CONTENTS
- Intracranial pressure
- 2D imaging of the brain:
- Transcranial Doppler
- Podcast
- Questions & discussion
- Pitfalls
optic nerve sheath diameter
- The optic nerve sheath is part of the dura surrounding the brain.(35001377) When intracranial pressure is elevated, the subarachnoid fluid surrounding the optic nerves swells – which widens the diameter of the optic nerve sheath. The optic nerve itself remains the same size.
- The most widely validated approach to estimating intracranial pressure involves measuring the optic nerve sheath diameter 3 mm behind the eye (figure below).
- Interpretation is roughly as follows:(31025061)
- <5 mm suggests normal intracranial pressure.
- 5-6 mm is a grey zone.
- >6 mm suggests elevated intracranial pressure.
- Two measurements should ideally be obtained in each eye (with the probe oriented in a sagittal plane and an axial plane), yielding a total of four values. This provides an estimation of the reliability of measurement within any individual patient. If all four measurements are consistently normal (<5 mm) or consistently abnormal (>6 mm), then that may increase confidence that the data is accurate.
- ⚠️ Precision may vary based on a number of different factors (e.g., whether the patient is moving their eyes, or the quality of the ultrasound machine). If measurements are inconsistent, then they aren't providing reliable information.
papilledema
- Papilledema may be noted on ultrasonography as a protrusion overlying the optic disc (figure above).
- Some studies have found that ultrasonographic papilledema has good performance for the detection of elevated intracranial pressure (e.g., sensitivity in the range of ~90%).(25190532, 27250852)
- Overall, papilledema is not as well validated as optic nerve sheath diameter. However, papilledema may remain a useful ancillary finding – especially among patients who have an equivocal optic nerve sheath diameter (e.g., values fluctuating between 5-6 mm).
- Papilledema should be distinguished from optic disc drusen (calcifications in the optic disc which appear bright; figure below) or inflammation due to optic neuritis (which is usually unilateral).
pulsatility index
- An elevated pulsatility index within the middle cerebral artery (MCA) may suggest elevated intracranial pressure. However, the pulsatility index probably relates most directly to cerebral perfusion pressure (CPP) rather than intracranial pressure. More on pulsatility index below.📖
- When beginning to perform 2D imaging of the brain via a temporal window, the key landmark is the midbrain (aka cerebral peduncles).
- Normally, the midbrain is surrounded by the perimesencephalic cistern. Contrast between the midbrain and its surrounding cistern creates a butterfly shape.(figure above)
- If the perimesencephalic cistern is compressed (e.g., due to herniation), then the normal butterfly configuration may be lost.(28261152) For example, if initial sonography shows a normal midbrain but then a follow-up scan shows that this landmark has disappeared, there should be concern for compression of the perimesencephalic cistern.
- Roughly ~10% of patients may have poor bone windows, which don't allow for the performance of transcranial ultrasound.(30894296)
technique
- Start by imaging the butterfly-shaped midbrain (as discussed in the section above).
- Tilting upwards by ~10 degrees reveals a transverse view of the third ventricle (figure below). The largest transverse diameter should be measured.
interpretation
- The normal size of the third ventricle may vary:(35001377)
- <60 years old: 1.2-5.1 mm
- >60 years old: 3.3-9.2 mm
- The diameter of the third ventricle can be followed in a serial fashion. It may also be compared to historical values from the patient's prior CT scans.
technique
- The distance from the ipsilateral temporal bone to the center of the third ventricle is measured from both sides of the head (using the same views that are utilized to evaluate for hydrocephalus, as shown in the last section). In the absence of hydrocephalus, the third ventricle is often fairly small (visible as a pair of parallel, hyperechoic ventricular walls; figure 1D above).(31580841)
- Midline shift may be evaluated as the difference between these distances divided by two (figure below). Division is necessary to avoid counting the midline shift twice.
interpretation
- Midline shift may be considered significant when it exceeds roughly ~5 mm. Trends in midline shift may be more accurate than individual values.
- One study of transcranial ultrasonography found that among patients with intracranial hemorrhage, midline shift >2.5 mm had 78% sensitivity and 89% specificity for the detection of a large intracranial hemorrhage.(31279352)
- Limitations to evaluation of midline shift:
- Skull defects (e.g., decompressive craniectomy or skull fractures).
- Chronic abnormality of intracranial anatomy.
- Intracranial hematomas will appear hyperechoic for about five days. Subsequently, they will become hypoechoic, with a surrounding hyperechoic halo.(28240783)
- Hyperechoic lesions on ultrasound may also be caused by arteriovenous malformations or tumors, so CT imaging is required to confirm the presence of hemorrhage. (28240783)
- Hematomas may be visible if they fall within the transtemporal window (e.g., many subdural hematomas or hypertensive hemorrhages).
- Sonography may be utilized to track hematoma volume (figure below).
This chapter assumes a general familiarity with transcranial Doppler ultrasonography. For a refresher of the basics, the following video is helpful:
precision & role of POCUS (versus formal study)
- Transcranial Doppler ultrasonography is best validated for evaluation of blood flow through the middle cerebral artery (MCA). Evaluation of the MCA is easiest and most robust, since this vessel is large and runs towards the skull (allowing Doppler measurements to be performed nearly parallel to the vessel).
