Keratin-filled cysts that grow medial to the tympanic membrane are considered to be congenital if they fulfill the following criteria: mass medial to the tympanic membrane,
normal tympanic membrane, no previous history of ear discharge, perforation or ear surgery.
Congenital cholesteatomas occur at three important sites: the middle ear, the Petrous apex, and the cerebropontine angle. They are most often found deep to the anterior aspect of the ear drum, and a vestigial structure, the epidermoid formation, from which congenital cholesteatoma may originate, has been identified in this area.
There was an interesting study on the middle ear volume, and it ended up being about 1.4 ml in non diseased individuals. In reality, you rarely get a full ml of fluid into the middle ear when you inject it. There is a number of reasons we put medications directly into the middle ear, such as Meneir’s Disease, sudden sensorineural hearing loss, etc… We are typically using an anti inflammatory such as dexamethasone or an ablative medication such as Gentamycin. Sometimes we combine the agents. When I use the Dexamethasone, I do no dilute, and I put in as much as the middle ear will handle. I typically anesthetize the ear with phenol or EMLA and have patient stay laying in the injection position without swallowing for 20 minutes if possible.