Atelectasis

Japanese: 無気肺

Definition/Concept Atelectasis is the English word for atelectasis, where ateles comes from the Greek word for incomplete and ektasis means stretching. It refers to a decrease in lung volume caused by a decrease in air in the alveoli. Atelectasis and collapse of lung are used almost synonymously, but collapse is used to refer to complete atelectasis.
Classification, causes, and pathogenesisAtelectasis is classified into four types based on the pathogenesis.
1) Resorption atelectasis:
This is the most common type of atelectasis, which occurs when air is reduced in the peripheral airways and alveoli due to bronchial obstruction. It is also called obstructive atelectasis. Depending on the location of obstruction, it can be total lung, lobar, or segmental, but atelectasis is less likely to occur distal to the segmental bronchi because of the presence of airway collaterals such as Kohn's foramen between the alveoli. Causes of obstruction include mucus plugs, foreign bodies, malignant tumors (lung cancer, especially squamous cell carcinoma), benign tumors, bronchial tuberculosis, swollen lymph nodes around the bronchi, and cardiomegaly. In cases where lung collapse progresses over time, such as in lung cancer, complete collapse is prevented by mucus plugs, inflammatory cells, exudate, and fibrosis. This is called obstructive pneumonitis.
2) Relaxation atelectasis:
It occurs when the lung adjacent to an occupying lesion in the thoracic cavity, such as pleural effusion, pneumothorax, lung tumor, or bulla, is compressed and the volume decreases. It is also called compression atelectasis or passive atelectasis.
Gravity dependent atelectasis appears as an unclearly bordered increase in density (dependent opacity), a subpleural line shadow just below the pleura, or discoid atelectasis. It is commonly seen in smokers and obese people, and can cause respiratory failure during and after surgery. The alveoli in the weight-bearing area are small, and alveolar collapse occurs due to surfactant damage and other factors. It can be differentiated from pulmonary parenchymal lesions by comparing the supine and prone positions.
Round atelectasis (also known as rounded atelectasis or folded lung) is characterized by localized pleural thickening and the appearance of a peripheral mass shadow just below the chest wall. Structural distortions such as an arc-shaped convergence of blood vessels and bronchi (comet tail sign) are observed. It often occurs when lung collapse persists despite the disappearance of pleural effusion. It is seen in conditions such as asbestos exposure, infectious pleural effusion, congestive heart failure, pulmonary infarction, malignant disease, and after cardiac surgery. In many cases, there is no change for several years.
3) Adhesive atelectasis:
Surfactant (a surface-active substance) maintains low surface tension, increases lung compliance, and prevents collapse, so its deficiency or inactivation can cause alveolar collapse. This is seen in neonatal respiratory distress syndrome (IRDS), acute respiratory distress syndrome (ARDS), acute radiation pneumonitis, pulmonary thromboembolism, and pneumonia.
4) Cicatrization atelectasis:
This refers to localized or diffuse volume loss due to fibrosis and destruction of the lung parenchyma. It is typically caused by chronic infection, and the internal bronchi of granulomatous disease or old pulmonary tuberculosis are dilated due to the surrounding fibrosis, which is called traction bronchiectasis. Shift of the mediastinum and hilum to the affected side, and compensatory hyperinflation of the surrounding lungs are observed.
Clinical symptoms, test results , and clinical findings vary depending on the underlying disease and its stage. Chief complaints include fever, cough, bloody sputum, dyspnea due to airway narrowing, and wheezing. Physical findings include decreased breath sounds and dullness on percussion in unilateral atelectasis. In lobar atelectasis, the other lungs expand in compensation, making it difficult to detect decreased breath sounds.
Diagnosis is made using chest X-rays and CT scans. Direct findings are due to the reduction in air content within the bronchi and alveoli, while indirect findings are changes other than a reduction in air content (Table 7-7-1). Atelectasis in each lobe has its own characteristics. Atelectasis in the upper lobes differs between the left and right, while atelectasis in the lower lobes follows the same pattern on both sides (Figure 7-7-1). Atelectasis in the entire lung is often caused by complete obstruction of the main bronchus. The healthy lung becomes hyperinflated and the entire mediastinum shifts towards the affected side (Figure 7-7-2).
1) Atelectasis of the right upper lobe:
The upper and lower interlobar fissures of the minor fissures on the frontal and lateral views and the major fissures on the lateral view are elevated. When atelectasis of the upper lobe occurs, a triangle extending upward from near the highest point of the mediastinal diaphragmatic surface is seen, which is called the juxtaphrenic peak (Figure 7-7-3).
2) Atelectasis of the left upper lobe:
Unlike the right upper lobe, there are no interlobular fissures, and the greater interlobular fissure is displaced anteriorly toward the mediastinum. Because atelectasis is displaced toward the anterior mediastinum, the silhouette sign on the left side of the cardiac shadow is positive in a frontal view. The hyperinflated upper part of the lower lobe appears crescent-shaped, so this is called the Luftsichel sign (air crescent). On a chest X-ray, atelectasis of the entire left upper lobe and atelectasis of only the upper segment show similar findings (Figure 7-7-4).
3) Atelectasis of the right middle lobe:
Since atelectasis is adjacent to the right atrium, the silhouette sign on the right side of the cardiac shadow is a clue. On a lateral view, the greater interlobar fissure, middle lobe and lower interlobar fissure move upward, and the lesser interlobular fissure moves downward (Figure 7-7-5).
4) Atelectasis of the lower lobes:
On a lateral view, atelectasis in the lower lobes on both the left and right sides is displaced inferiorly in the upper and lower interlobar fissures, and posteriorly in the middle interlobar fissure in the lower lobe. On a frontal view, the greater interlobar fissure is seen as a clear line extending from the hilum to the inferior and lateral side. On a frontal view, the greater interlobar fissure is displaced inferiorly toward the mediastinum (Figures 7-7-5, 7-7-6).
Differential diagnosis and treatment: Do not overlook the pattern of atelectasis on chest X-rays. If atelectasis is suspected, sputum tests, chest CT scans, bronchoscopy, etc. are performed to thoroughly investigate the cause. If it is obstructive, tumors, foreign bodies, swollen lymph nodes, mucus plugs, bronchial tuberculosis, etc. are suspected. Mucus plugs or foreign bodies are removed using a bronchoscope. If the cause is other than these, the underlying disease is treated. [Kuwano Kazuyoshi]
■ References
Fraser RG, Pare PD, et al: Diagnosis of Diseases of the Chest, 4th ed, pp513-560, WB Saunders, Philadelphia, 1999.
K. Hayashi: Atelectasis. Chest X-ray Diagnosis (K. Hayashi and H. Nakata, eds.), pp105-115, Shujunsha, Tokyo, 2002.

