Long-term evaluation of otosclerosis on temporal bone CT

Abstract

Purpose

To assess the long-term progression of otosclerosis lesions on temporal bone CT, particularly with regard to lesion expansion, distribution, and changes in density.

Materials and Methods

This retrospective study analyzed all patients who underwent HRCT or CBCT for the diagnosis of otosclerosis between 2012 and 2022. The study population was screened for the presence of follow-up imaging. Patients with available imaging over a period of five years were included in the study. Demographic data, clinical symptoms, and imaging findings were analyzed using descriptive statistics. The imaging findings were grouped according to otosclerosis subtype (fenestral, retrofenestral, or internal auditory canal (IAC)) and the long-term course of otosclerosis was assessed in terms of density and size.

Results

35 patients were included in a follow-up study with an average duration of 100 months (range: 62–168 months, 5 to 14 years ) for otosclerosis. A total of 65 ears were affected. The patients were on average 48 ± 12.1 (range: 11–74) years old. Women (n= 24, 69%) were more than twice as likely to be affected as men (n= 11, 31%). Retrofenestral otosclerosis was the most common form (54%), followed by fenestral otosclerosis (40%). Otosclerosis around the IAC was significantly less common, accounting for just 6% of cases. In both fenestral and retrofenestral otosclerosis, an increase in otosclerotic volume between the initial and last follow-up imaging scans was observed in fewer than a third of cases (16% vs. 20.6%; [Table 2]). The density increased in some cases over time, affecting 24% of fenestral and 38.2% of retrofenestral cases. If the IAC was affected, imaging showed no changes in extent or density over time.

Conclusion

Over the long term (five to 14 years), slight changes in density (increasing sclerosis) and size expansion can only be observed in approximately one third of patients. A progression from fenetral to retrofenestral otosclerosis was not documented in any of the cases.

Key Points

• There are no significant changes in density and volume increase in the long-term course of otosclerosis.

• Progression from fenestral to retrofenestral otosclerosis was not observed in any case.

• The slight morphological progression of CT lesions observed in individual cases in our study is consistent with the limited progression of hearing loss observed in two-thirds of the remaining patients in Ishai et al.ʼs study.

Citation Format

• Döring K, Satyavolu S, Durisin M et al. Long-term evaluation of otosclerosis on temporal bone CT. Rofo 2025; DOI 10.1055/a-2659-8853

 

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Zusammenfassung

Ziel

Bewertung des langfristigen Verlaufs von Otosklerose – Läsionen im Schläfenbein-CT, insbesondere hinsichtlich Läsionsausdehnung, Verteilung und Dichteveränderungen.

Materialien und Methoden

In dieser retrospektiven Studie wurden alle Patienten analysiert, die zwischen 2012 und 2022 zur Diagnose einer Otosklerose einer HRCT oder CBCT unterzogen wurden. Die Studienpopulation wurde auf das Vorliegen von Folgebildgebungen untersucht. Patienten mit Bildgebungen über einen Zeitraum von fünf Jahren wurden in die Studie aufgenommen. Demografische Daten, klinische Symptome und Bildgebungsbefunde wurden mittels deskriptiver Statistik analysiert. Die Bildgebungsbefunde wurden nach Otosklerose-Subtypen (fenestral, retrofenestral oder innerer Gehörgang (IAC)) gruppiert und der Langzeitverlauf der Otosklerose hinsichtlich Dichte und Größe beurteilt.

