A vascular subtraction method for improving the variability of evoked tympanic membrane displacement measurements.

OBJECTIVE: Evoked tympanic membrane displacement (TMD) measurements show a correlation with intracranial pressure (ICP). Attempts to use these measurements for non-invasive monitoring of ICP in patients have been limited by high measurement variability. Pulsing of the tympanic membrane at the cardiac frequency has been shown to be a significant source of the variability. In this study we describe a post processing method to remove the cardiac pulse waveform and assess the impact of this on the measurement and its repeatability. APPROACH: Three-hundred and sixteen healthy volunteers were recruited for evoked TMD measurements. The measurements were quantified by V m, defined as the mean displacement between the point of maximum inward displacement and the end of the stimulus. A sample of spontaneously pulsing TMDs was measured immediately before the evoked measurements. Simultaneous recording of the ECG allowed a heartbeat template to be extracted from the spontaneous data and subtracted from the evoked data. Intra-subject repeatability of V m was assessed from 20 repeats of the evoked measurement. Results with and without subtraction of the heartbeat template were compared. The difference was tested for significance using the Wilcoxon sign rank test. MAIN RESULTS: In left and right ears, both sitting and supine, application of the pulse correction significantly reduced the intra-subject variability of V m (p value range 4.0 × 10-27 to 2.0 × 10-31). The average improvement was from 98 ± 6 nl to 56 ± 4 nl. SIGNIFICANCE: The pulse subtraction technique substantially improves the repeatability of evoked TMD measurements. This justifies further investigations to assess the use of TMD measurements in clinical applications where non-invasive tracking of changes in ICP would be useful.

A model of in-vivo hydrocephalus shunt dynamics for blockage and performance diagnostics.

The accumulation of excess cerebrospinal fluid in the ventricles of the brain results in hydrocephalus, a condition that is fatal if left untreated. The usual remedy is to insert a shunt into the ventricles of the brain, which drains excess fluid away, moderated by a pressure dependent valve. It is important that the system functions properly so that a reasonable intracranial pressure is maintained. Unfortunately, pressure measurements in the ventricles are highly invasive, while pressure measurements in the shunt outside the skull may not detect any blockage in the catheter inside. Here we develop a model primarily aimed at detecting in vivo a blockage and other shunt malfunction using non-invasive measurements, so that shunt valves can be adjusted accordingly. The system offers a clear insight into how currently available clinical measurements may be utilized. We then extend this to investigate the phenomenon of 'chatter' (rapid opening and closing) and other mechanisms including intracranial pressure pulsatility. Although simple, the model offers a clear indication of what is required for successful regulation of both intracranial pressure and shunt flow.

Sonotubometry findings in children at high risk from middle ear effusion.

The functioning of the eustachian tube has an important role to play in the development of middle ear disease. It would be useful if a clinical test could assist in the identification of eustachian tube dysfunction, particularly if this is shown to be an indicator of persistent middle ear effusion. The aim of this study was to compare the results of sonotubometry using the MMS-10 instrument in children at high risk from middle ear effusion with a group of normal subjects. Forty-one subjects (age range 5-6 years) were allocated to one of two groups (experimental group, 21 subjects; control group 20 subjects) based on a questionnaire designed to identify subjects at high risk from middle ear effusion. The test protocol allowed each subject to swallow three times for each of two pure-tones (7 and 8 kHz) delivered by the nasal probe. Sonotubometry indicated opening of the eustachian tube on swallowing in around 80% of subjects. The incidence of positive findings varied greatly amongst subjects across both groups. In the control group, the mean increase in sound pressure level on swallowing was 11.5 dB (+/- 4.3) and 9.8 dB (+/- 2.5) for 7 and 8 kHz, respectively. The corresponding means for duration were 118 ms (+/- 47.9) and 137 ms (+/- 61.8). Sonotubometry failed to demonstrate a difference between the two groups of subjects. Hence, the clinical application of sonotubometry to identify subjects at high risk from middle ear effusion is not supported.

Quantitative assessment of intracranial pressure by the tympanic membrane displacement audiometric technique in children with shunted hydrocephalus.

