Pain in either shoulder may be caused by muscles rubbing against the top of the shoulder bone, usually from swelling and irritation from activities such as washing walls, hanging curtains, lifting heavy suitcases, gardening or repetitive athletic motions. Shoulder instability occurs when the joint is forced out of its normal position. To prevent more serious injury, sensations such as sliding, popping, stiffness, weakness or severe pain should be evaluated by a health care professional.
Inflammatory Conditions
Bursae are fluid filled sacs that act as cushions within the joint. Overuse, trauma, infection and arthritis may cause bursitis, or inflammation of the bursae. Tendons are bands of tissue that connect muscles to bones. Stretchy to allow movement, tendons become less flexible and increasingly prone to injury with age. Inflammation of the tendons, or tendinitis, usually results from repetitive movement. According to the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), two types of tendinitis can affect the shoulder. Biceps tendinitis produces pain in the front and side of the shoulder that may radiate down the arm to the elbow and forearm. Rotator cuff tendinitis causes pain at the top and upper outside of the arm, making reaching, pushing, pulling and overhead motions painful.
Torn Rotator Cuff
The rotator cuff is a group of tendons and muscles that surround the shoulder socket to hold and allow rotation of the arm bone. Injury to these connective tissues can occur in either shoulder and usually does not occur in both shoulders at the same time. Causes include calcium spurs that build up over time, lifting heavy objects, repetitive overhead motions or accidents. Rotator cuff injuries are extremely painful, especially when lying on the injured side or reaching into a back pocket. A guide for diagnosing rotator cuff tears published in the April 2005 issue of "American Family Physician" states that those over age 60 are more likely to have this type of injury than younger persons and complaints of nighttime pain are common. Limited range of motion, weakness and pain when lifting the arm alongside the ear are other typical symptoms. With the help of time and exercise, torn rotator cuffs usually heal spontaneously.
Referred Pain
The most serious cause of pain in the left shoulder is referred pain, which is pain felt next to or distant from the site of injury. This phenomenon is not well understood but left shoulder pain may occur in tubal pregnancy or pericarditis. The symptom can aid in the diagnosis of heart attack and ruptured spleen. The Mayo Clinic states a heart attack usually occurs when circulation to the heart muscle is blocked by a blood clot in a coronary artery. Pain and pressure may be felt in the left or mid-chest area, left neck, shoulder or arm.
Kehr's sign, or pain in the left shoulder when lying down with the legs elevated, indicates bleeding in the peritoneal cavity as the result of a ruptured spleen. The spleen is an organ involved in fighting infection and has been known to rupture spontaneously, usually following illness caused by infectious mononeucleosis.
