Anterior Cerebral Artery Disease

Anatomy

The ACA can be divided into a proximal or A1 segment, from its origin as the medial component of the internal carotid bifurcation to its junction with the ACoA, and a distal or postcommunicating artery segment ( Fig. 23.1 ). The distal segment has been variably subdivided by different authorities ^, : for example, into an A2 segment beginning at the ACoA and passing in front of the lamina terminalis as far as the junction of the rostrum and genu of the corpus callosum, an A3 segment passing around the genu of the corpus callosum, an A4 segment from above the corpus callosum to just beyond the coronal suture, and an A5 segment extending to the artery’s termination. The A2 and A3 segments have together been referred to as the ascending segment, and the A4 and A5 segments as the horizontal segment.

The A1 segment passes over the optic chiasm (in 70% of cases) or optic nerve (30%), varying in length from 7.2 to 18 mm (average, 12.7 mm). Its diameter ranges from 0.9 to4.0 mm (average, 2.6 mm) and is greater than 1.5 mm in 90% of brains. In 74% of brains, both A1 segments are larger than the ACoA, the diameter of which ranges from 0.2 to 3.4 mm (average, 1.5 mm).

The ACAs pass over the corpus callosum side by side in only a minority of cases, so the ACoA is most often directed obliquely or even anteroposteriorly; thus it is often best seen with angiography on oblique projections.

The recurrent artery of Heubner arises either at the level of the ACoA or just proximal or distal to it; in different series, it was described as arising most often from the A1 segment, from the A2 segment, or at the level of the ACoA. Usually the largest branch of the A1 or proximal A2 segment, the Heubner artery doubles back on the ACA for a variable distance and then, either as a single trunk or with as many as 12 branches, penetrates the anterior perforated substance above the ICA bifurcation or lateral to it in the Sylvian fissure; some branches enter the olfactory sulcus, the gyrus rectus, or more lateral inferior frontal areas. ^, Of obvious importance to the neurosurgeon is the fact that the Heubner artery most consistently supplies the head of the caudate, the anteroinferior part of the internal capsule’s anterior limb, the anterior globus pallidus, and parts of the uncinate fasciculus, olfactory regions, and anterior putamen and hypothalamus. ^a

a References 40, 44, 53, 55, 58, 62–64.

In addition to the Heubner artery, the A1 and A2 segments give off smaller basal perforating branches, up to 15 from each A1 segment ^{, ,} and up to 10 from each A2 segment. ^{, ,} One of these, called the short central artery, is considered more consistent than others, in some people supplying part of the caudate nucleus and anterior limb of the internal capsule. ^, Other proximal branches penetrate the anterior perforated substance and the optic tract and supply, variably, parolfactory structures, the medial anterior commissure, globus pallidus, caudate and putamen, and the anterior limb of the internal capsule; these vessels also commonly supply the genu and contiguous posterior limb of the internal capsule, part of the anterior nucleus of the thalamus, and most of the anterior hypothalamus. More distal A1 penetrating branches are smaller and supply the optic nerve, chiasm, and tract ^, ; gyrus rectus and inferior frontal lobe; anterior perforated substance; and suprachiasmatic area. Additional supply to the anterior inferior striatum and anterior hypothalamus comes from A2 segment branches, which can arise either separately or from a larger common trunk (the precallosal artery). Similar penetrating branches from the ACoA, 13 or fewer in number, ^, supply the suprachiasmatic and parolfactory areas, the dorsal optic chiasm, anterior perforated substance, inferior frontal lobe, septum pellucidum, columns of the fornix, corpus callosum, septal region, and anterior hypothalamus and cingulum. ^{, , ,}

Vascular anastomoses are less functional in the diencephalon and basal ganglia than elsewhere in the cerebral hemispheres, and the territories supplied by these ACA penetrating end-zone arteries are no exception. Capillary anastomoses, which are difficult to demonstrate by standard perfusion techniques, exceed arterial anastomoses. ^,

The distal ACAs, deep in the interhemispheric fissure, constitute the only example of major cerebral arteries running side by side, although, as noted, one (usually the left) is often posterior to the other. Because of the crossover of branches to the other hemisphere, occlusion of either artery can cause contralateral or bilateral infarction. Beyond the lamina terminalis, the main trunk of the ACA—the pericallosal artery—runs above the corpus callosum in the pericallosal cistern (or, less often, over the cingulate gyrus or in the cingulate sulcus ); it passes around the splenium of the corpus callosum and terminates in the choroid plexus of the third ventricle. Its posterior extent depends on the anterior extent of the posterior cerebral artery (PCA). ^, Except most posteriorly, the pericallosal artery lies below the free edge of the falx cerebri and can, therefore, shift across the midline.