The objective of this prospective study was to compare the clinical features at presentation, tympanic membrane displacement test results and direct intracranial pressure measurements in children with shunted hydrocephalus to procure a quantitative measure of the intracranial pressure by tympanic membrane displacement test. A prospective comparative evaluation of 61 clinical episodes of shunt malfunction was assessed by volume displacement of the tympanic membrane and direct intracranial pressure measurements in 40 patients with shunted hydrocephalus between January 1995 and June 1996. The volume displacement of the tympanic membrane (Vm) on stapedial contraction was inward for raised intracranial pressure in 27 episodes and ranged from -120 nl to -506 nl (mean = -250 nl). This was confirmed by direct intracranial pressure monitoring, which ranged from 23 to 40 mm Hg (mean = 29 mm Hg). The tympanic membrane displacement test measurement in 30 episodes of low intracranial pressure ranged from +263 nl to +810 nl (mean = +530 nl), and this was corroborated by direct intracranial pressure measurement ranging from 1 to 6 mm Hg (mean = 3.8 mm Hg). The normal baseline Vm values obtained when the subjects were asymptomatic ranged from +58 nl to +175 nl (mean = +115 nl). The tympanic membrane displacement test as a non-invasive diagnostic tool in predicting changes in intracranial pressure had a sensitivity of 93% and specificity of 100%. The predictive value of the test was 100%, and the negative predictive value was 73%. The kappa statistical analysis was used to measure the agreements between the groups. The strength of the agreement was very good, kappa = 0.88 and the P value was < 0.001. The objective measure of intracranial pressure by tympanic membrane displacement test with the Vm value of -200 nl and more negative was indicative of raised intracranial pressure and a Vm value of +200 nl and greater, for low intracranial pressure. The intracranial pressure measurements made on an individual subject basis were reliable and accurate. The test can therefore be used for regular assessment of shunted hydrocephalics to enable correlation of intracranial pressure with symptoms in individual patients.

Tympanic membrane displacement testing in regular assessment of intracranial pressure in eight children with shunted hydrocephalus.

OBJECT: The authors assessed the accuracy and repeatability of the tympanic membrane displacement (TMD) test, an audiometric technique that is used to evaluate changes in intracranial pressure (ICP) in children with shunted hydrocephalus. METHODS: A prospective comparative evaluation of 31 clinical episodes of shunt malfunction was made by using the serial TMD test and direct ICP measurement in eight children with shunted hydrocephalus between January 1995 and February 1996. The volume displacement of the tympanic membrane (Vm) on stapedial contraction was inward for raised ICP in 11 instances and ranged from -120 to -539 nl (mean -263.5 nl). This was confirmed by direct ICP monitoring, which showed values ranging from 20 to 30 mm Hg (mean 26 mm Hg). The TMD test measurement (Vm) in 18 instances of low ICP ranged from 263 to 717 nl (mean 431.3 nl); this was corroborated by direct ICP measurement, which ranged from 3 to 7 mm Hg (mean 4.2 mm Hg). The normal baseline Vm values obtained when patients were asymptomatic ranged from 98 to 197 nl (mean 110 nl). As a noninvasive diagnostic tool used in predicting changes in ICP, the TMD test had a sensitivity of 83% and specificity of 100%. The positive predictive value of the test was 100% and the negative predictive value was 29%. CONCLUSIONS: The TMD test can be used on a regular basis as a reproducible investigative tool in the assessment of ICP in children with shunted hydrocephalus, thereby reducing the need for invasive ICP monitoring. The equipment necessary to perform this testing is mobile. It will provide a useful serial guide to ICP abnormalities in children with shunted hydrocephalus.

Intracranial pressure at high altitude and acute mountain sickness.

1. Raised intracranial pressure has been noted in severe forms of acute mountain sickness and high-altitude cerebral oedema, but the role of intracranial pressure in the pathogenesis of mild to moderate acute mountain sickness is unknown. 2. Serial measurements of intracranial pressure were made indirectly by assessing changes in tympanic membrane displacement in 24 healthy subjects on rapid ascent to 5200 m. 3. Acute hypoxia at 3440 m was associated with a rise in intracranial pressure, but no difference was found in pressure changes at 4120 or 5200 m in subjects with or without symptoms of acute mountain sickness. 4. Raised intracranial pressure, though temporarily associated with acute hypoxia, is not a feature of acute mountain sickness with mild or moderate symptoms.