- Transcranial Doppler ultrasonography may be used to evaluate other arteries. The evidentiary basis for these evaluations is less robust than for the MCA. Performance when evaluating these arteries may be improved by the following:
- (1) Use of serial examinations to track changes over time.
- (2) Obtaining a “formal” transcranial ultrasound study by a technician who has extensive experience with transcranial ultrasonography.
defining the pulsatility index
- Pulsatility Index (PI) = (Peak systolic velocity – end diastolic velocity)/(mean velocity)
- A normal pulsatility index is roughly 0.5 – 1.2 (Nelson, 2020)
- One advantage of the pulsatility index is that it is robust to alterations in the angle of insonation of the vessel (since off-angle measurement will affect all velocities equally).
factors affecting the pulsatility index
- Pulsatility index (PI) is directly proportional to the pulse amplitude of systemic arterial blood pressure.(31279352; 28880397)
- VA-ECMO or an intra-aortic balloon pump may reduce the pulsatility index.
- Aortic regurgitation might increase the pulsatility index.
- Pulsatility index (PI) is inversely proportional to the cerebral perfusion pressure (CPP).
- As cerebral perfusion pressure (CPP) falls, this causes a disproportionate drop in the diastolic flow velocity – which increases the pulsatility index (figure below).
- The cerebral perfusion pressure (CPP) depends on the mean arterial pressure (MAP) and the intracranial pressure (ICP), based on the equation CPP = MAP – ICP.
- PaCO2
- Hypocapnia increases the pulsatility index.
- Hypercapnia decreases the pulsatility index.(22311229)
- Heart rate:
- Bradycardia allows the diastolic flow to drop lower, increasing the pulsatility index.
- Tachycardia allows less time for diastolic fall to decrease, which reduces the pulsatility index.
- Cardiac output:
- Higher cardiac output (e.g., fever, anemia) may decrease the pulsatility index.(28240783)
- Cerebrovascular resistance:
- A proximal stenosis or vasospasm may reduce the pulsatility index.
- Distal vasospasm may increase the pulsatility index.
- Arteriovenous malformation may reduce the pulsatility index.(28240783)
- Vascular compliance:
- Advanced age may increase the pulsatility index.(28240783)
clinical use of pulsatility index?
- The pulsatility index has been used in various formulas to calculate the intracranial pressure (ICP). Such equations have not been validated with sufficient accuracy to warrant clinical use.(28880397) Given the numerous factors which affect pulsatility index (listed above), it's unlikely that any simple formula can relate pulsatility index to intracranial pressure (ICP). Furthermore, the pulsatility index has been shown to correlate more closely with cerebral perfusion pressure (CPP) than with intracranial pressure (ICP).(22311229)
- In the context of normal systemic hemodynamics, a rising or elevated pulsatility index (especially >2) suggests low cerebral perfusion pressure (CPP).(35095741; 35001377; 22311229) Among patients with a normal mean arterial pressure (MAP), this would in turn imply an elevated intracranial pressure (ICP).(30894296)
(resistive index)
- Resistive Index (RI) = (Peak systolic velocity – end diastolic velocity)/(peak systolic velocity)
- >0.75-0.8 is abnormal.(Nelson, 2020; 30894296)
- Resistive Index is fundamentally similar to the pulsatility index (e.g., both will be elevated in patients with low cerebral perfusion pressure). However, the pulsatility index is utilized more broadly.
clinical background on vasospasm
- Vasospasm may occur as a form of delayed brain injury following subarachnoid hemorrhage. Although most notable in patients with aneurysmal subarachnoid hemorrhage, vasospasm can also occur following other forms of subarachnoid hemorrhage (e.g., traumatic or intraoperative).
- Further clinical discussion of vasospasm and delayed cerebral ischemia is here: 📖.
use of transcranial Doppler (TCD) for evaluation of vasospasm
- TCD may predict neurological deficits an average of 2.5 days prior to the onset of symptoms. (Nelson, 2020)
- If available, daily TCD studies are recommended in patients with aneurysmal subarachnoid hemorrhage, especially 3-10 days after onset. (Nelson, 2020)
- Trends over time may be more accurate than any single measurement.
- Increase in mean flow velocity of >50 cm/s since the prior day suggests vasospasm (even if the absolute flow velocity remains within a normal range).(31761060)
- TCDs are best for detection in the middle cerebral artery (MCA) , with lower performance in other arteries.(31279352) TCD misses vasospasm that is isolated to the distal arteries.
overall guide to normal mean flow velocity
- Middle Cerebral Artery (MCA) – normally ~~80 cm/s.
- <120 cm/s suggests an absence of vasospasm. (Wijdicks 2019)
- 120-150 cm/s is consistent with mild vasospasm. In the context of subarachnoid hemorrhage, a mean velocity >120 cm/s is 88% sensitive and 72% specific for vasospasm.(31922373)
- 150-200 cm/s is consistent with moderate vasospasm.
