1

Introduction

Surgical procedures of the ear are always a fine balance of work to improve the existing hearing or preserve what is left in the process of clearing the disease process. Hence it is important to minimise the iatrogenic damage to the anatomy during the surgical procedure. Drilling is an important component in many mastoid procedures. The shrill produced from the drill can produce sounds up to 107 dB well above the safe limit levels. The non-operated ear will be exposed to sounds just 5 dB less due to trans cranial attenuation and air conduction . This high intensity noise has the theoretical possibility of producing sensorineural hearing loss in the non-operated ear, many a times the only ear with intact hearing. This problem of noise induced acoustic trauma to the non-operated ear is a less researched topic. Pure tone audiograms in previous studies showed no changes in the hearing thresholds, but the fine changes and effects on the outer hair cells will be easily missed in such tests. DPOAEs can accurately monitor the outer hair cell functions and hence can be employed in their testing. In the present study we studied these possible deleterious effects by monitoring pre-operative and serial post-operative DPOAE measurements, and determined the duration of the effects and time taken for their recovery as well.

2

Materials and methods

A comparative and prospective study was conducted on the 49 patients who underwent modified radical mastoidectomy or cortical mastoidectomy (study group) and 49 patients who underwent myringotomy/myringoplasty surgeries (control group). Patients with features like cholesteotoma; attic-deep retractions, perforations, outer wall destructions; sigmoid sinus thrombus; tegmen erosion; chronic otitis with facial palsy were included in the unsafe type of chronic otitis media and subjected to canal wall down procedure. Normal preoperative pure tone audiometry and normal otoscopic findings in the non-operating ear in both groups were the inclusion criteria. Patient’s medical records were reviewed for age and the surgical technique used & the pre and post operative DPOAE amplitudes and the duration of surgeries were compared in each group and in between the two groups. DPOAEs were be obtained prior to surgery, immediately after surgery in the post op room, one day post operative period and one week post op. All the surgeries were done by the primary author. The drill used was Marathon Micro Motor N7R. The drill speed was between 20 and 40,000 RPM; the burr diameter was between 1 and 4.5 mm (cutting and diamond). Stimulus presentation data recording & spectrum analysis were carried out using the GSI AUDERA portable DPOAE measuring device. The F1 and F2 LEVELS were delivered at 65 dB. Frequencies F2 = 2, 3, 4 & 5 kHz & a frequency ratio of F2/F1 = 1.22. The values were compared and statistical analysis was done using student’s chi square test, with help of the institution’s statistician. Study was approved by the institutional ethical committee.

2

Materials and methods

A comparative and prospective study was conducted on the 49 patients who underwent modified radical mastoidectomy or cortical mastoidectomy (study group) and 49 patients who underwent myringotomy/myringoplasty surgeries (control group). Patients with features like cholesteotoma; attic-deep retractions, perforations, outer wall destructions; sigmoid sinus thrombus; tegmen erosion; chronic otitis with facial palsy were included in the unsafe type of chronic otitis media and subjected to canal wall down procedure. Normal preoperative pure tone audiometry and normal otoscopic findings in the non-operating ear in both groups were the inclusion criteria. Patient’s medical records were reviewed for age and the surgical technique used & the pre and post operative DPOAE amplitudes and the duration of surgeries were compared in each group and in between the two groups. DPOAEs were be obtained prior to surgery, immediately after surgery in the post op room, one day post operative period and one week post op. All the surgeries were done by the primary author. The drill used was Marathon Micro Motor N7R. The drill speed was between 20 and 40,000 RPM; the burr diameter was between 1 and 4.5 mm (cutting and diamond). Stimulus presentation data recording & spectrum analysis were carried out using the GSI AUDERA portable DPOAE measuring device. The F1 and F2 LEVELS were delivered at 65 dB. Frequencies F2 = 2, 3, 4 & 5 kHz & a frequency ratio of F2/F1 = 1.22. The values were compared and statistical analysis was done using student’s chi square test, with help of the institution’s statistician. Study was approved by the institutional ethical committee.

