The posterior portion of the circle of Willis is formed at earlier stages, when the fetal PCA turns into posterior communicating artery (PcomA) and branches from the fetal PCA fuse medially to form the distal end of the BA, while the adult PCA connects with the BA3. The most variable territory in arterial supply is the one usually fed by the paramedian thalamic-mesencephalic perforating branches. Gustave Percheron in 1973 described in a systematic classification these anatomical variants (figure 3).
Figure 3
Variants of the paramedian thalamic and midbrain supply according to Percheron1.
BA: basilar artery; M: midbrain; PCA: posterior cerebral artery; Th: thalamus
The most commonly known variant has been called “artery of Percheron” (AOP) and it is a single arterial trunk arising from the P1 PCA and supplying the paramedian territory of the bilateral thalami and the rostral midbrain1. However, the AOP naming is a misnomer, because Percheron described four different variants and the one known as AOP is better identified as the variant IIb. The attention raised by the variant IIB led to substantially neglect the other variants1,3,4, but they might also be relevant in neurovascular practice, as outlined by our case. An occlusion of the AOP usually causes an ischemic lesion involving the paramedian thalamus bilaterally and sometimes the rostral midbrain (57%) and/or the anterior thalamus (19%) in the territory usually supplied by tuberothalamic perforating arteries.4,5 Indeed the classical appearance of the AOP infarction, involving bilaterally both upper midbrain and paramedian thalamus, is rare in the published case series of AOP infarctions and defines a “V” sign in coronal plane.
While in the past, the possibility of defining the supplying pattern of the anterior thalamo-perforating arteries derived from cadaveric dissection studies1, actually DSA, in particular with three-dimensional rotational angiography (3D-RA) and Minimum Intensity Projection/MultiPlanar Reconstruction (MIP/MPR) allows to directly imaging perforating vessels6,7. In our case, the infarct’s location suggested a variant of the thalamic supply different from an AOP and the noninvasive neuroimaging techniques were not able to answer this question. DSA may directly image not only AOP but also the other variants of the thalamic supply. The presented (type III) (figures 2 and 3) is defined by the presence of an arcade of perforating branches arising from an artery bridging the P1 segments of both PCAs1,8. This variant is even rarer than AOP and occasionally reported in anatomical studies5, so it received less attention in the literature and its impact on the localization pattern of thalamus and midbrain infarction is less know. In the presented case an ischemic lesion involving the rostral midbrain, the paramedian thalamic and the anterior thalamic territory on the right side is showed with a neuroimaging appearance as single branch of the V sign described in AOP infarction4. DSA with 3D-RA allowed to image this variant and the anastomotic arcade between the P1 segments of the PCA on both sides with the main trunks of the perforating arteries are defined as arising from this arcade (figure 2). Moreover, it is possible to hypothesize that the presence of a type III variant prevented the bilateral extension of the thalamo-mesencephalic infarction during the acute phase, when the right branch of the anastomotic arcade is supposed to be occluded. At the time of the DSA the arcade was patent, so a spontaneous recanalization may be proposed as hypothesis. The involvement of the tuberothalamic arteries (or anterior polar artery1) territory might be because paramedian perforating arteries supply it, as in AOP variant. Although the tuberothalamic arteries are usually branches of the PComA, it is not rare their absence (one-third of individuals)5,8,9. In this case, the paramedian arteries or, even more commonly, the AOP, supply the anterior thalamic territory7. In our case, the type III variant supplied unilaterally the territory usually covered by AOP. In AOP infarction an asymmetric topography of the infarction among the two sides has been frequently reported (nearby to 90%)10, so multiple combinations of anatomic variants1,8 and variant territories5,9,10 may be supposed.