Monday, October 22, 2012

Carotid Dissection on Ultrasound

When talking about the carotid arteries, the mind jumps almost immediately to ultrasound as a modality.  Especially in the emergency setting -- what would be better than a fast, cheap, nonionizing test to rule out carotid dissection? You wouldn't have to radiate the patient's neck or wait hours for the MRI to finish up whatever lengthy cervical, thoracic, and lumbar scan is already in progress?

But is ultrasound effective as as a first line modality for carotid dissection?  To feel confident ruling out carotid dissection in the ED, you'd need to be sure that it has a high sensitivity. One source claims that in patients with their first-ever carotid territory symptoms, "normal ultrasound findings in the cervical ICA allowed the reliable exclusion of an underlying [spontaneous internal carotid dissection] reflected by sensitivity and NPV values of 96% to 97%" (ref 3). In this group of patients, ultrasound was shown to have a slightly higher false positive rate than other modalities, and the authors conclude that if detected by ultrasound, another modality (e.g. MRA) should be used for verification before initiating therapy.

Importantly, in patients with "local symptoms and signs on the side of dissection (eg, headache, neck pain, Horner syndrome, and cranial nerve palsy" ultrasound is only 69-71% sensitive... so ultrasound is not appropriate for ruling out dissection in a patient with Horner syndrome only and no carotid territory ischemic signs.

Grayscale and color Doppler images of a previously identified right carotid dissection.  The dissection membrane is fairly thick, and ultrasound was being used as a follow-up modality to assess progression/resolution.

The problem with using ultrasound as a screening modality in the ED is that old bugaboo -- operator dependence. Sensitivity and specificity are literally in the hands of your sonographer, so faith in their skills is paramount.  Compounding that problem is that most spontaneous or traumatic carotid dissections begin in the distal ICA, which is a little trickier area to evaluate for a novice sonographer.

Furthermore, although ultrasound could be used to rule out carotid dissection in certain cirucmstances, it certainly isn't the definitve modality for evaluation. A spontaneous dissection usually stops at the skull base, but if it doesn't, it'll be tough to know by ultrasound.  Maybe you have someone in your ED who can reliably perform transcranial Doppler of the ICA? I didn't think so.  So if the dissection is solely in the petrous portion of the carotid or above, you'll miss it completely on ultrasound.

A carotid dissection originating at the aortic arch is a little different beast. The aortic and common carotid portion of the dissection will be evaluated by CTA, but it doesn't seem unreasonable to image the higher cervical carotid with ultrasound to look for extent into the ICA.  Your positive predictive value is pretty high -- it's just a matter of finding the end point.

Part of the issue in dissection detection on ultrasound is the variety of dissection membranes.  Sometimes just the intimal layer dissects away, causing a thin dissection membrane.  Sometimes, the dissection involves a large chunk of the media, resulting in a thicker membrane which is easier to detect (on any modality, see example above). Dissection should be considered in any young patient with no visible plaque and smooth tapering of the internal carotid artery.  If the membrane does not dissect down to the point of visualization , the only detectable abnormality will be increased flow resistance on Doppler and possibly reduced flow velocity overall due to distal ICA obstruction.

If the initima is separated from the rest of the wall (see the second CTA example in 10/19/12), then it can flutter with each heartbeat, which is actually not too hard to detect on ultrasound, but if there's a thin flap, it can be hard to see on color Doppler imaging with color flow potentially overwriting the membrane.  Although sensitivity of detection of a dissection is better with Doppler and spectral sonography, visualization of the membrane is typically improved with the Doppler off.

Although not available for use yet in the U.S. the superior spatial resolution of contrast-enhanced ultrasound (CEUS), seems likely to improve detection of the dissection membrane (below) and hold promise for first-line evaluation of dissection in the ED.

a) Grayscale ultrasound does not clearly show the dissection membrane in this instance, b) and c) Color and Power Doppler hint at a dissection in the deep wall of the carotid, d) CEUS shows the flap clearly. (ref 5)

Carotid dissection is also frequently followed with ultrasound for resolution/progression (as in the example above). One group demonstrated on ultrasound that in patients being treated with the standard therapy of antiplatelets or antithrombotics, complete recanalization (normal ultrasound findings) occurred in 60% at 6 months.

1. "Rutherford's Vascular Surgery" Cronenwett and Johnston 7th ed (2010)
2. "Introduction to Vascular Sonography" Zweibel, Pellerito. 5th ed (2005)
3.  Benninger DH, Georgiadis D, Gandjour J, et al. "Accuracy of Color Duplex Ultrasound Diagnosis of
Spontaneous Carotid Dissection Causing Ischemia" Stroke. 2006;37:377-381.
4.  Arnold M, Baumgartner RW, Stapf C, et al. "Ultrasound Diagnosis of Spontaneous Carotid Dissection With Isolated Horner Syndrome" Stroke. 2008;39:82-86
5. Cleverta DA, Sommera WH, Zengelb P, et al. "Imaging of carotid arterial diseases with contrast-enhanced ultrasound (CEUS)" European Journal of Radiology. (2011), doi:10.1016/j.ejrad.2010.12.103
6. Rao AS, Makaroun MS, Marone LK, et al. "Long-term outcomes of internal carotid artery dissection" Journal of Vascular Surgery. (2010) Volume 54, Number 2