Distinguishing the internal carotid artery (ICA) from the external carotid artery (ECA) with ultrasound
In this video, you'll learn how to tell ICA and ECA apart using ultrasound and the special test you can use when they seem identical on ultrasaound.
Stenosis in the ICA is more likely to cause symptoms than a severe stenosis in the ECA, so being able to distinguish between these two is super important. In this video, you'll learn how to tell them apart using ultrasound and the special test you can use when they seem identical on ultrasaound.
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As we learned previously, the carotid bifurcation is a typical site for atherosclerosis. The presence of a stenosis in the ICA is a likely cause of the patient's symptoms, where as a significant stenosis in the ECA, is far less worrisome. Therefore, being able to distinguish between the ICA and ECA is an important skill.
In some cases it is very straightforward, but in other cases it can be more challenging, and you will need to apply everything that you learn from this lesson. Remember that the probe marker at the top left of the image should always represent the right side of the patient when imaging the carotid arteries in the transverse mode.
So in imaging the right side of the neck, the larger deeper artery on the left side of the image, that is the lateral side of the neck, is the ICA. The artery on the right side of the image, that is the medial side of the neck, is the ECA. When performing an ultrasound on the left side of the neck, the larger deeper artery on the right of the image, that is the lateral side of the neck, is the ICA.
Again, the artery on the left side of the image, that is the medial side of the neck, is the ECA. To help identify these arteries, first look at the right side of the patient's neck. Keep the probe in transverse, and try to locate the ECA. One key difference between the ECA and ICA is that the ECA has branches, whereas the ICA doesn't.
The color flow can be used to help identify the branches of the ECA. You may also be able to see the ECA branches in the longitudinal plane. The ICA and ECA also have different flow patterns, which can be identified using the pulse Doppler. The ICA is less pulsatile, and has a lower resistance waveform, which means it has higher end-diastolic flow.
In comparison, the ECA is more pulsatile, and has a higher resistance waveform, which means lower end-diastolic flow. As you can see, the differences in their waveforms are clear, but they also sound very different too. The ICA sounds less pulsatile and more continuous. In contrast, the ECA sounds much sharper, and the pulse is more pronounced.
You should learn how to recognize these differences. So don't forget to listen too, because your hearing can be more sensitive. Similarly, the color flow patterns reflect the differences between flow in the ICA and ECA. Although the differences are more subtle than those for the pulse Doppler.
Color flow and the ICA will appear more continuous and less pulsatile, so the color appears to fill the vessel throughout each cardiac cycle. In the ECA, it will appear more pulsatile, and almost reach zero between each pulse. Here there is very little color filling at the end of each cardiac cycle, with the vessel almost appearing black.
It's important to note that this video has been slowed down to more clearly show these differences. It will appear about twice as fast when assessing a patient. Remembering these key differences will help you clearly identify the ICA and ECA in most cases, but sometimes distinguishing between these two arteries is more difficult.
This is because the orientation of the vessel may be different, or the arteries might be very close together with minimal difference in the size or flow patterns. In these situations, the temporal tap test can be used to help identify the ECA. To perform this test, position the Doppler sample volume in what you think is the ECA, and switch on the pulsed Doppler.
Once you have a clear pulsatile waveform, use a finger from your other hand to locate the temporal artery in front of the patient's ear. Lightly tapping here while keeping the probe very still should produce a sawtooth pattern in the ECA waveform, but will have no effect on the ICA waveform.
This technique requires a lot of practice and experience, but it can also be really useful in difficult situations when disease is suspected. For example, where there is a long standing ICA occlusion and you can only see one vessel going up into the distal part of the neck in both transverse and longitudinal sections.
In this case, the ECA may be acting as a collateral vessel and have higher than expected diastolic flow. Using the temporal tap will confirm that this is the ECA. As a helpful tip, practicing this test in cases where you have already confidently identified each artery will help you develop this highly useful skill.