Shoulder Pain Maps
The shoulder complex involves multiple potential pain generators, including the glenohumeral joint (GHJ), acromioclavicular (AC) joint, subacromial space structures (bursa, rotator cuff tendons), and surrounding muscles.
Glenohumeral Joint
Mapping Methodology: Pain patterns have been mapped by analyzing pain diagrams from patients who experienced complete relief following diagnostic, image-guided intra-articular anesthetic injections into the GHJ.
Pain Referral Patterns (Kennedy et al. 2015):
- The most consistent finding (100% of responders) was pain located in the anterior and/or posterior aspect of the shoulder itself, often extending into the upper arm region.
- Referral to the lateral aspect of the neck was observed occasionally (18% of responders).
- Distal referral to the forearm (anterior 9%, posterior 8%) and hand (9%) occurred less commonly.
- Referral to the scapular region was rare (6%), as was referral to the medial neck.
- Certain combined patterns were notably absent in responders: pain involving both the medial neck and areas below the elbow; pain involving both the medial scapula and areas below the elbow; and pain involving both the medial scapula and the medial neck.
Acromioclavicular Joint
Mapping Methodology: Patterns were determined by experimentally provoking pain via hypertonic saline injections into the AC joints of healthy volunteers.
Pain Referral Patterns (Gerber et al. 1998 ):
- Pain localized directly over the AC joint itself.
- Referred pain to the anterolateral neck.
- Referred pain to the trapezius and supraspinatus region (superior aspect of the shoulder/scapula).
- Referred pain to the anterolateral deltoid region.
Notably, AC joint stimulation did not produce pain in the posterior aspect of the shoulder.
Subacromial Space
This space contains the subacromial bursa and rotator cuff tendons, and pathology here is often implicated in clinical diagnoses of "subacromial impingement syndrome".
Mapping Methodology: Patterns mapped via experimental hypertonic saline injection into the subacromial space of healthy volunteers.
Pain Referral Patterns (Gerber et al. 1998 ):
- Pain primarily localized to the region of the lateral acromion.
- Referral into the deltoid muscle region.
- Occasional referral distally into the forearm or fingers.
Importantly, subacromial space stimulation did not produce pain in the neck or the trapezius region.
Similar to the AC joint, stimulation did not cause pain in the posterior aspect of the shoulder.
Summary
While considerable overlap exists in shoulder pain presentations, these mapping studies suggest potential differentiating features based on the full distribution of pain. Pain originating from the AC joint often includes referral to the anterolateral neck and superior trapezius/supraspinatus region. In contrast, pain from the subacromial space tends to be more laterally focused over the acromion and deltoid, specifically lacking neck or trapezius referral. GHJ pain is primarily felt in the shoulder and upper arm, but uniquely among these three, it can sometimes refer distally to the forearm/hand or involve the lateral neck. The absence of posterior shoulder pain upon AC or subacromial stimulation suggests posterior pain may point towards GHJ or other sources (e.g., cervical spine, TrPs). Therefore, careful mapping of the entire pain field, including associated neck, scapular, or distal arm symptoms, may aid clinical reasoning in differentiating these common shoulder pain generators.
