Crouching Bunny, Hidden Stenosis

Carrying on with the innominate artery theme, Zweibel notes in his textbook that a biphasic (or "crouching bunny") right common carotid waveform can result from innominate stenosis.

First, a normal right common carotid waveform for comparison:

Normal right common carotid artery Doppler waveform:   Moderately broad systolic peaks and a moderate amount of flow throughout diastole. Uniphasic.

 

If there is enough stenosis in the innominate artery, then a steal phenomenon can result, with resulting biphasic waveforms, indicating back-and-forth blood flow.

If there is biphasic blood flow bilaterally, the best consideration is pulsus bisferiens (next post) from aortic insufficiency... but if it's only on the right side, it indicates a steal phenomenon in the right common carotid artery; during systole, blood flows forward into the carotid system and in diastole it flows backward into the right subclavian and arm.

In severe cases of innominate artery stenosis, there is complete flow reversal, but in less severe cases, the flow remains cephalad although an atypical biphasic waveform results... the "crouching bunny."

An example of the biphasic "crouching bunny" waveform showing midsystolic deceleration with antegrade late-systolic velocities .  Although this is is an image of the right vertebral artery, the principle and the Doppler waveform are the same in the carotid.

If the innominate artery were completely occluded, then obviously there would be complete flow reversal in the common carotid artery.

Two other points: 1) in a normal situation, carotid steal can only occur on the right side. 2) if you see abnormal right common carotid (or vertebral) waveforms, check the subclavian arteries and measure the BP in both arms.

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1. "Introduction to Vascular Ultrasonography" 5th ed.(2005) Zweibel, Pellerito, et al.
2. Tahmasebpour HR, Buckley AR, Cooperberg PL, Fix CH. "Sonographic Examination of the Carotid Arteries." November 2005 RadioGraphics, 25, 1561-1575.

Innominate Artery Aneurysm

Speaking of the innominate artery (a.k.a. the "brachiocephalic artery"), what is the likelihood of an innominate artery aneurysm?

Well, the first step would be to know what the normal diameter of the innominate artery is....

The reported normal size of the innominate/brachiocephalic artery is between 8-13 mm with 14 mm or greater considered dilatation.

The patient below is a 71Y lady who presented for evaluation of liver cysts.  During the exam dilatation of the innominate artery to 18 x 18 mm was also found, with dilatation of the origin of the right subclavian artery and kinking of the origin of the right common carotid. Although pacemaker wires were present for sick sinus syndrome, no cardiology report or prior imaging mentioned dilatation of the innominate artery.

71Y F with a right innominate artery/brachiocephalic artery aneurysm and kinking of the right common carotid artery. Incidentally, the diameter of the aorta is top normal at 3.9 cm.

My attempt at a Terrarecon reformat of the aneurysm.  There is artifact at the origin of the innominate and left common carotid artery from the overlying pacer wires in the left brachiocephalic vein.  This angle of view is meant to demonstrate the nondilated origin of the innominate artery corresponding to a class A aneurysm (below).

According to a recent series, innominate artery aneurysms that involve the origin of the artery (B) or involve the ascending aorta (C), are more common than those that only involve the artery itself (A).

Isolated aneurysms of the innominate artery are definitely uncommon events, although there does not seem to be a documented incidence in the literature. Conditions that are considered predisposing risk factors are the usual large artery arteriopathies: Takayasu's arteritis, syphilis, and of course, atherosclerosis.

In the 1950s, most innominate artery aneurysms were due to syphilis and presented at a late stage with compressive symptoms and few interventional options.

The most specific symptoms related to a compressive innominate artery aneurysm include dysphonia, dyspnea, or SVC syndrome.  Dysphonia is related to its proximity to the recurrent right laryngeal nerve, curling around the right subclavian artery (below).

Embolization of aneurysm thrombus material to the right upper extremity and right cerebral hemisphere are other documented sequelae of an innominate artery aneurysm.

Buckling or kinking of the innominate artery implies elongation,  tortuosity, and dilatation of the vessel, but the artery can buckle without being aneurysmal. This differentiation was more of a concern in interpretation of frontal radiographs, before the true size of the vessel lumen could be easily measured with cross-sectional imaging.

Treatment of an innominate artery aneurysm remains surgical.

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1. Honig, EI, Steinberg I., Dotter CT. "Innominate artery: angiocardiographic study," Radiology 58: 80, 1952.
2. Honig EI, Dubilier Jr. W, Steinberg I. "Significance of the Buckled Innominate Artery." Ann Intern Med. 1953 Jul;39(1):74-80
3. Kieffer E, Chiche L, Koskas F, Bahnini A. "Aneurysms of the innominate artery: Surgical treatment of 27 patients." Journal of Vascular Surgery 34:2, 2001.

The Cow Vessels

A common variant of the aortic arch is when the innominate artery and left common carotid artery arise from the same trunk, often misnamed the "bovine arch."

This configuration is reported in 13% of patients.  It is reported to have an ethnicity difference, occurring in 25% of African-Americans and 8% of Caucasians.

What is usually erroneously called the "bovine arch": a common origin of the innominate artery and left common carotid. (ref 1)

43Y M, CT of the chest with contrast: There is an incidental common origin of the innominate / brachiocephalic artery and the left common carotid artery.  The two share a common trunk, which differentiates this from the variant in which the left common carotid arises directly off the innominate (normal-sized innominate origin).

Also possible is a variant configuration in which there is no common trunk, and the left common carotid arises off the innominate artery (below). This is reported to occur in 9% of the population.

On the original radiograph, rightward deviation of the trachea was noted (green arrow).  On a follow-up CT, the deviation was seen to be due to mass effect from a T-shaped bifurcation of an innominate artery variant (green arrow) in which the left common carotid arises from the proximal innominate (not a common origin).

Why would reporting these variants matter? It probably doesn't most of the time and usually is merely a matter of accurately describing what one is seeing on CT or angiography. One situation in which the distinction might be significant could be with an intervention for an innominate artery injury.  If the anterior cerebral circulation is potentially threatened by the injury, the interventional approach might be modified.

So what is a true bovine arch?  In cows and buffalo, all the great vessels arise off the aorta from a single trunk. This long single trunk seems to be an adaptation related to the long distance from the aortic arch to the thoracic inlet in these animals (below).

What a true "bovine arch" would look like.  This is rarely, if ever, seen in humans. (ref 1)

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1. Laytona KF, Kallmesa DF, Clofta HJ, et al. Bovine Aortic Arch Variant in Humans: Clarification of a Common Misnomer. AJNR August 2006 27: 1541-1542
2. Mauney MM, Cassada DC, Kaza AK, et al. Management of Innominate Artery Injury in the Setting of Bovine Arch Anomaly. Ann Thorac Surg 2001;72:2134–6