Does general anaesthesia affect the child's auditory middle latency response (MLR)?

Auditory MLR and ABR were recorded simultaneously in 9 children under general anaesthesia, after electrocochleography had demonstrated a normal hearing threshold. In contrast to the ABR components which in all cases appeared to be clearly recognizable, and with latencies within the normal range, MLR revealed gross abnormalities represented by instability of the components and abnormal latency of the detectable peaks. Since such variations in the MLR pattern may be thought of as a direct consequence of general anaesthesia, it seems likely that these potentials reflect a neural activity which, in children, is affected by CNS drugs.

Auditory brain-stem response and hearing threshold in cerebellopontine angle tumours

The auditory brain-stem responses (ABR) in 19 patients with confirmed cerebellopontine angle tumours are evaluated in relation to the hearing threshold as obtained by pure-tone audiometry. ABR pathological alterations, in terms of either absence or presence of components, appear to relate to the suprathreshold stimulus intensity and to match the changes which can be observed in subjects with normal hearing, when stimulated at the equivalent suprathreshold intensity. Of the diagnostic indexes considered (I-V interval, IT5, delta V), IT5 and delta V could be evaluated in the largest patient population. They may be used in combination to enhance their sensitivity in retrocochlear lesion detection.

Simultaneous recording of auditory evoked potentials. Relationships among the fast, middle and long latency components

The whole pattern of the fast, middle and long latency auditory evoked potentials (AEP) was recorded simultaneously from the scalp surface of 13 normal-hearing adults. The individual responses were displayed on a nonlinear time axis in order to improve identification of the components. Stimulation consisted of 2048 unfiltered clicks, delivered monaurally at 80, 60, 40 dB HL with an ISI of 750 ms. Changes in mean latency and amplitude of each AEP component were statistically evaluated in relation to intensity and electrode montage (vertex-mastoid ipsi- and contralateral to the stimulated ear). The latencies of fast components I-VI and the slow P1 increase significantly with declining stimulus intensity. The amplitudes of the fast, I, II, III, V and the slow P1-N1, P2-N2 decrease significantly with intensity. As regards differences due to the electrode montage the contralateral recording causes significant changes in latency of the fast potentials up to wave IV, and changes in amplitude of the fast up to wave V, and of the slow P1-N1 and P2-N2. Therefore, as their latency and amplitude seem to be less closely related to the stimulus and electrode placement, the middle components behave differently, compared with the preceding and following components. Based on parametric comparisons of potentials ranging widely in latency, but each one evoked by an equal sensory input, this kind of AEP evaluation may be useful both for neurophysiological and clinical studies of the whole auditory pathway function.

Electrocochleography and brainstem potentials in the diagnosis of the deaf child

The deaf child must receive sound amplification before he reaches the age of two years. At this age the hearing threshold is best measured objectively by electrocochleography (ECochG) and auditory brainstem response (ABR) audiometry. When used correctly, both methods allow an exact threshold estimation which is informative enough for adequate hearing aid prescription. Both methods have advantages and disadvantages when used in children. The advantages of ECochG are: (a) a more exact threshold estimation and (b) strictly monaural evaluation. The advantages of ABR are: (a) ease of performance; it is not invasive and does not require general anaesthesia and (b) allows for exploration of higher levels in the auditory pathway up to the midbrain. We believe that ECochG and ABR are compatible and complementary in the diagnosis of childhood deafness. ABR could be used in first instance, while ECochG could be reserved for doubtful cases and for those who cannot be adequately sedated. Extra-audiological factors such as the availability of anaesthetists and varying hospital facilities, play a further role in determining the choice of electric response technique.

Evaluation of the monaurally evoked brainstem response in the diagnosis of sensorineural hearing loss

The auditory brainstem responses (ABRs) obtained in 47 subjects with asymmetric hearing loss (12 with surgically confirmed cerebello-pontine angle tumours, 35 without otoneurologic and/or neuroradiologic evidence of tumour) were evaluated by means of an index named delta V. The calculation of this index was based upon the patient's wave V absolute latency obtained at a fixed intensity of 90 dB HL and the value of latency predictable by means of the normative data. The index clearly separates retrocochlear from cochlear sites of lesion. Moreover delta V values obtained in defined cochlear lesions show a linear relation with the patient's pure tone hearing loss at 2 and 4 kHz; this behaviour is probably due to a reduction of the auditory dynamic range in the recruiting ears. delta V appears to have clinical usefulness because of two main points: first it is based upon an evaluation of the monaurally evoked ABR; second, it improves the diagnostic specificity of the responses. The rate of false positive results can be further reduced by combining delta V and IT5 values.

