Left shoulder pain frequently originates within the joint, but its causes are complex and can stem from distant areas of the body. The shoulder is a highly mobile ball-and-socket joint, but this range of motion compromises stability, making surrounding structures vulnerable to injury. Since pain signals travel along shared nerve pathways, discomfort felt in the left shoulder may not always be a muscular or skeletal issue.
Pain Originating Within the Shoulder Joint
Chronic shoulder discomfort often develops gradually from repetitive movements or age-related degeneration. The rotator cuff, a group of four muscles and their tendons, stabilizes the joint and controls rotation. Inflammation of these tendons is known as tendinitis, causing pain during movement or when lying on the affected side.
Tendinitis frequently occurs alongside subacromial bursitis, which is the inflammation of the fluid-filled sac (bursa) that cushions the rotator cuff tendons. When the space narrows, often during arm elevation, the tendons and bursa can get pinched, a condition termed impingement syndrome. Chronic irritation can lead to partial or complete tendon tears, resulting in weakness and limited range of motion.
Another source of persistent pain is arthritis, involving the breakdown of cartilage within the joint. Osteoarthritis (wear-and-tear arthritis) causes pain that worsens over time and is often felt acutely after periods of inactivity, such as in the morning. Rheumatoid arthritis, an autoimmune disorder, causes pain, swelling, and stiffness from systemic inflammation. These degenerative changes reduce the joint’s smooth gliding surfaces, leading to friction and a progressive loss of arm mobility.
Acute Traumatic Injuries
Sudden, intense left shoulder pain often signals an acute mechanical injury from a fall or direct impact. A glenohumeral joint dislocation occurs when the head of the humerus is forced out of its socket. This is a painful event that immediately deforms the shoulder’s contour. Most dislocations are anterior (the joint moves forward) and carry a high risk of recurrence, especially in younger individuals.
Fractures of the shoulder complex are common following trauma, involving the humerus, scapula, or clavicle. Clavicle fractures are frequent and can significantly impact shoulder function. A separated shoulder, or acromioclavicular (AC) joint separation, involves a tear in the ligaments connecting the collarbone and the shoulder blade. This injury results in acute pain over the top of the shoulder and may produce a visible bump depending on the ligament damage.
Pain Referred from the Cervical Spine
Pain felt in the shoulder can be a manifestation of a problem originating in the neck, known as cervical radiculopathy. This occurs when a nerve root in the cervical spine is compressed or irritated. Common causes include a herniated disc or the formation of bone spurs due to degenerative changes.
This nerve compression results in pain that starts in the neck and radiates down the shoulder and into the arm, often described as sharp or burning. The pain is accompanied by neurological symptoms, such as tingling, numbness, or weakness in the arm or hand, following the affected nerve root. Movements like extending the neck or turning the head toward the painful side usually exacerbate the discomfort, helping differentiate this from true shoulder joint pathology.
Systemic and Referred Organ Pain
The most serious causes of left shoulder pain originate from internal organs, a phenomenon known as referred pain. This occurs because sensory nerves from internal organs and nerves from the shoulder share common pathways entering the spinal cord, causing the brain to misinterpret the source.
Cardiac Events
A myocardial infarction, or heart attack, is a medical emergency where blood flow to the heart muscle is blocked, and it often presents with pain radiating to the left shoulder and arm. This pain is usually described as pressure, tightness, or squeezing in the chest, which may spread to the jaw, back, or neck. Angina, temporary chest pain due to reduced blood flow, presents with similar radiating pain, especially during physical exertion or emotional stress.
Left shoulder pain associated with a cardiac event is frequently accompanied by other systemic symptoms. These include shortness of breath, a cold sweat, nausea, light-headedness, or sudden fatigue. If left shoulder pain is sudden, severe, or occurs alongside any of these signs, immediate emergency medical attention is required.
Other Visceral Causes
Referred pain to the left shoulder can also signal pathology in abdominal organs, specifically those near the left hemidiaphragm. The phrenic nerve, which innervates the diaphragm, originates from the cervical spine (C3, C4, and C5), the same nerve roots that supply the shoulder area.
Irritation of the left side of the diaphragm causes pain to be perceived in the tip of the left shoulder. The most recognized example is Kehr’s sign, where irritating substances from a ruptured spleen accumulate under the left diaphragm. Splenic rupture, which can occur after trauma, presents with left upper abdominal pain and tenderness, often accompanied by this referred shoulder pain.