Serous otitis media - from Merck Manual

Serous otitis media is fluid accumulation behind the eardrum .

Serous otitis media often occurs after acute otitis media. The fluid that has accumulated behind the eardrum during the acute infection remains after the infection resolves. Serous otitis media may also occur without preceding infection, and may be due to gastroesophageal reflux disease or a blockage of the eustachian tube by infection or enlarged adenoids. Serous otitis media is extremely common in children between the ages of 3 months and 3 years.
Although serous otitis media is painless, the fluid can impair hearing, understanding of speech, language development, learning, and behavior.
Doctors diagnose serous otitis media by looking for changes in the color and appearance of the eardrum and by squeezing air into the ear to see if the eardrum moves. If the eardrum does not move but there is no redness or bulging and the child has few symptoms, then serous otitis media is likely.
Serous otitis media often does not improve when treated with antibiotics or other drugs, such as decongestants, antihistamines, or nasal sprays. The condition often resolves by itself after weeks or months.
If the condition persists without improvement after 3 months, surgery may help. In the United States, doctors perform myringotomy, in which they make a tiny slit in the eardrum, remove the fluid, and insert a small ventilating (tympanostomy) tube in the slit to provide drainage from the middle to the outer ear. Some doctors may perform a myringotomy to remove fluid but not to insert ventilating tubes; this procedure is called tympanocentesis.