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Axillary and Suprascapular Nerves
Introduction
The limited osseous stability of the shoulder allows for its extensive range of motion but makes the strength of the joint largely dependent on the musculotendinous rotator cuff. The complexity of the ligaments, tendons, and cartilaginous tissue that provide structural support for the shoulder, compensating for the shallowness of the glenoid fossa, creates a great deal of technical difficulty when considering the medical and surgical treatments of painful shoulder disorders. Chronic shoulder joint pain is a common, often debilitating problem reported by 18.7 million Americans annually, with a lifetime incidence of nearly 67%. While there are many treatments currently in use, there remains a need for improved pain reduction techniques for those cases that are refractory to standard therapies. There is burgeoning support in the literature for the benefits of peripheral neuromodulation of the shoulder, primarily targeting the suprascapular and axillary nerves, to treat a diverse array of painful shoulder pathologies.
Clinical Presentation
Chronic shoulder pain is the second most common musculoskeletal complaint after knee pain, and the differential diagnosis is vast. Although a wide variety of surgical and nonsurgical treatments exist, shoulder pain can often be recalcitrant to standard therapies, causing suffering and impaired quality of life. Here we cover the clinical presentation of common causes of shoulder pain that may be amenable to peripheral neuromodulatory therapies.
Osteoarthritis
Given the heavy daily use of the shoulder, it is unsurprising that the most common cause of chronic shoulder pain in older adults is osteoarthritis. Patients often present complaining of longstanding, progressive shoulder pain that is worse with activity. Physical exam may show a mildly tender joint and reduced range of motion. After exhausting conservative treatment options such as oral analgesics, steroid joint injections, and physical therapy, standard practice is to offer treatment with a shoulder arthroplasty to patients with persistent, moderate to severe symptoms. Even so, up to 22% of patients still experience persistent shoulder pain after complete replacement. Further, major complications occur in 13% to 15% of shoulder replacements within the first 2 years; these include adhesive capsulitis, infection, instability, and the need for revision surgeries. Osteoarthritis is, at its core, a musculoskeletal problem caused by degeneration of the joint from wear and tear over time. It is important to understand, however, that longstanding pain can become neuropathic overtime, and thus simply replacing the joint may not fully alleviate a patient’s pain. Neuromodulatory therapies for osteoarthritis, both before and after shoulder arthroplasty, have been promising.
Rotator cuff and labral pathology
Rotator cuff and labral surgeries are some of the most commonly performed shoulder surgeries in the United States. Typically, patients will present with pain and weakness in the affected shoulder. Pain occurs over the lateral deltoid, is exacerbated by overhead activities, and is worse at night. Weakness occurs in large or full-thickness tears, and the pattern of weakness will be dependent on the part of the rotator cuff that is torn. Labral tears often occur in patients with repetitive overuse, such as an athlete or manual laborer, though they can also occur due to acute injury. Importantly, patients with rotator cuff or labral pathology may or may not be able to identify the inciting event. A careful physical exam can identify where lesions to the shoulder have occurred.
Poststroke pain
Each year in the United States, approximately 795,000 people suffer a stroke, and hemiplegic shoulder pain affects up to 84% of survivors in the early poststroke period and up to 47% of patients 12 months afterward. Hemiplegic shoulder pain can cause a range of unpleasant sensations and is thought to be multifactorial in nature. Contributing factors may include subluxation of the glenohumeral joint, spasticity, soft tissue inflammation, reduced range of motion, and the development of pathologies such as adhesive capsulitis and shoulder-hand syndrome. In most patients with hemiplegic shoulder pain, the pain is aggravated or induced by movement, further contributing to a patient’s functional decline. Given the large number of patients suffering stroke annually and the frequency with which these patients develop new shoulder pain, poststroke pain syndromes contribute significantly to the social and functional decline incurred by many patients after a stroke. Another phenomenon experienced by approximately 10% of stroke survivors is central poststroke pain. In this disorder, patients experience symptoms in various parts of their bodies, described as unpleasant sensations including pins and needles, tingling, burning, numbness, electric shocks, painful cold, and itching. These sensations are thought to be due to a central nervous system process, likely involving the spinothalamic or trigeminothalamic pathways. In patients with central poststroke pain syndrome, almost half experience symptoms in the shoulder and upper extremity. Peripheral nerve stimulation (PNS) of both motor and sensory nerves have been studied and shown to reduce the pain associated with stroke survival. Stimulation of sensory nerves functions to reduce the perception of pain, while motor stimulation of the muscles of the shoulder aids in reducing pain by stabilizing the joint in the correct anatomic position.
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