The origin of pain in the Subacromial Pain Syndrome.

The Subacromial Pain Syndrome (SAPS) is the most common shoulder disorder in primary health care, for which 58/1000 persons seek medical help in the Netherlands yearly. SAPS is characterised by disabling shoulder pain that often results in ability to perform daily life activities and hence represents a great burden to our society (total costs per person within 6 months after first medical consultation: €689). With regards to the pathophysiology, several mechanisms have been suggested and these generally share a focus on (peri)articular changes leading to damage of subacromial tissue. Complaints often subside after treatment of damaged subacromial tissue, however, in up to 40% of patients, pain complaints become chronic. Especially in these patients, symptoms can often not be explained by (peri)articular changes, asking for better localisation of the origin of pain in SAPS. We aim to quantify the contribution of primary (peri)articular changes and secondary central sensitisation to pain, and to assess the role of altered muscle activation patterns, in SAPS. Three research questions will be addressed: 1) is high central pain sensitisation associated with a low effect of subacromial anaesthetics on pain?; 2) is there an effect of subacromial anaesthetics on central pain sensitisation?; 3) is there an association between central pain sensitisation and the effect of subacromial anaesthetics on muscle activation patterns?

We hypothesise that in the Subacromial Pain Syndrome (SAPS), a differentiation between primary peripheral (i.e. (peri)articular) and secondary pain due to altered processing of pain, i.e. central pain sensitisation, is needed. We further hypothesise that patients who adapt their muscle activation may be less viable for central pain sensitisation. Knowing the different origins of pain may result in patient-specific treatment and improved outcome.

There are two visits at the LUMC with two moments per visit (a, b: 3-3,5 hours per visit). Visit 1a) informed consent, demographics, questionnaires (e.g. VAS for pain, Pain Catastrophizing Scale), bilateral QST by means of Temporal Summation and Conditioned Pain Modulation and ARs. Visit 1b) US-guided subacromial infiltration (lidocaine or placebo), VAS for pain, anchor question (pain), bilateral QST and ARs. Visit 2a) VAS for pain, ARs, bilateral QST, US-guided subacromial infiltration (lidocaine or placebo). Visit 2b) VAS for pain, anchor question (pain), bilateral QST, ARs.

In a randomised sequence, participants will receive two US guided subacromial injections: one with anaesthetics (5cc lidocaine 1%, using a 50 mm 21 gauge needle) and one with a placebo (5cc NaCl 0.90%). There is a wash-out period of 1 week between both procedures.