pressure-pain thresholds. Moderate-to-strong deep pain intensity was experienced in the experimental and clinical part of the working session (six patients out of 12). The verbal descriptors used to describe the experimental or chronic pain qualities were partially similar, i.e. first of all verbal descriptors from the sensory class. The patterns of local and referred pain characteristics were similar (around the shoulder girdle), but it is difficult to evoke pain in extensive areas with hypertonic saline. Patients exhibited hypersensitivity or tenderness to pressure stimuli in both sides of the upper body, i.e. in the trapezius, supraspinatus, infraspinatus and pectoralis major muscles (p < 0.05). This hypersensitivity was only seen at the referred pain site in the healthy volunteers exposed to infraspinatus experimental muscle pain. In conclusion, the present study demonstrated similar sensory manifestations in the experimental and clinical neck-shoulder pain experiments. The presented experimental model can most likely be used to investigate the underlying pain mechanisms involved in work-related chronic musculoskeletal pain.