Reply to Letter to the Editor: Paget's Disease of the Temporal Bone
Regarding the histological study by Khetarpal and Schuknecht, the full quotation from the last paragraph in their discussion states: “Having failed to identify specific pathologic correlates for the conductive and sensorineural hearing losses, we suspect that impairment of compliancy of these systems may be the cause. The highly tuned middle and inner ear mechanisms could be altered by the changes in the density, mass, and form of the middle and inner ear bony structures that occur in Paget's disease (1).” Their conclusion that alterations in compliancy result in hearing loss was therefore the leading theory based on an otherwise lack of clear evidence to suggest another mechanism. While Khetarpal and Schuknecht's study is one of the largest series on histologic features of Paget's disease of the temporal bone (PDTB), an analysis of 26 temporal bones, only 12 of whom had paired audiometric data, can hardly be considered representative of the full spectrum of disease presentation in a very diverse and heterogeneous disorder such as Paget's. While later publications have theorized that multiple microfractures within the otic capsule may serve as a third window resulting in a conductive hearing loss of cochlear origin (2), Khetarpal and Schuknecht's original report also summarized the existing literature that identified numerous histological studies from Nager and others demonstrating cases of ossicular fixation, eustachian tube obliteration, tympanic membrane fibrosis, tympanic cavity fibrosis and otitis media, and round window obliteration (1). Further, the authors themselves report on several specific cases in their series where patients with mixed hearing loss demonstrated fibrous fixation of the footplate (case 1), fibrous replacement of ossicular bone (case 2), and tympanic membranes that are thickened by fibrous proliferation of the lamina propria (case 5) (1). Such findings have also been reported in more recent studies. In 2012, Dimitriadis et al. (3) performed a temporal bone histological study in eight patients with PDTB and reported the following findings that may account for hearing loss: fractured stapes footplate, stapes fixation, obliteration of Cotugno's canal, microfissures and microfractures of the otic capsule, bleeding in the scalae, strial atrophy, and cystic lesions in the spiral ligament. This study, along with many publications from other reputable histopathology labs, highlights the variability of potential causes for hearing loss in PDTB. While we agree that ossicular fixation is infrequently the cause and that a possible explanation for the air-bone gap (ABG) in some cases is a conductive hearing loss of cochlear origin, in an effort to be comprehensive, we listed other proposed mechanisms as reported in the literature (3,4).
Your original research on the mechanism of hearing loss in Paget's disease of the bone is unquestionably a landmark article on the topic (5). We do not disagree with your statement that there is an “underlying cochlear mechanism of pagetic hearing loss that is closely related to loss of bone mineral density in the cochlear capsule (5).” Rather we simply state (as others have also previously stated) (6) that perhaps correlation does not equal causation; it is unclear if the altered cochlear bone density actually accounts for the observed hearing loss, or if this merely serves as a marker of the disease effect. We are not aware that demineralization alone without dehiscence of the cochlear capsule is a common cause of inner ear conductive hearing loss.