Cystic collection. Cholesteatoma is caused by a poorly functioning eustachian tube leads from the back of the nose to the middle of ear, allowing air to equalizing ear pressure. Eustachian tube may not work properly due to chronic ear or, sinus infections, colds,etc. Partial vacuum may be middle ear. This can cause a section of your eardrum to be pulled into the middle ear, creating a cyst, that grows larger.
Can be serious. Although I agree with Dr. Mandell, it is important to know that cholesteatoma is a cyst of skin and in addition to being the result of negative pressure and collapse of the upper part of the ear drum (secondary acquired cholesteatoma), they can also be congenital and arise in the middle ear. Finally, the cholesteatoma is a serious disease process and can destroy bone and must be seen by ENT doc.