Detecting Changes in Intracranial Pressure Non-Invasively Using OtoAcoustic Emissions
Kevin N.
1Department
1
Sheth ,
Nicholas
2
Horton ,
Christopher
3
Shera ,
Jonathan
1
Rosand ,
Susan
4
Voss
of Neurology, Massachusetts General Hospital & Harvard Medical School, Boston, 2Department of Mathematics & Statistics, Smith College, Northampton, MA, 3Department
of Otology & Otolaryngology, Massachusetts Eye & Ear Infirmary, Harvard Medical School, 4Picker Engineering Program, Smith College, Northampton, MA
INTRODUCTION
RESULTS
Figure 1
• Intracranial pressure (ICP) changes lead to changes in intracochlear
pressure (ICoP), as the cochlear fluid is contiguous with the cerebral
spinal fluid. In turn, changes in ICoP affect the stiffness of the annular
ligament and consequently middle ear sound transmission (Figure 1)
Schematic of the Inner ear
with the cochlear aqueduct.
Salt, A. N. (2004)
http://oto.wustl.edu/cochlea/intro1.htm.
• Otoacoustic emissions (OAE’s) are involuntary sounds generated by
the cochlea and transmitted through the middle ear to the ear canal,
where they can be recorded by a sensitive microphone.
• There are many types of OAE’s. Distortion product OAE’s (DPOAEs)
are used here to detect changes in ICP via changes in middle-ear
transmission, because DPOAEs have large signal-to-noise ratios
relative to other types of OAEs.
• With DPOAEs, two pure tones at the two frequencies f1 and f2 are
presented simultaneously in the ear canal. In response, the nonlinear
cochlea generates tones at additional frequencies, known as distortion
product frequencies. Here, we monitor the response at the frequency
fdp=2f1 -f2 (Figure 2)
• Preliminary work has demonstrated systematic changes in DPOAEs
with changes in posture and presumably ICP (Figure 3)
• The goal of this work is to describe a non-invasive system that has
potential to detect changes in ICP through changes in middle-ear
transmission, determined as changes in DPOAEs.
METHODS
• Case series: Patients age > 18 years admitted to the Massachusetts
General Hospital Neuro-ICU who received either an intraparenchymal
or intraventricular ICP monitor
• Ears are inspected and consent is obtained from patient or
surrogate decision maker
• Discrete measurements of tympanometry, DPOAE’s (through Fourier
analysis), ICP are collected (Figure 4)
• DPOAE magnitudes were measured with an Etymotic ER-10c probe
using HearID v4.0 (Mimosa Acoustics). The frequency ratio was f2/ f1 =
1.3 and the tones were both at 75 dB SPL (Figure 4)
• The DPOAE magnitudes were measured at frequencies fdp=2 f1- f2 for
the frequencies f2=515, 609, 703, 843, 984, 1171, 1406,
1687, 2015, 2391, 2812, 3375, 3984 Hz.
Figure 4
Acoustic Probe and
Computer Setup
Figure 2
Two tones at the two
frequencies f1 and f2
presented to nonlinear
cochlea generate a
DPOAE at fdp, recorded
by microphone in the
ear canal.
Figure 3
Summary of DPOAE magnitude
measurements across five
measurement sessions for each of
seven healthy normal-hearing
subjects, in three postural positions.
The figure shows the least-squares
mean values for the DPOAE
magnitudes predicted with a
random effects statistical model,
controlling for day, position,
frequency, the interaction between
position and frequency, and
clustering within subjects.
Corresponding standard errors for
each value are all between 2 and
2.1 dB. The results suggest that
changes in ICP are reflected in
changes in DPOAEs (from Voss et
al., 2006).
Figure 5.
Examples of DPOAEs measured on patients undergoing ICP monitoring within the ICU. In each plot, the red and blue solid lines are
DPOAE magnitudes and the dotted lines are the corresponding noise floors, both plotted in units of dB sound-pressure level (SPL) as functions of
frequency f2. All data are plotted but generally those within 6dB of the noise floor will not be analyzed. Subjects 1 and 2 had an external ventricular
drain (EVD) and Subject 3 had an intraparenchymal monitor. In EVD subjects, solid red and blue lines represent clamped and unclamped
measurements, respectively. In the bolt subject, the measurements were taken one hour apart. The legends indicate the middle-ear pressure (MEP) in
dPa and the ICP in mm Hg that correspond to each measurement.
• In patients with external ventricular drains and intraparenchymal monitors, DPOAES were measured across a range
frequencies out of the noise floor.
• Measurements of DPOAEs at different times with similar ICP and MEPs generally appear repeatable. Future analyses
will quantify the repeatability of these measurements within the ICU environment.
• These measurements were quick (1-3 minutes), at the bedside, non-invasive and did not cause any reported discomfort or
harm.
• Future analyses will investigate how ICP changes correlate with DPOAE changes.
DISCUSSION
• DPOAE’s are a candidate noninvasive method for monitoring ICP changes
• Low frequency stimuli, at relatively constant middle ear pressures, may be best for
detecting ICP changes outside of the noise floor.
• DPOAE measurements can be completed in the neuro-ICU efficiently, non-invasively
and without harm.
• Future directions:
1. Determination of range of ICP over which these measurements correlate.
2. Quantify intra-subject variability
3. Quantify correlation to directly estimate ICP change from DPOAE
measurements
REFERENCES
Buki B., Avan P., Lemaire JJ, Dordain M, Chazal J, Ribari O. (1996) Otoacoustic emissions: a new tool for
monitoring intracranial pressure changes through stapes displacements. Hear. Res., 94:125-139
Buki B., de Kleine E, Wit HP, Avan P. (2002) Detection of intracochlear and intracranial pressure changes with
otoacoustic emissions: A gerbil model. Hear. Res., 167:180-191
Chapman PH, Cosman ER, Arnold MA. (1990) The relationship between ventricular fluid pressure and body position
in normal subjects and subjects with shunts: A telemetric study. Neurosurgery, 26:181-189.
Voss SE, Horton NJ, Tabucchi THP, Folowosele F, Shera CA. (2006) Auditory-based detection of changes in
intracranial pressure: Distortion-product otoacoustic emissions measurements Neurocritical Care, 4: 251-7.
ACKNOWLEDGEMENTS
This work was supported by the NIH National Institute of Neurological Disorders and Stroke and an American
Medical Association Seed Grant for KNS.