Hiroaki Funai

Experimental Study on the Effect of Inferior Colliculus Lesions upon Auditory Brain Stem Response

The relationship between the activity of the inferior colliculus and each component of the auditory brain stem response (ABR) was investigated in rats using an electrical coagulation method. The ABR seems to be a composite of slow and fast waves, and the central nucleus of the inferior colliculus is more important for the slow wave than for the fast waves. There was little effect on the shape and latency of the fast waves of the ABR when the destruction was limited to the central nucleus. When the lesion was extended to or located in the lateroventral part of the inferior colliculus, where the nerve fibers of the lateral lemniscus penetrate into the inferior colliculus, potentials 5 and 6 were abolished, but potential 4 remained with a slightly prolonged latency. These findings suggest that the structures peripheral to the inferior colliculus play an important role for the generation of potential 4, and the inferior colliculus, particularly its lateroventral part, is essential for generation of potentials 5 and 6.

Sweep-Frequency Tympanometry: Its Development and Diagnostic Value

This reports a newly developed tympanometric system using a sweep-frequency probe tone. For a sweep-frequency tone ranging from 220 to 2 000 Hz, measurements of sound pressure (P) and phase were performed at ear canal pressures of 0 and -200 mm H2O. The results were expressed as a sound pressure curve (P0-P-200 in decibels), a phase curve (formula: see text) and a polar curve (formula: see text) against probe tone frequency. Both the frequency at which the sound pressure curve crossed the 0-dB difference line and the peak frequency of the phase curve shifted lower than normal for ossicular disruption and higher than normal for ossicular fixation. Changes in the sound pressure curve and in the phase curve were exaggerated for ossicular disruption and limited for fixation. As the result of these, the polar curve showed an expanded type for disruption and a compressed type for fixation. A review of 220-Hz tympanograms and of the polar curves for 10 patients demonstrated that the latter permitted a better discrimination among ossicular disorders.

An electrophysiologic study of experimental perilymphatic fistula

This study was undertaken to elucidate the mechanism that causes sensorineural hearing loss in clinical cases with perilymphatic fistula. Perilymph was experimentally aspirated through the round window membrane in 17 guinea pigs. The extent of cochlear damage was examined electrophysiologically as well as histopathologically. Immediately after aspiration, several types of changes in summating potential (SP) were observed. Two animals without a polarity change of the SP showed only slight threshold changes in both cochlear microphonic and action potentials, and no specific histopathologic changes in the cochlea. Reversed polarity of the SP was observed in three animals, of which one showed a high-amplitude negative SP followed by rapidly progressive hearing loss. Bulging of Reissners membrane was confirmed histopathologically in this case. The SP disappeared in the remaining 12 animals. In animals with profound electrophysiologic changes, bulging or rupture of Reissners membrane and damaged hair cells were observed. These findings suggest that an abrupt change in perilymphatic pressure produces morphologic changes in the membranous labyrinth, causing changes in the vibration function of the cochlear partition and in the function of the organ of Corti. Abrupt pressure imbalance may be a causative factor of sensorineural hearing loss in the case of perilymphatic fistula.

Auditory Brainstem Responses in Leptomeningeal Metastatic Spread

Auditory brainstem responses (ABRs) were recorded in 2 patients with hearing loss caused by leptomeningeal metastatic spread. These recordings showed similar characteristic findings. The absolute latencies of wave V and interwave latencies I-V were exceedingly increased. Definite effects on the wave morphology and latency of wave V were observed with the use of high repetition rates. It is surmised that the hearing loss caused by leptomeningeal metastatic spread is mainly an effect on the auditory nerve and/or the cochlear nucleus. ABR examination is of clinical value in detecting functional abnormalities resulting from leptomeningeal metastatic spread.

Auditory threshold shift following meatal pressure change in ossicular disorders.

