Incidence of 2-kHz bone conduction threshold dip (2KBD) among various pathologic conditions.
Depth of 2-kHz bone conduction threshold dip (2KBD) among various pathologic conditions. Error bars indicate standard error.
Relationship between 2-kHz bone conduction threshold dip (2KBD) depth and air-bone gap (r = 0.34).
Postoperative recovery of 2-kHz bone conduction thresholds. Error bars indicate standard error, and the asterisk indicates a significant difference (P < .05, Bonferroni post hoc test). 2KBD indicates 2-kHz bone conduction threshold dip.
Kashio A, Ito K, Kakigi A, Karino S, Iwasaki S, Sakamoto T, Yasui T, Suzuki M, Yamasoba T. Carhart Notch 2-kHz Bone Conduction Threshold DipA Nondefinitive Predictor of Stapes Fixation in Conductive Hearing Loss With Normal Tympanic Membrane. Arch Otolaryngol Head Neck Surg. 2011;137(3):236-240. doi:10.1001/archoto.2011.14
To evaluate the significance of the Carhart notch (a 2-kHz bone conduction threshold dip [2KBD]) in the diagnosis of stapes fixation by comparing its incidence among ears with various ossicular chain abnormalities.
A total of 153 ears among 127 consecutive patients with a congenital ossicular anomaly or otosclerosis.
Main Outcome Measures
The 2KBD depth was defined as the threshold at 2 kHz minus the mean of thresholds at 1 and 4 kHz. The presence of 2KBD (depth, ≥10 dB), 2KBD depth, relationship between 2KBD depth and air-bone gap, and 2-kHz bone conduction recovery after operation were evaluated in a stapes fixation group (which included cases of otosclerosis and congenital stapes fixation), an incudostapedial joint detachment group, and a malleus or incus fixation group.
A 2KBD was present in 32 of 102 stapes fixation ears (31.4%), 5 of 19 incudostapedial joint detachment ears (26.3%), and 6 of 20 malleus or incus fixation ears (30.0%) (12 ears had other diagnoses). The mean (SD) 2KBD depths were 17.3 (5.2) dB in the stapes fixation group, 18.5 (2.2) dB in the incudostapedial joint detachment group, and 16.3 (2.1) dB in the malleus or incus fixation group. No statistically significant differences were noted among these 3 groups. No correlation was noted between 2KBD depth and air-bone gap extent. Recovery of 2-kHz bone conduction threshold in the stapes fixation group was less than that in the other 2 groups.
Incidence of 2KBD was similar among the stapes fixation, incudostapedial joint detachment, and malleus or incus fixation groups, implying that 2KBD is not a useful predictor of stapes fixation.
In 1950, Carhart1 reported bone conduction threshold elevation of approximately 2 kHz among patients with otosclerotic lesion–induced stapes ankylosis that disappeared after stapes surgery. Since then, this deceptive 2-kHz bone conduction threshold dip (2KBD) without inner ear damage has become a well-known indicator of stapes fixation (Carhart notch). However, results of studies2- 9 have suggested that elevation in bone conduction thresholds between 1 and 4 kHz can be caused by various factors that affect the conductive mechanism of the middle ear. In fact, it is not uncommon to encounter cases of Carhart notch in which hearing loss is caused by detachment of the incudostapedial joint. For Carhart notch to be used as a preoperative predictor of stapes fixation, it should be shown that the notch exists with stapes fixation but not with other ossicular chain disorders, such as disconnection; however, few clinical investigations have assessed this issue. In the present study, we evaluated the significance of 2KBD depth, defined as the threshold at 2 kHz minus the mean of thresholds at 1 and 4 kHz, in diagnosing various ossicular chain abnormalities in the setting of a normal tympanic membrane.
We studied 153 ears among 127 consecutive patients who had a congenital ossicular anomaly or otosclerosis that was confirmed during surgery between January 1997 and December 2007 at the University of Tokyo Hospital, Tokyo, Japan. On the basis of the diagnosis made during surgery, we assigned these ears to the following 3 groups: a stapes fixation group (which included cases of otosclerosis and congenital stapes fixation), an incudostapedial joint detachment group, and a malleus or incus fixation group. The medical records of these patients were retrospectively reviewed. Stapes fixation was observed in 102 ears (including 15 ears with congenital fixation), incudostapedial joint detachment without stapes fixation in 19 ears, and malleus or incus fixation without stapes fixation in 20 ears. The other 12 ears (including a combination of incudostapedial joint detachment and malleus or incus fixation or obstruction of the oval window) could not be classified into the aforelisted groups and were excluded from the analysis. The patients ranged in age from 6 to 72 years (mean [SD] age, 40  years). Postoperative diagnoses and the mean age and mean preoperative air and bone conduction thresholds (at 0.5, 1, and 2 kHz) are given in Table 1. Patients in the stapes fixation group were significantly older than those in the other 2 groups. No significant differences were noted among the 3 groups in preoperative air or bone conduction thresholds.
