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A R Thornton

Publications and source records attributed to A R Thornton.

At least 19 recordsLinked to original sources
PubMed2001

Comparison of transient evoked otoacoustic emission thresholds recorded conventionally and using maximum length sequences.

Presenting clicks according to maximum length sequences (MLSs) enables transient evoked otoacoustic emissions (TEOAEs) to be recorded at very high stimulation rates. Despite a decrease in TEOAE amplitude, the very large number of responses obtainable at high rates means that both signal to noise ratio (SNR) and detection sensitivity increase as the click rate increases. This study characterises conventional and MLS TEOAEs near threshold for a group of normally hearing adults. Stimulus presentation rates of 40 clicks/s (conventional) and 5000 clicks/s (MLS) were used. Compared to conventional recordings, the MLS technique enabled smaller responses to be detected, when averaged for the same time and to the same SNR. TEOAE amplitude recorded at detection threshold for MLS responses was 13 dB lower than that recorded conventionally. For each individual, MLS recording also produced clear, repeatable responses at stimulus levels below the detection threshold for conventional TEOAEs. The click level at TEOAE threshold was 12 dB lower for MLS compared to conventional emissions. These results suggest that TEOAE thresholds are not absolute but strongly related to the detection sensitivity of the recording system and physiological noise. The initial growth rates and the shape of input/output functions were found to be similar for the two recording techniques.

Acoustic Stimulation↗
PubMed2001

Temporal non-linearities of the cochlear amplifier revealed by maximum length sequence stimulation.

OBJECTIVES: The objective of this study was to examine whether temporal non-linearities of the cochlear amplifier, as reflected by otoacoustic emissions (OAEs), exist and are distinct from any recording system non-linearities. METHODS: Maximum length sequence stimulation, at various stimulus rates, was used to evoke OAEs from normally hearing subjects. Recordings from a 2cc cavity were also made. The data were analyzed to obtain the linear response and estimates of the slices of the 2nd and 3rd order Volterra kernels. This provided a measure of two and 3 click non-linear temporal interactions, respectively. RESULTS: The results showed that temporal non-linearities of OAEs do exist, are stable and repeatable within individuals and have properties that differ from those shown by the conventional linear response. Whilst some of the non-linear response properties conformed to the expected pattern, of increasing amplitude with increase in stimulus rate, there are some areas in which they show an unpredicted complexity. CONCLUSIONS: Whilst system non-linearities could be found, there was no difficulty in distinguishing between the physiological and system non-linear components. New areas of research and application may result from the use of these new OAE responses.

Acoustic Stimulation↗
PubMed2001

Endolymphatic hydrops in asymptomatic ears in unilateral Ménière's disease.

OBJECTIVES/HYPOTHESIS: The aim of our study was the objective assessment of endolymphatic hydrops in asymptomatic ears in unilateral Ménière's disease with a noninvasive electrophysiological test and investigation of significant clinical signs. The null hypothesis was that there would be no signs of endolymphatic hydrops in the asymptomatic ear. STUDY DESIGN: Prospective study using the traveling wave velocity test for endolymphatic hydrops. METHODS: The traveling wave velocity test was used in conjunction with standard audiological tests to investigate both ears of 181 Ménière's patients attending the Medical Research Council Institute of Hearing Research in Southampton, United Kingdom. The test uses derived auditory brainstem responses to estimate the velocity of the cochlear traveling wave that is altered in endolymphatic hydrops. Ménière's disease was assessed using Arenberg's five staging criteria. Significant correlations were evaluated using standard statistical methods. RESULTS: Of 100 patients with clinically unilateral Ménière's disease, 27% showed evidence of endolymphatic hydrops in their asymptomatic ear. There was a significant correlation between signs of hydrops and the mean air-conduction threshold at 500 Hz. CONCLUSIONS: We recommend that a full assessment of incipient disease in the asymptomatic ear in unilateral Ménière's disease should be undertaken before offering any treatment options to patients. Any suspicion of early disease in the asymptomatic ear in unilateral Ménière's disease should lead to full electrophysiological assessment to assess the evidence of endolymphatic hydrops in that ear.

Endolymphatic Hydrops↗
PubMed2000

Middle ear pathology can affect the ear-canal sound pressure generated by audiologic earphones.