Ergebnisse

35 Patienten wurden in die Studie eingeschlossen, das die durchschnittliche Follow-up-Zeit umfasst 100 Monaten (Bereich 62–168 Monate, 5 bis 14 Jahre). Insgesamt waren 65 Ohren betroffen. Das Durchschnittsalter der Patienten betrug 48 ± 12,1 Jahre (Bereich 11–74 Jahre). Frauen (n = 24, 69 %) waren mehr als doppelt so häufig betroffen wie Männer (n = 11, 31 %). Die retrofenestrale Otosklerose war die häufigste Form (54 %), gefolgt von der fenestralen Otosklerose (40 %). Sowohl bei der fenestralen als auch bei der retrofenestralen Otosklerose wurde in weniger als einem Drittel der Fälle (16 % gegenüber 20,6 %) eine Zunahme des otosklerotischen Volumens zwischen der ersten und der letzten bildgebenden Untersuchung beobachtet. In einigen Fällen nahm die Dichte im Laufe der Zeit zu, was 24 % der fenestralen und 38,2 % der retrofenestralen Fälle betraf.

Schlussfolgerung

Langfristig (fünf bis14 Jahre) sind nur bei etwa einem Drittel der Patienten leichte Veränderungen der Dichte (zunehmende Sklerose) und eine Volumenzunahme zu beobachten. Eine Progression einer fenestralen zur retrofenestralen Otosklerose konnten in keinem Fall nachgewiesen werden.

Kernaussagen

• Im Langzeitverlauf sind in Dichte und Volumenzunahme keine wesentlichen Veränderungen bei der Otosklerose zu verzeichnen.

• Ein Progress von einer fenestralen zu einer retrofenestralen Otosklerose konnte in keinem Fall beobachtet werden.

• Das allenfalls leichte morphologische Fortschreiten der CT-Läsionen, das in unserer Studie in einzelnen Fällen beobachtet wurde, stimmt mit dem begrenzten Fortschreiten der Schwerhörigkeit überein, das in der Studie von Ishai et al. bei zwei Dritteln der übrigen Patienten beobachtet wurde.

 

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Keywords

otosclerosis - long-term follow-up - SNHL - CBCT - HRCT

Abbreviations

AC: air conduction thresholds

BC: bone conduction thresholds

CBCT: cone beam CT

HRCT: high-resolution CT

IAC: internal auditory canal

SHNL: sensoneurinal hearing loss

 

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Introduction

Otosclerosis is typically characterized by progressive sclerosis of the bony labyrinth. The progressive bone remodeling disease, which is limited to the ear capsule of the temporal bone, occurs with an incidence rate ranging from 2.5% to 12%, with almost 85% of patients showing bilateral involvement [1] [2]. Three phases of bone remodeling are distinguished: the otospongiotic phase with increased osteoclast activity is followed by a translational phase in which osteoblasts begin to deposit spongy bone. This is followed by the osteosclerotic phase, which is characterized by increasing sclerosis. Otosclerosis results in the fixation of the stapes plate to the oval window, a condition known as stapes ankylosis. This results in conductive hearing loss due to reduced vibratory capacity of the ossicles. As the disease progresses, the second ear is often also affected. If left untreated, the disease slowly progresses to severe hearing loss. The preferred treatment is to replace the upper part of the stapes with a prosthesis (stapedotomy).

Despite its clinical significance, the exact underlying pathophysiology of otosclerosis remains unclear. The diagnosis of otosclerosis is still primarily based on clinical assessment [3]. However, radiological examination is becoming increasingly important in this context, not only for preoperative diagnosis and identification of anatomical challenges (e.g., an overhanging facial nerve) and exclusion of differential diagnoses of conductive hearing loss (e.g., a dehiscence of the upper auditory canal or atresia of the round window [4] [5] [6] [7]), but also for detection of otosclerotic foci. These foci can be identified using high-resolution computed tomography (HRCT) and cone beam CT (CBCT), with an accuracy of over 90% [8]. Nevertheless, isolated sclerotic foci may exhibit Hounsfield units that resemble regular bone density, making them easy to overlook radiologically [9] [10] [11]. Visible foci on HRCT and cone beam computed tomography (CBCT) are typically located anterior to the oval window in a region known as the fissula ante fenestram [12] [13], but may also be located retrofenestral or adjacent to the semicircular canals and internal auditory canal (IAC).

