Palavras-chave
ultrassom point-of-care - ultrassom na sala de emergência - ultrassom em pacientes
neurocríticos - ultrassom do diâmetro da bainha do nervo óptico - hipertensão intracraniana
- ultrassonografia point of-care à beira do leito
Introduction
For more than 50 years, ultrasound has been a safe and efficient method to make clinical
diagnoses. Currently, the equipment has become smaller, more precise, cheaper, with
immediate results, and without causing any trauma to the patient. Its use has spread
to practically every great reference center. It has been included in the syllabus
of medicine schools, and it has been considered “the stethoscope of the future.” Point-of-care
ultrasound is currently being used in emergency rooms in almost every medical specialty.
Images are obtained ate the bedside instantly, and can be reproduced as many times
as necessary, in real time, and recorded for posterior assessment, if needed.[1]
The concept for point-of-care ultrasound has been developed for non-radiology physicians
who wish to use another diagnostic tool at the bedside, in a non-invasive, safe, and
fast way. Up to date, the most prominent uses of this tool included: guided vascular
access in emergency rooms and intensive care, thoracenteses, paracenteses, as well
as for the assessment of the free liquid in cases of polytrauma, with the focused
abdominal sonography for trauma (FAST), having been replaced for the extended focused
assessment sonography for trauma (E-FAST), which includes a broader view, considering
pulmonary and cardiac assessments, apart from the windows already adjusted on the
FAST. In this new concept, lesions of the lungs, including hemothorax and pneumothorax,
as well as aortic and cardiac lesions, can also be diagnosed. More recently, however,
some questions that are common in emergency settings can be answered with point-of-care
ultrasound, such as: what is the patient's volemic state? Should we use inotropic
or vasoactive drugs? Does the respiratory insufficiency have pulmonary or cardiac
origins? Is there indication for decompressive craniectomy? Is there any midline shift?[2]
[3]
[4]
Objectives
The goal of the present article is to describe the current knowledge on the use of
point-of-care ultrasound to assess intracranial pressure measuring the diameter of
optic nerve sheath through the transorbital window in critical patients.
Materials and Methods
The present is a descriptive and quantitative research that was performed through
a literature narrative review on the Latin American and Caribbean Literature in Health
Sciences (LILACS, in Portuguese) and the National Library of Medicine (PubMed) databases
in July 2016, using the following descriptors: point-of-care ultrasound; ultrasound in emergency room; ultrasound in neurocritical care; intracranial hypertension; optic nerve sheath; and optic nerve ultrasound. The descriptors were combined for the search as follows: point-of-care ultrasound and optic nerve sheath; point-of-care ultrasound and neurocritical care; point-of-care ultrasound and emergency room; and point-of-care ultrasound and intracranial hypertension.
The study has as a guide the following question: can point-of-care ultrasound be used
to assess intracranial pressure in a neurocritical patient?
The inclusion criteria were: updated publications mostly from 2001 to 2016, with rare
exceptions, written in Portuguese, Spanish and English, with online access to the
full text. Duplicate articles were excluded.
For the analysis of the articles included in this review, the following aspects were
observed: the year of publication; the type of periodical; the place where the study
was conducted; the methodology used; and the main results.
Development
Intracranial hypertension (IH) in the emergency setting. Intracranial hypertension (IH) is a frequent problem in emergency rooms and intensive
care facilities, and in the case of an intracranial lesion, the recommendation is
to maintain the pressure below 20–25 mmHg. The techniques currently available for
this monitoring are invasive, with the introduction of an intraparenchymal and/or
intraventricular catheter, with risk of hemorrhage and/or associated infections. Aa
alternatives to the gold standard measurement, the invasive technique, there are options
such as computed tomography (CT) of the brain or magnetic resonance imaging (MRI)
of the encephalon, which will reveal indirect signs of IH with deletion of the cisterns
of the base, deletion of the cortical grooves, loss of the interface between the white
and gray matters, and midline deviation, but the accuracy of these imaging techniques
is still uncertain. In this context, point-of-care ultrasound is shown to be a fast,
reliable, safe and inexpensive method, which can be performed at the bedside. The
sheath around the optic nerve is a continuation of the dura mater, reflecting the
subarachnoid environment. The increase in intracranial pressure is immediately transmitted
to the interior of the sheath that covers the optic nerve, promoting its distension,
and, subsequently, papilledema. However, papilledema is a late event, which can take
from hours to days to manifest, unlike its acute character, which can take seconds
until the thickening of the optic nerve sheath. Point-of-care ultrasound can identify
this distension in the optic nerve sheath starting from a prefixed distance from the
retina, and it is an extremely useful tool in cases of cranioencephalic trauma, intracranial
hemorrhages, and other brain lesions.[5]
Optical nerve anatomy. The optic canal, which is located on the lower wing of the sphenoid bone, receives
the optic nerve (pair II of the cranial nerve) from the intraorbital region, bringing
retinal afferences. This nerve is wrapped by a meningeal sheath consisting of the
dura mater (pachymeninges), the arachnoid and the pia mater (both leptomeninges).
