Extracranial
Internal Carotid Artery (ICA)
The mechanism of occlusion of the
extracranial ICA is almost always formation of thrombus on an ulcerated atherosclerotic
plaque located at or near the carotid bifurcation. Sometimes embolism from
a cardiac source or carotid dissection is responsible, but this is quite rare.
The clinical syndromes that result are variable--they range from total infarction
of the entire cortical and deep territory of the middle cerebral artery (MCA)
and anterior communicating artery (ACA) affecting most functions of one entire
hemisphere to a small cortical MCA territory infarction to no neurologic deficit
whatsoever. This incredible variability reflects what sources of collateral
flow are available and how much blood the collaterals actually carried in
the particular patient at the time of occlusion.
| Question: |
Can
you think of three places where anastamotic flow might be established
that could nourish carotid territory if the ICA were occluded slowly and
progressively, allowing enough time for collaterals to increase their
diameter and hence the amount of blood they carry? |
| Four possible answers:
|
- Enlarged posterior communicating
artery (PCA) connecting the vertebral-basilar to the carotid circulation
via the circle of Willis
- Enlarged anterior communicating
artery (ACA) connecting the carotid circulation on each side via the
circle of Willis
- External to internal
carotid artery anastomoses, e.g., connections around the eye
- Cortical anastomoses
|
With all this variability in clinical presentation, you may be wondering how physicians
come to suspect an ICA occlusion. The answer is often in the patient's past
and present medical history.
Clues include:
- repeated transient ischemic attacks always in carotid territory
- a carotid bruit
or direct evidence of stenosis
from angiography or
- ultrasound studies showing reversed blood flow in ophthalmic artery branches.
| Question: |
If there
is a functional anastamosis between external and internal carotid using
ophthalmic artery branches, what direction would the blood be flowing? |
| Answer: |
- blood flow is inward when
the external carotid is nourishing internal carotid territory
|
