Determinants of hearing loss in perforations of the tympanic membrane

Abstract

Background: Although tympanic membrane perforations are common, there have been few systematic studies of the structural features determining the magnitude of the resulting conductive hearing loss. Our recent experimental and modeling studies predicted that the conductive hearing loss will increase with increasing perforation size, be independent of perforation location (contrary to popular otologic belief), and increase with decreasing size of the middle-ear and mastoid air space (an idea new to otology).

Objective: To test our predictions regarding determinants of conductive hearing loss in tympanic membrane perforations against clinical data gathered from patients.

Study design: Prospective clinical study.

Setting: Tertiary referral center.

Inclusion criteria: Patients with tympanic membrane perforations without other middle-ear disease.

Main outcome measures: Size and location of perforation; air-bone gap at 250, 500, 1,000, 2,000, and 4,000 Hz; and tympanometric estimate of volume of the middle-ear air spaces.

Results: Isolated tympanic membrane perforations in 62 ears from 56 patients met inclusion criteria. Air-bone gaps were largest at the lower frequencies and decreased as frequency increased. Air-bone gaps increased with perforation size at each frequency. Ears with small middle-ear volumes, < or = 4.3 ml (n = 23), had significantly larger air-bone gaps than ears with large middle-ear volumes, > 4.3 ml (n = 39), except at 2,000 Hz. The mean air-bone gaps in ears with small volumes were 10 to 20 dB larger than in ears with large volumes. Perforations in anterior versus posterior quadrants showed no significant differences in air-bone gaps at any frequency, although anterior perforations had, on average, air-bone gaps that were smaller by 1 to 8 dB at lower frequencies.

Conclusion: The conductive hearing loss resulting from a tympanic membrane perforation is frequency-dependent, with the largest losses occurring at the lowest sound frequencies; increases as size of the perforation increases; varies inversely with volume of the middle-ear and mastoid air space (losses are larger in ears with small volumes); and does not vary appreciably with location of the perforation. Effects of location, if any, are small.

Figures

FIG. 1

Tympanograms illustrating the estimation of middle-ear volume in an ear with a TM perforation. (Above) Tympanometric tracing from the normal (contralateral) ear. The ear-canal volume is 1.4 ml, measured at −400 daPa of ear canal pressure. (Below) A flat tracing in the ear with the perforation, with a total volume of 7 ml. Therefore, middle-ear volume in the ear with the perforation is estimated to be total volume in ear with perforation (7 ml) –ear-canal volume in contralateral normal ear (1.4 ml) = 5.6 ml.

FIG. 2

Scatterplot showing middle-ear volumes and areas of each perforation in the study population of 62 ears. Each ear is also grouped by location of the perforation: anterior, posterior, or both.

FIG. 3

Mean ABGs for the large-volume group (n = 39) and the small-volume group (n = 23) at each audiometric frequency. Error bars indicate ±1 standard error of the mean. Ears with small volumes had significantly larger ABGs at all frequencies except 2,000 Hz.

FIG. 4

Effects of area of perforation in the large volume group. Areas of perforations are in square millimeters. Error bars indicate ± 1 standard error of the mean.

FIG. 5

ABGs (in decibels) at each audiometric frequency for anterior versus posterior TM perforations. All ears had a large middle-ear volume, >4.3 ml. The last column indicates a four-tone (500, 1,000, 2,000, and 4,000 Hz) pure-tone average. Perforation sizes were similar in both groups. Error bars indicate ±1 standard error of the mean. NS, not statistically significant at the 5% level (p > 0.05).

FIG. 6

Predicted versus measured ABGs at 250 and 500 Hz (n = 62 ears). The solid lines indicate a slope of 1 (measured = predicted). The dotted lines represent a best-fit linear regression line through the data points. The correlation coefficient (r) was highly significant (p < 0.01) for both frequencies.

FIG. 7

Illustrative clinical cases from the present study comparing the ABGs in perforations with substantially different middle-ear volumes. The differences in hearing losses in these cases can be readily understood on the basis of differences in middle-ear volume rather than location. Ears with smaller volumes demonstrate larger ABGs. (A) An anterior and a posterior TM perforation are shown, with differing middle-ear volumes. The solid line representing a 9.6-mm2 posterior perforation with a greater than or equal to 5.9 ml middle-ear volume has smaller ABGs of 0 to 40 dB. The dashed line is a 3.1-mm2 anterior perforation with a 1.2-ml middle-ear volume and has larger ABGs of 15 to 35 dB. The differences in hearing losses between these two ears can be best explained on the basis of their differing volumes. (B) An anterior and a posterior TM perforation are shown, with differing middle-ear volumes. The solid line representing a 19.6-mm2 posterior perforation and a 0.9-ml middle-ear volume has large ABGs of 25 to 45 dB. The dashed line is an 11-mm2 anterior perforation with a greater than or equal to 5.7 ml volume and demonstrates smaller ABGs of 0 to 30 dB.