OBJECTIVE: To determine how the ear-canal sound pressures generated by earphones differ between normal and pathologic middle ears. DESIGN: Measurements of ear-canal sound pressures generated by the Etymtic Research ER-3A insert earphone in normal ears (N = 12) were compared with the pressures generated in abnormal ears with mastoidectomy bowls (N = 15), tympanostomy tubes (N = 5), and tympanic-membrane perforations (N = 5). Similar measurements were made with the Telephonics TDH-49 supra-aural earphone in normal ears (N = 10) and abnormal ears with mastoidectomy bowls (N = 10), tympanostomy tubes (N = 4), and tympanic-membrane perforations (N = 5). RESULTS: With the insert earphone, the sound pressures generated in the mastoid-bowl ears were all smaller than the pressures generated in normal ears; from 250 to 1000 Hz the difference in pressure level was nearly frequency independent and ranged from -3 to -15 dB; from 1000 to 4000 Hz the reduction in level increased with frequency and ranged from -5 dB to -35 dB. In the ears with tympanostomy tubes and perforations the sound pressures were always smaller than in normal ears at frequencies below 1000 Hz; the largest differences occurred below 500 Hz and ranged from -5 to -25 dB. With the supra-aural earphone, the sound pressures in ears with the three pathologic conditions were more variable than those with the insert earphone. Generally, sound pressures in the ears with mastoid bowls were lower than those in normal ears for frequencies below about 500 Hz; above about 500 Hz the pressures showed sharp minima and maxima that were not seen in the normal ears. The ears with tympanostomy tubes and tympanic-membrane perforations also showed reduced ear-canal pressures at the lower frequencies, but at higher frequencies these ear-canal pressures were generally similar to the pressures measured in the normal ears. CONCLUSIONS: When the middle ear is not normal, ear-canal sound pressures can differ by up to 35 dB from the normal-ear value. Because the pressure level generally is decreased in the pathologic conditions that were studied, the measured hearing loss would exaggerate substantially the actual loss in ear sensitivity. The variations depend on the earphone, the middle ear pathology, and frequency. Uncontrolled variations in ear-canal pressure, whether caused by a poor earphone-to-ear connection or by abnormal middle ear impedance, could be corrected with audiometers that measure sound pressures during hearing tests.

Acoustic Impedance Tests↗
PubMed1999

Combination therapy with tirofiban and enoxaparin in acute coronary syndromes.

BACKGROUND: Tirofiban, an intravenous glycoprotein IIb/IIIa antagonist, and enoxaparin, a low molecular weight heparin, have each been shown to be effective at reducing cardiac ischemic events compared to unfractionated heparin alone in separate trials of patients with unstable angina and non-Q-wave myocardial infarction. The combination of these agents may offer further therapeutic benefit. MATERIALS AND METHODS: Fifty-five patients with non-Q-wave myocardial infarction were randomized to receive double-blind treatment with tirofiban (0.1 microgram/kg/min i.v.) for 48-108 h coadministered with either enoxaparin (1 mg/kg sc q 12 h) (n=26) or unfractionated heparin (i.v. adjusted to activated partial-thromboplastin time) (n=27) to evaluate pharmacokinetics, pharmacodynamics, and safety. The primary objective of the study was to investigate the effect of unfractionated heparin versus enoxaparin on the plasma clearance of tirofiban. RESULTS: Coadministration of tirofiban and enoxaparin was generally well tolerated. Plasma clearance of tirofiban was 176.7+/-59.8 and 187.5+/-81.8 ml/min, respectively, for enoxaparin and unfractionated heparin-treated patients (P=NS). The mean difference was well within the prespecified criterion for comparability. Administration of tirofiban with enoxaparin vs. unfractionated heparin resulted in lesser variability and a trend towards greater inhibition of platelet aggregation using 5 microM adenosine phosphate agonist. More patients achieved target inhibition of platelet aggregation >70% in the tirofiban and enoxaparin group (84% vs. 65%, P=0.19). Median bleeding time was 21 min for tirofiban and enoxaparin vs. > or =30 min for tirofiban and unfractionated heparin (P=NS). For a given level of inhibition of platelet aggregation, bleeding time was less prolonged with tirofiban and enoxaparin than tirofiban and unfractionated heparin (adjusted mean bleeding time 19.6 vs. 24.9 min, P=0.02). Tirofiban plasma concentration and clearance were comparable whether coadministered with enoxaparin or unfractionated heparin. There were no major or minor bleeding events in either group by the TIMI criteria. INTERPRETATION: The more consistent inhibition of platelet aggregation and lower adjusted bleeding time of tirofiban and enoxaparin vs. tirofiban and unfractionated heparin support the therapeutic potential of combining these two agents. These data from the first clinical report of coadministration of a glycoprotein IIb/IIIa receptor antagonist and a low molecular weight heparin are consistent with prior data which show differential pharmacodynamic effects of enoxaparin and unfractionated heparin on platelet aggregation.

