The anatomy and clinical importance of the collateral circle between the external carotid arteries through an anastomosis between the superior thyroid arteries.

Three cases of obstruction of a single common carotid artery were incidentally discovered by Doppler ultrasonography. Interestingly, anterograde blood flow was demonstrable in the ipsilateral internal carotid artery (ICA), even though it was distal to the obstruction Anterograde blood flow was also observed in the intracranial portion of the internal carotid artery and in its branches that were explored (the ophthalmic and middle cerebral arteries). The blood flow was comparatively reduced and irregular with respect to that of the corresponding contralateral artery, but nonetheless anterograde. Further studies by color Doppler ultrasound revealed the presence of a collateral circle that passed through the superior thyroid arteries. The prominent blood flow in the ICA ipsilateral to the occlusion originated from the ipsilateral external carotid artery (ECA). In turn, the ECA was supplied by the contralateral ECA through a collateral circle that made use of an anastomosis between the superior thyroid arteries. In summary, the blood flow followed the following path: from the contralateral ECA and superior thyroid artery, through an anastomosis with the ipsilateral ECA. From this point, the blood flow reached the ipsilateral ICA. Blood flowed from the thyroid artery to the ipsilateral ECA, then flowed in a retrograde fashion toward the carotid bifurcation and toward the ICA, since this vascular territory offered lower resistance to flow as compared to that encountered in the distal ECA.(ABSTRACT TRUNCATED AT 250 WORDS)