<unk> s occur in two basic classifications: Acquired<unk> s, which are more common, are usually caused by pathological alteration of the ear drum leading to accumulation of keratin within the middle ear. Congenital<unk> s are usually middle ear epidermal cysts that are identified deep within an intact ear drum. Congenital<unk>  Keratin-filled cysts that grow medial to the tympanic membrane are considered to be congenital if they fulfill the following criteria (Levenson's criteria): * mass medial to the tympanic membrane * normal tympanic membrane * no previous history of ear discharge, perforation or ear surgery Congenital<unk> s occur at three important sites: the middle ear, the Petrous apex, and the cerebropontinio 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<unk>  may originate, has been identified in this area. Not all middle ear epidermal cysts are congenital, as they can be acquired either by metaplasia of the middle ear mucosa or by traumatic implantation of ear canal or tympanic membrane skin. In addition,<unk>  inadvertently left by a surgeon usually regrows as an epidermal cyst. Some authors have also suggested hereditary factors. Acquired<unk>  More commonly, keratin accumulates in a pouch of tympanic membrane which extends into the middle ear space. This abnormal folding or 'retraction' of the tympanic membrane arises in one of the following ways: *Wittmaack's theory : Invagination of tympanic membrane from the attic or part of pars tensa in the form of retraction pockets lead to the formation of<unk> . *Ruedi's theory : The basal cells of germinal layer of skin proliferate under the influence of infection and lay down keratinising squamous epithelium. *Habermann's theory: The epithelium from the meatus or outer drum surface grows into the middle ear through a pre-existing perforation and form<unk> .<unk>  may also arise as a result of metaplasia of the middle ear mucosa or implantation following trauma.
Cholesteatoma
Orbital cellulitis
Orbital lymphoma
Odontogenic keratocyst
Tympanosclerosis
Pseudolymphoma
Periorbital cellulitis
Rectovestibular fistula