Angina, Unstable↗
PubMed1999

Standardized format for depicting hearing preservation results in the management of acoustic neuroma.

The Committee on Hearing and Equilibrium of the American Academy of Otolaryngology-Head and Neck Surgery recently published guidelines for reporting hearing preservation in the treatment of acoustic neuromas. These suggestions included pretreatment and posttreatment pure-tone hearing thresholds, word recognition scores, and hearing classification. We present a standardized reporting format that addresses the Committee's recommendations and displays individual patient audiologic data as a simple, concise plot of posttreatment hearing results. To illustrate the use of the recommended format, preoperative and postoperative hearing data from our institution are reported. Such reporting criteria will facilitate comparative reviews of studies of hearing preservation after surgical or radiotherapeutic management of acoustic neuromas, while providing specific data for individual patient outcome analysis.

Audiometry, Pure-Tone↗
PubMed1998

The use of high stimulus rate auditory brainstem responses in the estimation of hearing threshold.

This normative study investigates the efficiency of using the maximum length sequence (MLS) technique applied to auditory brainstem evoked response (ABR) testing to estimate hearing thresholds. Using a commercially available system, ABRs were recorded in sixteen subjects at two conventional rates--9.1 and 33.3 clicks/s--and six MLS rates between 88.8 and 1000 clicks/s. Each subject was tested at five stimulus levels from 60 down to 10 dBnHL. The wave JV amplitude input-output (I/O) functions, relative signal to noise ratio (SNR) and speed of test were calculated for all conditions. The JV amplitude and detectability decrease as the stimulus rate increases and level decreases. The latency of JV increases as the stimulus rate increases and the intensity decreases. While the slope of the amplitude I/O function was maximal at 200 clicks/s, at 300 clicks/s it was comparable with that obtained at conventional rates. At higher rates, the slope of the I/O function decreases. When compared with the conventional recording rate of 33.3 clicks/s there is a small improvement in SNR for MLS rates between 200 and 600 clicks/s at levels above 30 dBnHL. The calculated speed improvement at 300 clicks/s is a factor between 1.4 to 1.6 at a screening level of 30-40 dBnHL. It is felt therefore that there may be a small advantage to using MLS in screening and that the optimal rate for this lies at around 200 to 300 clicks/s. However even at these rates, as a consequence of the adaptation of the response with both rate and level, the improvement in SNR or speed of test would be modest when estimating threshold.

Acoustic Stimulation↗
PubMed1998

Neonatal otoacoustic emissions recorded using maximum length sequence stimuli.

OBJECTIVE: Maximum length sequence (MLS) stimulation allows transient evoked otoacoustic emissions (TEOAEs) to be recorded at very high stimulation rates. Previous work has focused on recording from normally hearing adult subjects; the aim of this study was to obtain information about emissions recorded using this technique from newborns and to compare these results with those obtained from adults. The feasibility of recording from newborns on the postnatal wards also was addressed. DESIGN: The study comprised two parts. In the first, TEOAEs were collected at 13 stimulation rates from a selected group of babies. The second part of the study comprised only two stimulation rates, a conventional rate of 40 clicks/sec and the maximum MLS rate of 5000 clicks/sec. RESULTS: The neonatal MLS TEOAEs behave in a similar manner to those obtained from adult subjects. The morphology of the waveforms was similar for the conventional and MLS TEOAEs. As the stimulus rate increases, the amplitude of the emission decreases, reaching an approximate plateau by 1000 to 2000 clicks/sec. The absolute reduction in amplitude seen at the high MLS rate is related to the amplitude of the conventional TEOAE but is always approximately the same when expressed as a percentage or proportion of that amplitude. CONCLUSION: The theoretical advantages of speed and sensitivity seen for adult subjects also should hold true for the neonatal population. Although the system used to test was a prototype with none of the refinements found in commercial systems, it was possible to record adequate emissions from a ward-based population of newborns.

Acoustic Stimulation↗
PubMed1998

A study of the acoustic reflex using fast-rate otoacoustic emissions.

