Coughing in children is a defensive reflex movement that stops the aspiration of foreign bodies, prevents the accumulation of bronchial secretions, and removes secretions to avoid secondary respiratory tract infections. Cough can be caused by acute or chronic inflammation of the respiratory tract from any etiology. According to the course of the disease, it can be divided into acute cough (2 weeks), subacute cough (2 weeks to 4 weeks) and chronic cough (>4 weeks). The following is a discussion of the etiology and diagnosis of chronic cough in children: I. Respiratory infections and post-infection cough: Respiratory infections caused by many pathogenic microorganisms such as Mycobacterium pertussis, Mycobacterium tuberculosis, viruses (especially respiratory syncytial virus, parainfluenza virus, and cytomegalic inclusion virus), Mycoplasma pneumoniae, and Chlamydia are common causes of chronic cough in children, mostly in <5year-old preschoolers. Acute respiratory tract 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 squamous metaplasia of ciliated columnar epithelial cells and/or persistent airway inflammation with temporary airway hyperresponsiveness due to the infection. In recent years, Mycoplasma pneumoniae infection has shown a yearly increase, with cough being the prominent symptom of the disease. One of the characteristics of the disease is that the signs and symptoms are inconsistent with clinical manifestations such as severe cough and fever. The physical examination is only pharyngeal congestion, coarse breath sounds in both lungs, normal chest radiographs or only enhanced lung texture, and poor results with macrolides; therefore, mycoplasma infection should first be highly suspected, and clinical serum Mycoplasma pneumoniae antibody testing can clarify the diagnosis. Second, cough variant asthma (CVA): CVA is one of the common causes of chronic cough in children, especially in preschool and school-age children. i clinically no signs of infection or after a longer period of antibiotic treatment is ineffective, as chronic cough is mostly due to viral infection, respiratory allergy and repeated irritation of the airways by chemical irritation, often without wheezing, and wheezing sounds are mostly inaudible in the lungs and easily misdiagnosed. Therefore, in such children, detailed questions should be asked about the nature and pattern of the cough, the trigger for the attack, and the personal and family history of allergies. If necessary, a mutagen test and pulmonary function tests should be performed. If pulmonary ventilation is normal and bronchial excitation tests indicate airway hyperresponsiveness, bronchodilators may be used for diagnostic treatment if highly suspected. If treatment can provide significant relief of cough symptoms, the patient should be treated as asthma. History of allergic disease including drug allergy, and positive family history of allergic disease. Positive allergen testing may aid in the diagnosis. C. 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 cough. There is pressure pain in the sinus area, yellowish-white discharge from the opening of the sinus, obvious hyperplasia of the follicles in the posterior pharyngeal wall, cobblestone-like, and sometimes there is secretion attached to the posterior pharyngeal wall. If the sinusitis is caused by sinusitis, the corresponding changes can be seen in sinus x-ray or CT film. Targeted treatment such as antihistamines and leukotriene receptor antagonists, nasal glucocorticoids are effective. Gastroesophageal reflux cough: Gastroesophageal reflux is a physiological phenomenon in infancy and early childhood. The incidence is 40% to 65% in healthy infants and children, peaking at 1 to 4 months, with more natural relief at age 1. It becomes a disease when it causes symptoms and/or is accompanied by gastroesophageal dysfunction, i.e., GERD. The clinical features and diagnostic clues of the disease are: paroxysmal, sometimes violent cough, mostly at night; symptoms mostly appear after eating and drinking and feeding difficulties. Some children have epigastric or subxiphoid discomfort, retrosternal burning, chest pain, and sore throat; in addition to causing cough, infants may also suffer from asphyxia, bradycardia, and an arched back; it can lead to stagnant or delayed growth in children. V. Eosinophilic bronchitis: the clinical features and diagnostic clues of this disease include chronic irritant cough, normal chest radiograph and pulmonary ventilation, and no airway hyperresponsiveness; importantly, the relative percentage of eosinophils in