Intracranial pressure (ICP) is affected in many neurological
conditions. Clinical measurement of pressure on the brain currently requires
placing a probe in the cerebrospinal fluid compartment, the brain tissue, or
other intracranial space. This invasiveness limits the measurement to
critically ill patients. Because ICP is also clinically important in conditions
ranging from brain tumors and hydrocephalus to concussions, noninvasive
determination of ICP would be desirable. Our model-based approach to continuous
estimation and tracking of ICP uses routinely obtainable time-synchronized,
noninvasive (or minimally invasive) measurements of peripheral arterial blood
pressure and blood flow velocity in the middle cerebral artery (MCA), both at
intra-heartbeat resolution. A physiological model of cerebrovascular dynamics
provides mathematical constraints that relate the measured waveforms to ICP.
Our algorithm produces patient-specific ICP estimates with no calibration or
training. Using 35 hours of data from 37 patients with traumatic brain injury,
we generated ICP estimates on 2665 nonoverlapping 60-beat data windows.
Referenced against concurrently recorded invasive parenchymal ICP that varied
over 100 millimeters of mercury (mmHg) across all records, our estimates
achieved a mean error (bias) of 1.6 mmHg and SD of error (SDE) of 7.6 mmHg. For
the 1673 data windows over 22 hours in which blood flow velocity recordings
were available from both the left and the right MCA, averaging the resulting
bilateral ICP estimates reduced the bias to 1.5 mmHg and SDE to 5.9 mmHg. This
accuracy is already comparable to that of some invasive ICP measurement methods
in current clinical use.
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Further Reading TCD in ICP
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Non-invasive Methods of Estimating Intracranial Pressure
Transcranial Doppler Sonography
Because it is non-invasive, transcranial Doppler sonography
(TCD) has been used for multiple applications, including detection of changes in cerebral blood flow, vasospasm,
circulatory arrest, and the evaluation of TBI [traumatic brain injury] . The non-invasive evaluation of ICP using TCD has
also been studied. TCD generates a velocity–time waveform of cerebral blood flow from which the peak systolic
(PSV) and end-diastolic (EDV) flow rates can be measured. The mean flow velocity (MFV), resistance index (RI)
or Pourcelot index—an indicator of resistance of an organ to perfusion, and the
pulsatility index (PI) or Gosling index—a reflection of resistance encountered
with the cardiac cycle are commonly reported derivations from the waveform
display. (MFV = [(PSV + (EDV 9 x 2))/3], RI = (PSV - EDV)/PSV)], PI = (PSV - EDV)/MFV)
Klingelhofer et al. showed that ICP influenced TCD flow
patterns. With increasing ICP, there was a progressive reduction in the MFV and
EDV with an increase in RI.
Using a mathematical model, Schmidt et al. were able to predict
ICP trends based on arterial blood pressure and flow velocity with a mean
absolute difference of 4.0 ± 1.8 mmHg when compared to measured ICP. The above studies evaluated comatose or severely brain-injured patients
with impaired cerebral autoregulation, where the PaCO2 was held constant at
30–35 mmHg, limiting the generalizability of the findings . Despite those
limitations, in a series of 12 patients with severe head trauma, Goraj et al.
were able to demonstrate a similar relationship between the RI and ICP
trends, in patients with a PaCO2 range of 20–34 mmHg.
In a study of 81 adult and pediatric patients with a variety
of intracranial pathologies, Bellner et al. concluded that there was a linear correlation between the PI
and measured ICP, although the correlation decreased in sensitivity with higher
ICP. Similar findings were demonstrated in a study of 125 patients with TBI by
Moreno et al., where for each unit increase in ICP the PI increased by 0.03
unit.
In contrast, other groups failed to find a relationship between
PI and ICP. Behrens et al. used a lumbar infusion test to manipulate ICP in patients undergoing lumbar shunt placements.
They found that the TCD blood flow velocity was not a determinant of ICP.
Similarly, Figaji et al. examined the relationship between the PI and ICP in 34
children with TBI who underwent 275 TCD studies. The absolute PI value was not
a reliable correlate for ICP despite a weak tendency for higher PI values in
elevated ICP. They recommended that PI need not be used to detect ICP unless
the PI is very high.
Other studies have demonstrated equivocal results regarding
the usefulness of TCD in adult and pediatric populations with progressive hydrocephalus . In addition,
satisfactory TCD waveforms cannot be obtained in all patients.
