The vertebral artery shows normal antegrade flow pattern with waveform similar to the internal carotid artery. Vertebral artery is low resistance system supplying about 20%–30% blood flow to the brain. There are characteristic flow pattern changes in the vertebral artery secondary to ipsilateral subclavian stenosis proximal to vertebral artery origin. The severity of stenosis can be graded according to the abnormal waveform.
The earliest abnormality in the vertebral spectral waveform is transient sharp deceleration of blood flow after the first systolic peak producing a notch and giving rise to two systolic peaks with first being sharp and the second being rounded. With increasing stenosis the notch deepens, which means the first systolic peak becomes sharp and the second peak broadens. This represents in a shape of a crouching bunny (Figure 1). Eventually, the nadir of the notch reaches the baseline and eventually crosses the baseline causing flow reversal. This reversal is minimal and transient at first and gradually progresses to complete flow reversal which indicated high-grade stenosis.
The subclavian steal is due to flow being directed away from stenosed subclavian artery to the upper limb causing flow reversal in the vertebral artery. A similar phenomenon is also described in the coronary arteries. This is explained by Bernoulli's equation which relates the velocity and pressures in a system inversely, i.e., when the velocity increases in an artery, for example, secondary to stenosis the pressure decreases. This decrease in pressure is transmitted to the vertebral artery from which the blood is siphoned in the reverse direction. The pressure drop is greatest when the blood flow velocity is maximum across the stenosis which is at the peak systole. This pressure drop is reflected as the notch in the vertebral spectral waveform.
The subclavian steal waveforms have been classified into four types with various patterns showing two systolic peaks to flow reversal (Table 1 and Figure 2).
Table 1: Types of presteal waveforms in vertebral artery
||Degree of stenosis(%)
||Two systolic peaks with flow velocity at the nadir of the notch greater than the flow velocity at the end diastole
||Two systolic peaks with flow velocity at the nadir of the notch equal to or less than the flow velocity at the end diastole
||Two systolic peaks with flow velocity at the nadir reaches the baseline
||Two systolic peaks with flow velocity at the nadir below the baseline
Type 1: Two systolic peaks with flow velocity at the nadir of the notch greater than the flow velocity at the end diastole.
Type 2: Two systolic peaks with flow velocity at the nadir of the notch equal to or less than the flow velocity at the end diastole (Figure 3).
Type 3: Two systolic peaks with flow velocity at the nadir reach the baseline.
Type 4: Two systolic peaks with flow velocity at the nadir below the baseline.
Beyond this, there is flow reversal (Figure 4). Our patients presented with vertigo, transient altered speech and reading and other with prebypass graft surgery. This is a common workup for such patients. The carotid Doppler revealed two systolic peaks in one of the vertebral arteries. CTA revealed patent bilateral vertebral arteries (Figure 5). One disadvantage with CTA is that it does not show the flow direction. For flow direction, the Doppler examination is helpful. The MRA reveals signal loss on three-dimensional time-of-flight images which picks up signal in vessels with flow in the direction magnetic pulse and loss of signal from vessels with flow in reverse direction (Figure 6).
The identification of these presteal waveforms helps in informing the clinicians for further evaluation of posterior circulation flow abnormalities secondary to subclavian artery disease or other causes, before the development of frank retrograde flow. These changes can also be followed up with ultrasound or magnetic resonance imaging for response to the treatment.
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