Impedance measurement as a noninvasive technique for the monitoring of intracranial pressure variations

Unexpected relationship between tympanometry and mortality in children with nontraumatic coma

OBJECTIVE

We sought to further examine the relationship between tympanometry and mortality after noting an unexpected association on assessment of baseline data of a study whose primary aim was to investigate the utility of noninvasive tympanic membrane displacement measurement for monitoring intracranial pressure in childhood coma.

METHODS

We recruited children who presented with acute nontraumatic coma to the high-dependency unit of Kilifi District Hospital on the rural coast of Kenya. We excluded children with sickle cell disease, epilepsy, and neurodevelopmental delay. We performed tympanometry on the right ear before tympanic membrane displacement analyzer measurements. All children were managed according to standard World Health Organization guidelines.

RESULTS

We recruited 72 children with a median age of 3.2 years (interquartile range [IQR]: 2.0-4.3 years); 31 (43%) were female. Thirty-eight (53%) had cerebral malaria, 8 (11%) acute bacterial meningitis, 4 (6%) sepsis, and 22 (30%) encephalopathy of unknown etiology. Twenty (28%) children died. Tympanometry was normal in 25 (35%) children. Adjusting for diagnosis and clinical features of increased intracranial pressure, both associated with death on univariable analysis, children with abnormal tympanometry had greater odds of dying than did those with normal tympanometry (adjusted odds ratio: 17.0; 95% confidence interval: 1.9-152.4; P = .01). Children who died had a lower compliance (0.29 mL; IQR: 0.09-0.33 mL) compared with those who survived (0.48 mL; IQR: 0.29-0.70 mL) (P < .01).

CONCLUSIONS

Abnormal tympanometry appears to be significantly associated with death in children with acute nontraumatic coma. This finding needs to be explored further through a prospective study that incorporates imaging and intensive physiologic monitoring.

Ultrasonography of the optic nerve in neurocritically ill patients

The rapid diagnosis of intracranial hypertension is urgently needed for therapeutic reasons in various clinical settings. This can rarely be achieved without invasive procedures such as intracranial pressure (ICP) monitoring or neuroimaging. The optic nerve is surrounded by cerebrospinal fluid (CSF) and dura mater, which forms the optic nerve sheath (ONS). Because of the connection with the intracranial subarachnoid space, ONS diameter (ONSD) is influenced by CSF pressure variations. Bedside ultrasonographic measurement of ONSD has been proposed as a non-invasive and reliable means to detect raised ICP in neurocritically ill patients. In several studies, it proves to have a good correlation with the direct measurement of ICP and a low interobserver variability. However, no general consensus exists over the upper normal ONSD limit. We performed a review of the literature on the use of the ultrasonography of the optic nerve in the evaluation of patients with suspected intracranial hypertension. The aim of this review is to describe the technique and to assess the validity of this diagnostic method.

Posture systematically alters ear-canal reflectance and DPOAE properties. Hear Res. 2010;263:43-51. [PMID: 20227475 DOI: 10.1016/j.heares.2010.03.003]

Several studies have demonstrated that the auditory system is sensitive to changes in posture, presumably through changes in intracranial pressure (ICP) that in turn alter the intracochlear pressure, which affects the stiffness of the middle-ear system. This observation has led to efforts to develop an ear-canal based noninvasive diagnostic measure for monitoring ICP, which is currently monitored invasively via access through the skull or spine. Here, we demonstrate the effects of postural changes, and presumably ICP changes, on distortion product otoacoustic emissions (DPOAE) magnitude, DPOAE angle, and power reflectance. Measurements were made on 12 normal-hearing subjects in two postural positions: upright at 90 degrees and tilted at -45 degrees to the horizontal. Measurements on each subject were repeated five times across five separate measurement sessions. All three measures showed significant changes (p<0.001) between upright and tilted for frequencies between 500 and 2000 Hz, and DPOAE angle changes were significant at all measured frequencies (500-4000 Hz). Intra-subject variability, assessed via standard deviations for each subject's multiple measurements, were generally smaller in the upright position relative to the tilted position.

Posture-induced changes in distortion-product otoacoustic emissions and the potential for noninvasive monitoring of changes in intracranial pressure

INTRODUCTION

Intracranial pressure (ICP) monitoring is currently an invasive procedure that requires access to the intracranial space through an opening in the skull. Noninvasive monitoring of ICP via the auditory system is theoretically possible because changes in ICP transfer to the inner ear through connections between the cerebral spinal fluid and the cochlear fluids. In particular, low-frequency distortion-product otoacoustic emissions (DPOAEs), measured noninvasively in the external ear canal, have magnitudes that depend on ICP. Postural changes in healthy humans cause systematic changes in ICP. Here, we quantify the effects of postural changes, and presumably ICP changes, on DPOAE magnitudes.

METHODS

DPOAE magnitudes were measured on seven normal-hearing, healthy subjects at four postural positions on a tilting table (angles 90 degrees , 0 degrees , - 30 degrees , and - 45 degrees to the horizontal). At these positions, it is expected that ICP varied from about 0 (90 degrees ) to 22 mm Hg ( - 45 degrees ). DPOAE magnitudes were measured for a set of frequencies 750 < f2 < 4000, with f2/f1 = 1.2.

