Pain in the left shoulder when taking a breath is an alarming symptom that rarely points to a single cause. This discomfort often results from referred pain, where an internal organ shares nerve pathways with a distant area like the shoulder. The symptom signals potential issues ranging from simple muscle strain to severe conditions involving the lungs, heart, or abdominal organs. Understanding the anatomical connections between the chest and shoulder is the first step in assessment. This article explores the systems that can generate left shoulder pain in response to the mechanical action of breathing. This information is for educational purposes only and is not a substitute for professional medical diagnosis or treatment.
Musculoskeletal and Chest Wall Sources
The most common reasons for pain in the shoulder area while breathing originate in the structures of the chest wall, specifically the muscles, joints, and cartilage surrounding the rib cage. Since breathing relies on the expansion and contraction of the chest, any injury to these structures is immediately aggravated by inhalation. This pain is typically sharp, localized, and easily reproduced by pressing on the affected area.
The intercostal muscles, located between the ribs, move the rib cage during respiration. A strain in these muscles, often caused by vigorous exercise, a sudden twist, or a forceful cough, can produce sharp, localized pain that may radiate into the shoulder or upper back. This discomfort intensifies with deep breaths, coughing, or sneezing, as these actions force the injured fibers to stretch and contract.
Another common musculoskeletal cause is costochondritis, which involves inflammation of the cartilage connecting the ribs to the sternum. This inflammation often affects the upper ribs on the left side, creating a sharp, aching pain in the chest that can spread to the arm and shoulder. The discomfort worsens with deep breathing and movement, but unlike deep-seated organ pain, costochondritis pain is reproducible by applying pressure to the affected joints. Minor rib injuries, such as bruising or a slight subluxation, can also irritate surrounding nerves and muscles, causing pain exacerbated by the forceful movement of the rib cage during inspiration.
Pulmonary and Pleural Inflammation
Pain associated with breathing can stem from the lungs and their protective linings, causing a distinct type of sharp discomfort known as pleuritic pain. The lungs and the inner chest wall are encased by two thin layers of tissue called the pleura, which normally glide smoothly during respiration. When this lining becomes inflamed (pleurisy), the layers rub against each other, causing a sharp, stabbing pain.
This pain worsens significantly with deep breaths, coughing, or any movement of the torso, and it frequently spreads up to the shoulder. Pleurisy is usually a symptom of an underlying condition, most commonly a viral infection, but it can also signal bacterial pneumonia. If pneumonia is located in the lower lobes of the left lung, the resulting inflammation can directly irritate the parietal pleura, triggering referred shoulder pain.
A pneumothorax, or collapsed lung, is another pulmonary condition that creates sudden, sharp pain radiating to the shoulder. This occurs when air enters the pleural space, disrupting the negative pressure gradient that keeps the lung inflated. The accumulation of air puts pressure on the lung, causing it to partially or fully collapse. The sudden irritation of the parietal pleura results in characteristic pleuritic chest pain and shortness of breath, distinguishing it from the gradual onset of pain seen in many other causes.
Referred Pain via the Diaphragm
Pain originating in the abdomen or around the heart can manifest as left shoulder pain due to shared nerve pathways, especially when linked to breathing. The phrenic nerve controls the diaphragm, but its sensory origins trace back to the C3, C4, and C5 spinal nerve segments in the neck. Since the nerves supplying sensation to the shoulder also originate from these same segments, the brain misinterprets irritation of the diaphragm as shoulder pain.
This pathway explains Kehr’s Sign: pain felt at the tip of the left shoulder caused by irritation beneath the left side of the diaphragm. The classic example is a ruptured spleen, where irritants collect under the diaphragm, triggering the phrenic nerve and creating intense shoulder pain. This pain is exacerbated by deep inhalation because the diaphragm is forced downward, increasing contact with the source of irritation.
Pericarditis, the inflammation of the sac surrounding the heart, can also irritate the central part of the diaphragm where the phrenic nerve passes. The resulting sharp, stabbing chest pain is often made worse by lying flat or taking a deep breath. It frequently spreads to the left shoulder or neck via the phrenic nerve pathway. Other causes of subdiaphragmatic irritation include subphrenic abscesses, which are pockets of infection located below the diaphragm.
Critical Symptoms Requiring Immediate Care
While many causes of left shoulder pain are manageable, certain accompanying symptoms constitute a medical emergency. The most prominent concern is pain originating from the heart, as the discomfort of a myocardial infarction (heart attack) can radiate down the left arm and into the shoulder. Cardiac pain is often described as a heavy pressure, crushing sensation, or fullness in the chest, accompanied by other symptoms.
A pulmonary embolism (PE), a blockage in the pulmonary arteries, also demands urgent attention. PE causes sudden shortness of breath and sharp chest pain that worsens with inhalation, sometimes presenting atypically as severe shoulder or back pain. Other warning signs include:
- A rapid or irregular heartbeat.
- Coughing up blood.
- Signs of deep vein thrombosis (DVT) in the leg, such as swelling, redness, or tenderness.
Any sudden, severe onset of left shoulder pain with breathing accompanied by acute shortness of breath, dizziness, fainting, or signs of shock should be treated as a medical emergency.