The armpit, or axilla, is a complex anatomical space connecting the torso and the arm. It contains an intricate network of muscles, nerves, large blood vessels, and numerous lymph nodes. Due to this high concentration of sensitive tissues, pain in the left armpit is a common complaint. The cause can range from a minor, superficial irritation to a symptom of a deeper, systemic issue requiring medical evaluation.
Skin and Glandular Irritation
The most frequent sources of armpit pain originate on the skin surface or within the specialized glands of the axilla. This region is prone to inflammation due to friction and bacterial growth.
Folliculitis, an inflammation or infection of the hair follicles, is a common source of discomfort often caused by shaving or plucking. This results in small, painful, red bumps or ingrown hairs that are tender to the touch. Allergic reactions, known as contact dermatitis, can also occur from substances applied directly to the skin, such as deodorants, soaps, or laundry detergents.
The sweat glands can also be the focus of painful conditions. Hidradenitis suppurativa is a chronic inflammatory condition where blocked hair follicles and sweat glands lead to deep, painful nodules, abscesses, and scarring. Localized skin infections, such as erythrasma or fungal infections like ringworm, also thrive in the armpit’s warm environment.
Musculoskeletal Causes
Pain originating from the muscles and connective tissues surrounding the armpit is common, especially in physically active individuals. The axilla is bordered by powerful muscles, including the Pectoralis major and Latissimus Dorsi, which are susceptible to strain.
Forceful movements, such as heavy lifting or improper technique during exercise, can cause micro-tears in these muscle fibers, resulting in a strain. This muscular pain is typically sharp or aching and worsens when stretching the arm overhead or across the body. This type of pain often has a clear onset following a specific physical activity.
Pain can also be referred from the shoulder joint. Bursitis, the inflammation of the fluid-filled sacs cushioning bones and tendons, can radiate pain into the upper arm and axilla during movement. Acute muscle strains usually improve significantly with rest and anti-inflammatory medication within a week.
Lymph Node Swelling
The axillary lymph nodes are small, bean-shaped organs that filter pathogens and cellular debris as part of the lymphatic system. When the body fights an infection, these nodes swell and become tender, a condition known as lymphadenopathy.
This reactive swelling is the most common cause of painful armpit lumps. It can be triggered by a common cold, the flu, a localized skin infection, or a recent vaccination. The left armpit nodes specifically collect fluid from the left arm, chest wall, and breast tissue. These nodes usually feel movable and soft, and the pain typically subsides as the underlying infection resolves within one to two weeks.
Persistent or unusual lymph node swelling can signal a systemic or chronic condition. Autoimmune diseases, such as lupus, can cause axillary lymphadenopathy due to an overactive immune response. A hard, fixed, or rapidly growing lymph node that lacks signs of infection warrants immediate medical attention, as it may indicate a malignancy like lymphoma or metastatic cancer.
Referred Pain and Systemic Issues
The sensation of pain in the left armpit is sometimes referred from another location along a shared nerve pathway. This referred discomfort often stems from nerve impingement in the neck or shoulder.
Cervical radiculopathy, or a pinched nerve in the cervical spine, can cause sharp, burning, or electrical pain that radiates down the arm and into the posterior axilla. Pain can also originate from the brachial plexus, the complex network of nerves passing from the spine through the shoulder and into the arm. Conditions like brachial neuritis, which involves sudden nerve inflammation, may cause severe shoulder pain that transitions into weakness, numbness, and radiating pain.
Specific to the left side, pain can be a manifestation of cardiac ischemia, such as during angina or a heart attack. While chest pressure is the most recognized symptom, pain can radiate to the left arm, shoulder, jaw, and occasionally the left axilla. Additionally, pain can originate from mammary tissue that extends into the axilla, potentially related to hormonal changes, fibrocystic changes, or, rarely, inflammatory breast cancer.
Warning Signs and Seeking Medical Attention
While many instances of armpit pain are temporary and resolve with self-care, certain accompanying symptoms require prompt professional evaluation. Pain accompanied by crushing chest pressure, difficulty breathing, or severe shortness of breath necessitates immediate emergency medical care, as these are potential signs of a cardiac event.
A doctor should be consulted if a lump or swelling persists for longer than two weeks without a clear cause, or if it is hard, immovable, or rapidly increasing in size. Other concerning signs include pain that prevents arm movement, unexplained fever, persistent night sweats, or unintentional weight loss. A professional diagnosis is necessary to accurately determine the source of the pain and ensure any underlying serious conditions are addressed.