The function of the middle-ear air spaces as tuned resonators.

The middle-ear air-spaces (MEAS) not only act as an air reservoir and provide a buffering effect against negative intratympanic pressures caused by tubal dysfunction, but more fundamentally constitute a resonant cavity whose impedance is appropriately tuned to the frequency response of the auditory system of each individual species. When this ideal air volume is altered, either by pathology or by surgery, the response characteristics of the middle ear, and consequently, of the entire auditory system are modified. This concept was tested by measuring the compliance of artificial membranes coupled to closed rigid air spaces of varied volumes, and also by obtaining compliance-pressure functions (tympanograms) of guinea pig ears when the volume of their MEAS was surgically modified. A direct relationship between changes in the middle ear air volume and measurements of tympanic compliance was demonstrated in both models and real ears. This effect has implications on the sensitivity and frequency response characteristics of ears in which this volume has been significantly altered by either pathological processes or by surgical procedures.

Reversible hearing loss after meningitis. Prospective assessment using auditory evoked responses.

Hearing loss (HL) was prospectively studied in 236 children with meningitis using brainstem auditory evoked responses. Hearing loss was detected in 38 (16.1%) in the acute phase of the disease and in 10 (5.2%) of 193 patients who were tested 6 months later. Hearing loss was more frequent and more severe in patients with bacterial meningitis (25.4% in the acute phase, and 8% 6 months later) than in patients with viral and tuberculous meningitis. In half of the affected cases the auditory lesion was bilateral. Follow-up was possible in 32 patients with early postmeningitic HL. Ten (31.2%) had permanent HL while the remaining 22 (68.7%) recovered normal hearing. Severe initial losses tended to be permanent, whereas minimal and intermediate losses were generally reversible and patients recovered completely in 1 to 6 months. The high incidence of HL in bacterial meningitis suggests that any one of the existing methods of auditory screening should be performed in all patients during the convalescent period.

Hearing loss after Hemophilus influenzae meningitis. Follow-up study with auditory brainstem potentials.

Hearing loss was studied prospectively using auditory brainstem potentials in 34 children with Hemophilus influenzae meningitis treated with chloramphenicol and in 36 children with purulent meningitis caused by microorganisms other than H influenzae treated with ampicillin. Our results indicate that high doses of ampicillin are not associated with permanent hearing loss. Moreover, no statistically significant difference was found in the incidence of hearing loss following H influenzae meningitis and that following meningitis caused by other microorganisms, either during the acute phase or in the convalescent period. Neurological damage, when present, was probably located in the inner ear or in the auditory nerve. Finally, early hearing loss in purulent meningitis does not always constitute an irreversible lesion.

[Experimental study of post-labyrinthectomy compensation by means of the torsion pendulum]. / Estudio experimental de la compensación post- laberintectomía mediante el péndulo de torsión

Recovery patterns after unilateral labyrinthectomy were studied using a torsion pendulum to produce angular accelerations in unanaesthetized, immovilized cats. The oculomotor activity before and during angular accelerations was recorded and cuantified in 10 post-labyrinthectomized cats during 26 days of recovery both with and without environmental fixation (eyes open or covered). The integrated nistagmus obtained with rotation in each direction was used as a measure of oculomotor activity, as well as an indicator of directional preponderance. It was found this indicator to be far more sensitive than the raw electronystagmic tracings to detect asymmetries in the oculomotor response. The integrated movement of the eyes without ambiental fixation (eyes covered) was consistently 40% of the pendulum's displacement; while having both eyes open, the ocular displacement was equal (100%) to that of the environment. These relations changed in a predictable and characteristic way after labyrinthectomy. Approximately between 5 and 12 days post-labyrinthectomy, the responses to stimulus in both directions become of very small magnitude, indicating a depression of the reflexes originating in the remaining normal labyrinth.