Small Tympanic Membrane Perforations in the Inferior Quadrants Do Not Impact the Manubrium Vibration in Guinea Pigs

BACKGROUND: It has been believed that location of the perforation has a significant impact on hearing loss. However, recent studies have demonstrated that the perforation sites had no impact on hearing loss. We measured the velocity and pattern of the manubrium vibration in guinea pigs with intact and perforated eardrum using a laser Doppler vibrometer in order to determine the effects of different location perforations on the middle ear transfer functions. METHODS: Two bullas from 2 guinea pigs were used to determine stability of the umbo velocities, and 12 bullas from six guinea pigs to determine the effects of different location perforations on sound transmission. The manubrium velocity was measured at three points on the manubrium in the frequencies of 0.5-8 kHz before and after a perforation was made. The sites of perforations were in anterior-inferior (AI) quadrants of left ears and posterior-inferior (PI) quadrants of right ears. RESULTS: The manubrium vibration velocity losses were noticed in the perforated ears only below 1.5 kHz. The maximum velocity loss was about 7 dB at 500 Hz with the PI perforation. No significant difference in the velocity loss was found between AI and PI perforations. The average ratio of short process velocity to the umbo velocity was approximately 0.5 at all frequencies. No significant differences were found before and after perforation at all frequencies (p>0.05) except 7 kHz (p = 0.004) for both AI and PI perforations. CONCLUSIONS: The manubrium vibration velocity losses from eardrum perforation were frequency-dependent and the largest losses occur at low frequencies. Manubrium velocity losses caused by small acute inferior perforations in guinea pigs have no significant impact on middle ear sound transmission at any frequency tested. The manubrium vibration axis may be perpendicular to the manubrium below 8 kHz in guinea pigs

The average velocity responses recorded from 3 points on the manubrium of the intact TM (upper panel) and the PI quadrant perforated TM (lower panel).

<p>A similar pattern was found in the PI perforations compared to the AI perforations.</p

Frequency responses were measured at 3 points on the manubrium of intact and perforated TM.

<p>The perforations were made in the anterior-inferior quadrant of left ear and posterior-inferior quadrant of right ear. 1, 2, and 3 on the manubrium represent umbo, middle point of manubrium, and short process of manubrium respectively, and two circlets represent the perforation locations.</p

The umbo velocities recorded from 12 bullas with intact tympanic membrane are plotted.

<p>The individual measurements were drawn in gray. The thick black solid line represents the mean velocities at each of fourteen frequencies (the filled circles) while the thick black dashed lines represent one standard deviations from the means. The range of velocities was approximately 20 dB throughout the frequency range with two peaks at 2 and 4.5 kHz.</p

Average velocity responses recorded from 3 points on the manubrium of the intact TM (upper panel) and the AI quadrant perforated TM (lower panel).

<p>Shapes of the frequency responses were quite similar at all three points on the manubrium while the highest velocities were recorded at the umbo. The average ratio of the short process velocities to the umbo velocities was approximately 0.5 at all frequencies.</p

The umbo velocities throughout the frequency range within a time window and the umbo velocities of selected frequencies.

<p>These results demonstrate that measurement of the umbo velocity was repeatable over a period of 79- (a) or 83-minutes (b) at selected frequencies (2, 5, 8 kHz).</p

Mean velocities at the umbo from 12 bullas before and after perforation are presented.

<p>The upper and lower panels are mean velocities record from the perforations in the anterior-inferior quadrants of left ears (a) and in the posterior-inferior quadrants of right ears (b). Velocity losses were noticed below 1.5 kHz and negative losses at higher frequencies (2–8 kHz).</p