Vertebral Artery Ultrasound and Subclavian Steal: Protocol and Findings

The vertebral arteries are evaluated as part of a complete extracranial cerebrovascular assessment. Vertebral artery disease — particularly subclavian steal — is a clinically important diagnosis that ultrasound can identify directly. Here's how to approach it.

Why Evaluate the Vertebral Arteries?

The vertebral arteries (VA) supply the posterior circulation — the brainstem, cerebellum, and posterior cerebral hemispheres. Disease in the vertebral arteries or proximal subclavian arteries can cause:

• Posterior circulation TIA or stroke: Vertigo, ataxia, visual changes, drop attacks
• Subclavian steal: Reversed vertebral artery flow due to proximal subclavian artery stenosis or occlusion
• Vertebral artery dissection: Neck trauma, chiropractic manipulation
• Vertebral artery origin stenosis

Anatomy

Each vertebral artery arises from the subclavian artery and enters the bony transverse foramina at C6, coursing superiorly through C6–C1 before entering the skull at the foramen magnum. The two VAs join at the basilar artery.

The first segment (V1): From the subclavian origin to the C6 entry. This is where origin stenosis occurs.

The second segment (V2): Within the transverse foramina — this is what you typically image with ultrasound (interforaminal segment).

Scanning Technique

Probe: 5–12 MHz linear transducer.

Patient position: Supine, head turned slightly away from the side being examined.

Identifying the Vertebral Artery

1. Find the carotid arteries — you're already in the right neighborhood
2. The vertebral artery lies posterior to the common carotid artery
3. Look for the characteristic acoustic shadowing of the transverse processes — the C-spine vertebrae create alternating shadows like fence posts
4. The vertebral artery runs between these shadows, appearing as a pulsatile vessel in the acoustic windows between the shadows
5. The vertebral vein typically runs alongside (often visible on color Doppler)

The interforaminal segment (V2) is what you're imaging — the windows between the transverse processes give you glimpses of the artery at each level.

Doppler Sampling

• Sample in the interforaminal segment at mid-cervical level
• Angle correction ≤ 60°
• Get spectral waveform and color Doppler flow direction

Normal Vertebral Artery Findings

Direction: Cephalad (toward the head) — away from the subclavian, toward the brain. This is called antegrade or orthograde flow.

Waveform: Low resistance — similar to the internal carotid artery. Continuous forward diastolic flow reflecting the low-resistance posterior cerebral circulation.

Normal PSV: 20–60 cm/s (lower than ICA/CCA)

Diameter: 3–5 mm (may be asymmetric — a hypoplastic VA on one side is common)

Flow direction on color Doppler: Should be the same color (same direction) as the adjacent carotid arteries.

Subclavian Steal

Subclavian steal occurs when there is significant stenosis or occlusion of the proximal subclavian artery (before the vertebral artery origin). The ipsilateral arm is then perfused via retrograde flow through the vertebral artery — the vertebral "steals" blood from the posterior circulation to supply the arm.

Spectrum of Subclavian Steal

Pre-steal (latent):

• Normal antegrade flow at rest
• Systolic deceleration notch ("bunny ear" pattern) — waveform transiently decelerates at peak systole
• May become overt with arm exercise

Partial (transitional) steal:

• Bidirectional flow — antegrade in systole, retrograde in diastole
• Waveform alternates above and below the baseline

Complete steal:

• Fully retrograde flow — opposite direction to the carotid
• On color Doppler, the vertebral artery will be the OPPOSITE color from the carotid arteries

Provocative Testing for Latent Steal

If a bunny ear notch is seen, perform arm blood pressure cuff inflation:

1. Inflate the arm cuff on the affected side above systolic for 2–3 minutes
2. Release suddenly (reactive hyperemia)
3. Observe the vertebral artery — if complete flow reversal occurs after cuff release, subclavian steal is confirmed

Clinical Significance

Not all subclavian steal causes symptoms. Hemodynamic steal (retrograde VA flow) can be asymptomatic. Symptomatic subclavian steal causes symptoms with arm exercise (vertigo, visual changes, ataxia) — the exercising arm demands more blood and "steals" from the posterior circulation.

Always measure bilateral arm blood pressures (or note that they were measured clinically). A difference of > 15–20 mmHg between arms suggests significant subclavian stenosis.

Vertebral Artery Origin Stenosis

The origin of the VA from the subclavian is the most common site of stenosis.

How to image: The V1 segment (from subclavian to C6 entry) is technically challenging. Try a low supraclavicular approach — probe above the clavicle angled inferiorly. Optimize with color Doppler to find the subclavian and follow it to the VA origin.

Findings: Elevated PSV at the origin (> 100–120 cm/s), post-stenotic turbulence, and dampened waveforms in the V2 segment distally.

Vertebral Artery Hypoplasia

A hypoplastic VA (diameter < 2 mm, low velocity) is a normal variant, more common on the right. It doesn't cause steal but may contribute to posterior circulation insufficiency in some patients.

Distinguish from occlusion: A hypoplastic VA has flow (slow, low-volume). An occluded VA has no flow.

Key Images to Capture

1. Right and left vertebral arteries — color Doppler showing flow direction
2. Spectral waveform with angle-corrected velocity
3. Flow direction clearly demonstrated (cephalad = antegrade = normal)
4. If steal: waveform showing reversal or bidirectional flow
5. Arm blood pressure discrepancy (noted in report)

Reporting Language

• "Bilateral vertebral arteries patent with antegrade flow. PSV right 38 cm/s, left 42 cm/s. Normal low-resistance waveforms. No evidence of subclavian steal."
• "Left vertebral artery demonstrates complete reversal of flow (retrograde). Left arm systolic blood pressure 80 mmHg, right 138 mmHg — consistent with left subclavian steal. Retrograde flow confirmed with arm cuff provocation test."
• "Right vertebral artery: bunny-ear notch pattern on spectral Doppler — consistent with pre-steal physiology. Partial flow reversal demonstrated with arm cuff provocation. Findings suggest significant right proximal subclavian artery disease."

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SonoBuddy's carotid/vertebral protocol includes vertebral artery evaluation steps and normal Doppler values.