Internal carotid arteries behind the hyoid bone as a clue for identifying the flip-flop phenomenon.

A 62-year-old woman presented with sudden left hemiparesis. Magnetic resonance imaging (MRI) revealed scattered acute infarctions in the right middle cerebral artery territory. MR angiography (MRA) revealed no significant stenosis in the major intracranial or extracranial arteries, but a filling defect was noted at the origin of the right internal carotid artery (ICA). On day 5, MRA demonstrated a change in the positional relationship between the ICA and the external carotid artery. Although carotid ultrasonography initially revealed the ICA was located behind the hyoid bone, making plaque identification unclear, the ICA shifted laterally along the hyoid bone during swallowing, enabling visualization of a plaque. The flip-flop phenomenon, caused by dynamic compression of the ICA by the hyoid bone during swallowing, is a possible source of embolic stroke. When the ICA lies behind the hyoid bone, dynamic assessment and careful plaque evaluation are essential.