the sputum of such children is >3%; oral or inhalation glucocorticoid therapy is effective. Sixth, congenital respiratory disease: mainly seen in infants and children, especially within 1 year of age. These include congenital tracheoesophageal fistula, congenital vascular malformation compressing the airway, laryngotracheobronchial softening and/or stenosis, bronchopulmonary cysts, ciliary dyskinesia, and mediastinal tumors. VII. Psychogenic cough: The clinical features and diagnostic clues of psychogenic cough are: prevalence in older children; predominance of inter-H cough, which disappears when focusing on an event or resting at night; often accompanied by symptoms of anxiety; no organic disease, and other causes of chronic cough are excluded. VIII. Other etiologies: (1) Foreign body aspiration: cough is the most common symptom after aspiration of foreign bodies in the airway, and foreign body aspiration is an important cause of chronic cough in children, especially those aged 1-3 years. The cough usually manifests as paroxysmal violent choking cough, or it may only manifest as chronic cough with obstructive emphysema or pulmonary atelectasis. Once the foreign body enters the area below the small bronchus, there can be no cough, which is called the “silent zone”. I have diagnosed a case of sunflower seed aspiration for 5 years, and the cough disappeared after the foreign body was removed. A detailed history was taken of the child’s accidental aspiration at about 10 months of age while playing, so medical workers were reminded to pay close attention to this case. (2) Drug-induced cough: Angiotensin-converting enzyme inhibitors (ACEIs) are less commonly used in children, and some children with renal hypertension have cough induced by the use of ACEIs such as captopril. The mechanism may be related to bradykinin, prostaglandin, and substance P secretion. It usually presents as a chronic persistent dry cough, which is aggravated at night or when lying down, and can be significantly reduced or even disappeared by stopping the drug for 3-7 d. Adrenergic receptor blocking agents such as Tretinoin can cause bronchial hyperresponsiveness, so they may also lead to drug-induced cough. (3) Otogenic cough: 2% to 4% of the population have vagal ear branches. In this group, when the middle ear becomes diseased, the vagus nerve is stimulated and causes a chronic cough. Otogenic cough is a rare cause of chronic cough in children. The etiology of chronic cough in children is complex and varies by age. Chronic cough in infants under 1 year of age is predominantly post-inflammatory cough of the respiratory tract and inadvertent aspiration of breast milk in 57.58% of cases. The reasons for this are related to the young age of the infant, incomplete development of the immune system, low resistance and easy co-infection; another reason is that the infant’s brain is not yet well developed at this stage, and the swallowing function is not coordinated, which can easily cause breast milk misabsorption. Chronic cough in children aged 1 to 3 years is mainly caused by foreign bodies in the trachea and bronchi and coughing after inflammation of the respiratory tract (68.80%). At this stage, children are very active, crying, laughing, running or mistakenly inhaling foreign bodies in the mouth into the trachea and bronchi after external stimulation. With the role of breathing, the foreign body will fall into the distal bronchi, and cause choking, coughing, bruising, breathing difficulties, etc.. If the foreign body enters the trachea and is not detected and removed in time, it causes secondary infection with fever and cough, similar to bronchitis, pneumonia, or lung abscess; therefore, for unexplained chronic cough in children of this age, clinicians need to consider this condition, repeatedly take a medical history, and perform bronchoscopy and relevant imaging examinations. Respiratory tract infections are also an important cause of clinical chronic cough, manifested by recurrent infections, recurrent episodes of cough with an irritating dry cough or with a small amount of white mucous sputum, no abnormalities on chest radiographs, and normal lung function. The cause is thought to be the persistence of airway hyperresponsiveness promoted by airway damage and immunopathological changes that have not yet recovered from respiratory infections such as viral, bacterial or mycoplasma infections. Cough variant asthma and upper airway cough syndrome predominate in children older than 3 years of age with chronic cough (94.28%). Cough variant asthma, also known as