For audiometric evaluation, we measured air conduction thresholds at 0.125, 0.25, 0.5, 1, 2, 4, and 8 kHz. Bone conduction thresholds were measured at 0.5, 1, 2, and 4 kHz. Pure-tone audiometry was performed more than once on various days before surgery. The 2KBD was considered present when the bone conduction threshold at 2 kHz exceeded the mean of thresholds at 1 and 4 kHz by at least 10 dB. The 2KBD depth was calculated by subtracting the mean of thresholds at 1 and 4 kHz from the bone conduction threshold at 2 kHz.
Values were recorded as the mean (SD) unless indicated otherwise. Statistical analyses used χ2 test, Wilcoxon signed rank test, and 1-way analysis of variance with Bonferroni post hoc test.
The 2KBD was detected in 32 of 102 ears (31.4%) in the stapes fixation group, 5 of 19 ears (26.3%) in the incudostapedial joint detachment group, and 6 of 20 ears (30.0%) in the malleus or incus fixation group (Figure 1). The mean 2KBD depths were 17.3 (5.2) dB in the stapes fixation group, 18.5 (2.2) dB in the incudostapedial joint detachment group, and 16.3 (2.1) dB in the malleus or incus fixation group (Figure 2). No statistically significant differences were noted in 2KBD incidence or 2KBD depth among the 3 groups.
Table 2 gives age-related dip-positive rates. There were no significant differences in percentages of dip-positive cases among the different age groups.
Figure 3 shows the relationship between 2KBD depth and air-bone gap, indicating no correlation between the 2 variables. No apparent differences were observed among the 3 groups.
Table 3 gives 2-kHz bone conduction thresholds before and after surgery, as well as improvements obtained by surgery. Of 102 ears in the stapes fixation group, 3 ears were excluded in which ossicular reconstruction could not be performed. Of 19 ears in the incudostapedial joint detachment group, 1 was excluded because the patient dropped out during the postoperative follow-up period. Improvement in 2-kHz bone conduction thresholds among the stapes fixation group was less than that among the other 2 groups; the difference between the 2KBD-positive stapes fixation group and the 2KBD-positive malleus or incus fixation group was statistically significant (P < .05, Bonferroni post hoc test) (Figure 4).
Acute or chronic otitis media associated with perforation of the tympanic membrane and otitis media with effusion can be diagnosed easily; however, in patients with normal tympanic membrane, sufficient information is needed to diagnose the cause of hearing loss. An audiological feature of 2KBD, Carhart notch, is widely known and is traditionally believed to suggest stapes fixation10; however, few investigations have verified its usefulness. In the present study, we evaluated the significance of Carhart notch in predicting stapes fixation and found that 2KBD was ineffective as a predictive tool. Incidence of Carhart notch was almost identical among the stapes fixation, incudostapedial joint detachment, and malleus or incus fixation groups. Otologic surgeons should be aware of this fact and should be ready to adapt their procedures according to pathologic findings during surgery.
Supporting our present findings that Carhart notch is not specific to stapes fixation, bone conduction threshold elevation between 1 and 4 kHz has also been reported in various pathologic conditions that affect the conductive mechanism of the middle ear. This phenomenon has been described in fluctuations of otitis media with effusion,2,11- 13 chronic otitis media,5,6,14- 17 experimental creation of artificial conductive impairment by loading the tympanic membrane,8,18,19 occlusion of the round window or oval window,8,9 and disarticulation of the incudostapedial joint.9,20 Bone conduction threshold elevation has been reported principally between 1 and 4 kHz, with the largest being at 2 kHz.2,5,7,21
Bone conduction thresholds do not always represent a pure estimate of cochlear reserve, as many components are involved in bone conduction. The most important physical phenomena are believed to be (1) sound radiation into the ear canal, (2) inertial motion of the middle ear ossicles, and (3) compression and expansion of the bone encapsulating the cochlea.22,23 Ossicular chain deficiencies are closely related to these components. A change in the ossicular chain may result in less inertial motion energy transmitted into the inner ear and can cause impedance mismatch between the inner ear and the ossicular system, modifying (decreasing or increasing) the loss of bone-conducted sound pressure from the vestibule to the footplate. It is reported that the middle ear does not contribute to perception of bone conduction sound at frequencies lower than 1 kHz.8,20,24 At higher frequencies, the middle ear can affect bone conduction. Using human cadaver heads, Stenfelt20 reported that motion of the stapes with bone conduction sounds was decreased by 5 to 10 dB between 1.2 and 2.7 kHz after the incudostapedial joint was severed. Using cats, Kirikae8 reported a decrease in response at frequencies between 1 and 3 kHz after fixation of the stapes. With an intact ossicular chain, resonance frequency of the ossicular vibration with bone conduction stimulation is close to 1.5 kHz,20 which explains 2-kHz bone conduction threshold elevation in ossicular deficiency. Therefore, although the underlying mechanisms may differ, 2-kHz bone conduction threshold elevation may well occur in various impairments of the ossicular chain, including stapes fixation, incudostapedial joint detachment, and malleus or incus fixation. The reason for this phenomenon seems complex, but oversimplification is inappropriate; further fundamental studies are needed to clarify its mechanism.
