The epidermoid formation (EF) is a well-organized cell grouping in the human fetus, regularly present from 10 to 33 weeks of gestation, and less often present in neonatal temporal bones up to six months after birth. The position of the EF is consistently in relationship to the anterior bony annulus, insertion of the forming tympanic membrane, and lateral wall of the eustachian tube at the junction of the ciliated and cuboidal epithelia of the eustachian tube and middle ear respectively. The EF has a mean of 220 m (0.22mm), but can range up to 1400 m (1.4mm) in diameter. The EF is clearly not an artifact, and can be regularly found in about 40% of fetal temporal bones. The EF occurs within the anterior superior compartment of the middle ear bone bordered anteriorly by the eustachian tube, posteriorly by the tensor tympanic tendon, laterally by the malleus and tympanic membrane, and medially by the tensor tympani semicanal and middle ear floor.
The EF may serve as a developmental 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 by late fetal development. It may regress spontaneously, but in some cases, it appears to be budding off from its mucosal bed and, more probably, is normally swept down the eustachian tube. However, it is not difficult to imagine that in some rare cases the EF, instead of being propelled down the eustachian tube, becomes lodged in mesenchyme, or in the mucosal folds in the anterior superior compartment of the middle ear. The posterior migration of the EF would be limited by the tensor tympani tendon and the mucosal folds as described by Proctor bounding this space.
The EF is composed of large, eosinophillic hyaline-like cells and 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. In fact, McGill reported on a 6-week old child with a congenital cholesteatoma of nearly 3mm in size. It is reasonable to hypothesize that on rare occasions the EF may firmly implant or become trapped in the mucosal folds or residual mesenchyme of the anterior superior compartment of the middle ear, becoming a nidus for a congenital cholesteatoma.
Furthermore, one of us (MJL), based on surgical observations, is of the opinion that at least some, so-called acquired cholesteatomas may actually have their origin as congential cholesteatomas. Small and mid-sized congenitial cholesteatomas, when removed, generally demonstrate completely normal middle ear mucosa at surgery. When the congenital cholesteatoma ruptures with increasing size, the middle ear mucosa is invariably inflamed by the keratin debris in contact with it, despite the intact tympanic membrane. As the congenital cholesteatoma enlarges, it also thins 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 subsequent perforation, rapidly accelerating the growth and proliferation of the congenital cholesteatoma until, at surgery, the lesion becomes indistinguishable form an acquired cholesteatoma.
In summary, it appears that the EF outlives its organizing function in later fetal life and either regresses, or is expelled down the eustachian tube. However, it may remain in the anterior superior compartment of the middle ear and become lodged in mucosal folds or 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. Furthermore, based on the pattern of presentation of congenital cholesteatomas at surgery, at least some cholesteatomas previously considered as acquired are most likely congenital in origin.
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