Variations in intracranial vasculature are well known. We report a rare anatomic variation in a patient who underwent cerebral angiography for suspected intracranial aneurysm. Digital subtraction angiography revealed a bihemispheric posterior inferior cerebellar artery (PICA) and an azygous anterior cerebral artery (ACA). There was no evidence of any aneurysm or vascular abnormality. To our knowledge, this is the first reported case of a patient with a common PICA supplying both the cerebellar hemispheres and a common ACA supplying ACA territory bilaterally. It is important for the physician to be aware of these anatomical variations in order to differentiate a normal variant from a pathological condition.
Anatomical variations are patterns that deviate from the normal anatomical arrangement in the absence of functional impairment. The understanding of such variation is of the utmost importance when the vascular supply to the central nervous system is concerned. Not only understanding the common anatomical differences in vasculature and vascular territories is useful in the diagnosis of peculiar pathological processes, but also this knowledge is imperative when surgical procedures are being performed [
Two vessels with known anatomical variations are the posterior inferior cerebellar artery (PICA) and the anterior cerebral artery (ACA). Variations that occur are due to the embryologic development of these vessels [
To our knowledge, this is the first reported case of a patient presenting with both an azygos ACA and a bihemispheric PICA.
A 38-year-old white male presented with a sudden onset of diffuse headaches. On physical examination his vital signs were all within normal limits. He was alert and oriented to person, place, time, and situation with a Glasgow Coma Score of 15. His cranial nerves II through XII were intact. He had strength 5/5 in upper and lower extremities bilaterally, and his sensory examination revealed no deficit. His reflexes were 2+ throughout, and he did not have any pathologic reflexes including Hoffman’s, Babinski sign, clonus, or drift. A computed tomographic image without contrast showed no evidence for subarachnoid hemorrhage, but a computed tomographic angiogram (CTA) of the brain showed a possible right anterior cerebral artery aneurysm. To adequately evaluate for possible aneurysm, an angiogram was performed. An 18-gauge angiocath was used to access the right femoral artery; there was selective catheterization of the right and left common carotid arteries, internal carotid arteries, and vertebral arteries.
The right internal carotid artery showed a hypoplastic A1 segment, faintly filling the A2 segment; the left internal carotid artery showed a dominant A1 with a common short A2 segment (Figure
Left internal carotid injection. The arrowhead shows an azygous A2 from which both the pericallosal arteries arise.
Computed tomographic angiogram showing the azygous A2.
The right and left vertebral arteries were unremarkable for aneurysm or stenosis, but the left vertebral artery showed agenesis of the left PICA with no opacification of the corresponding cerebellar territory (Figure
Left vertebral artery injection. There is no filling of the PICA and the corresponding territory.
Right vertebral injection. The right PICA gives branches to supply the PICA territory in the left hemisphere.
Anatomical variation is important to understand especially when considering the vascular supply to the central nervous system. Many alternatives of neurovascular anatomy exist, and it is imperative for surgeons, radiologist, pathologists, and clinicians to have full understanding of the anatomical, pathological, surgical, and embryological implications of such variants. Two such deviations from the norm are the bihemispheric PICA and the azygos ACA.
The path and territory of the posterior inferior cerebellar artery is tortuous and variable [
The normal embryological development of the PICA, as described by Macchi et al., represents a recent phylogenic development. At day 30 of estimated ovulation age the cerebellar rudiment becomes present in very close proximity to the fourth ventricle. At this point in development, day 35, the only blood supply to the cerebellum is from the superior cerebellar arteries. It is not until day 44 that the PICA becomes apparent, as a small vessel that terminates in the choroid plexus, and it is not until much later that the definitive path of the PICA is established. This late development of the PICA could explain the large variability in its pathway as well as many of its deviations from the norm [
Cullen et al. report an absence of one PICA in up to 26% of patients and also state the most frequent PICA variation is PICA agenesis or hypoplasia. Usually in patients with this variation, the ipsilateral anterior inferior cerebellar artery (AICA) supplies blood to the posterioinferior part of the cerebellum. There have only been a handful of reported cases where a single PICA feeds both cerebellar hemispheres, and although the incidence of this anomaly is unknown [
Unlike vessels of the dura, intradural arteries rarely cross the midline and supply contralateral territories, but there are several proposed reasons for the occurrence of a bihemispheric PICA. One cause of this anomaly is the vessel crossing the midline via the cerebellar vermis to supply the contralateral territory [
In this case, the bihemispheric PICA was an incidental finding, and this is the standard in similar published cases. However, Gaida-Hommernick et al. report a case in which a bilateral cerebellar infarction is caused by the stenosis of a single PICA supplying both cerebellar hemispheres [
Understanding the complex nature of intradural arteries is essential to diagnosis and treatment of neurovascular pathology. The PICA is the most variable and complex of the cerebellar arteries [
The ACA arises from the internal carotid artery distal to the carotid syphon. It crosses the anterior perforating substance and gives rise to the medial striate vessels. At the longitudinal intercerebral fissure it connects with the contralateral ACA by the anterior communicating artery (AComA). After the AComA, the vessel follows the direction of the corpus callosum genu and parallels its contralateral vessel as it continues over the corpus colossus and terminates at the splenium. The ACA is responsible for the irrigation of oxygen and nutrients to both medial cerebral hemispheres and orbitofrontal portions of the brain [
The embryological development of this artery begins early when the internal carotid artery (ICA) reaches the forebrain and divides into the olfactory branch and a posterior branch [
The azygos ACA has been shown to occur with other congenital vascular anomalies. Cases of arteriovenous malformation and aneurysms have been shown in literature [
The bihemispheric PICA and the azygos ACA present in this patient could have a common phylogenic link. Although there are several explanations for their formation, they both can arise from midline fusion of vessels [
In the diagnosis of cerebrovascular pathology the PICA and its branches as well as the ACA are of the utmost importance. Together these two vessels supply a large portion of intracranial territory. As this case points out, multiple rare vascular variants can occur in a single patient. This illustrates that vascular anomalies should be considered when patients present with vascular pathology and before procedures are performed.
The authors declare that there is no conflict of interests regarding the publication of this paper.