Illustrated Encyclopedia of Human Anatomic Variation: Opus IV: Organ Systems: Respiratory System

Lungs and Trachea

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed

Variations of the Lobes and Fissures of the Lungs

An accessory inferior lobe is frequently found on the undersurface, extending up onto the medial surface in front of the pulmonary ligament. This lobe may be indicated by a shallow fissures or be clearly defined. It may present a tongue-like projection inward or may comprise the entire medial portion of the base, but usually consists of one-fifth to one-third of the base. The lobe may occur on either side or on both; it is larger and more frequently well-defined on the right, but more often present, or at least indicated, on the left. It was found in 47% of 210 lungs.

Occasionally absence of a fissure marking off the middle lobe has been reported. An irregular fissure may subdivide the left lung into three lobes, and both lungs may further subdivided, especially the right. In one study, a single pair of lungs was divided into 11 clearly defined lobes with fully developed pleural fissures. However, one more frequently finds the commonly described five lobes not separated by fissures.

The most detailed study of the human lung has been performed by Boyden and associates.The brief summary that follows is from some of that work. The lung may show various degrees of developmental arrest: (a) agenesis, absence of one or both lungs; Congenital absence of a lung has an incidence of 0.007% (2/30,000) of autopsies (Ellis,1917) (b) aplasia, formation of rudimentary bronchi: and (c) hypoplasia, incomplete development of the lung. Short trachea has been reported (Wells et. al.) to occur in 36% of patients (31/87) with myelomengocele. It is also reported by Wells et. al. to be associated with a number of syndromes including: DiGeorge anomaly (77%), interrupted aortic arch (83%), hypoplastic left heart complex (63%), congenital cardiovascular disease (25%), clinical brevicollis, including Klippel-Feil and Jarcho-Levin syndromes (57%), skeletal dysplasias, including Ellis-Van Creveld syndrome, Jeune syndrome, osteogenesis imperfecta, achondrogenesis type 2, Kniest syndrome, hypophosphatasia (spur-limbed type), mesomelic dwarfism (Langer type), and short rib-polydactylia (59%), diaplacental rubella (40%), congenital hemidysplasia and ichthyosiform erythroderma with limb defects (CHILD) syndrome, Möbius sequence, Nager acrofacial dyostosis, partial chromosome 3 trisomy, and generalized GM1 gangliosidosis type 1.

Thomas and Boyden reported three cases of agenesis of the right lung with numerous variations of the left lung. In two cases, the left lung had three lobes and in a third it had no lobulation. In addition, there are cases of variations in bronchopulmonary segments, left eparterial bronchus, and vascular anomalies including coarctation of the aorta, persistent ductus arteriosus, and left post-tracheal or bronchial pulmonary artery.

Hypoplasia of only a single lobe or the entire lung has been described. In one case, the right lung appeared to have two lobes, but within the oblique fissure a small bronchus was found to supply a small piece of lung tissue; the bronchus originated at the expected site of the middle bronchus.

Rudimentary lung tissue may be found arising in ectopic positions such as (most commonly) the trachea. Examples of these so-called supernumerary lungs are described by Boyden; they arise from tracheal diverticula, and are not to be confused with displaced lobar bronchi. In one study, five specimens of this type were found in 6,000 autopsies (0.08%).

Another type of supernumerary lung is the Rokitansky lobes, isolated structures, located in the lower thorax. They occur most frequently on the left side, usually above the diaphragm. In one study, in 27 of 41 cases the lobes were located above the diaphragm and in 5 cases below the diaphragm. These isolated masses of lung tissue are usually separated from both the tracheobronchial tree and the pulmonary vascular system. It has been reported that these isolated masses are subject to the same diseases that affect lungs. Anomalous lung tissue has been reported as an outgrowth of the wall of the stomach.

Aeby (1880) first characterized the human lung as asymmetrical because of the presence of a right eparterial bronchus. Boyden noted that in very rare instances, a left eparterial may also occur (five cases, with only one case in 100 consecutive dissections). Although this variation has an incidence of "no more" than 1%, it is of surgical importance that it be recognized when present.

Lobar bronchi may be displaced to a point higher or lower than usual on the primary lung sacs. Cases are known in which the whole right upper lobe bronchus originates immediately above the middle lobe bronchus. The right upper and middle lobe bronchi may arise from a single stem located at the usual site of the middle lobe. The middle lobe bronchus may arise from a normally placed right upper lobe bronchus, either from the main stem or from the proximal part of the anterior segmental bronchus. The right upper lobe bronchus sometimes originates from the right lateral wall of the trachea.

Variations in the patterns of the bronchial trees are, for the most part, due to displacement of segmental and subsegmental bronchi. These variations are important considerations in resection of bronchopulmonary segments. One such variants, which supplies all or a part of the apical segment, arises on the right side of the trachea or from the lateral wall of the right primary bronchus before the origin of the lobar bronchus. Boyden has termed this the "pre-eparterial bronchus." In addition, one of the upper lobe bronchi may originate at a lower level.

The right anterior bronchus, so-called B², may arise from the middle-lobe stem. Boyden has termed this the "postarterial bronchus." Usually the left medial basel (B⁷) and the left anterior basal (B⁸) bronchi arise from a common stem. In 7% of 100 specimens studied, Boyden found that B⁷arises independently, at a higher level on the tree. In the left lower lobe, the posterior ramus of the anterior segment (B^2a) may be absent (35% of cases studied). When present, it occupies the site at which the absence of the left upper lobe most frequently occurs. Other examples and a detailed analysis of variations in the lung can be found in the numerous works of Boyden and co-workers.