Thus, the liquor content is also present in the subarachnoid space, reflecting its
intracerebral pressure. The thickness of the optic nerve sheath directly translates
the pressure of this liquor into the subarachnoid space, correlating directly to the
measurement of the intracranial pressure.[4] The closer to the eyeball, the greater the meningeal distensibility found, and this
establishes a dilation visualized through the ultrasound as having a bulbous aspect.
While papilledema may take a period ranging from days to weeks to appear, the distension
of this bulbar region of the optic nerve sheath can be visualized almost instantaneously,
in seconds, after the elevation of the intracranial pressure.[6]
[7] The first ultrasonographic description of the eyeball was performed in 1956 to estimate
intracranial pressure in cadavers. However, the low quality of the equipment and the
lack of methodology made its interpretation quite difficult. Only in 1996, with more
sensitive equipment, it was possible to identify the dilation of the optic nerve sheath
and its appropriate point of observation, at 3 mm posterior to the retina: this is
the point most sensitive to ultrasound with the use of linear transducers.[4]
[7]
Ultrasonographic appearance of the optic nerve. At the transorbital ultrasound, the eyeball is visualized as round, darken (black)
and full of fluid. The anterior chamber is anechoic and corresponds to the lens, while
the iris is observed as bright or echogenic. The choroid and the retina are visualized
as a thin greyish layer on the posterior face of the eyeball. The optic nerve reveals
itself as a black band running out of the eyeball, in the posterior direction, and
it is necessary to position it in the center of the ultrasound screen. The optic nerve
sheath is visualized through the ultrasound with higher reflectivity, that is, it
will be clearer compared with the nerve itself. Some authors describe the signs of
dilation of the optic nerve bulb, 3 mm from the retina, as a sign of the crescent,
indicating the presence of IH.[8]
[9]
Technique recommended to estimate intracranial pressure. Although there still is no universal consensus, some general principles should be
observed for optimization and to be able to draw comparisons between professionals.
The transducer used should be of linear high frequency (7–10 MHz), preferably with
a good quality resolution equipment and viewing capacity of structures at depths of
5–6 cm. The examiner should apply gel on the closed eyelids, supporting his hand with
the transducer on a rigid bone structure, to avoid pressure on the eyeball. Initially,
the transducer will be placed in the transverse position, and will be slowly shifted
to the parasagittal position; the optic nerve should be centered on the ultrasound
screen, with concomitant visualization of the lens and iris; Both eyes should be evaluated
in this exam. Once properly positioned, the image is frozen, and the transducer is
removed from the eyelid; the examiner locates the spot 3 mm from the retina and assesses
the diameter of the sheath of the optic nerve ([Fig. 1]). The authors usually recommend that this measurement be made at least 2 or 3 times
for greater reliability of the results. It is also important to emphasize that there
is no greater precision in measurement made with the transducer positioned parasagittally
(longitudinally) or transversally. The patient's head can be in horizontal ventral
decubitus, or even in decubitus elevated at 20–30°.[2]
[5]
[7]
[10]
Fig. 1 Recommended technique to measure the diameter of the optic nerve sheath and anatomical
structures of the eye and the nerve correlated with the ultrasound image.
Reference values of the optic nerve sheath diameter and its correlation with intracranial
pressure. The value of the diameter of the sheath of the optic nerve that can reflect intracranial
hypertension is subject to debate, and has not yet been clearly defined in the literature.
Based on the maximum normal value for intracranial pressure of 15 mmHg, with IH defined
when this value is ≥ 20 mmHg, several authors believe that 5 mm of thickness for the sheath is a reasonable limit.
However, many studies (with small samples) don't agree with this value: Rajajee et
al[5] use 5.2 mm; Soldatos et al,[4] 5.7 mm; and Bäuerle and Nedelmann,[11] 5.8 mm. Shevlin[7] observed that the variation in the limit in the several studies to this moment is
between 4.8 mm and 6.0 mm, which is a complicating factor to define IH. Several studies
that proposed these different reference values cannot be compared, since they use
different methodologies and were performed on different populations, making new prospective
and multicentric studies directed to this end necessary. ([Table 1]).[5]
[11]
[12]
[13]
[14]
[15]
[16]
Table 1
Studies that propose a cut-off point for the optic nerve sheath diameter that would
best correspond to intracranial pressure > 20 mmHg, with their respective sensitivity
and specificity
|
Study
|
Optic nerve sheath diameter
|
Patients (n)
|
Sensitivity
|
Specificity
|
|
Blaivas et al[10]
|
5.0 mm
|
35
|
100%
|
95%
|
|
Goel et al[17]
|
5.0 mm
|
100
|
98.6%
|
92.8%
|
|
Tayal et al[18]
|
5.0 mm
|
59
|
100%
|
63%
|
|
Kimberly et al[19]
|
5.0 mm
|
15
|
88%
|
93%
|
|
Moretti et al[20]
|
5.2 mm
|
63
|
94%
|
76%
|
|
Geeraerts et al[14]
|
5.9 mm
|
37
|
90%
|
84%
|
|
Soldatos et al[12]
|
5.9 mm
|
76
|
74.1%
|
100%
|
|
Shirodkar et al[21]*
|
4.8 mm
|
101
|
75%
|
100%
|
|
Wang et al[22]**
|
4.1 mm
|
279
|
95%
|
92%
|
Notes: Source: Adapted from Shevlin[7] and Ochoa-Pérez and Cardozo-Ocampo.[2]*Indian population. **Chinese population.
