Keywordstemporal bone, cholesteatoma, middle ear, external auditory canal
A cholesteatoma is a cystic mass filled with keratin and lined by stratified squamous epithelium. Cholesteatoma is not a neoplasm and can be thought of most simply as skin in the wrong place. Although a cholesteatoma is histologically identical to an epidermoid or epidermal inclusion cyst, the term “cholesteatoma” is preferred for a lesion located in the middle or external ear ( Fig. 42.1 ).
Cholesteatomas may be either congenital (~2%) or acquired (~98%). Acquired middle ear cholesteatomas are further subcategorized into those associated with the pars flaccida (more than 80%) of the tympanic membrane and those associated with the pars tensa (<20%). Although the pathogenesis of acquired cholesteatoma remains controversial, the most widely accepted theory is the “retraction pocket theory,” which states that dysfunction of the eustachian tube leads to the buildup of negative pressure in the middle ear such that a segment of the tympanic membrane (most commonly the pars flaccida) is drawn into the middle ear and becomes sequestered, followed by accumulation of the trapped keratin.
Histologically, cholesteatoma is divided into three layers—the cyst contents, the matrix, and the perimatrix. Keratinous debris represents the primary component of the central cyst, and it is the central cystic contents that give the lesion its mass. The central cyst is surrounded by the matrix layer, which consists of stratified squamous epithelium. Although not neoplastic, this squamous epithelium is hyperproliferative, and the keratinized matrix is commonly colonized by bacterial biofilm. The perimatrix is the outermost layer and consists of granulation tissue containing collagen, fibrocytes, and inflammatory cells such as histiocytes and neutrophils.
Osseous erosion ( Fig. 42.2 ) is the hallmark of cholesteatoma and occurs primarily as the result of osteoclastic activity. Additionally, pressure effects promoting osseous remodeling and enzymatic activity of the perimatrix may play a role. Osteoclastic activity is believed to be upregulated by proinflammatory cytokines released by the perimatrix inflammatory cells in response to the presence of bacterial biofilm and/or existing inflammation. In addition to stimulating osteoclastic bone resorption, these cytokines also stimulate further squamous proliferation, feeding a vicious cycle of cholesteatoma enlargement and further osseous erosion.
The symptoms and complications of cholesteatoma are primarily related to the associated osseous erosive changes. The ossicles are most commonly eroded, resulting in conductive hearing loss. As the cholesteatoma progresses, the facial nerve canal, tegmen, and lateral semicircular canal may become involved, which can result in facial paralysis, cephalocele, and labyrinthine fistula, respectively.
The treatment for cholesteatoma is complete surgical removal. If any squamous epithelium is left behind or introduced at the time of surgery, the cholesteatoma will recur.
Temporal Evolution: Overview
The skin is continuously regenerating and is estimated to turn over every 40 to 60 days. Normally, the replaced skin cells are shed to the environment. When skin fragments are confined to a space, such as the middle ear, the normal process of regeneration may continue, but the dead cells and keratin cannot be shed. As a result, the natural course of cholesteatoma is to slowly enlarge with time, progressively filling the space in which it is confined. Left untreated, cholesteatoma will fill the middle ear and mastoid in a predictable manner based on (1) the site of origin and (2) the path of least resistance.
Temporal Evolution: in Greater Depth
Pars flaccida cholesteatomas begin in Prussak space ( Fig. 42.3 ), which is bordered by the pars flaccida laterally, the neck of the malleus medially, the lateral process of the malleus inferiorly, the lateral malleal ligament superiorly, and the anterior malleal ligament anteriorly.
Enlarging cholesteatomas follow characteristic paths of least resistance determined by boundary-forming mucosal folds, ossicular suspensory ligaments, and the ossicles themselves ( Fig. 42.4 ). There is no posterior boundary to Prussak space, and enlarging cholesteatomas most commonly first extend posteriorly. Upon reaching the posterolateral wall of the attic, further expansion of the cholesteatoma is deflected superiorly toward the aditus ad antrum and mastoid antrum. As the cholesteatoma fills the aditus ad antrum, the adjacent lateral semicircular canal is at risk. Erosion of the bony covering of the lateral semicircular canal results in a labyrinthine fistula ( Fig. 42.5 ). This complication occurs in 5% to 10% of patients with cholesteatoma and must be recognized and reported by the radiologist, as its presence alters both the operative plan and the prognosis.
As the cholesteatoma enlarges, it may extend inferiorly from Prussak space between the tympanic membrane and the posterior malleal fold to reach the posterior mesotympanum, typically filling the oval window niche, round window niche, sinus tympani, and facial recess ( Fig. 42.6 ). As the cholesteatoma fills the oval window, involvement of the adjacent tympanic segment of the facial nerve by cholesteatoma may result in facial paralysis. Additionally, the bony covering of the tympanic segment of the facial nerve is frequently congenitally dehiscent. If such a dehiscence is not recognized and reported prior to surgical removal of an adjacent cholesteatoma, iatrogenic facial paralysis may occur.