Two Lesser Known but Useful Signs of Neonatal Pneumothorax

Transcription

1 Two Lesser Known but Useful Signs of Neonatal Pneumothorax LEONARD E. SWISCHUK1 Two lesser known signs of neonatal pneumothorax are presented : the large, hyperlucent hemithorax sign and the medial stripe sign. In a review of 113 consecutive cases of proven neonatal pneumothorax, the large, hyperlucent hemithorax was the presenting sign in 29 and the medial stripe in 12 (25% and 11%, respectively). A leftsided preponderance of both signs but especially of the large, hyperlucent hemithorax remained unexplained. The practical significance of the signs, differential diagnosis, and influence of the Mach effect are described. Because anterior pneumothoraxes in infants have a somewhat atypical roentgenographic appearance, they are frequently overlooked. In addition, medial pneumothoraxes are usually misinterpreted as pneumomediastinum or pneumopericardium because of their location. This communication presents two signs associated with anterior and medial pneumothonaxes in infants. The Mach effect and its influence on perception of medial pneumothoraxes is also considered. Material and Results A review of consecutive cases of neonatal pneumothroax was undertaken. Of the cases, 72 (64%) demonstrated the classic patterns of neonatal pneumothorax : 18 were bilateral, 25 on the night, and 29 on the left. The remaining 41 infants (36%) demonstrated one or the other of the signs described in this communication. To qualify for these latter categories none of the classic signs of pneumothorax could be present. Twenty-nine (25%) of the infants demonstrated the large, hypenlucent hemithorax sign, 22 of which were on the left. Twelve (1 1 %) demonstrated the medial stripe sign : six on the left, four on the right. and two bilateral. Discussion Neonatal pneumothorax is a well known condition ; its diagnosis is usually readily established with chest roentgenognams [1-5]. However, when the pneumothorax is of small volume on less than classic configuration, it can elude the observer. This is especially true when films are obtained with the infant in supine position. In this regard, pneumothoraxes collecting over the anterior aspect of the lung or along its medial border are of special significance [6, 7]. Those lying anterior to the lung frequently present with the large, hyperlucent hemithorax sign, while those lying medial to the lung present with the medial stripe sign. Large, Hyperlucent Hem/thorax Sign This sign results from free air accumulating over the anterior aspect of the lung of an infant examined in the supine position. When such collections extend around the lateral aspect of the lung, they are not difficult to recognize since the free edge of the lung is readily visible. With lesser volumes of air, however, the free lateral edge of the lung is not seen and the hyperlucent hemithorax can be totally overlooked, misinterpreted, or simply dismissed as being due to an artefact of positioning. Further complicating matters in these cases is the fact that the mediastinum is not usually grossly displaced, and the involved hemidiaphragm is only slightly depressed (fig. 1). Increased lucency of the involved hemithorax results from the presence of extra air in the pleural space. The increased size of the hemithorax, albeit subtle in many cases, is a manifestation of net volume gain. In other words, the partially aerated lung and the air trapped in the pleural space occupy more volume than is normally available. Consequently the hemithorax enlarges. There is contralateral mediastinal shift and depression of the ipsilateral diaphragmatic leaflet ; overall, a large, hyperlucent hemithorax results. The degree of enlargement and hyperlucency depends upon the volume of free air present, and both findings are accentuated if the chest roentgenogram is obtamed in partial or complete expiration. Once a pneumothorax is suspected because of the large, hyperlucent hemithorax sign, a cross-table lateral [8] or regular lateral view should be obtained to confirm its presence. The cross-table lateral view is useful in demonstrating the free air layered over the anterior surface of the lung, but such air can also often be seen on regular lateral views (fig. 1 ). In those cases where uncertainty persists, a decubitus film with the involved side up is helpful, since the free air then tends to rise and outline the free edge of the lung. Anterior pneumothoraxes must be differentiated from pneumomediastinal air collections. On frontal view, there is usually little difficulty in differentiating the two since the typical central collections of air in the superior mediastinum, outlining or elevating the thymus gland, are characteristic of pneumomediastinum. On lateral view some confusion may arise. However, pneumomediastinal air on this view is almost always confined to the upper part of the retrosternal space, while anterior pneumothoraxes tend to extend down to the lower retrosternal space (fig. 1). In addition to the large, hyperlucent hemithorax sign it is important to note that the ipsilateral mediastinal edge appears unusually sharp or crisp in these infants (fig. 1 ). This is a subtle but very useful finding which results from the fact that free air, instead of aerated lung, is located next to the mediastinal structures. When normal aerated lung abuts these structures, the bronchovascular structures in the lung Presented as part of a scientific exhibit at the annual meeting of the American Roentgen Ray Society, San Francisco, September Departments of Radiology and Pediatrics, University of Texas Medical Branch, Galveston, Texas Am J Roentgenol 127: ,