Table 7-7-1
Imaging findings of atelectasis ">

Table 7-7-1

Figure 7-7-1
Atelectasis patterns in each lobe

Figure 7-7-1

Source : Internal Medicine, 10th Edition About Internal Medicine, 10th Edition Information

Japanese:

定義・概念
　無気肺は英語でatelectasisとよぶが，atelesはギリシャ語でincomplete，ektasisはstretchingの意味である．肺胞内の空気が減少したことによって肺容積が減少することをいう．無気肺と肺虚脱（collapse of lung）はほぼ同意語で使用されるが，虚脱は完全な無気肺に対して使用される．
分類・原因・発症機序
　無気肺は発症機序によって4つに分類される．
1）吸収性無気肺（resorption atelectasis）：
気管支閉塞によって末梢気道および肺胞の含気が減少することによって生じる最も多い無気肺である．閉塞性無気肺（obstructive atelectasis）ともいう．閉塞部位によって全肺，肺葉性，区域性となるが，区域気管支より末梢には肺胞間にKohn孔などの気道側副路が存在しているため無気肺は生じにくい．閉塞の原因として粘液栓，異物，悪性腫瘍（肺癌，特に扁平上皮癌），良性腫瘍，気管支結核，気管支周囲の腫脹したリンパ節，心拡大などがある．肺癌のように時間をかけて肺の虚脱が進行する場合は粘液栓や炎症細胞，浸出液，線維化によって完全な虚脱に至らない．これが閉塞性肺炎（obstructive pneumonitis）である．
2）弛緩性無気肺（relaxation atelectasis）：
胸水，気胸，肺腫瘍，ブラなど胸腔内の占拠性病変に接する肺が圧迫され容積が減少することによって生じる．圧迫性無気肺（compression atelectasis），受動性無気肺（passive atelectasis）ともいう．
　荷重部無気肺（gravity dependent atelectasis）は，境界不明瞭な濃度上昇（dependent opacity）または胸膜直下の線状影（subpleural line），板状無気肺として認める．喫煙者，肥満者に認めやすく，手術中後の呼吸不全の原因ともなる．荷重部の肺胞は小さく，表面活性物質の障害などが加わり肺胞虚脱が生じる．仰臥位と腹臥位を比較することで肺実質病変と鑑別が可能である．
　円形無気肺（round atelectasis，rounded atelectasis， folded lung）は，局所的な胸膜肥厚を伴い胸壁直下に末梢性の腫瘤影を呈する．血管，気管支の円弧状の集束（comet tail sign）など構造のねじれが認められる．胸水消失にもかかわらず肺の虚脱が残存したために生じることが多い．アスベスト暴露，感染性の胸水，うっ血性心不全，肺梗塞，悪性疾患，心臓手術後などで認められる．多くは数年間にわたり変化がない．
3）癒着性無気肺（adhesive atelectasis）：
サーファクタント（界面活性物質）は維持表面張力を低く抑え，肺のコンプライアンスを増加させ虚脱を防いでいるため，その不足や不活性化によって肺胞の虚脱が生じる．新生児の呼吸促迫症候群（IRDS），急性呼吸促迫症候群（ARDS），急性放射性肺炎，肺血栓塞栓症，肺炎などにみられる．
