1. Malfunction of the eustachian tube: The eustachian tube is the only tube that communicates between the middle ear and the external environment. The eustachian tube has the functions of regulating the air pressure inside the tympanic chamber, maintaining the balance of air pressure with the outside world, cleaning and defending, and preventing sound. It is now believed that several other malfunctions of the eustachian tube in addition to the sound prevention function may be an important cause of secretory otitis media. (1) Eustachian tube obstruction: Under normal circumstances, the air pressure inside and outside the middle ear is basically equal. When the eustachian tube becomes dysfunctional due to various reasons, the gas in the middle ear is absorbed by the mucosa, resulting in negative pressure in the middle ear, which leads to dilatation of the veins of the middle ear mucosa, increased permeability, and leakage of serum to accumulate in the middle ear, thus forming middle ear effusion. Special infections such as tuberculosis, syphilis and AIDS can cause the disease due to direct compression, blockage of the pharyngeal opening or affecting lymphatic return, and swelling of the mucosa in the lumen of the eustachian tube. Non-mechanical obstruction: In children, the muscles of the palatofantoid muscle, palatofantoid muscle and pharyngeal muscle are weak and contracted, and the cartilage of the eustachian tube is not mature enough and less elastic. Bacterial infection causes a decrease in the active material on the surface of the eustachian tube, resulting in increased resistance to opening the eustachian tube, which is also considered to be one of the causes of secretory otitis. (2) Cleaning function: The “mucus cilia transport system” of the eustachian tube may not properly discharge secretions, pathogenic microorganisms, and toxins from the middle ear and the canal cavity due to bacterial exotoxins, radiation damage, immature development of the eustachian tube in infants and children, congenital respiratory mucosal cilia dyskinesia, and primary cilia dyskinesia. This leads to otitis media. (3) Defensive dysfunction: Poor one-way activation of the mucosal folds of the eustachian tube due to various reasons such as the passage of connective tissue in the elderly, reduced elasticity of the elastic fibers below the mucosa of the eustachian tube, and scar traction at the pharyngeal opening of the eustachian tube may cause otitis media. 2.Infection: It is currently believed that secretory otitis media may be related to low virulence bacterial infection or endotoxin produced by bacteria, viral infection and incomplete treatment of bacterial infection. Immune reaction: Type I and III allergic reactions may cause secretory otitis media, which may be related to allergy-induced edema of the mucosa of the eustachian tube and obstruction of the lumen.