DOI: 10.1148/radiol.2472050864
(Radiology 2008;247:589-590.)
© RSNA, 2008
The Incomplete Fissure Sign1
Meghan G. Lubner, MD
1 From the Mallinckrodt Institute of Radiology, Washington University School of Medicine, 510 S Kingshighway Blvd, Box 8131, St. Louis, MO 63110. Received May 23, 2005; revision requested July 9; revision received November 1; final version accepted January 16, 2006.
Address correspondence to the author (e-mail: lubnerm{at}mir.wustl.edu).
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APPEARANCE
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The incomplete fissure sign (1) is seen on the frontal chest radiograph and consists of a clear area of perihilar lucency laterally circumscribed by a sharp concave edge and varying degrees of opacification peripheral to this edge (Fig 1). This sign is more frequently seen on the right side of the patient.
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Figure 1: Supine chest radiograph illustrates incomplete fissure sign as a medial lucency and lateral opacity in the middle portion of left lung. Arrows = edge formed by pleural fluid adjacent to aerated lung.
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EXPLANATION
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This appearance is caused by pleural fluid lateral to aerated lung in a patient with an incomplete major fissure (Fig 2). The perihilar lucency is aerated lung. The lateral concave edge is produced by fluid around the aerated lung.
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Figure 2: Transverse computed tomographic (CT) image of the chest in the same patient confirms an incomplete major fissure on the left with pleural fluid seen along the border of the aerated lower lobe.
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DISCUSSION
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Pulmonary fissures divide the lungs into lobes by means of infolded reflections of the visceral pleura (2) and are important anatomic landmarks in thoracic radiology. It is essential to know the anatomy of the fissures to recognize their appearance on conventional radiographs and CT images and to accurately assess the presence and extent of disease (3).
The major fissure separates the upper and middle lobes from the lower lobe on the right and the upper lobe from the lower lobe on the left. On conventional frontal chest radiographs, Proto and Ball (4) described the major fissure as a curving contour, typically an edge with medial lucency and lateral opacity, along the superior segment of the lower lobe, termed the superolateral major fissure. This curving contour was identified in 14% of patients. The lateral opacity is believed to be due to extrapleural fat; however, pleural thickening or fluid could cause a similar or more exaggerated appearance. Fisher (5) described the following five factors that contribute to visibility of the major fissure on conventional frontal chest radiographs: volume changes causing fissural rotation, an incomplete or variant fissure with fluid, pleural thickening, adjacent consolidation, and variation in fissural course. An early study (6) of CT images of the major fissure found that it is seen most often as an avascular lucent band, less often as a line, and least often as an opaque band. On CT images, the appearance of the fissure is also dependent on section thickness.
The major pulmonary fissure is highly variable and often incomplete. The degree of incompleteness ranges from nearly complete absence to nearly complete presence of the fissure (7–10). An incomplete major fissure is seen more commonly on the right side, and the degree of incompleteness is more variable on this side (9). Glazer et al (8) found an incomplete major fissure on the right in 64% and on the left in 52% of the 50 patients studied. Similarly, Aziz et al (9) reported that in 622 patients, 48% of the right and 43% of the left major fissures were incomplete on at least one CT image. Both of these studies evaluated thin-section CT images of patients' lungs. Otsuji et al (10) reported an incomplete interlobar fissure in 83% of right lungs and 50% of left lungs in a study of CT images of both patient and cadaver lungs. Raasch et al (11) performed a detailed study of fixed and inflated lung specimens (n = 100; 50 right, 50 left) and reported fissural incompleteness in about 70% of cases.
There are several clinical implications of an incomplete major fissure. When there is an incomplete fissure, the lung parenchyma of adjacent lobes fuse and can allow the spread of disease and collateral air drift (4). Disease processes, such as pneumonia, can spread between adjacent lobes via the Kohn pores and the canals of Lambert. Collateral air drift can be clinically important in the setting of lobar bronchial obstruction. The air drift can allow continued aeration of the lobe with the obstructed bronchus via the adjacent unobstructed lobe across an incomplete major fissure. Otsuji et al (10) found that more than 95% of incomplete interlobar fissures are associated with bronchovascular structures traversing the fused segments. Most commonly the structure is a pulmonary vein, and less commonly it is a pulmonary artery or bronchus, which may also influence the clinical course of various lung diseases. Knowledge of the presence of an incomplete fissure can be important in the planning of lobar resection as there is increased risk of air leak in lobar fusion (12).
Pleural effusion is a common clinical entity with a variety of causes. For example, the incomplete fissure sign is commonly seen in critically ill patients who have been imaged in the supine position. Given the prevalence of both pleural effusion and an incomplete fissure, the incomplete fissure sign can be a useful tool in daily practice to avoid confusing aerated lung with a pathologic air collection in the chest.
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FOOTNOTES
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Author stated no financial relationship to disclose.
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References
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- Dandy WE. Incomplete pulmonary interlobar fissure sign. Radiology 1978;128:21–25.[Abstract]
- Hansell DM, Armstrong P, Lynch DA, McAdams HP. Pleural disorders. In: Imaging of diseases of the chest. 4th ed. St. Louis, Mo: Elsevier Mosby, 2005; 1023–1042.
- Proto AV. The chest radiograph: anatomic considerations. Clin Chest Med 1984;5:213–246.[Medline]
- Proto AV, Ball JB Jr. The superolateral major fissures. AJR Am J Roentgenol 1983;140:431–437.[Abstract/Free Full Text]
- Fisher MS. Significance of a visible major fissure on the frontal chest radiograph. AJR Am J Roentgenol 1981;137:577–580.[Abstract/Free Full Text]
- Proto AV, Ball JB Jr. Computed tomography of the major and minor fissures. AJR Am J Roentgenol 1983;140:439–448.[Abstract/Free Full Text]
- Hayashi K, Aziz A, Ashizawa K, Hayashi H, Nagaoki K, Otsuji H. Radiographic and CT appearances of the major fissures. RadioGraphics 2001;21:861–874.[Abstract/Free Full Text]
- Glazer HS, Anderson DJ, DiCroce BS, et al. Anatomy of the major fissure: evaluation with standard and thin-section CT. Radiology 1991;180(3):839–844.[Abstract/Free Full Text]
- Aziz A, Ashizawa K, Nagaoki K, Hayashi K. High resolution CT anatomy of the pulmonary fissures. J Thorac Imaging 2004;19(3):186–191.[CrossRef][Medline]
- Otsuji H, Uchida H, Maeda M, et al. Incomplete interlobar fissures: bronchovascular analysis with CT. Radiology 1993;187:541–546.[Abstract/Free Full Text]
- Raasch BN, Carsky EW, Lane EJ, O'Callaghan JP, Heitzman ER. Radiographic anatomy of the interlobar fissures: a study of 100 specimens. AJR Am J Roentgenol 1982;138:1043–1049.[Abstract/Free Full Text]
- Venuta F, Rendina EA, De Giacomo T, et al. Technique to reduce air leaks after pulmonary lobectomy. Eur J Cardiothorac Surg 1998;13:361–364.[CrossRef][Medline]
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