Otitis Media with Effusion (OME): This condition is characterized by the accumulation of fluid in the middle ear without signs or symptoms of acute infection. It is a common cause of fluid behind the ear, especially in children, and can follow an upper respiratory tract infection.

Acute Otitis Media: An infection of the middle ear that can cause fluid accumulation and is often accompanied by symptoms such as ear pain, fever, and hearing loss.
Eustachian Tube Dysfunction: The Eustachian tube connects the middle ear to the back of the nose and throat. Dysfunction of this tube can lead to fluid accumulation in the middle ear.
Allergic Rhinitis: Allergies can cause Eustachian tube dysfunction, leading to fluid buildup behind the ear.

Mastoiditis: An infection of the mastoid bone, which is located behind the ear. It can be a complication of untreated otitis media and requires prompt medical attention.
Brain Abscess: Although rare, a brain abscess can be a life-threatening complication of otitis media or mastoiditis, especially if the infection spreads to the brain.
Lateral Sinus Thrombosis: A rare but serious complication of mastoiditis where a blood clot forms in the lateral sinus, potentially leading to severe neurological consequences.

Cholesteatoma: A type of skin cyst located in the middle ear that can be destructive to the surrounding bone and tissue, potentially leading to complications such as hearing loss, vertigo, and neurological problems.
Tumors (e.g., Acoustic Neuroma): Rarely, fluid behind the ear can be a symptom of a tumor affecting the ear or the nerves surrounding it.
Labyrinthitis: An inner ear disorder characterized by inflammation of the part of the inner ear responsible for balance and hearing, which can lead to symptoms including vertigo and hearing loss.