The prevalence of perilymphatic hypertension in subjects with tinnitus: a pilot study.

This study investigated the prevalence of perilymphatic hypertension (raised perilymphatic pressure) in a population of subjects with tinnitus. A review of the literature showed how changes in perilymphatic pressure could affect tympanic membrane displacement measurements. This review also revealed that perilymphatic hypertension was more likely to occur in young females (less than 45 years) than in other subjects. An experiment was designed to test 32 subjects, who were divided into four groups according to their age and sex. These subjects underwent several routine audiological tests and were then tested with the tympanic membrane measurement system to determine the perilymphatic pressure of both ears. Statistical analysis of the experimental results showed that the young females had raised perilymphatic pressure. This was significantly higher than the perilymphatic pressure of the other test groups and of that of a normal population. The young females also exhibited other symptoms indicative of raised perilymphatic pressure. The raised pressure was thought to be due to an increase in fluid pressure which is more likely to occur in females due to variations in the levels of circulating hormones with menstrual irregularities, pregnancy and the menopause.

Long-term assessment of intracranial pressure using the tympanic membrane displacement measurement technique.

The tympanic intracranial pressure (ICP) measurement technique was used to assess intracranial pressure over several months in two patient populations. In the first study, 43 shunted hydrocephalic children, aged 4-17 years, were tested over a period of 18 months when clinically well. Of these 11 (26%) were later admitted with symptoms suggesting acute shunt blockage. The tympanic ICP measurement correlated with clinical and/or operative findings in 10 cases. In the second study, illustrated by a case report, repeated testing over a period of 5 months in children with chronic symptoms suggestive of periods of increased ICP, demonstrated a correlation between symptoms and ICP and helped influence management decisions. Tympanic ICP measurement in shunted children is a valuable tool in the assessment of acute and chronic shunt malfunction. Serial testing was shown to be clinically useful in the long-term management of these patients as a diagnostic indicator of pressure variation and shunt dysfunction.

Non-invasive assessment of ventricular shunt function using tympanic membrane displacement measurement technique.

A technique which was originally developed for measuring cochlear fluid pressure, has been under trial for the past two years as a means of assessing ventricular shunt function. The principle of the technique has been reported in previous papers (1, 2, 3) and has been shown to provide a reliable measure of intracranial pressure (ICP) in terms of induced displacement of the tympanic membrane. This study concentrates on the assessment of shunt blockage in spina bifida patients who subsequently underwent shunt revision. The tympanic displacement technique is shown to be of value to this patient group in three respects. Firstly as a research technique to study group-averaged ICP measurements where the ICP needs to be known but invasive measurements cannot be justified. Secondly for assessing shunt dysfunction in individual patients and, finally, in determining the success of shunt revision surgery. The technique was found to be extremely sensitive to relative changes in ICP with shunt revision or any subsequent blockage. The technique is more reliable diagnostically in instances where the patients act as their own controls and a comparison with baseline measurements can be made. Three case reports are illustrated which were selected from those patients tested to emphasise the importance of serial measurements of ICP over period of several days post-revision. The tympanic displacement technique is shown to provide a practical and acceptable method by which this can be undertaken.

Cochlear and cerebrospinal fluid pressure: their inter-relationship and control mechanisms.

The patency of the cochlear aqueduct is a key factor in intra-cochlear hydromechanics. If patent, the cerebrospinal fluid (CSF) provides the reference pressure for the perilymph and also to a large extent the endolymph, since Reissner's membrane can only withstand a relatively small pressure differential. The aqueduct often becomes sealed as a natural process of ageing. In this instance the reference pressure is from a source, its position unknown, within the boundaries of the cochlea itself. Relatively large and rapid changes in the cerebrospinal fluid pressure may result from everyday events such as coughing (ca. 175 mm saline) and sneezing (ca. 250 mm saline). The resistive nature of the cochlear aqueduct and the mechanical compliance of the cochlear windows are probably important factors in limiting the amount of stress, and therefore possible damage, which may occur to the cochlea and cochlear windows for a given pressure change within the CSF system. A narrow aqueduct and compliant cochlear windows reduce the risk of structural damage. In practice, this should mean that the risk of structural damage will be increased by any process which reduces the compliance of one or both of the cochlear windows, for example, extremes of middle ear pressure perhaps brought about by Eustachian tube dysfunction or rapid barometric pressure changes. Techniques are now available which provide non-invasive indirect measures of perilymphatic pressure and CSF-perilymphatic pressure transfer. The tympanic membrane displacement measurement technique has been used to provide reliable measures of perilymphatic pressure and CSF-perilymphatic pressure transfer on an individual subject basis.(ABSTRACT TRUNCATED AT 250 WORDS)

