Subscapular nerve entrapment is a condition involving the compression or irritation of the subscapular nerves, which leads to impaired function in the muscles they control. While less common than other shoulder neuropathies, it presents a challenge in diagnosing the cause of shoulder pain and functional loss. Recognizing this specific nerve issue is often necessary for effective management of chronic shoulder problems that have not responded to treatments for common diagnoses, such as rotator cuff tears.
The Role and Location of the Subscapular Nerve
The subscapular nerves originate from the posterior cord of the brachial plexus, a complex network of nerves stemming from the C5 and C6 spinal nerve roots in the neck. These nerves are generally divided into the upper and lower subscapular nerves. The upper subscapular nerve is a short structure that primarily provides motor innervation to the superior portion of the subscapularis muscle.
The lower subscapular nerve is longer and supplies the inferior portion of the subscapularis muscle, in addition to the teres major muscle. The subscapularis muscle is the largest and strongest of the four rotator cuff muscles, acting as the primary internal rotator of the shoulder. The teres major muscle works alongside it as a strong adductor and internal rotator of the arm. Damage to these nerves directly impairs the shoulder’s ability to rotate the arm inward and pull it toward the body.
Mechanisms Leading to Entrapment
Entrapment of the subscapular nerve is typically caused by direct trauma, traction injuries, or the presence of a mass that physically compresses the nerve. Because the nerve is relatively short and travels deep within the shoulder, it is particularly vulnerable to injury from forceful mechanisms. Blunt trauma to the shoulder or a severe stretch injury, known as neuropraxia, can damage the nerve fibers and interrupt communication with the muscles.
Mass compression can occur from adjacent soft tissue lesions, such as a tumor, or from scar entrapment following a previous injury or surgery in the shoulder area. In throwing athletes, a repeated stretching injury to the lower subscapular nerve can occur, leading to a temporary nerve dysfunction called neuropraxia. Inflammatory conditions, such as Parsonage-Turner syndrome, can also cause denervation changes in the subscapularis and teres major muscles, mimicking an entrapment syndrome.
Recognizing the Signs of Subscapular Nerve Entrapment
The defining sign of subscapular nerve entrapment is a distinct pattern of muscle weakness that affects shoulder internal rotation. Patients often report an inability to generate power when attempting to turn the arm inward against resistance, or when performing movements that require a strong internal rotation, such as a throwing motion. In athletes, this weakness may manifest as a noticeable decrease in throwing velocity or power.
Over time, denervation of the muscle fibers leads to visible changes in the shoulder contour, manifesting as muscle atrophy, or wasting. This atrophy specifically targets the inferior subscapularis and the teres major muscles. While shoulder pain is a common complaint, it is often deep and aching, sometimes felt in the posterior shoulder. The presence of atrophy and weakness in this muscle distribution is a strong indicator that the subscapular nerve is involved.
Diagnostic Procedures
A healthcare provider begins the diagnostic process with a comprehensive physical examination, specifically testing the strength of the subscapularis and teres major muscles. Objective confirmation of nerve involvement relies heavily on specialized imaging and electrodiagnostic studies.
Magnetic Resonance Imaging (MRI) is a key tool, primarily to visualize the indirect effects of nerve compression on the muscle tissue. Early nerve injury causes muscle belly swelling, which appears as a bright signal on certain MRI sequences. In more long-standing cases, the MRI will reveal signs of chronic denervation, such as muscle atrophy and the replacement of muscle tissue with fat. The MRI is also essential for identifying a direct cause of compression, such as a mass lesion or scar tissue.
Electrodiagnostic testing, which includes Nerve Conduction Studies and Electromyography (EMG), provides the most definitive confirmation of nerve damage. These tests measure the electrical activity and speed of nerve signals, localizing the injury to the subscapular nerve and confirming denervation in the subscapularis and teres major muscles.
Treatment Approaches
Initial management for subscapular nerve entrapment typically focuses on conservative measures to relieve pressure and allow the nerve to recover. This approach includes a period of rest and specific activity modification, particularly avoiding the movements that exacerbate stretching or compression of the nerve. Non-steroidal anti-inflammatory drugs (NSAIDs) may be used to manage pain and reduce any associated inflammation around the nerve.
Physical therapy plays an important role, concentrating on maintaining the range of motion and strengthening the surrounding, unaffected musculature to compensate for the weakness. If the nerve injury is due to a reversible traction mechanism, such as neuropraxia, cessation of the offending activity can lead to recovery. Surgical decompression becomes necessary when conservative treatment fails or if imaging reveals a clear, persistent source of compression, such as a tumor, cyst, or significant scar tissue. The goal of surgery is to physically free the nerve from the source of compression, though the reversibility of long-term muscle atrophy may be limited.