- >200 cm/s is consistent with severe vasospasm. In the context of subarachnoid hemorrhage, a mean velocity above >200 cm/s may be up to 98% specific and 73% sensitive for vasospasm.(31922373)
- Vertebral Artery – normally ~40 cm/s. >80 cm/s is more specific for vasospasm.(Torbey, 2019)
- Basilar Artery – normally ~40 cm/s. >95 cm/s is more specific for vasospasm.(Torbey, 2019)
factors affecting flow velocity
- Factors increasing flow may mimic vasospasm:(28240783; 31580841)
- Global hyperdynamic state (e.g., sepsis, fever, anemia).
- Arterial hypertension, preeclampsia.
- Chronic intracranial arterial stenosis (e.g., atherosclerotic plaque).
- Hypercapnia.
- Factors decreasing flow may obscure vasospasm:(28240783)
- Decreased cerebral perfusion pressure (CPP):
- Elevated intracranial pressure (ICP).
- Reduced mean arterial pressure (MAP) or global hypodynamic state (e.g., due to aortic stenosis or left ventricular failure).
- Hypocapnia.
- Hypothermia.
- Wrong angle of insonation.
- Decreased cerebral perfusion pressure (CPP):
ratios of flow
- Lindegaard ratio (LR) to assess flow in the middle cerebral artery (MCA).
- This is defined as the ratio of MCA mean flow velocity divided by the extracranial internal carotid artery mean flow velocity. For a patient with increased flow velocity, this ratio may help differentiate focal vasospasm involving the MCA from globally hyperdynamic blood flow:
- Elevated flow velocity & LR <3: Suggests hyperdynamic flow (e.g., sepsis, fever, anemia).
- Elevated flow velocity & LR >3 suggests vasospasm.(35095741)
- LR 3 – 4.5 suggests mild vasospasm.
- LR 4.5 – 6 suggests moderate vasospasm.
- LR > 6 suggests severe vasospasm.
- To determine the extracranial internal carotid flow velocity, a phased-array probe may be used in attempts to align the internal carotid artery in parallel with the probe (using a retromandibular window at the angle of the jaw; figure below).(31580841) The internal carotid artery is located lateral to the external carotid and lacks cervical branches.(28240783)
- The Sloan ratio may be used to assess flow in the anterior cerebral artery (ACA). This is the ratio of the ACA mean flow velocity divided by the extracranial internal carotid artery mean flow velocity.
- The Soustiel ratio may be used to assess flow in the basilar artery. This is the ratio of the basilar artery mean flow velocity divided by the extracranial vertebral artery mean flow velocity. A Soustiel ratio >3 with basilar artery velocity >85 cm/s has >90% sensitivity & specificity for a basilar artery narrowing of >50%. (Nelson, 2020)
findings
- As intracranial pressure increases, blood flow decreases as shown above.
- The final three patterns shown above are consistent with brain death:(31580841)
- Oscillating flow: Brief forward flow or systolic spikes with diastolic flow reversal.
- Systolic spikes: Brief systolic flow spikes with no diastolic flow. Systolic spikes should be short (<200 ms duration) and small (<50 cm/s peak velocity).(35001377)
- No flow, in a patient in whom flow was previously visible via TCD. Blood flow should still be visible extracranially (e.g., in the external carotid artery).
- ⚠️ The patient's blood pressure must be adequate while performing the study, to exclude the possibility of transiently reduced flow due to transient hypotension.(35095741)
clinical utility
- Studies have found sensitivity and specificity values >90%, indicating performance which is very good but not perfect. Test performance will vary depending on exactly what findings are obtained.
- If available, a nuclear flow scan is preferred over transcranial Dopplers, given that a nuclear flow scan is less operator-dependent and provides more unequivocal evidence of brain death.
- Ultrasonography may be helpful in determining an appropriate time to order a nuclear flow scan. For example, if ultrasonography demonstrates preserved cerebral blood flow then it would be premature to obtain a nuclear flow scan.(29030715)
- A series of TCD studies showing progressive loss of flow over time may constitute the strongest evidence to support brain death. Alternatively, if the first study detects no flow, then this is less convincing (as it could reflect poor bone windows rather than true absence of flow).(Wijdicks 2019) Power mode may also be helpful in searching for blood vessels if these are difficult to find; one study suggested that power mode could improve the performance of TCD for indicating brain death.(31279352)
- Further discussion of the diagnosis of brain death is here: 📖
- Transcranial Doppler might be used to detect large vessel occlusion (LVO) (e.g., with sensitivity and specificity >90% for occlusion of the middle cerebral artery).(30894296)
- Acute ischemic stroke management is generally directed primarily by advanced imaging (e.g., CT and/or MRI). However, it is conceivable that within some stroke systems transcranial Doppler might be used as a tool to facilitate early detection of large vessel occlusion (thereby fast-tracking patients to interventional radiology thrombectomy).
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- Beware that numerous factors affect transcranial Doppler parameters, such as the pulsatility index (PI). Interpretation should take into account clinical context.
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References
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