Vasomotor reactivity in the ophthalmic artery

Transcranial Doppler ultrasonography (TCD) is a noninvasive technique in which sudden cerebral blood flow velocity (CBFV) changes in the basal cerebral arteries can be evaluated. The most important feature is the ability to monitor these sudden changes. It is assumed that the velocity changes during breathholding are caused by varying resistance secondary to diameter changes in small vessels distal to the M2 segment of the middle cerebral artery (MCA). These changes observed in the velocity are considered to be indicative of the cerebro-vascular reserve of cerebral vascular structures formed in response to hypoxia (1). The vasodilation capacity of cerebral arterioles can be measured indirectly by changes in the BFV in the main cerebral arteries. This capacity, defined as a cerebrovascular reserve or vasomotor reactivity (VMR), can be evaluated by carbon dioxide inhalation, acetazolamide injection, and breath-holding and vasodilator stimulation (2-4). The ophthalmic artery (OA) is the first branch of the internal carotid artery responsible for arterial supply of the eye and other structures in the orbita. The MCA is one of the three main vessel pairs responsible for the arterial supply of the cerebrum. The MCA originating from the internal carotid artery continues its course in the lateral sulcus and gives many branches to the lateral cerebral cortex. The vasomotor features of the M1, M2, and posterior cerebral arteries of the MCA have been previously discussed (1,5).