Fast-rate otoacoustic emissions (OAEs) were used to determine the inward and outward transmission change produced by the stapedial muscle reflex. The subjects were otologically normal adult volunteers. Satisfactory recordings were obtained from a total of 16 ears. Runs of 16 click-evoked OAEs were recorded at a rate of 4282 clicks/s using the maximum length sequence (MLS) technique, with a contralateral tone presented at 10 dB above the contralateral acoustic reflex threshold during the recording of OAEs 5-12 in each run. The OAEs recorded with the contralateral tone were compared with those recorded without the tone to determine the effect of the acoustic reflex. An analysis of the OAEs in 0.5 kHz frequency bands suggested that the reflex produced significant attenuation of low frequency sound with smaller effects at high frequencies, which included amplification for some subjects. For eight of the 16 ears there was an increase in OAE amplitude with the acoustic reflex for at least one frequency band. At low frequencies (1, 1.5 kHz) the inward transmission change produced by the acoustic reflex appeared to be similar in magnitude to the outward transmission change.

Acoustic Stimulation↗
PubMed1997

Effect of olivocochlear bundle section on evoked otoacoustic emissions recorded using maximum length sequences.

Presenting clicks according to maximum length sequences (MLS) enables transient evoked otoacoustic emissions (TEOAE) to be recorded at very high stimulation rates. As the click rate is increased from 40 clicks/s up to a maximum rate of 5000 clicks/s there is a reduction in TEOAE amplitude that reaches an approximate asymptote at 1500 clicks/s. One hypothesis put forward to explain this MLS 'rate effect' is that ipsilateral efferent activity is involved. To test this hypothesis TEOAEs were recorded from both ears of five patients who had undergone a unilateral vestibular nerve section--a surgical procedure which also entails sectioning the olivocochlear bundle. TEOAEs were recorded conventionally at 40 clicks/s and using MLS stimulation at 5000 clicks/s. Increasing the rate from 40 to 5000 clicks/s was found to reduce the amplitude of the TEOAEs by equivalent amounts in ears ipsilateral and contralateral to a vestibular nerve section as well as in the ears of normal-hearing adults. Since an ear ipsilateral to a vestibular nerve section should have no efferent innervation the hypothesis that efferent activity is the major mechanism involved in the MLS rate effect is rejected. Instead, the possibility that intracochlear processes are the underlying mechanism will now be investigated.

Acoustic Stimulation↗
PubMed1997

Transient evoked otoacoustic emissions recorder using maximum length sequences as a function of stimulus rate and level.

OBJECTIVE: The recently developed technique of recording transient evoked otoacoustic emissions (TEOAEs) using clicks presented according to maximum length sequences (MLSs) enables very high stimulation rates to be used. The aim of this study was to provide normative data on the relationship between TEOAEs recorded conventionally (at 40 clicks/sec) and those recorded using the MLS technique (at 11 maximum rates between 100 and 5000 clicks/sec) to establish a baseline for future clinical studies. DESIGN: TEOAEs were recorded at 12 rates from 12 normally hearing adult ears at click levels decreasing in 5 dB steps from 68 dB peSPL. RESULTS: The morphology of the waveforms and the pattern of the input/output functions with latency were similar for conventional and MLS TEOAEs. The only major difference between TEOAEs recorded at the different rates was in their absolute amplitude. As the click rate was increased from 40 clicks/sec there was a reduction in amplitude that reached a near asymptote at approximately 1500 clicks/sec. When expressed as a percentage reduction in amplitude compared with that recorded at 40 clicks/sec, this MLS "rate effect" was independent of stimulus level over all but the lowest test level (38 dB peSPL SPL). CONCLUSION: Over a wide range of amplitudes of conventionally recorded TEOAEs (21 to 450 microPa for the 9 to 13 msec section of the otoacoustic emission), the mechanism involved in the MLS rate effect seems to act in a way that reduces the amplitude by an almost constant proportion, whatever its original size.

Acoustic Stimulation↗
PubMed1997

Maximum length sequences and Volterra series in the analysis of transient evoked otoacoustic emissions.

Previous work from this group (IHR, Southampton) has shown the feasibility of using maximum length sequence (MLS) stimulation to obtain evoked otoacoustic emissions (OAE). Because an MLS is one of a set of inputs that enables the Volterra series to be computed, we investigated its use with OAE. We wanted to see if the Volterra series could model the system and if we could extract the higher order kernels. In order to realise a practicable MLS system, a variant of the MLS has been used which permits real time recovery of the response and so enables the rejection of noisy epochs. This paper shows that the MLS used does enable the higher order kernels to be measured. The problems of selecting the correct sequences are discussed and the necessary system checks described.

Acoustic Stimulation↗
PubMed1996

Early hearing testing of still critically ill neonates.

