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Serratus Anterior Muscle Syndrome
Chest wall pain syndromes are commonly encountered in clinical practice. Some occur with relatively greater frequency and are more readily identified by the clinician, such as costochondritis and Tietze syndrome.
Others occur so infrequently that they are often misdiagnosed, resulting in a less-than-optimal outcome. Serratus anterior muscle syndrome is one such infrequent cause of anterior chest wall pain. The syndrome is a constellation of symptoms consisting of pain overlying the fifth to the seventh ribs in the midaxillary line, with referred pain that may radiate down the ipsilateral upper extremity into the palmar aspect of the ring and little finger.
Serratus anterior muscle syndrome can mimic the pain of myocardial infarction and is frequently misdiagnosed as such. It is a myofascial pain syndrome. The intensity of the pain associated with serratus anterior muscle syndrome is mild to moderate and is described as having a deep, aching character. The pain of serratus anterior muscle syndrome is intermittent.
What are the Symptoms of Serratus Anterior Muscle Syndrome?
On physical examination, the patient with serratus anterior muscle syndrome will exhibit myofascial trigger points overlying the fifth to seventh ribs in the midaxillary line, with referred pain that may radiate down the ipsilateral upper extremity into the palmar aspect of the ring and little fingers.
Pain is reproduced with palpation of these trigger points rather than with movement of the chest wall and shoulders. A positive jump sign will be present when these trigger points are stimulated.
Trigger points at the lateral border of the scapula may be present and amenable to injection therapy. As mentioned, movement of the shoulders and chest wall will not exacerbate the pain.
How is Serratus Anterior Muscle Syndrome diagnosed?
Plain radiographs are indicated in all patients with suspected serratus anterior muscle syndrome to rule out occult bony pathological processes, including metastatic lesions. Based on the patient’s clinical presentation, additional testing may be indicated, including complete blood cell count, prostate-specific antigen level, sedimentation rate, and antinuclear antibody testing.
Magnetic resonance imaging (MRI) of the chest is indicated if a retrosternal mass such as thymoma is suspected or if trauma to the serratus anterior muscle itself has occurred. Electromyography is indicated in patients with serratus anterior muscle syndrome to help rule out cervical radiculopathy or plexopathy that may be considered because of the referred arm pain.
Injection of the serratus anterior muscle with a local anesthetic and steroid serves as both a diagnostic and therapeutic maneuver.
Differential Diagnosis
As mentioned, the pain of serratus anterior muscle syndrome is often mistaken for pain of cardiac origin and can lead to visits to the emergency department and unnecessary cardiac workups. If trauma has occurred, serratus anterior muscle syndrome may coexist with fractured ribs or fractures of the sternum itself, which can be missed on plain radiographs and may require radionucleotide bone scanning for proper identification.
Tietze syndrome, which is painful enlargement of the upper costochondral cartilage associated with viral infections, can be confused with sternalis syndrome, as can costosternal syndrome.
Neuropathic pain involving the chest wall may be confused or coexist with costosternal syndrome. Examples of such neuropathic pain include diabetic polyneuropathies and acute herpes zoster involving the thoracic nerves. ‘The possibility of diseases of the structures of the mediastinum remains ever present and at times can be difficult to diagnose. Pathological processes that inflame the pleura, such as pulmonary embolus, infection, and tumor, also should be considered.
Treatment
Initial treatment of serratus anterior muscle syndrome should include a combination of simple analgesics and the nonsteroidal antiinflammatory agents or the cyclooxygenase-2 (COX-2) inhibitors. The local application of heat and cold may be beneficial to provide symptomatic relief of the pain of serratus anterior muscle syndrome. The use of an elastic rib belt may help provide symptomatic relief in some patients. For patients who do not respond to these treatment modalities, injection of the trigger areas located in the sternalis muscle using a local anesthetic and steroid may be a reasonable next step.
Complications and Pitfalls
The major problem in the care of patients thought to have serratus anterior muscle syndrome is the failure to identify potentially serious pathological conditions of the thorax or mediastinum. Given the proximity of the pleural space, pneumothorax after injection of the serratus anterior muscle is a distinct possibility, as is injury to the mediastinal and intrathoracic structures. Approximately 25% of patients will report a transient increase in pain after this injection technique and should be warned of this.CLINICAL PEARLS
Patients with serratus anterior muscle syndrome will often go the emergency department, fearing they are having a heart attack. The syndrome is also frequently misdiagnosed as a cervical radiculopathy because of the referred arm pain. Electromyography will help delineate the cause and extent of neural compromise.
This injection technique is extremely effective in the treatment of serratus anterior muscle syndrome. Coexistent costosternal or manubriosternal arthritis may contribute to anterior chest wall pain and may require additional treatment with a more localized injection of an anesthetic and depot steroid. This technique is a safe procedure if careful attention is paid to the clinically relevant anatomy in the areas to be injected. Pneumothorax can be avoided if shorter needles are used and the needle is not advanced too deeply. Care must be taken to use sterile technique to avoid infection, and universal precautions must be used to avoid risk to the operator. The incidence of ecchymosis and hematoma formation can be decreased if pressure is placed on the injection site immediately after injection. The use of physical modalities, including local heat and gentle range-of-motion exercises, should be introduced several days after the patient undergoes this injection technique for shoulder pain. Vigorous exercises should be avoided because they will exacerbate the symptoms. Simple analgesics and nonsteroidal anti inflammatory agents may be used concurrently with this injection technique.
