Carotid sinus is a slight dilation in the carotid artery at its bifurcation into external and internal carotid arteries. It contains baroreceptors (pressure sensors/nerve endings) sensitive to pressure, which when stimulated cause a reflexive slowing of the heart rate, a drop in blood pressure (via dilation of blood vessels), and a subsequent decrease of blood flow to the brain. The symptoms of this sudden decrease of blood to the brain are collectively called syncope, which includes a sudden loss of muscle tone and strength, making it difficult to stand erect, and may cause a loss of consciousness.
Attacks are the result of pressure on the carotid sinus, but may occur spontaneously as a result of a tight collar, shaving the neck area over the sinus, or even turning the head to the side. All of these can aggravate the carotid sinus. Carotid sinus syncope lasts only seconds to minutes.
Unilateral carotid insufficiency is a different condition that may mimic the same symptoms as carotid sinus syndrome.