Letter to the Editor: Regarding Deep N, Besch-Stokes J, Lane J, Driscoll C, Carlson M, “Paget's Disease of the Temporal Bone
Early individual case reports and small series correctly showed that PDTB was characterized by a high-frequency sensorineural hearing loss (SNHL) and a low-frequency air-bone gap (ABG), but were adversely affected by selection bias toward dramatic advanced cases, and had limited evidence bearing on the mechanisms of HL.
Khetarpal and Schuknecht's (2) histopathologic study of 26 ears showed a lack of cochlear disorders to account for SNHL and a lack of ossicular abnormalities despite audiometric ABGs. They concluded with respect to both the SNHL and ABG “that the hearing losses in Paget's disease are caused by changes in bone density, mass, and form that serve to dampen the finely tuned motion mechanics of the middle and inner ears.” Their study refuted the hypothesis of multifactorial causation of HL speculated in early studies (auditory nerve compression or stretching, vascular shunts, ossicular damage, etc.) and demonstrated that the ABG in PDTB, like the ABG in superior canal dehiscence, is not due to defects in the sound conducting mechanism, i.e., that an ABG does not equate with conductive hearing loss in all diseases.
Deep et al.'s imaging techniques were limited to qualitative examination of computed tomography (CT) images, which is unable to detect loss of bone mineral density until about half the density is lost. We developed a robust method of quantitative CT with digital image analysis, which we applied to a well-characterized series of 37 patients with 71 pagetic temporal bones (by the way, larger than the series of Deep et al.), showing strong and significant correlations between both the high-frequency SNHL and the low-frequency ABG in PDTB cases with a wide range of pagetic involvement (3). We also showed that in 56 evaluable ears with PDTB that there were no abnormal ABRs, indicating that the SNHL of PDTB is due to cochlear mechanisms.
Runx2+/− mice and D4 mice are models of human cleido-cranial dysplasia, which is a bone disease associated with SNHL, mixed HL, and ABG. Affected animals have hearing loss demonstrated by ABR and compound action potentials, though normal histologic anatomy. Using nanoindentation, Alliston et al. (4) demonstrated that the cochlear capsule bone of Runx2+/− mice and D4 mice have reduced hardness and elastic modulus relative to wild type mice. X-ray tomography showed a quantitative shift in the distribution of mineral content in Runx2+/− mice and showed reduced mineral content in D4 mice relative to wild type. In both Runx2+/− mice and D4 mice the reduced elastic modulus correlated strongly with reduced ABR thresholds. Rescue of the defective material properties of bone matrix in Runx2+/− was sufficient to restore normal hearing. This seminal work demonstrated that precise regulation of the material properties of extracellular bone matrix is essential for normal hearing, and shows how alteration of bone properties could cause HL in PDTB as well. Schuknecht's hypothesis, supported by our work and Alliston's, must now be accepted as the leading explanation for the predominant mechanism of HL in PDTB.
In contrast to the pessimistic conclusions of Deep et al. based on qualitative CT, our quantitative CT studies revealed that PDTB is very dynamic, progresses in almost all untreated cases, and responds quickly and effectively to aggressive medical therapy. Although hearing loss does not recover under treatment, it progresses little if at all, so that hearing losses are stabilized in most cases.