3

Results

The mean age in the case group was 32 and the mean age in the control group was 27. In the case group 26 males and 23 females were included. In the control group 29 males and 20 females were included. Of the 49 patients in the case group 30 patients were having safe type of CSOM who underwent cortical mastoidectomy with myringoplasty, whereas 19 patients presented with unsafe type of CSOM who underwent modified radical mastoidectomy procedures. Of the 49 patients in the control group 34 patients who presented with safe type of CSOM underwent myringoplasty and 3 patients with ASOM and 12 patients with secretory otitis media underwent myringotomies, both procedures which lack drilling.

Of the 49 patients in the case group the duration of mastoid drilling in the cortical mastoidectomy procedures was under 60 min, whereas the MRM procedures were longer. 5 procedures were ranging between 60 and 180 min and 12 procedures were longer than 180 min.

3.1

Changes in the DPOAE amplitudes in case and control group pre and post operatively (till 7th Post op day)

There was a change in the amplitudes of DPOAEs in the immediate post-operative period over all the 4 frequencies in the case group. All the changes were statistically very significant (p < 0.001) observed over 45 cases. 4 cases did not show any change in the DPOAEs. Maximum changes in the pre op and immediate post op amplitudes of DPOAEs were recorded in 2 and 4 kHz. Recovery was noticed by post operative day 1 and complete recovery was seen by post operative day 7 ( Table 1 and Chart 1 ).

Table 1

Differences between pre op and post op period DPOAEs in case group.

Group	Dependent Variable	(I) time	(J) time	Mean Difference (I − J)	p
Cases	DPOAE	Pre	Post immed	18.755	.000
	2 kHz		Post 1 day	7.959	.000
			Post 7 days	1.122	1.000
		Post immed	Post 1 day	− 10.796	.000
			Post 7 days	− 17.633	.000
		Post 1 day	Post immed
			Post 7 days	− 6.837	.000
	3 kHz	Pre	Post immed	8.367	.000
			Post 1 day	2.510	.004
			Post 7 days	.286	1.000
		Post immed	Post 1 day	− 5.857	.000
			Post 7 days	− 8.082	.000
		Post 1 day	Post immed
			Post 7 days	− 2.224	.015
	4 kHz	Pre	Post immed	11.918	.000
			Post 1 day	4.592	.000
			Post 7 days	.714	1.000
		Post immed	Post 1 day	− 7.327	.000
			Post 7 days	− 11.204	.000
		Post 1 day	Post immed
			Post 7 days	− 3.878	.000
	5 kHz	Pre	Post immed	2.959	.000
			Post 1 day	1.000	.017
			Post 7 days	.286	1.000
		Post immed	Post 1 day	− 1.959	.000
			Post 7 days	− 2.673	.000
		Post 1 day	Post immed
			Post 7 days	− .714	.194

DPOAE changes in case and control group from preop to post op 7th day.

Pre operative amplitudes of the DPOAEs in the control group were similar to those of the case group without any statistically significant differences between them. No changes of statistical significance were noticed in the control group over all the frequencies in the post-operative period ( Table 2 and Chart 1 ).

Table 2

DPOAEs in the control group pre and post operatively.

Descriptives
group			N	Mean	Std. Deviation
Controls	DPOAE	Pre	49	20.94	1.049
	2 kHz	Post immed	40	20.75	.899
		Post 1 day	49	18.69	5.554
		Post 7 days	49	20.61	1.151
	3 kHz	Pre	49	17.18	.808
		Post immed	40	16.98	.800
		Post 1 day	49	16.76	1.128
		Post 7 days	49	17.08	.759
	4 kHz	Pre	49	12.88	.807
		Post immed	40	12.93	.764
		Post 1 day	49	11.86	3.208
		Post 7 days	49	12.78	1.066
	5 kHz	Pre	49	10.27	.730
		Post immed	40	10.30	.648
		Post 1 day	49	10.18	.858
		Post 7 days	49	10.29	.540

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