cough variant asthma, was first reported and named variant asthma by Gluser in 1972. Cough variant asthma is a specific type of asthma in which chronic cough is the predominant or only clinical manifestation. In the early stages of asthma onset, about 5-6% have a persistent cough as the main symptom, mostly occurring at night or in the early morning, often irritating and often misdiagnosed as bronchitis. Cough variant asthma is a form of asthma with the same pathophysiological changes as bronchial asthma, with a persistent airway inflammatory response and airway hyperresponsiveness. Clinical manifestations include: ① persistent cough or recurrent attacks for more than 1 month, mostly at night or in the early morning, aggravated by exercise, and sputum; ② laboratory or other tests showing no obvious signs of infection or ineffective after long-term antibiotic treatment; ③ bronchodilators can reduce symptoms or reduce attacks; ④ clear history of allergy, eczema, urticaria, allergic rhinitis, etc., family history of allergy; ⑤ exercise, cold air, allergens or viral infection can trigger an asthma attack; 7) no obvious organic changes on chest radiograph. Upper airway cough syndrome (UACS): that is, postnasal drip syndrome (PNDS) refers to the chronic inflammatory state of the nasal cavity and sinuses in which the purulent secretions produced by the inflammatory site flow backwards into the nasopharynx, oropharynx, and hypopharynx through the posterior nostril. The long-term stimulation of such purulent secretions causes secondary inflammation and related symptoms in the above-mentioned areas, which is often one of the root causes of chronic clinical cough. However, it has been reported that at least 20% of patients do not feel postnasal drip, which makes the diagnosis difficult. Common diseases causing UACS include chronic rhinitis, chronic sinusitis, chronic rhinopharyngitis, and allergic factors. In pediatrics, the main causes are allergic rhinitis, sinusitis, and adenoidal inflammation. Our guidelines for the diagnosis and treatment of cough recommend the following diagnostic criteria for UACS: ① episodic or persistent cough with a predominantly daytime cough and less coughing after sleep; ② a postnasal drip and/or a sense of mucus adhesion to the posterior pharyngeal wall; ③ a history of rhinitis, sinusitis, nasal polyps or chronic pharyngitis; ④ examination reveals mucus adhesion and cobblestone-like changes in the posterior pharyngeal wall; ⑤ cough relief after targeted treatment. Clinicians should take this disease into full consideration when seeing children in the sub-age group, pay attention to pharyngeal symptoms and nasopharyngeal examination, and make a correct etiologic diagnosis.3.2 Other causes of chronic cough Other causes of chronic cough include gastroesophageal reflux, congenital developmental abnormalities, allergic, drug and psychogenic cough, passive smoking, and thymic hypertrophy. The 1998 cough guidelines issued by the American College of Chest Physicians (APPC) clearly state that gastroesophageal reflux (GER) is the most common cause of chronic cough, along with postnasal drip syndrome (PNDS) and cough variant asthma (CVA). Patients with cough caused by GER often present with paroxysmal cough, mostly at night, accompanied by nocturnal discomfort in the upper abdomen and subxiphoid process, burning sensation behind the sternum, chest pain and pharyngitis, or simply with a chronic cough that does not resolve. Psychogenic cough, also known as psychogenic cough, manifests as heavy daytime and light nighttime cough symptoms that can be relieved with psychological treatment. 23% of chronic coughs have been reported to have psychogenic causes, which requires clinicians to pay attention to this cause. 2006 U.S. Cough Diagnosis and Management Guidelines recommend replacing PNDS with upper airway cough syndrome (UACS), which is one of the common causes of chronic cough. In addition to nasal disease, upper airway cough syndrome UACS is often associated with diseases of the pharynx, larynx, and tonsils, such as allergic or non-allergic pharyngitis, chronic tonsillitis, and laryngitis. In conclusion, chronic cough is a common pediatric symptom with complex etiology. Clinicians should fully grasp the etiology of chronic cough, follow the diagnosis and procedures provided in the guidelines for diagnosis and treatment of chronic cough in children in China, diagnose it meticulously, analyze the causes according to cough characteristics, ancillary tests, and response to treatment, and achieve the right treatment for the cause.