Auditory threshold shifts for air conduction following meatal pressure changes were examined in 30 normal ears and in 27 ears with ossicular abnormalities. Negative and positive changes in meatal pressure caused threshold elevation at frequencies lower than 1,000 Hz in all normal ears. In the ears with ossicular fixations, air pressures were less effective in attenuating hearing as compared with normal ears. Clearly different results were obtained in the patients with incudostapedial disconnection without stapes fixation. Thresholds were elevated by positive pressure, although marked threshold gains were measured by negative pressure at low frequency tones. Differences in threshold levels between the air pressures of +200 and -200 mm H2O were 26 to 40 dB at 250 Hz. This marked reverse effect in direction of the threshold shifts cannot be explained only by relative compliance changes in the tympanic membrane. Possible mechanisms underlying this phenomenon are discussed.

Congenital Stapedial Tendon Ossification; A Case Report.

A ten-year-old girl with right conductive hearing loss was found to have unilateral congenital stapedial tendon ossification. Exploratory tympanotomy revealed absence of the incus long crus, anomaly of the stapes superstructure and a stapedial tendon ossification. After excision of the tendon and insertion of the cartilage piece between the malleus neck and the superstructure, her hearing was almost normal.

Primary Cyst in the Mastoid

Formation of cysts in the paranasal sinuses is common but in the mastoid cavities, it is rare. A 49-year-old female had a primary cyst in a mastoid cavity. Operation confirmed that the aditus was completely obliterated by the cyst wall and a cholesterol granuloma. There was no communication between the mastoid cavity and the epitympanic cavity. The etiology of mastoid cysts has been uncertain, but obstruction of the air ventilation in the middle ear space at the aditus is thought to play an important role in the etiology. Mastoid cyst is diagnosed by aspiration of the cyst, but it is important to differentiate it from meningoencephalocele and meningocele protruding into the mastoid cavity. Radiologic imaging, especially CT and MRI, should be done before the aspiration.

The use of intraoperative auditory evoked brainstem responses in the treatment of congenital ossicular anomalies

Intraoperative auditory brainstem responses (ABRs) were monitored in 3 patients undergoing middle ear surgery for congenital ossicular anomalies. It was investigated whether this technique was capable of providing useful information to the operating surgeon. After the induction of anaesthesia and intubation, standard disc electrodes were attached to the vertex (active), ipsilateral mastoid (reference), and forehead (ground). These electrodes were draped out of the surgical field. The general anaesthesia was maintained by inhalation of enflurane. Stimulus intensity was decreased from 80 dBnHL in 10 dB steps until no response was obtained. Thus latency-intensity functions for wave V of ABR were obtained. First (A) recording was performed before the incision. The tympanomeatal flap was then elevated and the bony annulus overloading the oval window was drilled out. This enabled to visualize and confirm the anomalies of the ossicles. Before reconstructing the ossicles, the tympanomeatal flap was returned to its normal position and second (B) recording was obtained. During the reconstruction, recordings (C1, C2,...) were repeated as indicated. Then it was confirmed whether the effective reconstruction was obtained for auditory function. The results suggest that latency of wave V was significantly decreased when the effective reconstruction of the ossicular chain was done. Latency-intensity function is shifted to the left of A and B functions. Intraoperative auditory assessment using ABR in the reconstructive surgery for the ossicular channel is promising in providing useful information for auditory function and in predicting postoperative hearing improvement.

An electrophysiological study on the effect of laser irradiation of round window membrane in the guinea pig.

An argon laser beam was used to irradiate the round window in 17 guinea pigs. Each animal was examined electrophysiologically with an electrode placed on the round window (RW) membrane 3 or 30 days after the irradiation, after which the cochlea was examined morphologically. The RW membrane was found intact in 16 out of 17 animals. All potentials (SP, CM, CAP) were abolished in 8 of the 17 animals. Histologically, perforations of the basilar membrane and Reissners membrane were observed in 7 of 8 cases. In 4 of the 17 cases, a small effect of irradiation was evident both physiologically and histologically. Significant elevation of thresholds was observed in the remaining 5 animals, where amplitudes of CM, +SP, and CAP decreased at all intensities. Varying degrees of membranous labyrinth destruction can be used without perforating the RW membrane by the present method.