The Clinical Syndrome

The latissimus dorsi muscle is a broad, sheetlike muscle whose primary function is to extend, adduct, and medially rotate the arm; its secondary function is to aid in deep inspiration and expiration. The latissimus dorsi muscle originates on the spine of T7; the spinous processes and supraspinous ligaments of all lower thoracic, lumbar, and sacral vertebrae; the lumbar fascia; the posterior third iliac crest; the last four ribs; and the inferior angle of the scapula. The muscle inserts on the bicipital groove of the humerus and is innervated by the thoracodorsal nerve.

The latissimus dorsi muscle is susceptible to myofascial pain syndrome, which most often results from repetitive microtrauma to the muscle during such activities as vigorous use of exercise equipment or tasks that require reaching in a forward and upward motion ( Fig. 83.1 ). Blunt trauma to the muscle may also incite latissimus dorsi myofascial pain syndrome.

Latissimus dorsi syndrome is usually caused by repetitive microtrauma to the muscle during such activities as vigorous use of exercise equipment or tasks that require reaching forward and upward.

Myofascial pain syndrome is a chronic pain syndrome that affects a focal or regional portion of the body. The sine qua non of myofascial pain syndrome is the finding of myofascial trigger points on physical examination. Although these trigger points are generally localized to the part of the body affected, the pain is often referred to other areas. This referred pain may be misdiagnosed or attributed to other organ systems, thus leading to extensive evaluation and ineffective treatment.

The trigger point is pathognomonic of myofascial pain syndrome and is characterized by a local point of exquisite tenderness in the affected muscle. Mechanical stimulation of the trigger point by palpation or stretching produces not only intense local pain but also referred pain. In addition, involuntary withdrawal of the stimulated muscle, called a jump sign, often occurs and is also characteristic of myofascial pain syndrome.

Taut bands of muscle fibers are often identified when myofascial trigger points are palpated. Despite this consistent physical finding, the pathophysiology of the myofascial trigger point remains elusive, although trigger points are believed to be the result of microtrauma to the affected muscle. This trauma may result from a single injury, repetitive microtrauma, or chronic deconditioning of the agonist and antagonist muscle unit.

In addition to muscle trauma, various other factors seem to predispose patients to develop myofascial pain syndrome. For instance, a weekend athlete who subjects his or her body to unaccustomed physical activity may develop myofascial pain syndrome. Poor posture while sitting at a computer or while watching television has also been implicated as a predisposing factor. Previous injuries may result in abnormal muscle function and lead to the development of myofascial pain syndrome. All these predisposing factors may be intensified if the patient also suffers from poor nutritional status or coexisting psychological or behavioral abnormalities, including chronic stress and depression. The latissimus dorsi muscle seems to be particularly susceptible to stress-induced myofascial pain syndrome.

Stiffness and fatigue often coexist with pain, and they increase the functional disability associated with this disease and complicate its treatment. Myofascial pain syndrome may occur as a primary disease state or in conjunction with other painful conditions, including radiculopathy and chronic regional pain syndromes. Psychological or behavioral abnormalities, including depression, frequently coexist with the muscle abnormalities, and management of these psychological disorders is an integral part of any successful treatment plan.