PhiliPPine Journal of otolaryngology-head and neck Surgery Vol. 36 no. 1 January – June 2021 PhiliPPine Journal of otolaryngology-head and neck Surgery Vol. 36 no. 1 January – June 2021 PhiliPPine Journal of otolaryngology-head and neck Surgery 6564 PhiliPPine Journal of otolaryngology-head and neck Surgery FROM THE VIEWBOX Philipp J Otolaryngol Head Neck Surg 2021; 36 (1): 65-66 c Philippine Society of Otolaryngology – Head and Neck Surgery, Inc. Fenestral Otosclerosis: A Subtle Lesion Easily Missed Nathaniel W. Yang, MD Department of Otolaryngology-Head and Neck Surgery College of Medicine - Philippine General Hospital University of the Philippines Manila Department of Otolaryngology -Head and Neck Surgery Far Eastern University - Nicanor Reyes Medical Foundation Institute of Medicine Correspondence: Dr. Nathaniel W. Yang Department of Otolaryngology - Head and Neck Surgery Ward 10, Philippine General Hospital University of the Philippines Manila Taft Avenue, Ermita, Manila 1000 Philippines Phone: (632) 8526 4360 Fax: (632) 8525 5444 Email: nwyang@up.edu.ph The author declared that this represents original material, that the manuscript has been read and approved by the author, that the requirements for authorship have been met by the author, and that the author believes that the manuscript represents honest work. Disclosures: The author signed a disclosure that there are no financial or other (including personal) relationships, intellectual passion, political or religious beliefs, and institutional affiliations that might lead to a conflict of interest. A 29-year-old Filipina of Chinese descent presented with progressive bilateral conductive hearing loss of several years’ duration. While working overseas, she consulted with an otolaryngologist and underwent computerized tomographic (CT) imaging of the temporal bone as part of her evaluation. She was informed that no abnormalities were identified in the imaging exam, and she was offered exploratory middle ear surgery with possible stapes surgery. She then sought a second opinion, with the intention of obtaining a more definitive diagnosis prior to any invasive medical intervention. A review of the CT imaging study, with particular emphasis on looking for radiologic evidence of otosclerosis, revealed the presence of a focal region of bone demineralization in the region of the fissula ante fenestram. (Figure 1) This finding is consistent with a diagnosis of fenestral otosclerosis. Creative Commons (CC BY-NC-ND 4.0) Attribution - NonCommercial - NoDerivatives 4.0 International Figure 1. Axial CT image at the level of the stapes footplate / oval window, using a bone window setting for temporal bone imaging (WW 4000, WL 1000). A focal area of demineralization can be identified in the area where the fissula ante fenestram (FAF) is located. The landmarks for this area include the cochleariform process (CP), from which the tensor tympani tendon arises to attach to the malleus (MH); and the oval window, where the thin bone of the stapes footplate is located (FP/OW). PhiliPPine Journal of otolaryngology-head and neck Surgery Vol. 36 no. 1 January – June 2021 PhiliPPine Journal of otolaryngology-head and neck Surgery Vol. 36 no. 1 January – June 2021 PhiliPPine Journal of otolaryngology-head and neck Surgery 6766 PhiliPPine Journal of otolaryngology-head and neck Surgery FROM THE VIEWBOX Otosclerosis is one of the main differential diagnoses for a patient presenting with bilateral conductive hearing loss and no other visible evidence of otologic disease. Although it is more common in the Caucasian population,1 it must remain as one of the considerations in the Asiatic population, including Filipinos. High-resolution CT is the imaging technique of choice in the evaluation of conductive hearing loss.2 When evaluating a scan for evidence of otosclerosis, it must be remembered that the most common location of involvement is the bone just anterior to the oval window, in a small cleft known as the fissula ante fenestram. It is this relationship that gives rise to the term fenestral otosclerosis. The fissula is a thin fold of connective tissue Figure 2. Axial CT image of a normal temporal bone at the same level and bone window setting for comparison, showing the dense bone in the region of the fissula ante fenestram (FAF). The density of this region normally matches that of the otic capsule that surrounds the cochlea. extending through the endochondral layer, located in the region between the oval window and the cochleariform process, where the tensor tympani tendon turns laterally toward the malleus.3 (Figure 2) Since the average length of the stapes footplate along its short axis is around 1.5 mm, it is highly recommended that submillimeter image slice thickness be routinely ordered for the CT imaging study, in order to maximize the opportunity to identify the oftentimes small and subtle areas of focal demineralization. At a slice thickness of 0.5 mm, such a lesion might only be identified by an astute clinician in 2-3 sequential axial imaging slices. REFERENCES 1. Altmann F, Glasgold A, Macduff JP. The incidence of otosclerosis as related to race and sex. Ann Otol Rhinol Laryngol. 1967 Jun;76(2):377-392. DOI:10.1177/000348946707600207. PubMed PMID: 6028664. 2. Curtin HD. Imaging of conductive hearing loss with a normal tympanic membrane. AJR Am J Roentgenol. 2016 Jan;206(1):49-56. DOI: 10.2214/AJR.15.15060. PubMed PMID: 26491893. 3. Lee TC, Aviv RI, Chen JM, Nedzelski JM, Fox AJ, Symons SP. CT grading of otosclerosis. AJNR Am J Neuroradiol. 2009 Aug;30(7):1435-9. DOI: 10.3174/ajnr.A1558. PubMed PMID: 19321627 PubMed Central PMCID: PMC7051554