 

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Materials and Methods

Our ethics committee approved this retrospective study and waived the requirement for informed patient consent.

Patient selection

In a tertiary referral center, all patients treated for otosclerosis between 2012 and 2022 at the Department of Otolaryngology were scanned for the existence of follow-up temporal bone CT scans, regardless of age and gender. Only patients with a minimum CT imaging interval of 5 years were included in the study. The exclusion criterion was low image quality. Demographic data and additional clinical information were added.

 

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Imaging of the temporal bone

All patients underwent HRCT or CBCT of the temporal bone. Different types of HRCT scanners (HiSpeed Advantage RP, HiSpeed Advantage and LightSpeed16; all GE, Milwaukee, WI, U.S.A.) were used over the long period of our study and slice thickness ranged from 0.625 to 1 mm. CBCT scans were performed using MiniCAT IC (Xoran Technologies; MI, USA). The acquired images were uploaded to our current PAC system (GE, Milwaukee, WI, U.S.A.) and VISAGE Imaging (Visage 7, Pro Medicus Limited, Berlin, Germany).

 

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Radiological assessment of otosclerosis manifestation

Two neuroradiologists specializing in head and neck imaging and emergency neuroradiology reviewed all imaging studies for progression of the size and density of the lesions. The following locations were examined in greater detail: the fissura ante fenestram (in front of the oval window), the stapes footplate, the round window, the pericochlear region (excluding the fissura ante fenestram), the anterior border of the internal auditory canal (IAC), and the areas surrounding the semicircular canals (SCC). The separation and integrity of the ossicular chain were also evaluated. If the extent of otosclerosis increased during the period under review, this was measured in the plane of greatest extent. Imaging findings were recorded by consensus.

 

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Statistical analysis

Statistical analysis was performed using SPSS, Version 29.0. Descriptive statistics were used to analyze the study population, the type and quality of neuroimaging, imaging findings, patient status, and clinical symptoms. Continuous variables, such as patient age, are expressed as means, medians, and ranges. Categorical variables and qualitative parameters, such as patient sex, mechanism of strangulation, context of strangulation, type of imaging study performed and number of injuries reported, are expressed as total count (n) and percentages. Means and standard deviation (SD) were calculated for metric variables (i.e., age).

 

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Results

Retrospective research identified a total of 566 patients with otosclerosis who underwent follow-up temporal bone CT examinations in the database. Of these patients, 210 underwent more than one imaging procedure in the form of HRCT or CBCT. In most cases (n = 175), the second imaging procedure was performed within five years of the initial diagnosis. Only 37 patients had a follow-up period between 5 and 14 years. Two patients were excluded due to low image quality, leaving 35 patients for the study. The study population evaluated in this manuscript was based on these patients.

Baseline characteristics of the study population

The 35 patients included in the study were aged between 11 and 74 years. The mean age was 48 ± 12.1 years. Women (n= 24, 69%) were more than twice as likely to be affected as men (n= 11, 31%) (see also [Table 1]).

Table 1 Baseline data.

Label	Value	Unit	Percentage	Description
Patient cohort	35	count
Sex	11/24	count	31%/69%	(m/w)
Age	48.9±12.1 11–74	years		mean±std, range
Total ears affected	65	count
Bilateral	30	count	85.7%
Unilateral	5	count	14.3%

The mean follow-up period was 100 months (approximately eight years), ranging from 62 to 168 months. Sixty-three ears (90%) were affected by otosclerosis. Retrofenestral otosclerosis was the most common form (54%), followed by fenestral otosclerosis (40%). Isolated otosclerotic lesions affecting the IAC accounted for 6% of cases.