RESULTS

For the low-frequency range of 750 <or=f2<or= 1500, the differences in DPOAE magnitude between upright and - 45 degrees were highly significant (all p < 0.01), and above 1500 Hz there were minimal differences between magnitudes at 90 degrees versus - 45 degrees. There were no significant differences in the DPOAE magnitudes with subjects at 90 degrees and 0 degrees postures.

CONCLUSIONS

Changes in ICP can be detected using the auditory-based measurement of DPOAEs. In particular, changes are largest at low frequencies. Although this approach does not allow for absolute measurement of ICP, it appears that measurement of DPOAEs may be a useful means of noninvasively monitoring ICP.

Skull vault growth in craniosynostosis

BACKGROUND

Since its first description by Virchow, the principle of abnormal skull growth due to restriction of skull growth at the fused sutures, and the realisation by Moss that the sutures at the skull base are equally affected, have been the main intellectual driving forces behind the majority of cranial expansion procedures performed currently in children with craniosynostosis.

CURRENT OBSERVATIONS

Despite original impressions that craniosynostosis leads to craniostenosis, many studies have demonstrated that in the majority of patients with craniosynostosis there is normal skull volume in those over the age of 6 months. In Apert syndrome, skull volume is invariably larger than normal. Some studies have shown that intracranial pressure is independent of intracranial volume, and can exist in the presence of normal volume, or indeed after cranial expansion. These observations imply that cranial expansion procedures create a state of artificially increased skull volume, in the quest to improve appearance and function.

FUTURE ADVANCES

This creates a new angle of view through which skull growth abnormalities are seen. It is becoming clearer that in most patients with craniosynostosis, there is regional imbalance of skull growth, which co-exists with a variety of other equally important factors, such as genetic defects, raised intracranial pressure, venous hypertension, and other brain parenchymal anomalies such as hindbrain hernia or hydrocephalus. It is becoming increasingly obvious that the type of surgical treatment currently practised in most cases is conceptually incorrect. Recent modifications such as the use of springs or distraction do not escape from the underlying philosophy of cranial expansion. With that in mind, it is hoped that advances in the fields of genetics and molecular biology will provide treatments for the cause of craniosynostosis rather than the symptomatic relief that surgery offers currently.

CONCLUSION

Until then, there is a need to develop better ways of quantifying regional abnormalities of skull growth.

The behavior of evoked otoacoustic emissions during and after postural changes

Click-evoked and stimulus frequency otoacoustic emissions (CEOAEs and SFOAEs, respectively) were studied in humans during and after postural changes. The subjects were tilted from upright to a recumbent position (head down 30 deg) and upright again. Due to the downward posture change, CEOAEs showed a phase increase (80 deg at 1 kHz) and a level decrease (0.5 at 1 kHz), especially for frequency components below 2 kHz. For SFOAEs, the typical ripple pattern showed a positive shift along the frequency axis, which can be interpreted as a phase shift of the inner-ear component of the microphone signal (90 deg at 1 kHz). This also occurred mainly for frequencies below 2 kHz. The altered posture is thought to cause an increase of the intracranial pressure, and consequently of the intracochlear fluid pressure, which results in an increased stiffness of the stapes system. The observed emission changes are in agreement with predictions from a model in which the stiffness of the cochlear windows was altered. For CEOAEs, the time to regain stability after a downward turn was of the order of 30 s, while this took about 20 s after an upward turn. For SFOAEs, this asymmetry was not found to be present (about 11 s, both for up- and downward turns).

Otoacoustic emissions: a new tool for monitoring intracranial pressure changes through stapes displacements

Changes in hydrostatic intracranial pressure (ICP) are thought to be transmitted to cochlear liquids, thereby altering the mechanical load on the stapes footplate. Hence the stiffness of stapes' annular ligament is expected to reflect ICP changes. Corresponding middle-ear transmission changes have been assessed using click-evoked otoacoustic emissions (EOAE) in two experiments. The first one was performed in 22 normal human subjects submitted to posture changes. In the second one, controlled ICP increases were applied to 18 patients in neurosurgery operating theater. EOAEs were monitored in these subjects throughout the experiments and their phases and amplitudes were analyzed as a function of frequency. ICP affected EOAEs in a systematic manner in both experiments, i.e., the main effect of ICP increase was phase lead of EOAE components below 2 kHz without any significant amplitude or frequency shift. Their variations were much more evident than the modifications in middle-ear impedance assessed by standard impedance or admittance measurements. The second experiment led to a quantitative linear relationship between ICP and EOAE phases. It demonstrated that the sensitivity of EOAE phase monitoring technique proves to be quite high, i.e., 55 daPa increases in ICP are readily detected in all subjects. The results are consistent with the predictions of classical middle-ear models as to the transmission alterations induced by ICP-related changes in the stiffness of the annular ligament of the stapes. Non-invasive monitoring of ICP in patients with hydrocephalus treated with ventriculo-peritoneal shunts might be attempted with this technique, although it is restricted to patients with detectable EOAEs (i.e., about 50% in this series).