The pain experienced on the left side under the armpit, medically known as the axilla, can originate from a wide array of underlying issues. This area contains a concentrated network of muscles, nerves, blood vessels, sweat glands, and lymph nodes. Discomfort here can signal anything from a minor skin irritation to a serious condition. While many causes are localized and benign, persistent or accompanied pain warrants medical evaluation.
Localized Skin and Muscle Pain
The most common sources of discomfort in the axilla are superficial issues involving the skin and surrounding musculature. Skin conditions often result from the warm, moist environment of the armpit, which is prone to friction and bacterial growth. Folliculitis, an infection of the hair follicles, typically presents as small, red, pimple-like bumps that are itchy and painful to the touch. This condition is frequently triggered by shaving, which allows bacteria to enter the follicle opening.
Contact dermatitis results from an inflammatory reaction to an external irritant or allergen. Chemicals in deodorants, antiperspirants, soaps, or laundry detergents can cause a painful, burning, or intensely itchy rash. The discomfort is usually limited to the skin surface and may be accompanied by redness or small blisters. Addressing the irritant generally resolves this localized pain.
Pain may also stem from a strain in the powerful muscles near the armpit, such as the pectoralis major or the serratus anterior. These muscle strains usually present as a dull, localized ache or a sharp pain that increases with specific movements, such as lifting the arm or pushing. Overexertion from weight training or repetitive overhead activities can stretch or tear these muscle fibers.
Pain originating from the nerves feels distinctly different from muscle soreness or skin irritation. Intercostal neuralgia involves the nerves running between the ribs, causing a sharp, burning, or stabbing sensation that wraps around the chest wall. This neuropathic pain can be aggravated by actions that expand the rib cage, such as taking a deep breath, coughing, or sneezing. The sensory disturbance may also manifest as tingling or numbness.
Causes Related to Swollen Glands and Lymph Nodes
Pain that feels deeper or presents as a throbbing lump often points to an issue within the glandular or lymphatic structures of the axilla. The armpit is home to a cluster of lymph nodes, which function as filters for the lymphatic system. When the body is fighting an infection, these nodes swell and become tender, a condition known as lymphadenopathy.
Common infections like the cold, flu, or strep throat can cause reactive swelling in the lymph nodes as they filter pathogens. The pain results from the rapid enlargement stretching the node’s outer capsule; the nodes typically feel soft and movable. A bacterial infection like cat scratch fever, caused by the bacterium Bartonella henselae, is a specific cause of highly painful and visibly swollen axillary lymph nodes that can persist for weeks or months.
Another source of deep, painful lumps is Hidradenitis Suppurativa (HS). This chronic inflammatory skin condition affects areas rich in apocrine sweat glands and hair follicles, including the armpit. HS causes recurrent, deep-seated nodules and abscesses that can rupture, drain, and lead to painful tunnels and scarring. Unlike a simple boil, HS lesions tend to be recurrent and progressive.
In women, pain in the left armpit can sometimes be a referred symptom of benign changes in breast tissue. Fibrocystic breast changes cause tenderness, lumpiness, and fullness, often in the upper, outer quadrant of the breast, extending into the axilla. This discomfort is typically cyclic, worsening in the days leading up to the menstrual period due to fluctuating hormone levels.
Urgent Signals: When Left-Sided Pain is More Serious
Certain characteristics of left-sided armpit pain, especially when accompanied by other symptoms, signal a condition requiring immediate medical attention. Pain referred from the heart is a serious concern, as nerve pathways are shared between the chest and the left arm or axilla. Angina, or heart attack, may manifest as crushing, squeezing, or heavy pressure in the chest that radiates to the left armpit, jaw, neck, or back.
Pain of cardiovascular origin is often accompanied by distressing symptoms, including shortness of breath, profuse sweating, dizziness, or nausea. Chest discomfort that is sudden, severe, and does not subside with rest should prompt an immediate call for emergency medical services. Heart attack symptoms in women can sometimes be less typical, presenting as stomach pain, back pain, or fatigue.
A different category of serious pain involves compression of the major nerve and blood vessel structures passing through the thoracic outlet (the space between the collarbone and the first rib). Thoracic Outlet Syndrome (TOS) can cause pain, numbness, and tingling in the arm and hand, frequently worsening when the arm is held elevated. Compression of the blood vessels, particularly the vein, can also lead to swelling, heaviness, or discoloration of the arm.
The brachial plexus, a network of nerves originating from the spinal cord that controls arm movement and sensation, also runs through the axilla and can be affected by trauma or disease. Injuries here may result in a sudden, severe, shooting, or burning pain that travels down the arm, potentially causing muscle weakness or loss of function in the hand. While most axillary lumps are benign, a lump that is hard, fixed (non-moving), or persistently growing raises suspicion for malignancy. These suspicious nodes may also be painless and can be accompanied by systemic signs such as unexplained weight loss, night sweats, or a persistent fever.