CONGENITAL CHOLESTEATOMA AND THE EPIDERMOID FORMATION
by Mark J. Levenson, M.D., F.A.C.S.
The epidermoid formation (EF) is a well-organized cell grouping, found in the forming middle ear of the fetus. It is usually present from the 10th to the 33rd week of pregenancy. Sometmes it is present in the temporal bones of the newly born, even up to six months after birth. Its position is consistent, relative to the insertion of the forming ear drum, as well as the wall of the eustachian tube.
Its size can be as large as 1.4 millimeters in diameter (10 of these millimeters do NOT even equal 1/2 inch). Most are much smaller, measuring less than .2 of a millimeter. EF is not a rare occurrence; EFs are found in as many as 40% of the temporal bones of all unborn fetuses. The EF may serve as a development organizer, since it is located where the cells of the first branchial groove meet the cells of the first branchial pouch.
Under normal circumstances, the EF disappears late in fetal development. It may regress spontaneously, but in some cases, it appears to bud off from its mucosal bed and then is normally swept down the eustachian tube. In some rare cases, the EF becomes lodged in mesenchyme or in the mucosal folds of the front-top compartment of the middle ear instead of being propelled down the eustachian tube.
This migration of the EF can be limited by the tensor tympani tendon and the mucosal folds. The EF is composed of large, eosinophillic hyaline-like cells. A keratin cap is often noted on its outer surface. Some epidermoid formations are very large (1.4mm), close in size to the smallest clinically identified congenital cholesteatomas. It is reasonable to hypothesize that the EF may firmly implant or become trapped in the mucosal folds of residual mesenchyme of the front compartment of the middle ear, becoming a nidus for a congenital cholesteatoma on rare occasions.
Further, surgical observations lead us to the opinion that at least some so-called "acquired" cholesteatomas may actually have their origins as congenital cholesteatomas. When generally removed, small and mid-sized congenitial cholesteatomas demonstrate completely normal middle ear mucosa at surgery.
When the CG does rupture because of its increasing size, the middle ear mucosa is invariably inflamed by the keratin debris in contact with it, despite the intact tympanic membrane. As the CG enlarges, it also becomes thinner and atrophies the overlying tympanic membrane.
In this setting, even a mild bout of otitis media would be likely to result in tympanic membrane rupture and perforation. This would accelerate the growth and proliferation of the CG rapidly, until at surgery, the lesion becomes indistinguishable form an acquired cholesteatoma.
In summary, it appears that the EF outlives its usefulness as an organizing entity and is gone in later fetal development. Either it regresses, or it is expelled down the eustachian tube. However, it can remain in the front compartment of the middle ear and become lodged in mucosal folds of mesenchyme. Since a clear potential exists for keratin production, the EF may serve as a nidus for the formation of a congenital cholesteatoma in this compartment.
Finally, based on the pattern of presentation we see at surgery, some of the cholesteatomas previously considered as "acquired" are most likely congenital in origin.
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