cough

Japanese: 咳

Concept/Mechanism: Cough is one of the most common chief complaints of patients visiting general practitioners. Coughing is a type of respiratory movement in which the vocal cords are momentarily closed after deep inspiration, and then the glottis is opened when airway pressure has risen sufficiently, resulting in explosive exhalation. The mechanism by which coughing occurs is shown in Figure 2-35-1. In most cases, it occurs reflexively, but it can also be performed consciously by command from the cerebral cortex. It is essentially an important biological defense symptom that eliminates foreign objects in the airway, inhaled harmful substances, phlegm, etc. The airway clearing effect of coughing is largely dependent on the expiratory flow rate. Causes of reduced expiratory flow rate during coughing include central or peripheral nerve disorders that control the contraction of the expiratory muscles, muscle disorders, chest wall pain, and abdominal muscle pain. In addition, in asthma, the airway clearing effect of coughing is inhibited by airway spasm and airway obstruction due to thick sputum, while in emphysema and cartilage abnormalities of the trachea and bronchi, the airway becomes narrower than normal when breathing out (dynamic compression).
Complications Excessive coughing can cause problems. Each cough consumes approximately 2 kcal of energy, and if it continues for a long period of time, it can be physically draining. It can also cause severe chest pain due to damage to the intercostal and abdominal muscles and rib fractures. It can also put strain on the vocal cords, causing sore throat and hoarseness. Increased abdominal pressure can also induce incontinence and vomiting. Intrathoracic pressure can rise to a maximum of 100-300 mmHg, reducing blood return to the heart, causing temporary hypotension and fainting (cough syncope). High intrathoracic pressure causes air to enter the interstitium from the periphery of the lungs, causing pneumomediastinum and eventually subcutaneous emphysema. Coughing is generally more frequent at night, causing sleep disorders.
Causes Most respiratory diseases can cause cough. If abnormal findings are found on a chest X-ray or CT scan, it is a priority to investigate whether this is related to cough. However, in routine medical practice, there are many cases where no shadows are found. The main causes of acute cough, which improves naturally within about three weeks, are acute bronchitis caused by various bacteria, viruses, mycoplasma, and chlamydia. These are also important causes of persistent cough, which lasts for about three to eight weeks. The main causes of chronic cough that lasts for more than eight weeks are sinus-bronchial syndrome, cough asthma, atopic cough, gastroesophageal reflux disease, and chronic bronchitis.
A cough that is accompanied by phlegm is called a productive cough, while one that is not is called a dry cough. Typical diseases that cause wet cough include chronic bronchitis, diffuse panbronchiolitis, and bronchiectasis. Typical diseases that cause dry cough include cough asthma, atopic cough, interstitial pneumonia, cough caused by angiotensin-converting enzyme inhibitors, and psychogenic cough.
Treatment Cough is often a very painful symptom for patients. It is necessary to appropriately suppress it with cough suppressants and bronchodilators. However, in the case of wet cough, the presence of phlegm is one of the causes of coughing, so excessive cough suppression may promote the accumulation of phlegm and may actually worsen the condition. The same is true for cough caused by silent aspiration of saliva in elderly people. [Yamaguchi Etsuro]

Figure 2-35-1
Mechanism of coughing

Figure 2-35-1

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

Japanese:

概念・機序
　咳は家庭医を訪れる患者の主訴の中で最も多いものの1つである．咳とは深吸気後に一瞬声帯を閉じて気道内圧が十分上昇したところで声門を開放することにより，爆発的に息を吐き出す一種の呼吸運動である．咳の発生機序を図2-35-1に示す．ほとんどの場合反射的に発生するが，大脳皮質の指令により，意識的に行うこともできる．本来は気道内異物，吸入性有害物質，喀痰などを排除する重要な生体防御症状である．咳の気道清浄化作用は，呼気流速に大きく依存する．咳の呼気流速が低下する原因には，呼気筋の収縮を制御する中枢ないし末梢神経疾患，筋疾患や，胸壁痛，腹筋痛などがある．また喘息では気道攣縮や粘稠な喀痰による気道閉塞により，肺気腫や気管・気管支の軟骨異常症では，呼気時に通常より気道が狭細化する（動的圧縮）ことにより，やはり咳の気道浄化作用が阻害される．
合併症
　咳は過剰となると障害をもたらす．すなわち1回の咳でおよそ2 kcalのエネルギーを消費し，長期に続くと体力が消耗する．また肋間筋や腹筋の障害や肋骨骨折により強度の胸痛の原因となる．声帯に負担がかかり，喉頭痛や嗄声の原因となる．腹圧の上昇のために，失禁や嘔吐を誘発することもある．胸腔内圧は最大100～300 mmHgまで上昇するため，心臓への血液還流が低下し，一過性の低血圧が生じて失神に至る咳失神（cough syncope）が発生することがある．高い胸腔内圧のために，肺末梢から間質に空気が入り込み，縦隔気腫ひいては皮下気腫の原因となる．咳は一般に夜間に多く，睡眠障害の原因となる．
原因
　ほとんどの呼吸器疾患が咳の原因となる．胸部X線写真やCTで異常所見が認められるなら，それと咳との関連を検討することが優先される．しかし日常診療では陰影を認めない例も多い．そのうち3週間程度で自然軽快する急性咳嗽のおもな原因は，各種の細菌，ウイルス，マイコプラズマ，クラミジアによる急性気管支炎である．これらは3～8週間程度持続する遷延性咳嗽の原因としても重要である．8週間以上続く慢性咳嗽のおもな原因疾患は，副鼻腔気管支症候群，咳喘息，アトピー咳嗽，胃食道逆流症，慢性気管支炎などである．
　痰を伴う咳は湿性咳嗽（productive cough），伴わないものを乾性咳嗽（dry cough）とよぶ．湿性咳嗽をきたす代表的疾患として慢性気管支炎，びまん性汎細気管支炎，気管支拡張症がある．乾性咳嗽をきたす代表的疾患には，咳喘息，アトピー咳嗽，間質性肺炎，アンジオテンシン転換酵素阻害薬による咳，心因性咳などがある．
治療
　咳は患者にとってしばしば大変つらい症状である．鎮咳薬や気管支拡張薬により適宜抑制する必要がある．しかし，湿性咳嗽では痰の存在が咳の一因であるので，過度な咳の抑制は痰の貯留を助長し，むしろ病態を悪化させる可能性がある．高齢者の唾液の不顕性吸引による咳の場合も同様である．［山口悦郎］

図2-35-1
咳の発生機序">

図2-35-1

出典　内科学第10版内科学第10版について　情報

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