The relationship between intracranial pressure and tympanic membrane displacement.

Transmission of intracranial pressure (ICP) to the perilymph of the cochlea may occur via the cochlear aqueduct and possibly other routes. Indirect measurement of perilymphatic pressure may be investigated by observing tympanic membrane (TM) displacement during stapedial reflex contraction. In a previous study we investigated the effects of changes in ICP on perilymphatic fluid pressure in three patients who underwent ventriculo/lumbar-peritoneal shunt operations. The TM displacement technique proved extremely sensitive and revealed marked changes in cochlear fluid pressure brought about by changes in ICP (Marchbanks et al., 1987). The study has been extended to 58 patients with hydrocephalus, intracranial tumours and other neurological conditions associated with abnormal ICP. Significant differences in the TM displacement were found between patients with raised and normal ICP. We have shown that changes in ICP can affect the hydrostatic pressure of the cochlea and influence the peripheral auditory system. The finding that ICP can be correlated with TM displacement strengthens the association between an abnormal TM displacement and abnormal cochlear hydrostatic status, irrespective of cochlear aqueduct patency. We suggest that the TM displacement technique provides a useful non-invasive method for the assessment of perilymphatic fluid pressure.

Comparison of intracranial pressure between spina bifida patients and normal subjects using a non-invasive pressure assessment technique.

A comparison of mean intracranial pressure (ICP) between 36 spina bifida patients and 24 patients with clinically normal ICP was undertaken. A new non-invasive method of assessing ICP was used throughout this study. The technique relates tympanic membrane displacement to ICP and has been shown to be a reliable measure of mean ICP by comparison with direct measures via ventricular catheters and reservoirs, subdural catheters and lumbar punctures. Using this non-invasive method the results of this study indicated that the patients with spina bifida and without a ventricular shunt have a higher than average mean ICP. The results demonstrated a significant difference (99.9%) in the measures of ICP between the spina bifida and normal populations. A significant difference (99.9%) in the ICP was also found between patients with spina bifida and with and without ventricular shunts, and thus presumably those with and without diagnosed hydrocephalus. It is suggested that patients with spina bifida and without ventricular shunts probably have higher than average mean ICP when compared with the normal population.

Effects of posture and age on tympanic membrane displacement measurements.

This study used changes in posture to investigate the transmission of cerebro-spinal fluid (CSF) pressure changes to the cochlear fluids in subjects falling into two age groups. Results were obtained from 32 subjects: 16 aged 19 to 32 years and 16 aged between 40 and 63. Measurements were made with the subject sitting upright and then at 63 degrees to the vertical, using a technique which measures displacement of the tympanic membrane in response to acoustic stimulation of the stapedial reflex. Changes in such measurements are believed to represent changes in perilymphatic fluid pressure. Statistical analysis of the results showed the magnitude of changes in Tympanic Membrane Displacement (TMD) measurements with postural change to be smaller in the older group than in the younger age group. In addition, a larger proportion of the older subjects showed no change in TMD measurements with change in posture. The proportion of subjects in each age group who showed no change in TMD measurements with change in posture was similar to the proportion of temporal bones of subjects of similar ages that had non-patent cochlear aqueducts as determined by a previous histopathological study. Audiometric testing revealed a link between hearing threshold and presumed patency of the communication routes between cochlear and cranial fluids. The magnitude of the tympanic membrane displacement was found to decrease with age in agreement with previous studies showing decrease of stapedial reflex magnitude with increasing age. It is suggested that the communication between cochlear and cranial fluids may be important in the pathogenesis of auditory dysfunction in several patient groups.