Phonometric study of the human first heart sound

The first heart sound was studied in 20 normal subjects. The phonocardiogram (PCG) was recorded from apical and mid-precordial areas using microphone with a flat response curve from 0.2 to 8,000 Hz. It was stored, together with a simultaneous electrocardiogram, on an FM analog tape recorder (linear frequency response from 0 to 4,000 Hz), fitted with a filter with weighting curve B according to the American National Standard Institute. A linear (SPL) recording was also made. The signal was fed through a digital converter into a minicomputer and the frequency distribution of the first heart sound was analyzed using Fast Fourier Transform. These data were stored and the average spectra were calculated for both B and SPL. The SPL spectra from both apex and mid-precordium showed a maximum intensity of about 80 dB between 12-20 Hz, decreasing progressively to a constant level of 35 dB between 110-120 Hz. The spectra obtained from both areas using filter B showed a maximum intensity of 40-50 dB between 20-60 thereafter. It is important to emphasize that the dB values in B and SPL are absolute, since they refer to a standard reference weighting. It appears that the SPL recording is more valuable in that it allows the study of all components of the PCG signal. The spectra obtained in this study will be used as a standard for future research in various pathological conditions.

Clinical and audiological findings in a case of auditory agnosia

A case is reported of severe agnosia for verbal and non-verbal sounds without associated aphasic disorder. A CT scan revealed bilateral, temporal lobe lesions from two ischaemic accidents that had occurred 9 months apart. The search for subtle deficits in the patient showed normal sensitivity to changes in the intensity and frequency of simple sounds; in contrast, his ability to discriminate sound duration and musical note sequences was severely impaired. The simultaneous recording of the whole auditory-evoked response pattern revealed no abnormality in the early components, which reflect the activation of the auditory nuclei and pathways of the brain stem. However, the middle and late components were delayed and slowed. These results and others in the literature suggest that the neocortex in man, as in other mammals, plays an essential role in the temporal aspects of hearing. Also, the two main ingredients commonly recognized in auditory agnosia, i.e. word deafness and the inability to interpret non-verbal sounds, are caused by the disruption of elementary, bilaterally represented cortical functions which start the processing of every kind of auditory information.

Logarithmic display of auditory evoked potentials

Auditory evoked potentials (AEP) can be simultaneously recorded on-line as a succession of 11 waves, through a single input channel of a mini-computer. Since the response waves differ widely in frequency, a computing routine has been developed to display the whole response pattern in a single picture. Based upon a non-linear samples reduction of the digitized response, this routine allows a logarithmic transformation of the time axis. The method improves the identification of the AEP components and provides an objective estimate of the central auditory pathway for both neurophysiological and neuroclinical studies.

Habituation and rate effect in the auditory cortical potentials evoked by trains of stimuli

The effects of the stimulus repetition rate over the habituated auditory cortical evoked responses were studied. The stimulation pattern consisted of trains of pure tone bursts with interstimulus interval (ISI) of 1 s, and intertrain interval (ITI) of 5 s, delivered with constant time and intensity parameters during 93 min. The analysis of the responses was based upon across averaging of the trains, each single response being evaluated in the latency and amplitude parameters. Two time-dependent factors affected the responses in a distinct way: the habituation throughout the whole stimulation and the rate effect within the train. The linear regressions of the time/amplitude functions of the responses were calculated in relation to the duration of ISI and ITI. By introducing a correction factor depending on the repetition rate it was possible to evaluate the relationships between habituation and repetition rate. Changes in the repetition rate do not have any effect on the habituation process. The two phenomena are completely distinct, and they probably have neurophysiologic substances corresponding to different levels in the central nervous system (CNS).

The auditory brainstem response to binaural delayed stimuli in man

The brainstem responses (BER) evoked by binaural clicks (0.1 ms) with interaural time differences (delta t) from 0 to 3.5 ms were studied in 6 normal subjects. The responses analysis was carried out via computer in two different ways: (a) Comparison between binaural BER and templates obtained with the addition of two monaural BERs. (b) Extraction from the binaural BER of the second delayed pattern. The results suggest that an algebraic addition of the responses from the two stimulated sides is the main mechanism of the binaural BER generation. In comparison with the templates, the binaural response shows some differences which are evident at delta t = 0--1.5 ms and are limited in the region of VI and VII waves. Such differences are caused essentially to an amplitude decrease of the second pattern--that is response to the delayed stimulus--mainly at delta t 1.5 ms. However these results do not allow us to say if the phenomenon recorded at the surface is due to central adaptation or to specific mechanism of binaural interaction.

The fast Fourier transform in the analysis of the normal phonocardiogram

The present study is focused on spectrum analysis as a useful method of processing the PCG in order to obtain the frequency spectral distribution of normal heart sounds. Thirty normal subjects aged from 17 to 34 were studied. PCG was recorded on the fourth intercostal space at the left sternal border using a sound level meter coupled with a standard 6 ml cavity. The microphone had a linear response from 0.2 to 8,000 Hz. The signal was filtered with the standard B network according to the ANSI specifications and was registered on a four track FM tape recorder. A four channel analyzer with a microprocessor for off-line elaborations was used with 10 KHz sampling frequency. The PCG signal was triggered by a QRS detector on the R wave of the simultaneously recorded ECG. Fast Fourier transform was performed employing either a four channel analyzer with a microprocessor, or an A/D converter with a computer. Finally the results of the analysis were statistically elaborated. The described procedures permit to obtain a direct and exact tracing of the acoustic features of the heart, thus representing an attempt to come closer to the standardization and automatic analysis of the phonocardiographic technique.