The development of the middle ear in neonatal chinchillas II. Two weeks toadulthood.

Studies of auditory function in the human neonate indicate adult-like heari ng sensitivity, mature cochlear function and well-developed responses in th e auditory pathway. Paradoxically, measurements of middle ear function are characterized by responses that would be interpreted as abnormal in older s ubjects. Consequently, there is not an accepted clinical test for middle ea r disease in the newborn population. Like human neonates, chinchillas have normal hearing sensitivity at birth, but middle ear function tested by mult ifrequency tympanometry is abnormal compared to the adult. A previous study from our laboratory indicated that the newborn chinchilla middle ear is fr ee of mesenchyme and other debris. Over the first 2 weeks of life there wer e no significant changes in tympanic membrane thickness and diameter, tympa nic membrane to promontory distance and stapes footplate length. There were small changes in mastoid bulla area and perimeter and in mastoid bulla bon e thickness. The most striking difference between the newborn and adult tem poral bone was in bone composition, the newborn bone having a less dense, s pongy appearance. Impedance characteristics of the newborn chinchilla ear, measured by multifrequency tympanometry, were abnormal relative to adult an imals and did not change over the first 2 weeks of life. This investigation is an extension of the previous study, designed to better understand the r elationship between middle ear function, hearing sensitivity and the struct ural changes of the newborn chinchilla middle ear. Twenty animals, aged 2-8 weeks, were studied. Additional adult animals were used as controls. Middl e ear function was assessed by a wideband reflectance/impedance system. Hea ring sensitivity was measured by auditory brainstem response in 2- and 8-we ek-old animals. Structural characteristics of the temporal bone were analyz ed using histopathologic preparations. There was an orderly progression in middle ear impedance and reflectance characteristics as the chinchilla ear matured from 2 to 8 weeks of age. At 8 weeks of age, impedance and reflecta nce patterns approached, but did not match, those of the adult animal. Hear ing sensitivity was unchanged throughout this maturational period. Finally, histological analysis demonstrated no age-related changes in distance from the tympanic membrane (TM) to the promontory and in stapes footplate lengt h. There was a small significant decrease in the TM thickness from 2 weeks to adulthood. The most significant developmental changes were a reduction i n mastoid bone thickness and concomitant increases in the perimeters and ar eas of the middle ear and posterior bulla.