2 - :.Th -:, -, Fig. 1 -Large, hyperlucent hemithorax sign with sharp ipsilateral mediastinal edge showing varying degrees of abnormality. A, Supine view showing left hemithorax hyperlucent compared to right ; left mediastinal edge sharper and crisper than on right. B, Standard lateral view showing large volume of air layered over anterior surface of left lung (arrows). C, Supine view of another infant with hyperlucent left hemithorax and increased sharpness of left mediastinal edge. Findings could be misinterpreted as due to rotation to left. D. Standard lateral film confirming presence of air anterior to left lung (arrows). E, Supine view showing more subtle changes in third infant whose chest roentgenogram was obtained during expiration (note emptied right lung). There is mediastinal shift to right and left mediastinal edge is sharper than right. F, Standard lateral view confirming presence of large volume of air anterior to left lung (arrows).

3 . NEONATAL PNEUMOTHORAX 625 Fig. 2.-Cloaking of the lung sign at various locations. A. Supine view of infant with respiratory distress. Presence of bilateral pneumothonaxes could be overlooked, especially on right. On left, radiolucent strip along left cardiac border could be misinterpreted as pneumomediastinal air. This is medial stripe sign of medial pneumothorax. B, Diagrammatic representation of free air locations seen in part A. 1, Over apex of lung; 2. along base of lung anteriorly; 3, along base of lung posteriorly in posterior costophrenic sulcus; and 4, along medial edge of lung. produce a slight blurring of the mediastinal-pulmonary interphase, resulting in less sharpness of the mediastinal edge. When free air is present, the lung and its bronchovascular markings are displaced posteriorly, and the mediastinal edge becomes more clearly defined. Unusual sharpness of the mediastinal edge can also be seen with pneumomediastinum and, in some cases, where the lung itself is overdistended. Consequently some caution must be exercised in interpreting this sign. Nevertheless it remains a useful adjunctive finding which can further heighten suspicion about the presence of free air in the pleural space. It is especially useful with bilateral anterior pneumothoraxes. If a chest tube is inserted to decompress the pneumothorax, it is important that it be placed anteriorly. If it remains posterior, it is unlikely to completely decompress the anterior pneumothorax. Medial Stripe or Medial Pneumothorax Sign This sign has recently been discussed [6]. We previously called attention to it as part of the cloaking of the lung phenomena [7] seen in some cases of pneumothorax. Briefly stated, cloaking of the lung refers to free air accumulating around the lung (fig. 2). However, it is the medial collection of free air, especially when seen alone, that is most subject to misinterpretation. In most cases it is misdiagnosed as a pneumomediastinum or pneumopenicardium [6, 7]. A clue to the fact that such air represents a pneumothorax is that it is unilateral and extends in virtual parallel fashion along the entire mediastinal edge (fig. 3). Indeed, it frequently extends to the inferiormost aspect of the car- Fig. 3.-Medial stripe sign of medial pneumothorax. Note the following: wide stripe of free air along medial aspect of left lung (arrows) ; ipsilateral mediastinal edge sharper than that on right ; free air extending along the entire mediastinum, down to infeniormost aspect of cardiac silhouette ; and some free air along inferior aspect of left lung. diac silhouette. This does not usually occur with pneumomediastinal air collections, even large ones. However, even if pneumomediastinal air should extend down to the infeniormost aspect of the cardiac silhouette, it seldom remains unilateral in its distribution. In most cases the air will also be seen on the other side or over the top of the heart. In this latter position it usually elevates or completely sur-

