The atlas loop segment (V3) is created by a curved course of the artery around the atlas. The intracranial segment V4 is the section of the vertebral artery after penetrating the atlantooccipital membrane, dura mater and arachnoidea. At the clivus the right and left vertebral artery merge to form the basilar artery, which is a part of the intracranial posterior circulation. The diameter of vertebral arteries varies from 1.5 to 5.0 mm. Identical width of VA occurs in 25% of the population, in 65% the left vertebral artery is wider, whereas in the remaining 10% the

right vertebral artery is larger DAPT [3]. Khan et al. found dominance of the left vertebral artery in 50%, and of the right vertebral artery in 25% in regard to the diameter

of the vessel [4]. The following congenital anatomic variations of the vertebral artery are described in the literature: vertebral artery aplasia and vertebral artery hypoplasia (VAH). mTOR inhibitor Aplasia of VA occurs in about 1% of the population [5]. Vertebral artery hypoplasia (VAH) is classified as a vessel with a diameter in the entire course of less than 2 mm [6], respectively less than 3 mm [7], or with a side difference equal or greater than 1:1.7 [8]. Additionally to the vessel diameter, another criterion contains reduced blood flow velocity and increased resistance index values in the ultrasonographic findings [1] and [9]. There is a tendency of compensatory increase in the vessel diameter of the contralateral vertebral artery of more than 5 mm [1]. These various definitions of the incidence of VAH are based on subsequent characteristics: a diameter of less than 2 mm was observed by the method of duplex ultrasonography by the authors Delcker and Diener in 1.9% of the population [6], a diameter of less than 3 mm was described by Touboul et al. in 6% of the

population [7]. Trattnig et al. set a side asymmetry in the ratio 1:1.7 for more than 10% of patients examined by ultrasonography [8]. Frequency of VAH (diameter equal or less than 2 mm) in the general population is 26.5% in unilateral and 1.6% in bilateral hypoplasia of the vertebral artery [10]. In terms of side difference, the right hypoplastic vertebral artery occurs in 6.2% of the population, while left vertebral hypoplasia is present less frequently in 4.5% most [2]. Visualization of vertebral artery is possible by ultrasonographic examination, by invasive or non-invasive angiography (MRA, CTA), and also by autopsy findings. As mentioned previously, a more narrow vessel lumen is present in the ultrasonographic image in vertebral artery hypoplasia, and additionally, blood flow parameters are defined by a reduced diastolic flow velocity associated with higher peripheral resistance. The resistance index (RI) is equal to or greater than 0.75. The peak systolic velocity (PSV) is usually less than 40 cm/s [1] and [5].