Many previous studies have examined the sensitivity of CT for the diagnosis of otosclerosis using clinical data for comparison [2
]. All of these studies compared CT evaluations to history and physical exam, audiometric data, and/or intraoperative findings. Typical audiometric findings may be absent early in the course of the disease or may not be bothersome enough to the patient to prompt evaluation. Surgical intervention allows the opportunity to confirm stapes fixation and/or gross plaques consistent with otosclerosis. While intraoperative findings are reliable for confirming the diagnosis of otosclerosis, only patients with disease that has progressed enough to merit surgical intervention and patients that elect surgery can be included in such studies. Clinical data has been used for comparison to CT findings in otosclerosis since it is not possible to evaluate histology of the otic capsule during life.
This study compares CT imaging on a series of temporal bone specimens with otosclerosis to histology of the same temporal bones. Histology is the most accurate way to determine the diagnosis of otosclerosis. It allows confirmation of the diagnosis even when the disease has not caused typical audiometric findings, and has not led to surgical intervention. Comparison of CT findings to histology allows evaluation of temporal bones with otosclerosis at all stages in the disease course. The extent of the disease determined on CT and histology can be compared because individual foci of otosclerosis can be specifically examined. Furthermore, direct comparison of the CT image to the corresponding histologic slide provides information on why CT evaluations may differ from histology.
In this study, the sensitivity of CT for the diagnosis of otosclerosis when compared to histology was 80%. Several contemporary studies, that compared CT findings to clinical data, found the sensitivity of CT to be greater than 90% for the diagnosis of otosclerosis [5
]. This small difference may be accounted for by the limited number of temporal bones with otosclerosis that met inclusion criteria for the study. Additionally, one missed diagnosis in this study occurred in a patient with a small, subtle focus of otosclerosis in the pericochlear region, and no otosclerosis anterior to the oval window. This patient with early histologic otosclerosis would not have been included in studies comparing radiology to clinical data because she had no associated hearing loss. Regardless, comparison of CT to histology provides the most compelling data as histology is the gold standard for determining the presence or absence of the disease process.
An unanticipated finding from this study was the false positive radiologic diagnosis of otosclerosis in three temporal bones due to areas of connective tissue and vessels anterior to the oval window. The findings in these three control temporal bones possibly represent a variant of normal histology. Otologists and radiologists should be aware that a lucency in the antefenestral area is not always due to otosclerosis,.
The sensitivity of CT for identifying otosclerotic foci in various locations around the otic capsule was highest in the area anterior to the oval window (zone 1) and lowest in the round window niche (zone 3). Because, in this phase of the study, radiologists were aware of the diagnosis of otosclerosis (though blinded to the location), they may have been biased toward a positive finding of otosclerosis for zone 1 since this is the most common location for otosclerosis. The higher rate of missed foci in the round window niche is partly due to the extremely small size of foci at that location in three of the temporal bones. These small foci of otosclerosis would not be expected to have any clinical sequelae, and thus failure to identify tiny foci of otosclerosis at the RW niche on the CT would not impact clinical management. Additionally, slight differences in the plane of tissue between a CT slice and the best corresponding histologic slide may lead to discrepancies with identification of very small foci.
When otosclerosis reaches the endosteal margin of the cochlea, deposition of a hyalin material in the spiral ligament may ensue and cause sensorineural hearing loss[1
]. Identification of patients with otosclerosis in whom the sensorineural component is due to otosclerosis is important for patient counseling. In the future, identifying these patients may become essential as medical therapies (e.g. bisphosphonate treatment) for otosclerosis-related sensorineural hearing loss are developed. Multiple authors have compared CT findings with audiometry to determine whether endosteal margin involvement is associated with otosclerosis-related sensorineural hearing loss[3
]. The average bone conduction threshold among patients with endosteal involvement on CT has been found to be significantly elevated compared to those without endosteal involvement[5
]. This implies generally good correlation between positive findings of endosteal involvement on CT and the pathology leading to sensorineural hearing loss. The present study confirms excellent correlation: all temporal bones with endosteal margin involvement on CT demonstrated endosteal margin involvement on histology. Guneri et. al detected radiolucent areas in 45% of patients suspected to have cochlear otosclerosis by history and audiometry[13
]. This poor correlation could be attributed to an error in the diagnosis of cochlear otosclerosis or failure of the CT to detect radiolucent foci with endosteal margin involvement. In the present study, the false negative rate for endosteal margin involvement on CT was 37% (3/8), which would help explain the poor correlation in the study by Guneri et al. Thus, CT may be used to confirm endosteal margin involvement, but is not as accurate in ruling out involvement.
Although the protocol used for imaging these temporal bones was as close to clinical standards as possible, technical issues related to imaging of a temporal bone in isolation may have altered the quality of images obtained. The CT images were examined at the orientation which best matched the histological sections but which may not have been the most optimal angle to visualize subtle otosclerotic changes. Additionally, there was some minor artifact related to fixation material. The images were also slightly degraded due to presence of air in soft tissue structures.