Distortion Product Otoacoustic Emissions in Otosclerosis Intraoperative Findings

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R. Filipo, G. Attanasio, M. Barbaro, M. Viccaro, A. Musacchio, G. Cappelli, E. De Seta

Department of Neurology and Otolaryngology, University 'La Sapienza', Rome, Italy

Abstract

The aim of the study was to investigate changes in middle ear dynamic characteristics caused by both otosclerosis and stapes surgery (platinotomy, prosthesis positioning, ossicular chain maneuver) and to evaluate distortion product otoacoustic emissions (DPOAEs) before and following surgery. The study included 15 patients (12 women, 3 men; mean age 51 years; range 32-69 years) with advanced otosclerosis. All the patients were evaluated with the use of pure-tone audiograms (preoperatively, 5 and 30 days after surgery), stapedial reflexes (preoperatively), and DPOAE recordings (preoperatively, at the end of surgery, and 5 and 30 days after surgery). Changes in the hearing thresholds and in the DPOAE amplitudes were compared. Preoperative tests showed conductive hearing loss, with a mean air-bone gap of 36.6 dB HL ranging from 0.25 to 1 kHz, and no stapedial reflexes were detected. DPOAEs were not measurable preoperatively, and they were detected only in 2 patients at the end of surgery, with low amplitudes in a narrow frequency range. No significant changes occurred in DPOAEs 5 days postoperatively. A month after surgery, improvement in conductive hearing loss was observed; the mean air-bone gap from 0.25 to 1 kHz was 12.9 dB HL, whereas the higher frequencies were still affected by the disease. DPOAEs increased in amplitude in 4 patients, but this was not significant. It remains unclear why DPOAEs are not detected despite a subjective hearing improvement and a sufficiently closed air-bone gap at least in middle and low frequencies. The results of our study show that DPOAEs cannot replace behavioral threshold tests; they may only be included in a battery of tests for a complete clinical follow-up for efficiency monitoring after stapes surgery.

Copyright © 2007 S. Karger AG, Basel

Otoacustic emissions (OAEs) are objective, noninvasive measures for cochlear outer hair cell function. OAEs are the product of biomechanical motility of hair cells; they are divided into spontaneous and evoked emission [1].

Distortion product otoacustic emissions (DPOAEs) occur because of the nonlinear nature of outer hair cells. DPOAEs are produced when two tones of different, but related, frequencies (Fj and F2) are presented to the cochlea simultaneously; in response to these two tones, a normal cochlea will generate tones related to Fj and F2.

OAEs are usually used to evaluate, analyze and check several diseases that may involve the cochlea, including otosclerosis [2].

Otosclerosis is a primary localized disease of the bony otic capsule. It has a predilection for the oval window and fixes the stapedial footplate, but it can also involve the whole otic capsule, causing conductive or sensorineural hearing loss. Usually stapes surgery, and particularly stapedotomy, is the first-choice treatment.

The aim of the study was to investigate changes in middle ear dynamic characteristics caused both by the disease and the surgical steps (platinotomy, prosthesis positioning, ossicular chain maneuver) and to evaluate their influence on DPOAE responses in patients with otosclerosis [3, 4].

A group of 15 patients (12 women and 3 men) affected by advanced otosclerosis was included in the present study. The patients were admitted to the ENT clinic and candidates for stapedotomy. Preoperatively, pure-tone audiograms, stapedial reflexes and DPOAEs were recorded. At the end of surgery, after piston prosthesis insertion and tympanomeatal flap repositioning, DPOAEs were recorded. Five days thereafter, before patient discharge, DPOAEs were measured again. The patients were asked to come back to the clinic 30 days after surgery and they underwent pure-tone audiogram and DPOAE measurements.

Regarding DPOAEs, stimulus presentation, data recording and spectrum analysis were carried out using a Labat Otoacoustic Emission Test Instrument Model Eclipse. The F1 and F2 levels were both 75 dB SPL, they were constant at all frequencies tested and DPOAEs were plotted for different frequencies (DP-gram). DP-grams were collected in 1-octave steps from 500 Hz to 1 kHz and in 1/3-octave steps from 1 to 8 kHz.

Preoperative tests showed conductive hearing loss, with a mean air-bone gap of 36.6 dB HL ranging from 0.25 to and 1 kHz; no stapedial reflexes were detected. DPOAEs were not measurable preoperatively and they were evident only in 2 patients in the intraoperative test, with low amplitudes in a narrow frequency range. No significant changes were observed 5 days postoperatively. A month after surgery, conductive hearing loss had improved; the mean air-bone gap from 0.25 to and 1 kHz was 12.9 dB HL. The DPOAE amplitude increased in 4 patients, but it was not significant. Figures 1 and 2 show the mean audio-metric and DP-gram results.

Air-bone gaps in pure-tone audiograms were reduced in all operated patients limited to the frequency range from 0.25 to 1 kHz, while the higher frequencies were still affected by the disease.

Fig. 1. Mean tonal audiograms of the 15 patients included in the study, collected a day before and 30 days after stapedotomy. A = Air conduction threshold; B = bone conduction threshold.
Fig. 2. Mean DP-grams recorded just before surgery (pre), intraoperatively (intra), 5 days later (peri) and 30 days after stapedotomy (post).

The results of the present study showed that despite a subjective hearing improvement and the fact that the air-bone gap is sufficiently closed, at least in middle and low frequencies, OAEs cannot be recorded in most cases. However, it could be observed that the surgical maneuvers in the middle ear and around the oval window do not significantly affect the outer hair cell physiology even at the level of the basal cochlear turn.

References

1 Kemp DT: Stimulated acoustic emissions from within the human auditory system. J Acoust Soc Am 1978;64:1386-1391.

2 Balkany TJ, Telishi FF, McCoy MJ, Lonsbury-Martin BL, Martin GK: Otoacustic emissions in otologic practice. Am J Otol 1994;15:29-38.

3 Osterhammel PA, Nielsen LH, Rasmussen AN: Distortion product otoacustic emissions. The influence of middle ear transmission. Scand Audiol 1993;22:111-116.

4 Herzog M, Shehata-Dieler WE, Dieler R: Transient evoked and distortion product otoacoustic emission following successful stapes surgery. Eur Arch Otorhinolaryngol 2001;258:61-66.

Prof. Roberto Filipo

Department of Neurology and Otolaryngology, University 'La Sapienza'

Viale del Policlinico 155

IT-00161 Rome (Italy)

E-Mail [email protected]

Arnold W, Häusler R (eds): Otosclerosis and Stapes Surgery. Adv Otorhinolaryngol. Basel, Karger, 2007, vol 65, pp 137-145

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