Confirmation of IH through different ultrasound methods. The measurement of intracranial pressure by intraventricular catheterization is considered
the gold standard method in the evaluation of IH. However, the risk of infection and
hemorrhage is inherent to the procedure, as well as to the clinical status of the
patient. The correlation between the increase in the diameter of the optic nerve sheath
and the intracranial pressure measured by intraventricular catheter has not been well
established, due to the aggressiveness of the procedure and the clinical condition
of the patients. Rajajee et al[5] assessed 536 patients with intraventricular catheter and correlated the data with
the diameter of the sheath of the optic nerve, finding a strong correlation between
IH > 20 mmHg and a mean sheath diameter of 5.3 mm (5.1–5.7 mm), while intracranial
pressure < 20 mmHg correlated to the mean of 4 mm in diameter of the optic nerve sheath
(p < 0.0001). It is important to highlight that the authors did not find a statistically
significant difference between patients submitted to mechanical pulmonary ventilation
and those who did not intubate.[5] Neuroimaging criteria to confirm increased optic nerve sheath thickness (> 5 mm)
findings were used and correlated with midline shifts (> 3 mm), ventricle III deletion,
mass effect, hydrocephaly, and signs of early or late cerebral edema, and significant
results, with 100% of sensitivity and 95% of specificity, were found when the cranial
CT was used.[5]
[10] Ochoa-Pérez and Cardozo-Ocampo[2] emphasize that, in addition to the measurement of the optic nerve sheath diameter
to estimate IH, a bone window performed through the temporal bone scale with a low
frequency transducer (1–5 MHz) is able to detect the midline shift. The technique
is essentially the same used in transcranial color Doppler, except for the fact that
the structures are visualized in ultrasound B (brightness) mode. The compressions
of the lateral ventricles and midline shifts resulting from ischemic vascular accidents,
sub- and extradural hematomas, as well as hemorrhages from the basal nuclei, may be
visualized. Shunts can be placed with the help of this technique. However, due to
the interference of the bone, the authors report that, in ∼ 15% of the cases, the
images of the cerebral parenchyma are not satisfactory. In those undergoing decompressive
craniectomies, the use of point-of-care ultrasound becomes an even more useful tool,
with the area of absence of bone as the most appropriate window.[2]
[9]
Arguments in favor of and contrary to the use of the optic nerve sheath diameter for
the evaluation of intracranial pressure. The advantages of using ultrasound at the bedside are: reproducibility, non-invasiveness,
low cost, portability, rapid execution, absence of ionizing radiation, and possibility
of performance without transfer of the patient out of the emergency room or intensive
care unit. Among the factors considered to be disadvantageous, one can mention: lack
of preparation of intensivist and emergency physicians in the use of point-of-care
ultrasound at the bedside, which is a new technique; the need for a learning curve,
since ultrasound is a technique dependent on the operator's ability; lack of reference
values based on large prospective and multicenter population studies; relative risk
of ocular globe injury due to excess pressure of the transducer on the patient's eyelid;
risk of thermal injury by the absorption of the apparatus energy and heat transformation;
and inability to use it in cases of suspicion of lesions of the eyeball.[4]
[7]
Conclusion
Point-of-care ultrasound is a reality in large, high complexity centers around the
world. Its characteristics contribute to its dissemination: it is a low-cost exam,
with good accuracy, fast, non-invasive, recommended for the non-specialist physician,
with no ionizing radiation, and it can be performed at the bedside. Several medical
specialties have been using point-of-care ultrasound, and neurology and neurosurgery
are no exception. The idea of estimating intracranial pressure using at the bedside
a tool that is non-invasive and fast has been gaining space in emergency medicine.
The transorbital window enables the visualization of the optic nerve sheath bilaterally,
enabling the estimation of the intracranial pressure transmitted by the liquor to
this structure. Its diameter measured 3 mm posterior to the retina seems to represent
an intracranial pressure within the limits of normality when lower than 5 mm. However,
there are still no large prospective and multicenter studies that validate these values.
It is also not known for certain whether all intracranial injuries, in victims of
polytrauma, may have the same reference value. Another possibility is through the
transtemporal window, in which midline shifts, the size of the lateral ventricles,
and the presence of hematomas can be observed. Despite the need for further studies
to validate the reference values, point-of-care ultrasound in neurology is an extremely
promising tool for neurocritical patients.
