Carotid doppler sonography (CDS) represents a minimally invasive screening test that yield hemodynamic information very suggestive of DAVF,
such as low resistance and high velocity flow in external carotid arteries (ECA).
These hemodynamic changes can be useful for follow-up and evaluation of treatment effectiveness.
On the other hand,
the most important limitation is that it only shows the hemodynamic changes in the feeding arteries without showing the actual size and patterns of the venous drainage.
As CDS has been widely used to evaluate the severity of atherosclerosis in carotid arteries,
simultaneous analysis of the ECA Resistance index (RI) during routine CDS may detect occult DAVF in some patients.
During follow-up after DAVF diagnosis or treatment,
conventional angiography should be performed to revaluate the status of the lesion,
in all patients presenting deterioration of CDS results (e.g.,
lowering of the ECA RI).
Some authors advocate that follow-up using CDS should be carried out at least once per year or any time the clinical picture of the patient changes.
6.1) Rationale of using ultrasonography in DAVF diagnosis
Diagnosis of DAVF by ultrasonography depends on hemodynamic changes in the feeding arteries and drainage veins as well as shortened cerebral circulation time.
6.1.1) Arterial findings
As most DAVFs are feed from branches of the ECA,
pathological hemodynamic changes can be accessed in cervical arteries and translate into:
- Increased flow volume;
- Increased Peak-systolic velocity (PSV);
- Increased End-diastolic velocity (EDV);
- Reduction in flow resistance (due to direct shunting in the absence of capillary beds)*.
(*Resistance index (RI) = (PSV− EDV)/PSV)
6.1.2) Venous findings
In normal conditions,
the venous system displays low flow velocity (without pulsatile waveforms,
as seen in arteries).
High flow through the AV shunts may also induce hemodynamic changes in the cerebral sinuses and drainage veins:
- Arterialized pulsatile waveforms;
- Increased flow velocity in cervical veins;
- Venous dilatations;
- Reversed blood flow.
6.2) CDS in follow-up after treatment
Some studies showed that CDS could be used to evaluate effectiveness of DAVFs treatment.
In order to do so,
CDS should be performed before and immediately after endovascular therapy and the baseline hemodynamic status should be registered,
so changes during follow-up can be noticed.
It was found a good correlation between serial ECA RI changes and the evolution of clinical symptoms related to dural AVFs.
In patients with total resolution of the DAVF,
the ECA RI increases after embolization,
while the same is not true in lesions that were only partially embolized.
6.3) Ultrasound equipment
Looking at literature,
the most reported systems have been the HP 4500 (Hewlett-Packard Co.,
Palo Alto,
CA),
the VST Master (Diasonics,
Tirat Carmel,
Israel) and the SSD-3000 (Aloka Co.,
Ltd.,
Tokyo,
Japan),
but virtually any ultrasound system with carotid doppler capability can be used.
Regarding to transducers,
those reported usually range from 3 to 11 MHz real time B-mode-transducers associated with 3 to 6 MHz pulsed-doppler-transducers.
6.4) CDS parameters for DAVFs
Establishing the optimal cut-off values for the screening,
diagnosis and follow-up of DAVFs is the critical point,
as those values are slightly different across studies.
The table below summarizes some significant data:
|
No.
of DAFVs studied |
Proposed parameters |
Conclusions |
Tsai et al.
(2004) |
35 |
RI of ECA < 0.72 (right) or < 0.71 (left) |
RI of ECA is the best parameter for predicting DAVFs (74% sensitivity and 89% specificity).
|
Tsai et al.
(2005) |
29 |
Change in RI of ECA ≥ 0.1
(correlates with DAVF clinical status at follow-up) |
RI of ECA is correlated with the effectiveness of treatment and clinical evolution of DAVFs. |
Arning et al.
(2005) |
17 |
Low RI of ECA branches
(versus CCA and ICA) |
Studying ECA branches is more diagnostic (positive predictive value 100%). |
Tsai et al.
(2009) |
63 |
RI of ECA < 0,7
and
ICA-to-ECA ratio of RI > 0,9
(validated in 20400 subjects) |
Combined CDS parameters can be used for screening of DAVF (51% sensitivity and 99% specificity). |
Yeh et al.
(2010) |
67 |
RI of ECA < 0,72
and
EDV of ECA > 21 cm/s
|
RI and EDV of ECA have high diagnostic sensitivity for DAVFs in
patients with pulsatile tinnitus (sensitivity 92% to 95%). |
Tee et al.
(2013) |
24 |
RI of Occipital artery <0.76 |
RI of Occipital artery can be used to screen for DAVFs having the occipital artery as one of feeding arteries (about half of DAVFs) (96% sensitivity and 97% specificity). |
Although the parameter RI of ECA seems the most sensitive for predicting DAVF,
the combination of ECA RI and ICA-to-ECA ratio of RI have highest specificity (99%) and positive predictive value (97%).
Therefore,
if the diagnosis of DAVF is suspected in patients with equivocal symptoms (such as tinnitus),
that set of parameters can be used to enhance our confidence to arrange more invasive and expensive studies.
But,
since the diagnostic sensitivity is not high (51%),
that set of parameters is only recomended as a screening tool and NOT to confirm a diagnosis of DAVF.
Fig. 10: 42-year-old woman with left pulsatile tinnitus and bilateral occipital bruit. Carotid duplex sonography showed low resistance index (RI = 0.48) and high end diastolic velocity (EDV = 128 cm/s) in the left ECA (B) with an ICA to ECA RI ratio of 1.3 (C). Analyzes of the left Internal Jugular Vein (D) revealed a pseudoarterialized flow pattern and an increased flow velocity. Similar findings were recorded in the right side.
[Patient E in Fig. 10 and Fig. 11]
Fig. 11: 42-year-old woman with left pulsatile tinnitus and bilateral occipital bruit (seen in fig. 10). Conventional angiography showed a high flow dural arteriovenous fistula, Borden 2, with feeders from the left occipital artery, posterior branches of the middle meningeal artery, neuromeningeal division of the ascending pharyngeal artery (A, left ECA, left lateral view), as well as participation from the right occipital artery (B, right ECA and ICA, right lateral view), and from dural branches of the right vertebral artery (C, right vertebral artery, right lateral view). Venous drainage showing early venous filling of the internal jugular vein and retrograde flow throw cortical veins (D, left ECA, anteroposterior view).
To summarize the best CDS signs of DAVF:
(a) ECA RI < 0.7
and
ICA-to-ECA ratio of RI > 0.9
and
no other CDS abnormality (cervicofacial vasogenic pathology,
cervical tumors feed by ECA, carotid/vertebral artery stenosis)
(b) ECA end dyastolic velocity > 21 cm/s
To summarize CDS warnings during follow-up:
(a) CDS should be performed once per year or whenever the patient clinical picture changes
(b) a change in ECA RI ≥ 0.1 over time should prompt additional investigation
When,
based on CDS findings,
a DAVF is highly suspected or the status of a previous known DAVF changes,
cerebral angiography should be performed.