4 626 SWISCH U K Fig. 4.-Mach effect erroneously suggesting presence of medial pneumothoraxes in infant with hyaline membrane disease and poor inspinatory effort. A, Vague halo of radiolucency (more pronounced on left) surrounding heart suggesting bilateral medial pneumothoraxes. B, With cardiac silhouette blocked out, radiolucent halo disappears indicating no pneumothoraxes. Findings confirmed by densitometer which showed no increase in radiolucency around cardiac edge.. _i. 4# =.srr-- - Fig. 5.-Mach effect accentuating perception of medial pneumothorax in infant. A, Supine view showing stripe of free air paralleling mediastinal structures from top to bottom and some free air over apex of left lung. Ipsilateral edge of mediastinum sharper than on right. B. With cardiac silhouette blocked out. radiolucency of medial stripe sign markedly diminished. Patient had proven pneumothorax on left ; presence of free air along left cardiac border confirmed with densitometer. rounds the thymus gland. mediastinal or pneumopericardial air, such shifting and Air in the pericardial sac is less of a problem since in the rising does not occur. neonate such collections are usually of large volume, and the heart becomes completely surrounded by air. In addi- Mach Effect tion, the pericardium itself is visualized. Nonetheless, if con- In view of the recent interest in the Mach effect [9], its fusion persists, a lateral decubitus view with the involved influence on perception and enhancement of a medial side up becomes useful. If the air represents that of a medial pneumothorax should be considered. The Mach effect ocpneumothorax, it will shift and rise to the top. With pneumo- curs when two nearly equal densities are viewed in juxta-

5 NEONATAL PNEUMOTHORAX 627 position (e.g., the heart and a collapsed or infiltrated lung). In such cases the interface between them seems to become more radiolucent, and a medial pneumothorax is suggested (fig. 4A). However, by covering the cardiac silhouette with a piece of black paper, it can be demonstrated that none is present (fig. 48). The apparent medial pneumothorax disappears completely. This optical illusion occurs because of normal retinal blocking of that portion of the image constituting the interface between the two densities. The net result is a radiolucent halo or stripe around the heart. A densitometer will show that no radiolucent stripe and, hence, no pneumothorax exists. If a small pneumothorax is present and a true medial stripe sign is visualized, the Mach effect can enhance its perception to a surprising degree (fig. 5). The enhancement is more pronounced when the stripe is thinner, or, in other words, when the pneumothorax is smaller. This can be confirmed by blocking out the cardiac silhouette in figures 3 and 5 and comparing the enhancement of the pneumothorax; enhancement is more pronounced in figure 5 where the pneumothorax is smaller. REFERENCES 1. Chassler CN : Pneumothorax and pneumomediastinum in the newborn. Am J Roentgenol 91 : , Chernick V. Avery ME : Spontaneous alveolar rupture in newborn infants. Pediatrics 52 : , Emery JL : Interstitial emphysema, pneumothorax, and airblock in the newborn. Lancet 1 : , Steel RW, Metz JR. Bass JW, DuBois JJ : Pneumothorax and pneumomediastinum in the newborn. Radiology 98 : , Strouji MN : Pneumothorax and pneumomediastinum in the first three days of life. J Pediatr Surg 2 : , Moskowitz PS, Griscom NT: The medial pneumothorax. Radiology. In press, Swischuk LE, Fagan CJ: Newborn chest findings: uncommon findings in common disease. Scientific exhibit presented at the annual meeting of the American Roentgen Ray Society, San Francisco, September MacEwan DW, Dunbar JS, Smith RD, St J Brown B: Pneumothorax in young infants: recognition and evaluation.j CanAssoc Radiol 22: , Lane EJ, Proto AV, Phillips TW: Density perception. Scientific exhibit presented at the annual meeting of the Radiologic Society of North America, Chicago, December 1975