The mean age of patients in the stapes fixation group was significantly older than that of patients in the incudostapedial joint detachment group and the malleus or incus fixation group. In our cohort, patients in the latter 2 groups with normal tympanic membranes were mainly young adults or children with congenital malformations of the ossicular chain. This may explain the younger mean ages of these groups. Aging raises bone conduction thresholds at high frequencies, and elevation of 4-kHz bone conduction thresholds results in underestimation of potential 2KBD depths in older patients with stapes fixation. However, in our series, no significant differences were noted in 2KBD incidence among age groups. This suggests that age differences among the 3 study groups did not affect 2KBD incidence.
Several investigations have focused on 2KBD, but its definitive criteria have not yet been established. However, previously reported mean 2KBD depths in otosclerosis ranged from 2.4 to 12.5 dB.20,24,25 These studies included all cases and not just those classified as dip positive by certain criteria. The overall mean 2KBD depth, including dip-negative cases, was 8.5 dB in our stapes fixation group, which is within the range previously reported. For the 2KBD-positive cases, all 3 groups showed similar depths. These results suggest that an apparent elevation in bone conduction thresholds caused by a middle ear deficiency is similar regardless of the cause. Moreover, no correlation was observed between air-bone gap and 2KBD depth, suggesting that the depth may not be influenced by the degree of middle ear deficiency.
Carhart1 originally reported postoperative bone conduction improvements of 5 dB at 500 Hz, 10 dB at 1 kHz, 15 dB at 2 kHz, and 5 dB at 4 kHz. Ginsberg et al21 confirmed the finding of optimal bone conduction improvement at 2 kHz. Àwengen25 showed an improvement of 4 to 12 dB for bone conduction at 2 kHz in otosclerosis after stapedectomy. In our series, the mean recovery of 2-kHz bone conduction was 4.6 to 6.3 dB, and the value was 4.3 to 19.2 dB when we limited the analysis to only dip-positive cases. Recovery trends in the stapes fixation group were worse than those in the other 2 groups. Gerard et al26 proposed that there is less postoperative improvement in bone conduction with increasing age; this suggests that the aging cochlea is more susceptible to surgical damage. Because the mean age of patients was significantly older in the stapes fixation group, this may have influenced their recovery. When we evaluated 33 younger patients (about one-third of the group; mean age, 33 years) from the stapes fixation group, the overall 2KBD recovery was 7.9 (1.8) dB, which was comparable to that of the other 2 groups. Surgical procedures used in stapes surgery are more invasive and involve opening of the inner ear. Cook et al27 reported a weak (r = 0.28) but significant (P < .05) correlation between bone conduction recovery at 2 kHz and air conduction recovery after stapes surgery. We also investigated this issue and found that bone conduction recovery at 2 kHz had a weak correlation with the mean air conduction recovery (r = 0.38, P < .05) but observed that preoperative air-bone gap and 2-kHz bone conduction threshold recovery had no significant correlation (r = −0.11, P > .05). These facts imply that air and bone conduction threshold elevations are related somewhat but that the underlying mechanisms of these phenomena may not be simple.
Correspondence: Ken Ito, MD, Department of Otolaryngology, Teikyo University School of Medicine, 2-11-1 Kaga Itabashi-ku, Tokyo, Japan (firstname.lastname@example.org).
Submitted for Publication: March 29, 2010; final revision received September 10, 2010; accepted October 18, 2010.
Author Contributions: Drs Kashio, Ito, Karino, Iwasaki, Sakamoto, Suzuki, and Yamasoba had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis. Study concept and design: Kashio and Ito. Acquisition of data: Kashio, Ito, Karino, Iwasaki, Sakamoto, Suzuki, and Yamasoba. Analysis and interpretation of data: Kashio, Ito, Kakigi, Sakamoto, Yasui, and Yamasoba. Drafting of the manuscript: Kashio, Ito, and Yasui. Critical revision of the manuscript for important intellectual content: Kashio, Ito, Kakigi, Karino, Iwasaki, Sakamoto, Suzuki, and Yamasoba. Statistical analysis: Ito and Yasui. Administrative, technical, and material support: Kashio. Study supervision: Ito, Kakigi, Karino, Iwasaki, Sakamoto, Suzuki, and Yamasoba.
Financial Disclosure: None reported.