Frequent S100 and PAX8 expression in otitis media: Unexpected findings and potential sources of diagnostic confusion in middle ear pathology.

The middle ear is an anatomic region that many pathologists may not feel comfortable reviewing. A variety of neoplastic and non-neoplastic lesions may occur in this anatomically complex space, but specimens are uncommon so the full spectrum of alterations remains obscure. This study was prompted by anecdotally encountered cases of unexpected S100 and PAX8 expression in middle ear specimens which led to diagnostic confusion. Twelve cases of otitis media were studied. In each case there was no clinical or radiological suspicion for neoplasia. Immunohistochemistry for S100 and PAX8 were performed on each case, with SOX10 and CD1a also done on a subset. Varying degrees of S100 immunoexpression was seen in the middle ear stroma of 12 cases, while the same cells were negative for SOX10 (0 of 11) and CD1a (0 of 3) in all cases tested. PAX8 was positive in the cuboidal middle ear epithelium in 11 of 11 tested cases. Expression was seen both in the lining surface components as well as in areas of glandular metaplasia. S100 is unexpectedly positive in middle ear stroma, a finding which can lead to diagnostic confusion, particularly for nerve sheath tumors. PAX8, on the other hand, is unexpectedly positive in middle ear epithelium, a pitfall for diagnosing various neoplasms, especially endolymphatic sac tumor of the inner ear. Awareness of these immunoexpression patterns will help avoid misdiagnoses. In particularly difficult cases, additional immunohistochemistry and clinical/radiographic correlation can clarify the diagnosis.