Muscle:Sternalis

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Template:Muscle stub Sternalis is an anomalous superficial muscle of the anterior chest whose trigger points (TrPs) produce a deep substernal ache that is remarkably independent of body movement and that closely mimics the substernal pain of myocardial infarction or angina pectoris. The muscle is present in approximately 1 in 20 individuals and is clinically significant out of all proportion to its small size.

Anatomy

The sternalis muscle is highly variable in its presence, symmetry, length, bulk, attachments, and innervation. It may occur bilaterally or, more often, unilaterally, on either side of the sternum. The two muscles may occasionally fuse across the sternum. It was found in 1.7% to 14.3% (median 4.4%) of cases across studies of at least 10,200 bodies; in 48% of anencephalic specimens; and in 4.3% of 2,062 cadavers summarised by Christian. A unilateral muscle is as common on the right as on the left, but active TrPs appear to be more common on the left side, probably because of their activation as satellite TrPs within the zone of referred pain from the heart.

Attachments: The fibres are superficial to the pectoralis major and generally lie parallel to the margins of the sternum. Above it may attach to the sternum, to the fascia over either the pectoralis major or sternocleidomastoid muscle, or it may form a continuation of those muscles. Below it may attach to the third through seventh costal cartilages, the fascia covering the pectoralis major, and/or to the sheath of the rectus abdominis muscle. The muscle may be as thick as 2 cm (¾ in) over the sternum, a depth through which it is difficult to palpate the features of pectoralis major TrPs.

Innervation: Based on the innervation patterns of 26 sternalis muscles in 20 cadavers, the sternalis muscle was considered a variant of either the pectoralis major or the rectus abdominis muscle. Sixteen of 26 sternalis muscles (62%) received their innervation from intercostal nerves (anterior primary divisions of thoracic spinal nerves) and were considered homologous to the rectus abdominis. The remaining 38% received their innervation from the cervical plexus, usually via the medial pectoral nerve (from spinal nerves C8 and T1), and were considered homologous with the sternal portion of the pectoralis major.

Function: No skeletal movement is attributed to this muscle. No electromyographic data or clinical reports of muscular contraction of the sternalis were located; thus, if, when, or why it contracts is unresolved.

Referred Pain Patterns

The referred pain pattern of the sternalis usually includes the entire sternal and substernal region, and may extend on the same side across the upper pectoral area and front of the shoulder to the underarm and to the ulnar aspect of the elbow. This pattern closely mimics the substernal ache of myocardial infarction or angina pectoris. The chest pain arising from this muscle has a terrifying quality that is remarkably independent of body movement.

The left-sided pattern of the sternalis differs from the referred pain of the left pectoralis major in that the latter is more likely to extend beyond the elbow into the ulnar aspect of the left forearm and hand.

Sternalis TrPs usually occur over the upper two-thirds of the sternum and are most likely to be found as central TrPs slightly to the left of the midline at the mid-sternal level. A TrP located at the confluence of the sternalis, pectoralis major, and sternal division of the sternocleidomastoid muscles can be the source of a dry, hacking cough. Penetration of this TrP with a needle, in whichever muscle it lies, activates the cough momentarily and then relieves it.

Activation and Perpetuating Factors

It is important to realise that patients with either acute myocardial infarction or angina pectoris are likely to develop active TrPs in both the sternalis and left pectoralis major and minor muscles. A sternalis TrP that was activated by an episode of myocardial ischaemia is likely to persist long after this initiating event.

Right or left sternalis muscles may develop satellite TrPs when the sternalis lies within the zone of pain referred downward from the lower portion of the sternal division of the sternocleidomastoid muscle.

Activation of TrPs may also result from direct trauma to the costosternal area.

Clinical Examination

Range-of-motion tests are negative, since the pain is neither relieved nor aggravated by any musculoskeletal activity, such as movement of the shoulder girdle, deep breathing, or stooping. This movement independence is a key distinguishing feature from pectoralis major TrP pain.

Trigger Point Examination

Sternalis TrPs are found by systematic palpation against the underlying sternum and costal cartilages. Firm pressure elicits focal deep tenderness at the TrP and projection of referred pain, but rarely elicits a local twitch response. On examination, the patient has difficulty in distinguishing between the local and the referred pain that is elicited from this muscle, unless the pain radiates not only to the sternum, but also to the shoulder or arm. Referred pain responses due to needle penetration of the TrP are more clearly distinguishable.