In both fenestral and retrofenestral otosclerosis, an increase in otosclerotic volume between the initial and last follow-up imaging scan was observed in fewer than a third of cases (16% vs. 20.6%; see [Table 2] and [Fig. 1], [Fig. 2], [Fig. 3], [Fig. 4], [Fig. 5], [Fig. 6], [Fig. 7]). The density increased in some cases over time, affecting 24% of fenestral and 38.2% of retrofenestral cases. Fenestral otosclerosis severely affects the following areas in descending order of frequency: the fissula ante fenestram (84%), the stapes footplate (28%), and the round window (16%). By contrast, the distribution of lesions in retrofenestral otosclerosis varies considerably (in descending order of frequency): internal acoustic canal (IAC) (91.2%), fissula ante fenestram (85.3%), pericochlear (58.8%), stapes footplate (52.9%), round window (44.1%), and superior canal (SCC) (23.5%).

Table 2 Distribution of the affected regions in the different otosclerosis subtypes.

			Increased		Involvement of
Location of otosclerosis		Number of ears	Extent	Density	Fissula ante fenestram	Stapes footplate	Round window	IAC*	Semicircular canals	Pericochlear lesions
*Internal auditory canal
I	Fenestral	25	16%	24%	84%	28%	16%	0%	0%	0%
II	Retrofenestral	34	20.6%	38.2%	85.3%	52.9%	44.1%	91.2%	23.5%	58.8%
III	Isolated involvement of IAC	4	0%	0%	0%	0%	0%	0%	100%	0%

Fig. 1 The patient is now 34 years old, with 12 years having passed since the two images were taken (above vs. below) (154-month follow-up). Neither an increase in the volume of the preexisting osteosclerosis lesion nor any changes in density are visible in the images.

Fig. 2 A 60-year-old female patient: five years elapsed between the two images (above vs. below) (63-month follow-up). Neither an increase in the volume of the preexisting osteosclerosis lesion nor any changes in density are visible in the images.

Fig. 3 51-year-old female patient with 8-year follow-up (105 months) (above vs. below) (63-month follow-up). An increase in volume of the preexisting osteosclerosis lesion cannot be seen. The anterior part of the circular sclerotic area (white arrow) is slightly wider on the follow-up scan, indicating very slight progression of sclerosis. This also shows the difficulty with respect to judging the lesions using different techniques.

Fig. 4 Subtle sclerosis in a case of fenestral otosclerosis in a 45-year-old patient with bilateral fenestral otosclerosis over a period of 12 years and one month (white arrows). At the second time point, the lesion is barely visible (bottom image). An increase in the volume of the preexisting osteosclerosis lesion is not visible.

Fig. 5 Density increased over a period of eight years (98 months) in a 67-year-old male patient with unilateral retrofenestral otosclerosis. The contralateral side has a lesion at the fissura ante fenestram, which is not shown. An increase in the volume of the preexisting osteosclerosis lesion is not visible.

Fig. 6 This image shows the increasing size of the left lesion in a 79-year-old female patient with retrofenestral otosclerosis over a period of 12 years. The increase is subtle: looking at the bony separation between the carotid canal and the lesion (white arrows), it decreases.

Fig. 7 A left-sided, increasing lesion adjacent to the semicircular canals in a 45-year-old female patient after seven years of follow-up.

 

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Discussion

Over the period of 5 to 14 years, neither a significant increase in size nor a significant change in the density of the otosclerotic lesions was detected. Slight changes in density were detected in only one third of cases. In none of the cases a progression from fenestral otosclerosis to retrofenestral otosclerosis was observed. Automatic registration, a Visage Imaging tool that enables 1:1 superimposition of images over a long period of time, was particularly helpful in this regard. Although it could only be used in 42% of cases (n = 15), it led to improved intra- and interobserver agreement and made it easier to reach a consensus in unclear cases.

Lee et al. reviewed inter- and intraobserver agreement using the standard CT classification system for otosclerosis according to Symmons and Fanning. They cited cases in which patients underwent follow-up examinations several years later, during which no obvious progression of otosclerosis was observed [14]. This is consistent with our long-term observations and suggests that, after an initial short progression phase, the sequelae remain relatively stable. Progression is so gradual that follow-up observation over several decades is necessary to detect changes.

