In contrast to extracranial diagnostic vascular examinations, transcranial ultrasound procedures cannot access to imaging modalities, because resolution of Duplex techniques is not sufficient to image vessel anatomy or structure. Thus TCD and TCCD can determine only blood flow velocities, which allows indirect conclusions on stenosis or occlusion.
Frequently early stages of stenosis are not detectable because they become notable not before the grade of narrowing exceeds more 50% (ICA > 70%).
Additionally measurement of narrowing is restricted since increased BFV can also rely on hyperperfusion, which can be caused by disturbed cerebral autoregulation (see special chapter). Thus, detection of intracranial vascular stenosis relies largely on indirect criteria.
Atherosclerotic processes are in a majority of cases the reason of intracranial occlusions. In contrast to that, intracranial increase of blood flow velocity can rely on different reasons, which efforts a note of caution in differential diagnostic considerations: E.g. vasospasm after subarachnoidal haemorrhage, autoimmune or infectious vasculitis, isolated atherosclerotic narrowing caused by diabetes, hypoplasia of single vessel segments, anemia and vascular dissection are frequent reasons of BFV accelerations.
TCD – diagnostic examinations
1. Diagnosis of extracranial carotid artery stenosis
Using TCCD > takes advantage due to imaging of plaque morphology. TCD plays only a minor role.
2. Diagnostic criteria of high-grade stenosis (> 70%) in intracranial carotid artery (ICA)
Modified from “Doppler- und Duplexsonographie der hirnversorgenden Arterien”, Tab. 16.2, Widder et Görtler, Springer Verlag, 6. Auflage.
Diagnosis of stenosis of distal ICA within the carotid siphon seems to be technically difficult since in usual examination depths of 60 to 70 mm signals of different vessels are detectable (ACOM, PCOM, P1, sometimes contralateral segments), which can imitate stenosis originally generated by turbulent collateral flow signals. Neuronavigated TCD could help to minimize this problem since it allows even an estimation of each insonation angle and visualisation of the vascular tree.
3. Diagnostic criteria of MCA stenosis
Because of the smaller diameter of MCA (2.0-2.5-3.0 mm) Digital Subtraction Angiography (DSA) does not allow the estimation of MCA stenosis. Up to now these narrows can only be determined by measurement of BFV.
Stenosis less than 40% (120-160 cm/s) are characterized as “low grade”, 40-70% (160-220 cm/s) as “medium” and more than 70% (≥ 220 cm/s) as “high-grade”. Possible hyperperfusion needs to be excluded prior to diagnosis. Therefor, it is necessary to determine the Lindegaard-Ratio (vMCA/vICA). Values less than 2 point to the presumption of hyperperfusion rather than stenosis. In contrast to that, values higher than 3 as well as BFVs higher than the double normal velocity presume stenosis firstly.
Likewise, comparison of pre- and poststenotic BFVs as well as bilateral velocity comparison provides useful information.
Modified from Doppler- und Duplexsonographie der hirnversorgenden Arterien, Tab. 16.2, Widder et Görtler, Springer Verlag, 6. Auflage.
High-grade stenosis in the segment of proximal MCA in a patient suffering from TIA.
From: Doppler- und Duplexsonographie der hirnversorgenden Arterien, Tab. 16.2, Widder et Görtler, Springer Verlag, 6. Auflage.
4. Intracranial ICA stenosis
Detection of ACA Stenosis within the A1-segment is challenging, because the poststenotic demand of blood can be covered by collateralisation by ACOM. Thus accelerations of BFVs in A1 are relatively rare, which causes missing detection of ACA-Stenosis ( negative results). Since aplasia as well as hypoplasia of A1 are common (2-3%) a missing flow signal is not an accurate proof for occlusion of A1-segment.
Hence, only an increase of BFV in ACOM is a clear ultrasonic clue for A1 occlusion.
5. Diagnostic criteria of PCA stenosis
The same applies in the same manner as described for ACA above for P1, providing a working PCOM segment, but because hypoplasia of P1 is more common (10-11%) than for A1, a missing P1 signal is not sufficient for diagnosis of occlusion. In the same manner only the PCOM signal provides further indirect information. In case of aplastic or hypoplastic PCOM, which account for the most frequent intracranial vessel anomaly (17-18%), similar criteria apply for PCA as for MCA (30% lower as for MCA).
- False negative results caused by poor ultrasonic bone windows
- Difficult differentiation from hyperperfusion
- Confusion caused by mistaken vessel labelling in depth of > 55mm
Paramountly, the most important preventive measure is to reduce atherosclerotic risk factors.
That means abstinence from smoking, adequate treatment of hypertension, hypercholesterolemia and diabetes as well as reduction of obesity.
The therapy of intracerebral vessel narrowing is depending on the grade of stenosis as well as neurological complications.
The guideline of the German Association of Neurology concerning “cerebral ischemia” regards in case of asymptomatic stenosis or TIA as initial manifestation the treatment with 100-300 mg acetylsalicylic acid per day. With the occurrence of neurological recurrent symptoms, angioplasty and, if possible, stenting is indicated for anterior circulation; additional to acetylsalicylic acid and Clopidogrel (grade of evidence C).
Because of introduction of intracerebral angioplastic procedures (2002), the value of bypass surgery becomes less important.
From: Stentimplantation zur Behandlung intrakranieller Stenosen Klein GE, Zeitschrift für Gefäßmedizin 2007; 4 (1), 4-10.
This text belongs closely to:
Doppler- und Duplexsonographie der hirnversorgenden Arterien, Widder et Görtler, Springer Verlag, 6. Auflage, Kapitel 16.