4）瘢痕性無気肺（cicatrization atelectasis）：
肺実質の線維化と破壊による限局性またはびまん性の容積減少をいう．慢性感染症によるものが典型的であり，肉芽腫性疾患や陳旧性肺結核の内部の気管支は，周囲の線維化によって拡張しており，牽引性気管支拡張（traction bronchiectasis）とよばれる．縦隔や肺門の患側への偏位，周囲の肺の代償性過膨張が認められる．
臨床症状・検査成績
　臨床所見は，原因疾患，およびそれらの病期によって異なる．主訴は，発熱，咳，血痰，気道狭窄による呼吸困難，喘鳴などがある．身体所見としては，片肺の無気肺であれば呼吸音の減弱と打診にて濁音を認める．肺葉性の無気肺ではほかの肺葉が代償性に膨張するため呼吸音の減弱は認めにくい．
診断
　胸部X線写真とCTによって診断する．直接所見は，気管支や肺胞内の含気の減少そのものによる所見であり，間接所見は含気の減少以外の変化である（表7-7-1）．各肺葉の無気肺には特徴がある．上葉の無気肺は左右で異なり，下葉の無気肺は左右同じパターンである（図7-7-1）．片肺全部の無気肺は主気管支の完全閉塞が原因であることが多い．健常肺は過膨張となり縦隔全体が患側へ偏位する（図7-7-2）．
1）右上葉の無気肺：
正面および側面像における小葉間裂（minor fissure）と側面像における大葉間裂（major fissure）の上葉下葉間裂が拳上する．上葉の無気肺が生じると，縦隔側横隔膜面の最上点付近より上方へ伸びる三角形が認められ，傍横隔膜尖頭（juxtaphrenic peak）という（図7-7-3）．
2）左上葉の無気肺：
右上葉と異なり小葉間裂がなく，大葉間裂が前方縦隔側へ偏位する．無気肺は前縦隔へ偏位するため正面像では心陰影左側のシルエットサインが陽性となる．過膨張した下葉上部は三日月様にみえることからLuftsichelサイン（空気の三日月）とよばれる．胸部X線写真では，左上葉全体の無気肺と上区のみの無気肺は同様の所見を示す（図7-7-4）．
3）右中葉の無気肺：
無気肺が右心房に接するため，心陰影右側のシルエットサインが手掛かりである．側面像では，大葉間裂の中葉下葉間裂が上方へ，小葉間裂が下方へ移動する（図7-7-5）．
4）下葉の無気肺：
側面像では，下葉の無気肺は左右共に，大葉間裂の上葉下葉間裂は下方へ，下葉中葉間裂は後方へ偏位する．正面像では大葉間裂が肺門から下方側面へ伸びる明瞭な線として認められる．正面像では大葉間裂は縦隔側下方へ偏位する（図7-7-5，7-7-6）．
鑑別診断・治療
　胸部X線写真での無気肺のパターンを見逃さないことである．無気肺が疑われれば，原因の精査のために，喀痰検査，胸部CT，気管支鏡など行う．閉塞性であれば，腫瘍，異物，リンパ節腫脹，粘液栓，気管支結核などを疑う．粘液栓や異物であれば気管支鏡を用いて除去する．その他の原因では，原疾患の治療を行う．［桑野和善］
■文献
Fraser RG, Pare PD, et al: Diagnosis of Diseases of the Chest, 4th ed, pp513-560, WB Saunders, Philadelphia, 1999.
林　邦昭：無気肺．胸部単純X線診断（林　邦昭，中田　肇　編），pp105-115, 秀潤社，東京，2002.

表7-7-1
無気肺の画像所見">

表7-7-1

図7-7-1
各肺葉の無気肺のパターン">

図7-7-1

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