Definition: A cough with symptoms lasting more than 4 weeks is called a chronic cough. The following 1, 2 and 3 are common causes of chronic cough in children 1. Respiratory infections and post-infectious cough: Respiratory infections caused by many pathogenic microorganisms such as Mycobacterium pertussis, Mycobacterium tuberculosis, viruses (especially respiratory syncytial virus, parainfluenza virus, cytomegalic inclusion virus), Mycoplasma pneumoniae, and Chlamydia are common causes of chronic cough in children, mostly in preschool children <5 years old. Acute respiratory infections with cough symptoms lasting more than 4 weeks can be considered post-infectious cough. The mechanism may be disruption of airway epithelial integrity and/or squamification of ciliated columnar epithelial cells and/or persistent airway inflammation with temporary airway hyperresponsiveness as a result of the infection. Clinical features and diagnostic clues for post-infectious cough are (1) a recent history of definite respiratory infection, (2) an irritating dry cough or a small amount of white mucous sputum, (3) no abnormalities on chest radiography, (4) normal pulmonary ventilation, (5) a cough that is usually self-limiting, and (6) except for other causes of chronic cough. If the cough lasts longer than 8 weeks, other diagnoses should be considered. 2. Cough variant asthma (CVA): Cough variant asthma, also known as allergic cough, is one of the most common causes of chronic cough in children, with cough as the only or main manifestation, not accompanied by significant wheezing. The clinical features and diagnostic clues of cough variant asthma are: (1) cough lasting >4 weeks, often with episodes or exacerbations at night and/or early morning, with a predominantly dry cough; (2) no clinical signs of infection or ineffective with prolonged antibiotic therapy; (3) effective diagnostic treatment with anti-asthmatic drugs; (4) exclusion of other causes of chronic cough; (5) positive bronchial excitation test and/or PEF daily variability (continuous monitoring for 1 to 2 weeks) ≥ 20%; (6) personal history of atopic diseases such as eczema, allergic rhinitis or asthma, or positive allergen test, either personally or in first- or second-degree relatives. Items 1 to 4 above are the basic conditions for diagnosis. 3. Upper airway cough syndrome (UACS): Various rhinitis (allergic and non-allergic), sinusitis, chronic pharyngitis, chronic tonsillitis, nasal polyps, adenoid hypertrophy and other upper airway diseases can cause chronic coughing, which was previously diagnosed as postnasal drip (flow) syndrome, referring to coughing caused by nasal secretions flowing backwards through the postnasal orifice to the pharynx. The name “postnasal drip syndrome” is now usually replaced by “upper airway cough syndrome”. The clinical features and diagnostic clues of upper airway cough syndrome include: (1) chronic cough with or without sputum, which is worse in the early morning or when the position is changed, often accompanied by nasal congestion, runny nose, dry throat with foreign body sensation, repeated clearing of the throat, and a feeling of mucus adhesion to the posterior pharyngeal wall, with a few children complaining of headache, dizziness, and low fever; (2) examination of the sinus area may have pressure pain, and there may be yellowish-white secretions flowing from the sinus openings, and the follicles of the posterior pharyngeal wall (3) Targeted treatment such as antihistamines and leukotriene receptor antagonists, and nasal glucocorticoids are effective; (4) If the sinusitis is caused by sinusitis, the corresponding changes can be seen on sinus x-ray plain film or CT film. 4. Gastroesophageal reflux cough (GERC): GERC is a physiological phenomenon in infancy and early childhood. The incidence of GERC in healthy infants is 40% to 65%, peaking at 1 to 4 months of age and mostly resolving spontaneously at 1 year of age. It becomes a disease when it causes symptoms and/or is accompanied by gastroesophageal dysfunction, i.e., gastroesophageal reflux disease. The prevalence of GERD in children is about 15%. The clinical features and diagnostic clues of GERD are (1) paroxysmal cough, sometimes severe, mostly at night; (2) symptoms mostly appear after eating and drinking and feeding difficulties. Some children have epigastric or subxiphoid discomfort, retrosternal burning sensation, chest pain, and sore throat; (3) in addition to causing cough, infants may also suffer from asphyxia, bradycardia, and an arched back; (4) it may lead to arrested or delayed growth of the child. 5. Eosinophilic bronchitis (EB): Eosinophilic bronchitis was first reported by Gibso in 1989, and a recent prospective study revealed that eosinophilic bronchitis accounts for 13.5% of chronic cough patients in adults. Eosinophilic bronchitis is considered an important cause of chronic cough in adults, but its prevalence in children is unclear [E/B]. Clinical features and diagnostic clues of eosinophilic bronchitis are (1) chronic irritant cough; (2) normal chest X-ray; (3) normal pulmonary ventilation without airway hyperresponsiveness; (4) relative percentage of eosinophils in sputum > 3%; and (5) effective oral or inhaled glucocorticoid therapy. 6, congenital respiratory disease: mainly seen in infants and children, especially within 1 year of age. Gormley’s study reported that 75% of children with tracheal softening (second only to congenital vascular malformation) had persistent cough, and the mechanism may be related to tracheal softening obstructing the discharge of secretions and inflammatory damage to the terminal bronchi. The mechanism may be related to the obstruction of secretion drainage by softened airways and inflammatory damage to the terminal bronchi. This condition is often misdiagnosed as asthma. 7. Psychogenic cough: Psychogenic cough in children can only be diagnosed when tic disorders are excluded and the cough improves after behavioral interventions or psychotherapy; cough characteristics are only suggestive of psychogenic cough and do not have a diagnostic role. The clinical features and diagnostic clues of psychogenic cough are: (1) it is more common in older children; (2) the cough is predominantly daytime and disappears when focusing on an event or resting at night; (3) it is often accompanied by anxiety symptoms; and (4) it is not accompanied by organic disease and other causes of chronic cough are excluded. 8. Other etiologies: (1) Foreign body aspiration: coughing is the most common symptom after aspiration of foreign bodies from the airway, and foreign body aspiration is an important cause of chronic cough in children, especially those 1-3 years old. Studies have found that 70% of patients with airway foreign body aspiration present with cough, and other symptoms include decreased breath sounds, wheezing, and history of asphyxia. Once the foreign body enters the area below the small bronchus, there can be no cough, i.e. the so-called “silent zone”. (2) Drug-induced cough: Angiotensin-converting enzyme inhibitors are less commonly used in children, and some children with renal hypertension may have a cough induced by the use of such drugs as captopril. The mechanism may be related to bradykinin, prostaglandin, and substance P secretion. The cough is usually chronic and persistent, aggravated at night or when lying down, and can be significantly reduced or even disappeared after 3-7 d of drug withdrawal. β-adrenergic receptor blockers, such as benztropine, can cause bronchial hyperresponsiveness, so they may also lead to drug-induced cough. (3) Otogenic cough: 2-4% of the population has an auricular branch of the vagus nerve (arnold nerve). In this group, when the middle ear is diseased, the vagus nerve is stimulated and causes chronic coughing. Otogenic cough is a rare cause of chronic cough in children.