Auditory Brainstem and Middle-Latency Responses in Bell's Palsy: Les réponses du tronc cérébral et de latence moyenne dans la paralysie de Bell

Auditory brainstem response

The auditory brainstem response (ABR), consisting of five to six vertex-positive peaks with separation of about 0.8ms, is very sensitive to factors that affect conduction velocity and hence ABR wave latencies in the brainstem auditory pathways. In addition, disorders causing dissynchronization of neural activity result in an amplitude decrease or disappearance of ABR peaks. The opposite effects occur in the maturation process, which takes about 2 years postterm; here conduction velocity increases quickly to its adult value, but synaptic delays being sensitive to synchronous release of transmitter substance take considerably longer. In neurological disorders, those that cause dissynchrony, such as auditory neuropathy and vestibular schwannoma, Gaucher disease, and Krabbe disease, the (longer latency) ABR peaks are reduced or absent. Effects on neural conduction, resulting in increased ABR interwave latencies, are found in vestibular schwannomas, Bell's palsy, Duane retraction syndrome, Marcus Gunn ptosis, and various encephalomyopathies. These measures allow an assessment of the parts of the brainstem that are involved.

Evaluation of the acoustic (stapedius) reflex test in children and adolescents with peripheral facial nerve palsy

INTRODUCTION

The stapedius nerve is one of the branches of the facial nerve in the temporal bone. It supplies the stapedius muscle, which is responsible for the attenuation reflex that protects the inner ear from loud noises. The stapedius (acoustic) reflex (SR) test is useful in identifying the site of facial nerve injury. The return of the SR (acoustic) to normal after an injury is a good prognostic factor in the treatment of facial nerve palsy.

OBJECTIVE

The aim of this study was to evaluate the effect of FNP on the SR (acoustic) response and determine the acoustic reflex threshold (ART) levels on the affected side.

MATERIAL AND METHOD

In this study, 70 patients, 3-7 years old, were screened. The study population consisted of 26 boys (37%) and 44 girls (63%). Follow-up tests were performed 3-18 months after the initial tests.

RESULTS

Most patients in the study population had a negative SR (acoustic) response on the affected side. In other patients, mean ART values were statistically higher on the affected side. There was no statistically significant relationship between a reflex response and the time from the onset of facial nerve palsy.

DISCUSSION

In the available literature, the SR (acoustic) testing is limited in determining whether or not the reflex is present without stimulus frequency or ART measurements. It is estimated that the reflex response is negative with ipsilateral stimulation on the affected side in 35-80% patients.

CONCLUSIONS

The SR (acoustic) is absent in most patients on the affected side. The ART value was statistically higher on the affected side. The SR (acoustic) response was statistically time independent.

[Analysis of morphology of auditory brainstem with idiopathic peripheral facial nerve paresis responses in patients]

INTRODUCTION

Idiopathic facial nerve paresis still remains a challenge for laryngologists and neurologists. The etiology of idiopathic peripheral facial nerve paresis has not been explained so far. There is a group of patients that complain about coexisting hearing abnormalities.

AIM

Analysis of audiological abnormalities in patients with idiopathic peripheral facial nerve paresis on the basis of pure tone audiometry and auditory brainstem responses.

MATERIAL AND METHODS

The retrospective analysis of 35 patients, aged under 40 years, with idiopathic peripheral facial nerve paresis hospitalised in Department of Otolaryngology of Warsaw Medical University-Poland (2004-2009). Control group consisted of age-matched, 23 healthy subjects. Each patient underwent audiometric evaluation that included pure tone audiometry and auditory brainstem responses. Statistical analysis was performed with Student's t-test.

RESULTS

In the analysed group of 35 patients (17 male + 18 female), age ranged 13-40 years (mean 26.7 +/- 7.1) significantly prolonged latencies of wave III and V and III-V and I-V intervals in comparison with control group where observed (p < 0.05). We found also that 48.6% patients with idiopathic peripheral facial nerve paresis had high-frequency hearing loss.

CONCLUSIONS

The auditory brainstem responses in this study showed an association between retrocochlear pathology of the auditory system and idiopathic peripheral facial nerve paresis. Further investigations are necessary to estimate the etiopathology of these coexisting abnormalities.

Re-examination of the role of the human acoustic stapedius reflex

The "rollover" seen in the word recognition performance scores of patients with Bell's palsy (facial nerve paralysis) has historically been taken as an indicator of the role of the stapedius reflex in the protection from upward spread of masking. Bell's palsy, however, may be a polyneuropathy, so it is not clear that the poor word recognition performance at high levels is necessarily attributable specifically to impaired facial nerve function. The present article reports two new experiments that probe whether an isolated impairment of the stapedius reflex can produce rollover in word recognition performance-intensity functions. In experiment 1, performance-intensity functions for monosyllabic speech materials were obtained from ten normal listeners under two listening conditions: normal and low-frequency augmented to offset the effects of the stapedius reflex on the transmission of low-frequency vibrations to the cochlea. There was no effect of the spectral augmentation on word recognition for stimulus levels up to 107 dB SPL. In experiment 2, six patients who had undergone stapedectomy were tested for rollover using performance-intensity functions. None of the patients showed rollover in their performance-intensity functions, even at stimulus levels in excess of 100 dB HL. These data suggest that if the stapedius reflex has a role in protection from upward spread of masking, then this role is inconsequential for word recognition in quiet.

Multiple-channel stimulation of the cochlear nucleus

To further test the feasibility of a central nervous system auditory prosthesis, the characteristics of the electrically evoked middle latency response were studied in a series of acutely anesthetized pigmented guinea pigs, with multi-channel penetrating cochlear nucleus electrodes placed into the cochlear nucleus under direct visualization. These stimulating electrodes consisted of a silicone substrate, with five stimulating pads each, sputtered with iridium. Monopolar and bipolar stimulation were used. Threshold, latency, and input-output functions of the electrically evoked middle latency response were studied. Systematic differences were observed, depending on the site and parameters of stimulation. Principally, higher currents were required to produce waves of equal amplitude when the electrodes were closely spaced. For near electrode pairs, the maximum wave amplitudies obtainable within the limits of tissue safety were much lower than for distant electrode pairs. The slope of the growth function curve was steeper for widely spaced electrodes than for adjacent sites. Monopolar stimulation demonstrated maximum wave amplitudes with the lowest current intensity, implying current spread to the entire cochlear nucleus with this stimulation montage. In some cases, threshold differences were observed, higher thresholds being associated with closely spaced electrodes. These findings are consistent with simple models of the electric fields expected to be generated by these electrode arrays. The results support the hypothesis that activation of subpopulations of auditory brainstem neurons with multi-channel penetrating microelectrodes is possible.