OBJECTIVE: To use a newly applied hearing screening technique for early measurement in neonatal intensive care unit (NICU) patients to learn more about the high incidence of hearing loss in this population. METHODS: An automated, portable infant hearing screener that measures the auditory brain-stem response at the bedside was used at the NICU of the Hospital Nacional de Niños, San Jose, Costa Rica. Patients were evaluated early, even if they were on a ventilator. The screener tested with a 40-dB hearing level click stimulus to each ear over a bandwidth of 750 to 3000 Hz. RESULTS: During a 15-month period, 92 newborns underwent 226 auditory brain-stem response tests (range, one to six tests; mean, 2 1/2 tests). Before discharge from the NICU or death, each infant was successfully screened. Of 72 infants discharged from the hospital alive, 68 passed bilaterally and four failed bilaterally, a 6% failure rate. Of 20 infants who died, 15 failed bilaterally, a 75% failure rate. Persistent bilateral failure of the test was detectable from each infant's first test and showed an association (chi-square, P<.001) with death. The overall bilateral failure rate was 21%. CONCLUSIONS: Simple bedside auditory brain-stem response screening of all NICU infants was consistently possible regardless of clinical status, the early onset of hearing loss suggests that NICU treatment was not ototoxic and the unexpectedly high overall bilateral test failure rate resulted from the inclusion of patients who would have died untested if conventional testing had been done.

Audiometry, Evoked Response↗
PubMed1996

Audiometric changes in patients undergoing coronary artery bypass surgery.

The incidence of severe hearing loss following coronary artery bypass surgery has been estimated as one per thousand (Plasse et al., 1981). To evaluate the audiological changes that may be caused by the operation, audiograms were taken before and after surgery. The aim was to detect losses that were not necessarily severe and to examine the factors associated with any auditory insult. Patients without a past medical history of hearing problems or ototoxic drugs were examined on the day prior to the coronary bypass operation and audiometry carried out. Six days post-operatively they were re-examined and asked about their hearing and any tinnitus. Various operative details were taken and the post-operative drug record examined. Patients who were too ill for retesting, or who had been given sedative or ototoxic drugs or opiate analgesia, were dropped from the study. Twenty patients completed the tests. The whole procedure was repeated on a control group of 20 patients who were having thorocotomy but without being placed on a cardiopulmonary bypass. From the bypass sample of 40 ears, five ears (four individuals) had an individually statistically significant high-frequency hearing loss. The results show a statistically significant difference between the bypass group and the control group, with the bypass patients having worse threshold shifts following the operation. Discriminant analyses have shown that those patients suffering hearing impairment may be discriminated principally with four variables: the patient's age, minimum temperature and minimum blood pressure during the operation, and the time on bypass.

Audiometry↗
PubMed1996

Correlation of hearing loss and radiologic dimensions of vestibular schwannomas (acoustic Neuromas).

A retrospective analysis was performed of puretone audiograms, speech-discrimination scores, and gadolinium-enhanced magnetic resonance imaging scans of 75 patients with vestibular schwannomas (acoustic neuroma). Sensorineural hearing loss was analyzed for low frequencies (250-500 Hz), midfrequencies (1,000-2,000 Hz), and high frequencies (4,000-8,000 Hz). The largest tumor diameter in the cerebellopontine angle and the lateral extent of invasion by tumor into the internal auditory canal were calculated from magnetic resonance images. There were statistically significant correlations between the largest tumor diameter and the severity of low-frequency sensorineural hearing loss (p = 0.001). However, no significant correlations were found between the following: largest tumor diameter and the severity of mid-frequency or high-frequency sensorineural hearing loss or speech-discrimination scores and lateral extent of invasion of the internal auditory canal and sensorineural hearing loss at all frequencies or speech-discrimination scores (p > or = 0.05). The findings suggest that nerve compression is not the only cause of hearing loss in vestibular schwannoma.

Adult↗
PubMed1996

Otoacoustic emissions recorded at high rates in patients with confirmed acoustic neuromas.

Otoacoustic emission testing was carried out on 39 patients with confirmed acoustic neuromas, and satisfactory emissions were recorded from the neuroma ear of 59% of them. A comparison of the patients with and without emissions showed no significant differences in low-frequency or high-frequency hearing loss, optimum speech discrimination score, or canal paresis in the affected ear between the two groups. Emissions were recorded at stimulus rates up to 5,000 clicks/s by using the maximum length sequence (MLS) technique. The decrease in the emission amplitude with increase in click rate (rate suppression) was significantly less than the amount that would be expected from normal subjects for several of the neuroma patients. However, one patient showed normal suppression despite having a large neuroma and no measurable hearing. This would suggest that efferent suppression may not be the only mechanism involved.

Acoustic Stimulation↗