Back pain localized to the left side is common, with origins spanning the muscular, skeletal, and internal organ systems. Since the spine and vital organs share nerve pathways, pain signals can be localized to the back even when the source is visceral. Determining the cause requires investigating the pain’s location, accompanying symptoms, and factors that make it better or worse.
Common Causes Originating in the Spine and Muscles
Most left-sided back pain stems from mechanical issues involving the lumbar region’s vertebrae, discs, ligaments, or muscles. The most frequent culprit is a simple muscle strain or ligament sprain, often resulting from poor lifting technique, twisting, or prolonged poor posture. This pain is typically a dull ache localized left of the spine and worsens with specific movements or exertion.
The left Quadratus Lumborum (QL) muscle is a specific source of unilateral pain, connecting the lowest rib to the pelvis and aiding in side-bending. Strain often presents as a deep, persistent ache concentrated along the waistline, just above the hip bone, sometimes radiating into the buttock or hip. QL pain typically increases when standing for long periods or when bending sideways away from the painful left side.
Pain originating from the spine’s bony structures often involves the intervertebral discs. A herniated disc in the lower left lumbar spine can compress a spinal nerve root, causing radiating pain known as sciatica that travels down the left buttock and leg. This nerve pain is typically sharp or burning and may include tingling, numbness, or weakness in the affected limb.
Degenerative changes, such as left facet joint arthritis, also cause localized back discomfort. Facet joints are small pairs of joints on the back of each vertebra that allow for spinal movement. Arthritis in the left joint often produces pain that is worse when twisting or bending backward, as these motions compress the irritated surfaces. This discomfort may feel like a dull ache and can spread into the left thigh or buttock, but it usually does not extend below the knee.
Referred Pain from the Lower Abdomen and Urinary Tract
If left-sided back pain is not linked to movement or injury, the kidneys and lower digestive tract are primary areas of concern due to their anatomical proximity. The left kidney lies behind the abdominal cavity near the flank, making it a frequent source of referred pain.
A kidney stone (renal colic) moving down the ureter causes intense, sharp pain that often begins in the flank and moves toward the groin. This colicky pain comes in waves and is typically severe, making it impossible to find a comfortable position. A kidney infection (pyelonephritis) also causes left flank pain, usually presenting as a constant, dull ache accompanied by systemic signs like fever, chills, and painful or frequent urination.
The sigmoid colon is located in the lower left quadrant of the abdomen. Inflammation of small pouches in the colon lining, called diverticulitis, can cause abdominal pain that radiates to the lower left side of the back. This back pain often presents alongside digestive symptoms like fever, nausea, or a change in bowel habits, distinguishing it from a muscle strain. The inflammation irritates nearby nerve fibers, causing referred lower back pain that may feel like a persistent, dull ache.
Referred Pain from the Upper Abdomen and Chest
Pain originating from organs in the upper left abdomen or chest cavity can be perceived in the left side of the back, often in the mid-back or shoulder blade region. The pancreas, located behind the stomach, is a common source of this upper visceral referred pain.
Pancreatitis (inflammation of the pancreas) typically causes severe, persistent pain in the upper abdomen that radiates to the mid-back or below the left shoulder blade. The pain may worsen after eating fatty foods and can be temporarily eased by leaning forward. Splenic issues, such as an enlarged or ruptured spleen, cause intense discomfort concentrated in the upper left abdomen and flank. A classic sign of a ruptured spleen is the Kehr sign, where diaphragm irritation causes pain felt specifically at the tip of the left shoulder.
Certain cardiac issues can manifest as referred pain in the left shoulder blade and back. Angina or a heart attack can produce pain that radiates from the chest to the left shoulder, neck, and back due to shared sensory pathways. This pain is usually described as pressure or squeezing and is not worsened by movement, which helps differentiate it from musculoskeletal causes. Pericarditis, inflammation of the sac surrounding the heart, can also cause sharp pain that spreads to the left shoulder or back, often worsening with deep breaths or when lying flat.
When to Seek Medical Evaluation
While most left-sided back pain resolves with conservative care, certain warning signs, known as “red flags,” indicate a need for prompt medical attention. Any sudden, severe back pain unrelieved by rest or changes in position should be evaluated by a healthcare professional.
Immediate care is required if the pain is accompanied by fever, chills, or unexplained weight loss, which may signal an infection or systemic illness. Urgent attention is needed for new neurological symptoms, such as weakness, numbness, or tingling in the legs, or difficulty controlling bladder or bowel function. Back pain linked with chest pain, shortness of breath, dizziness, or a rapid pulse should be treated as a medical emergency.