Objective tinnitus and tympanic membrane displacement.

A young girl with objective tinnitus is presented. Using tympanic membrane displacement, the probable origin of the tinnitus has been demonstrated. The patient has responded well to treatment with a tinnitus masker.

Mean intracranial pressure monitoring by a non-invasive audiological technique: a pilot study.

Intracranial pressure is normally transmitted to the perilymph of the cochlea via the cochlear aqueduct. The relationship between perilymphatic pressure, indirectly measured by tympanic membrane displacement, and mean intracranial pressure defined either clinically or by direct measurement has been examined in 58 patients (aged 5-77 years), with hydrocephalus, benign intracranial hypertension, intracranial tumours, subarachnoid haemorrhage and head injuries. The most consistent results were obtained in young patients with hydrocephalus and benign intracranial hypertension. However, the technique was not suitable when the stapedial reflex was absent as a result of middle ear/brainstem dysfunction and did not reflect intracranial pressure when the cochlear aqueduct was not patent. This pilot study suggests that the tympanic membrane displacement technique may provide a useful non-invasive method for serial monitoring of intracranial pressure in young patients with hydrocephalus or benign intracranial hypertension.

The effect of raised intracranial pressure on intracochlear fluid pressure: three case studies.

The effects of changes in intracranial fluid pressure (ICP) on cochlear fluid pressure have been studied in 3 patients who underwent ventriculo/lumbar-peritoneal shunt operations. The operations were performed in order to alleviate problems caused by an abnormally raised ICP. Indirect measurements of perilymphatic pressure were made before and after surgery using a non-invasive technique which measures tympanic membrane displacement. This technique proved extremely sensitive and revealed changes in cochlear fluid pressure brought about by changes in the ICP. The results emphasise the need to consider audiological measurements in the context of ICP abnormalities. These findings have important implications in that an audiological technique may be used for the simple and non-invasive serial monitoring of changes in ICP and may even assist differential diagnosis in a number of neurological patients.

Middle ear muscle activity during vocalization in normal speakers and stutterers.

Although it has been established that the middle ear muscles contract prior to vocalization, it is not known which muscle is mainly responsible for this activity. Also, there have been contradictory reports about whether this activity in stutterers differs from that of normal speakers. To tackle these questions, extratympanic pressure measurements prior to vocalization are reported for normal speakers and stutterers. This measure allows activity deriving from the two middle ear muscles to be differentiated and for the temporal course to be followed more accurately than by impedance measurement. To establish which muscle is primarily responsible for the pre-vocalization activity, the measured pressure changes prior to vocalization are compared with activity measured in tasks known to involve the stapedius or tensor tympani alone. These data show that in normal speakers and stutterers, the activity that is measured prior to vocalization resembles that of the tensor tympani. Contrary to other reports, there is no difference between normal speakers and stutterers in the time course of this activity.

A computer-based test system for implementing pure-tone audiometry, acoustic immittance and tympanic membrane displacement measurements.

Valuable information on functional features of the auditory system may be gained by measuring the fine structure and time course of the acoustic stapedial reflex. This type of measurement invariable needs a computer for averaging the reflex response and the analysis of its salient features. This paper describes a 'Computerised Audiometric System' (CAS) which allows an integrated approach to certain audiometric tests, including pure-tone audiometry and tympanometry. In particular, 'tympanic membrane displacement' and acoustic immittance measurements have been combined to allow a comprehensive study of the reflex characteristics to be made. The purpose of this paper is twofold; first to describe the CAS in respect of reflex measurements; secondly, to describe the design of a practical computerised audiometric system which may be used for a wide variety of clinical testing and research. Features of special mention are the modular design strategy used, which makes the system an extremely versatile research tool as well as a potentially valuable addition to the audiology clinic, and menu-driven software written in FORTH, which allows complex test paradigms to be learnt and reliably implemented with a minimum of training. The prototype facility has been used extensively for research over the past 18 months, and a second is now installed in the Wessex Regional Audiology Centre. Both facilities will be linked to a patient data bank now under development.