Sternalis central TrPs are most commonly found to the left of the midline at the mid-sternal level. Attachment TrPs are sometimes also found close to the attachment region at an end of the muscle belly.

During injection, TrPs on the front of the sternum may be found as deep as 2 cm (¾ in) beneath the skin surface — such deep TrPs may be attachment TrPs of the pectoralis major rather than sternalis fibres. The needle sometimes penetrates two layers of muscle, a superficial one and a deeper one, either or both of which may contain TrPs.

Entrapment

None are attributed to this muscle.

Differential Diagnosis

When multiple areas of spot tenderness are found over the costochondral junctions without the referred pain feature of sternalis TrPs, the examiner should consider costochondritis or Tietze's syndrome. Tietze's syndrome is identified by upper anterior chest pain with tender, nonsuppurative swelling in the area of the costal cartilages or the sternoclavicular junctions. Multiple lesions are more frequent than single lesions and usually involve adjacent articulations. Also, in Tietze's syndrome, systemic manifestations are absent and radiographic and laboratory studies are normal, except for occasional reports of increased calcification at affected sites.

In addition to costochondritis and cardiac disease, the clinician should consider gastroesophageal reflux, oesophagitis, and an anginal presentation of a C7 radiculopathy.

One rarely observes sternalis TrPs alone, without the presence of active TrPs in the pectoralis major muscle. The possibility that a sternalis TrP represents a satellite of a distant key TrP makes it important to examine the lower portion of the sternal division of the sternocleidomastoid muscle, which may refer pain downward over the sternum.

Treatment

Trigger Point Release

Stretch of the sternalis muscle is not practical except for myofascial release; however, application of vapocoolant spray is occasionally effective in the treatment of these myofascial TrPs. Application in a crisscross pattern while the patient holds a deep breath has been the most successful spray technique for TrPs in this muscle.

The sternalis TrPs are responsive to trigger point pressure release against the underlying bone, and the TrPs are easily injected. Deep friction massage applied to the muscle fibres in the region of the TrP is also beneficial.

Local treatment of the sternalis myofascial pain syndrome is not complete until active TrPs in the pectoralis major, or in the lower end of the sternal division of the sternocleidomastoid muscle, have been inactivated — often by trigger point release (see Muscle:Pectoralis Major and Muscle:Sternocleidomastoid). The patient is less likely to experience recurrence of pain due to TrPs in the sternalis muscle if these other two muscles are released prophylactically, even though they contain only latent TrPs which are clinically silent with respect to pain.

Trigger Point Injection

A TrP in the sternalis is identified by flat palpation and is then fixed between two fingers, probed, and precisely infiltrated. When a sternalis TrP is encountered by the tip of the needle, the patient reports projection of pain under the sternum and sometimes across the upper pectoral region and down the ulnar aspect of the arm as far as the elbow. Injection has not usually been observed to induce a local twitch response in this muscle.

Both sides of the sternum must be checked for sternalis TrPs. The needle is aimed toward bone.

Moist heat is applied promptly after injection of TrPs. This muscle cannot be stretched except by massage.

Relief of sternal pain by the spray does not rule out a cardiac aetiology of the pain.

Corrective Actions

The patient should learn to perform trigger point pressure release on their own sternalis TrPs, followed by application of moist heat. The patient selects a tender spot and presses on it steadily with one finger to the point of discomfort and holds it until it fully releases. This release is assisted by slow relaxed exhalation. When the previously tender spot of muscle at the TrP becomes normosensitive, it is no longer a source of referred pain. It may remain quiescent indefinitely, unless the TrP is reactivated, as by recurring angina pectoris.

Satellite Trigger Points

  • Pectoralis major — one rarely observes sternalis TrPs alone without active TrPs in the pectoralis major; the two are almost always active together
  • Sternocleidomastoid (sternal division, lower end) — may refer pain downward over the sternum; sternalis TrPs may be satellite to this key TrP
  • Pectoralis minor — active in the same pain zone; commonly active alongside pectoralis major

References

  • Travell JG, Simons DG. Myofascial Pain and Dysfunction: The Trigger Point Manual, Volume 1: The Upper Half of Body. 2nd ed. Baltimore: Williams & Wilkins; 1999. Chapter 44.
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