Cochlear otosclerosis is characterized by a combination of conductive and sensorineural hearing loss (SHNL). From the basal to the apical turn of the cochlea certain frequencies are mapped to different areas: low-frequency sounds are most effectively amplified at the apical turn, while high-frequency sounds are most effectively amplified at the basilar turn. Among others, Shin et al. successfully established a correlation between bone level thresholds and the radiological extent of otosclerosis within the ear capsule [15]. This suggests a local effect of the otosclerotic foci on this part of the cochlea [16] [17]. However, other authors attribute sensorineural hearing loss to cytotoxic enzymes that cause hyalinization of the spiral ligament [18] [19] [20]. If this hypothesis is accepted, there would be no correlation between the exact location of a lytic focus on the cochlea and the frequency at which hearing loss occurs.

In their study to determine the long-term incidence and degree of SNHL in otosclerosis, Ishai et al. concluded that, over a period of 10 years, the average long-term SNHL due to otosclerosis was statistically significantly higher at every frequency than when age alone was taken into account [10]. However, the average values obtained (ranging from 2.6 to 4.6 dB across the tested frequencies) were not considered clinically significant. Therefore, of the 357 ears examined, one third showed no clinically significant progression of the sensorineural component of hearing loss. The slight CT morphological progression of the lesions of our study is consistent with the relatively limited progression of hearing loss observed in the remaining two-thirds of the patients in Ishai et al.ʼs study. Redfors et al. also evaluated hearing outcomes in otosclerosis patients over a long period of 28 to 30 years after stapedectomy [21]. They found that, in both the early and late stages of the disease, hearing loss was significantly greater than in the reference population for both air conduction (AC) and bone conduction (BC) thresholds. They also confirmed that otosclerosis patients had significantly more pronounced hearing loss than the normal-hearing population, as measured at both AC and BC thresholds, both after surgery and 28–30 years later. This deterioration in hearing was primarily caused by sensorineural hearing loss, which was not reflected in the lesions detected in radiological examinations [21].

Otosclerosis is described in the literature as being present in both ears in 85% of cases [1] [2] [15] [16] [17] [18] [19] [20] [21]. This figure corresponds well with the results of our study, in which CT findings consistent with otosclerosis were present in both ears in 30 out of 35 patients (85.7%). The percentage of lesions found in each region largely corresponds to the data reported by Amold et al. [22]. Only the number of lesions anterior to the IAC was significantly higher in our study. Further epidemiological results are also consistent with current data [1] [2]. With an average age of 48 (range: 11–74), the typical peak incidence is observed in the 20–40 age group. The typical gender distribution of 2:1 women to men is also reflected in our study group, which was not preselected in this respect: 69% women and 31% men.

Limitations

In addition to the retrospective, monocentric study design, the long observation period is also a limitation. The latter also necessitated the use of different CT scanners. Furthermore, the fact that it was often only possible to compare CT and DVT images limited the immediate one-to-one assessment, particularly with regard to critical details such as density in small areas. Artifacts caused by the implanted cochlear implant (CI) or, less frequently, by the stapes prosthesis, made assessment difficult and, in some cases, impossible.

 

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Conclusion

Over a period of 5 to 14 years, only slight progression of otosclerotic lesions was observed. This correlates with the long-term progression of sensorineural hearing loss (SNHL), which, in most patients with otosclerosis, is not clinically significantly worse than age-related presbycusis alone (Ishai et al.). No clinically significant progression of the sensorineural component of hearing loss is represented by imaging. Progression from fenestral to retrofenestral otosclerosis was not documented in any of the cases.

 

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Conflict of Interest

The authors declare that they have no conflict of interest.

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Publication History

Received: 12 June 2025

Accepted after revision: 11 July 2025

Article published online:
30 July 2025