Ultrasound of the ONS (optic nerve sheath)
Ultrasound of the ONS is a very well-studied modality for
the non-invasive assessment of ICP. Ophthalmic ultrasound typically uses a frequency between 5 and 10.5 MHz to
evaluate the eye and orbit [79, 80].
Hansen and Helmke [81] used ultrasound in a cadaver study to
demonstrate that in the area just behind the eye-ball, elevated pressure can
increase the sheath diameter by more than 50%. In another study, the same
authors used intrathecal infusion tests to prove that the optic nerve sheath
diameter (ONSD) varies with alteration of lumbar CSF pressure [82]. A similar
study was done by Tamburrelli et al., who showed that the ONS begins to expand
when the diastolic ICP is increased to greater than 13–14 mmHg. Beyond that point,
a linear correlation is seen between the enlargement of the ON sheath and
simultaneous increases in ICP [83]. These changes in the ON sheath occur before
changes in the nerve are visible on fundoscopic examination [84]. Using 4.5 mm
as the cutoff for normal, Tamburrelli et al. [83] found a sensitivity of
ultrasound to identify an ICP greater than 15 mmHg of 88% and a specificity of
90%.
Several studies have directly correlated ONSD measurements
on ultrasound with ICP measured invasively.
The cut-off value for normal ONSD, measured 3 mm posterior
to the globe, ranges from 5.2 to 5.9 mm. The sensitivity is 74–95% and the specificity is 74–100% to
identify ICP >20 mmHg, as shown in Table 3 [85–90].
Ultrasound of the ONS has also been compared to findings of
increased ICP on CT, such as changes in ventricle size, basilar cistern size,
sulci size, degree of transfalcine herniation, and gray/white matter
differentiation [20]. Grisgin [91] showed that the mean ONSD in Neurocrit Care
patients with signs of brain edema on CT is larger than that of controls. Other
authors have shown that with a cut-off of normal ONSD ranging from 5.0 to 5.9
mm, the ONSD predicts findings of increased ICP on CT with a sensitivity of
74–100% and a specificity of 63–95%, as shown in Table 4 [92–95].
Studies of ultrasound to measure the ONSD have also been
performed in children. Malayeri et al. [96] showed that there is a significant difference between the ONSD in
children with evidence of increased ICP on CT or transcranial ultrasound and
the ONSD in controls. Two studies showed that the upper limit of normal is 4.0
mm in infants aged less than 1 year, and 4.5 mm in older children [97, 98].
Helmke and Hansen [99] agree that the ONSD should be considered enlarged when
it is over 5.0 mm in children aged over 4 years. However, another study showed
higher baseline values in asymptomatic patients with a history of hydrocephalus
[100]. Taking the upper limit of normal to be 4.0 mm in children aged less than
1 year and 4.5 mm over 1 year, Le et al. [101] found a sensitivity of 83% and
specificity of 38% for predicting increased ICP as seen on CT or lumbar puncture
opening pressure. Beare et al. [102] used a cut-off of 4.2 mm in children, and
found a sensitivity of 100% and specificity of 86% for predicting increased ICP
seen on CT.
Several studies have examined variables affecting the
accuracy of ultrasound in measuring ONSD. Romagnuolo [103] showed that the ONSD does not change with patient
position. Inter-observer variation is quite low [104], and the measurements are
highly reproducible [105], even for novice operators taught in a single
training session [106].
However, ultrasound is subject to artifacts [107], and
measuring the ONSD in off-axis will result in an erroneous value [108]. In
order for ultrasound to be used reliably, a standardized technique must be used
[104].
Doppler
Color Doppler ultrasonography has also been used as a way to
measure ICP non-invasively. The technique is similar to standard ultrasound,
but the pressure on the globe must be minimized to prevent a decrease in blood
flow velocity [109]. The ophthalmic artery and the central retinal artery and
vein can reliably be detected [110, 111]. Doppler spectral analysis is accurate
and reproducible [110], and blood velocity in the major retinal vessels varies
minimally [112]. Querfurth et al. showed that the arterial resistance rises
with increasing ICP in the mild-moderate range; however, the flow seems to
normalize with more severe elevations of ICP. The authors speculate this may be due to
local autoregulatory vascular changes and/or diversion of cerebral blood flow
into the ophthalmic circulation [113].
Miller et al. [114] showed that the central retinal artery
systolic blood flow velocity is significantly reduced in children with increased ICP compared to controls. A study
which combined ophthalmodynamometry to evaluate venous flow and color Doppler to
measure flow in the ophthalmic and central retinal arteries showed that
combining both parameters resulted in a better correlation with absolute ICP (r
= 0.95) than either parameter alone [62].