The most common variation of the lungs is the presence of supernumerary fissures. These fissures do not always separate segments but may enter subsegmental or interbronchial planes. The most common site for a supernumerary fissure is the plane between the medial basal (B⁷) and anterior basal (B⁸) segments of the right lower lobe. This partially segregates a cardiac or infracardiac lobe. In one study, it was found in 35% of 180 human lungs, and in another study it was found in 38% of 50 injected right lungs. These variations would have been found in surgical exploration and, most of them, in radiologic examinations. A comparable fissure occurs, infrequently, in the left lung. Another common fissure segregates, more or less, the superior segment of the right or left lower lobe (so-called "dorsal lobe of Nelson"). In a study by Dévé it was found bilaterally in 12 cases, in the right lower lobe in 40, and in the left lower lobe in 14. In this study, the separated part was called the "posterior lobe."

Yet another common cleavage is the left horizontal fissure. This subdivides the left upper lobe into two almost equal parts, the lower part being called the "left middle lobe." Four types of left middle lobe have been described by Boyden: (a) a true middle lobe separating normal upper and lower division segments, (b) a compressed lingular, (c) an expanded lingular, and (d) an ectopic pulmonary type, which always separates the sector of an eparterial bronchus from the left upper lobe.

Another common form of lung variation includes the absence of fissures. In a study of 277 lungs, the horizontal fissure was absent in 21% and incomplete in 67%. Incomplete oblique fissures occur in about 30% of both right and left lungs.

Certain segments of the upper and middle lobes have a predilection for disease processes. In the right lung, subsegment B^3a is the site of secondary Assmann's focus of pulmonary tuberculosis. Segment B³on the right side and subsegment B^2a on both the right and left sides are common sites of lung abscesses. Segments B⁴and B⁵are common sites of bronchiectasis. In the lower lobes, the superior segment (B⁶), the subsuperior bronchus (B*), and accessory subsuperior bronchi from B⁹and B¹⁰are common sites of lung abscess.

A small process of the right lung just above the base, behind the termination of the inferior vena cava, rarely becomes isolated as the lobus cava. The vena azygos may be displaced outward, so that, instead of curving over the root of the lung, it makes a deep fissure in the upper part of the right lung, called the azygos lobe. This variation may be recognized radiographically and has the shape of an inverted "teardrop" or a comma. In one study its incidence was about 0.5% (7 of 1400 specimens examined); in another study its incidence was about 0.5% (7 of 1400 specimens examined); in another study of 50,000 roentgen examinations its incidence was about 0.26%. All sources combined (323,641 examinations) indicate the incidence to be 0.57%.

The lobe of the azygos has been known since 1777; it was described by Wrisberg in the cadaver of a 3 year boy. In this first case the lobe appeared bilaterally. At least 11 other cases of left-sided lobes have been reported. The bronchial supply of the azygos lobe has been identified as the apical segment B1a and B3a, the apical rami of the apical and posterior segments. Azygos radiographic image was first seen by Wessler and Jaches. Velde named it the azygos lobe of the lung.

Rarely a segmental bronchus arises from the trachea. An anomalous lobe of the lung may also arise from the esophagus.

The absence of a lung has been reported.

The right lung is usually heavier than the left by about 50 to 100gm. It is also usually shorter and wider.

The trachea usually divides at the level of the fifth or sixth thoracic vertebra and much less commonly at the seventh thoracic.

The shape of the trachea is variable.

In short trachea, the tracheal rings are reduced to 15 or less from a normal 17.01 +/- 1.28 according to Wells, et al.

Variations in Pulmonary Blood Supply

Although vascular variations are catalogued in part 2, the Cardiovascular System, the inclusion of pulmonary vasculature would seem appropriate here as well.

Accessory bronchial arteries may arise from the left aorta (embryologic), subclavian arteries, highest intercostal artery, and the right internal thoracic artery.

Accessory pulmonary arteries may arise from the thoracic aorta below the hilum of the lung and atypically from the derivatives of the embryonic arches. In the 25 recorded cases, they arise from the ascending aorta and the arch of the aorta in 10, from the innominate artery in four, from the right subclavian in two, from the left subclavian in five, and from the decsending aorta below the hilum of each lung in four. The variations are usually associated with malformation of the heart and developmental anomalies of the aortic arch.

The pulmonary veins may also retain connections with the derivatives of the anterior cardinal veins or other embryonic vessels. In one study summarizing 106 cases, the drainage from the lungs passed entirely into the right atrium or its tributaries in 36% and partially in the remaining 64%. Of the 64%, the site of drainage was (in order of frequency) superior vena cava, right atrium, and left innominate vein. Pulmonary venous anomalies occur about twice as frequently on the right as on the left. Other unusual sites of drainage include the coronary sinus, inferior vena cava, azygos vein, left subclavian vein, portal vein, and left persistent superior vena cava.

Variations in the mode of entrance of pulmonary veins into the left atrium are of surgical importance. In one study, in 3% of 148 specimens, the right lung drained into a common right pulmonary vein before emptying into the left atrium. On the left side, the frequency may be as high as 25%. The right pulmonary vein may also be represented by three veins, which arise from the usual three lobes of the right lung and empty into the left atrium. In one reported case, however, the middle lobe of the three veins drained a portion of the right middle lobe and also a part of the lower lobe. One may not assume therefore that each of these three veins is entirely restricted to a particular lobe.

Sappey, Traité d’anat. descript. Tome IV, p. 421, Paris, 1859.
Cruveilheir, Traite d’anat. descript., 5th ed., tome II, p.271, Paris, 1874
Letulle, Bull. et Mem. d. l. Soc. Anat. d. Paris, 1885(2):132-133.

Variations in Peripheral Segmentation of Right Lung

Pulmonary Lobe of Azygos Vein

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