The serratus anterior muscle on chest radiographs

Our plain-English summary of The serratus anterior muscle on chest radiographs, by D. Gilmartin (Radiology, 1979).

The study

• Author: D. Gilmartin
• Journal: Radiology
• Published: 1979;131(3):629-35
• Cited by: 10 articles, as counted by NIH iCite
• Read it: PubMed · Publisher
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 002077).

Abstract

The radiographic appearance of the serratus anterior muscle shadow is analyzed. In frontal and near-frontal views, it produces a "bowling-pin" silhouette. When the muscle is well developed, the medial edge of this silhouette may be superimposed upon the air shadow of the lung in a variety of ways. When it overlies the apex of the lung, it gives rise to the companion shadow; when overlying the midlateral lung edge and costophrenic angle it may mimic pleural and/or extrapleural disease. Recognition of the various possible patterns is important to prevent overdiagnosis of disease, particularly asbestosis.

In plain English

This 1979 article, from the Veterans Administration Hospital in Providence, Rhode Island, described how a normal chest-wall muscle, the serratus anterior, appears on chest X-rays. It drew on X-rays of selected patients seen over six months, mostly at the VA hospital. 2 patients also had CT scans; patient numbers were not stated. The author described the muscle as forming a "bowling-pin" outline on front views. When well developed, its inner edge can overlap the side of the lung as a "midsection shadow" or the lower corner as a "costophrenic curtain." The author believed, without final proof, that it also causes the "companion shadow" seen at the top of the lung. The author wrote that the midsection shadow may sometimes be indistinguishable from pleural thickening linked to old rib fractures or asbestosis. In one patient, X-rays could not settle the question; a CT scan showed pleural plaques, which were attributed to asbestosis. The author concluded that uncalcified pleural or extrapleural asbestosis may be overdiagnosed unless this muscle shadow is considered, but cautioned that telling the two apart can be difficult.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Descriptive radiographic study of a normal anatomical shadow, illustrated with patient X-rays and CT scans (no statistical analysis)
• Who was studied: Selected patients seen over a six-month period, most from the VA Hospital (Providence, R.I.). Number not stated. Standard and oblique chest X-rays were taken, and 2 patients also had thoracic CT scans.
• Exposure: No asbestos exposure was studied or measured. The subject is the normal serratus anterior muscle shadow and how it can mimic pleural disease, including what the author calls asbestosis (uncalcified pleural or extrapleural plaques).
• Main results:
  • On frontal and near-frontal X-rays, the three parts of the serratus anterior muscle form a "bowling pin" shape. This shape is usually not superimposed on the lung shadow.
  • The author described three patterns formed by the inner edge of the "bowling pin" shadow: the companion shadow, the midsection shadow, and the costophrenic curtain.
  • The author believed that the companion shadow at the top of the lung is caused by the upper part of the muscle overlapping the lung apex. The discussion calls this the probable cause, not a proven one.
  • The midsection shadow can be about 1 cm thick when the muscle is well developed. It overlies the side of the lung from the third rib to several centimeters above the costophrenic angle.
  • The midsection shadow may sometimes be indistinguishable from pleural or extrapleural thickening associated with old rib fractures or asbestosis.
  • With the costophrenic curtain, the lower corner of the lung stays sharp (not blunted) but appears shifted toward the middle.
  • In one patient (Figure 4), CT scans showed no explanation inside the chest for the various shadows.
  • In one patient (Figure 9), oblique X-rays could not tell whether a shadow was inside the chest or was muscle. A CT scan showed a pleural plaque at that site and other uncalcified plaques elsewhere, and the changes were attributed to asbestosis.
• The authors' conclusion: The author concluded that uncalcified pleural or extrapleural asbestosis may be overdiagnosed unless the "bowling pin" muscle shadow is taken into account. The author also cautioned that the difficulty of telling the two apart should not be underestimated.
• Limitations the authors state:
  • The muscle explanation for the companion shadow is called probable, not proven. Final proof may depend on the shadow disappearing after the muscle is surgically removed.
  • The author did not check how the position of the arms and shoulders affects the midsection shadow.
  • With simple views of varying angle, it was sometimes hard to tell whether the front or back edge of the muscle produced the shadow.
  • Without a CT scan, many published cases of asbestosis would be hard to distinguish from serratus anterior shadows.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Asbestosis, Pleural plaques and thickening.

Medical terms in this study, explained in our Lexicon: Chest X-Ray, Costophrenic angle.

Related studies

• Asbestosis and the serratus anterior muscle — J.D. Collins, R.K. Brown, et al. (1983)
• A comparative evaluation of intrathoracic latissimus dorsi and serratus anterior muscle transposition — M.K. Widmer, T. Krueger, et al. (2000)
• Increased body mass index is related to apparent circumscribed pleural thickening on plain chest radiographs — Y.C. Lee, C.K. Runnion, et al. (2001)
• Comparative immunohistochemical staining for desmin and muscle-specific actin. A study of 576 cases — S. Rangdaeng, L.D. Truong (1991)
• Expression of muscle actins in diffuse mesotheliomas — I.T. Kung, V. Thallas, et al. (1995)
• The usefulness of computed tomography in detecting asbestos-related pleural abnormalities in people who had indeterminate chest radiographs: the Libby, MT, experience — O.I. Muravov, W.E. Kaye, et al. (2005)

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