The pericallosal artery has been variably defined as beginning at the ACoA or at the point where the ACA gives off the callosomarginal artery; however, the callosomarginal artery is absent in 18%–60% of brains. ^, The callosomarginal artery has been defined as that branch of the ACA traveling in or near the cingulate sulcus and giving off at least two major cortical branches. It originates from just beyond the ACoA to the genu of the corpus callosum, most often from the A3 segment, and can be of the same diameter, larger, or smaller than the pericallosal artery. Any or all of the callosomarginal artery’s usual branches can arise from the pericallosal artery ; these branches supply the inferior frontal lobe (including the gyrus rectus, the orbital part of the superior frontal gyrus, the medial part of the orbital gyri, and the olfactory bulb and tract), the medial surface of the hemisphere (including the cingulate gyrus, the superior frontal gyrus, the paracentral lobule, and the precuneus), and the superior 2 cm of the lateral convexity (including the superior frontal, precentral, central, and postcentral gyri), anastomosing there with branches of the middle cerebral artery (MCA). (These border zones of shared arterial territory are of clinical importance: In a radionuclide study of 365 consecutive patients with stroke, infarction occurred in the “watershed” between the ACA and the MCA in 5% of patients compared with the MCA territory in 28% and the ACA territory in 1%.) The band of lateral convexity supplied by the ACA is wider anteriorly than posteriorly and may extend into the middle frontal gyrus.

Although variable in number and in whether they arise directly from the pericallosal artery or from its callosomarginal branch, eight major cortical branches of the distal ACA can usually be defined. ^{, ,} The orbitofrontal artery arises from the A2 segment except, infrequently, when it shares a common trunk with the frontopolar artery or arises just proximal to the ACoA. Running forward in the floor of the anterior fossa as far as the planum sphenoidale, the orbitofrontal artery supplies the gyrus rectus, olfactory bulb and tract, and orbital surface of the frontal lobe. The frontopolar artery arises from the A2 segment (or, uncommonly, from the callosomarginal artery), passes to the frontal pole along the medial hemispheric surface, and supplies parts of the medial and lateral surfaces of the frontal pole.

The anterior, middle, and posterior frontal arteries arise separately from the A2, A3, and A4 segments of the pericallosal artery or from the callosomarginal artery; infrequently they arise from a common stem. ^, They supply the anterior, middle, and posterior parts of the superior frontal gyrus and the cingulate gyrus. The paracentral artery, arising from A4 or the callosomarginal artery, supplies premotor areas and the paracentral lobule.

The superior parietal artery—arising anterior to the splenium of the corpus callosum from A4, A5, or the callosomarginal artery—passes through the marginal limb of the cingulate sulcus and supplies the superior part of the precuneus. The inferior parietal artery, subdivided by some authorities into the precuneal and parieto-occipital arteries, ^, is the most commonly absent cortical branch of the ACA (36% of brains in one series ); it arises from the A5 segment (or rarely from the callosomarginal artery) just above the splenium of the corpus callosum and supplies the posterior inferior part of the precuneus and portions of the cuneus.

The rostrum, genu, body, and splenium of the corpus callosum are supplied by short callosal arteries, pericallosal artery branches that pass through the callosum to supply, additionally, the septum pellucidum, anterior pillars of the fornix, and anterior commissure. ^, Posteriorly, the pericallosal artery extends around the splenium of the corpus callosum (the posterior pericallosal artery) and then passes forward, ending on the inferior surface of the splenium or extending all the way to the foramen of Monro.

Of obvious importance in interpreting symptoms and signs is the normal variability of the boundaries (or border zones) between the anterior, middle, and PCAs. Fig. 23.2 , which is based on postmortem injection studies of 25 healthy brains, illustrates the range of cortical distribution of the ACA. In those with the most extensive ACA distribution, the primary motor and sensory cortices were supplied by the ACA, not only medially but also over the convexity as far as the inferior frontal sulcus. In those with the least extensive ACA distribution, the ACA supplied little or none of the primary motor cortex, even medially.

Anomalies and Species Differences

The anatomy of the anterior circle of Willis is so varied among otherwise healthy people that whether a variation should be called an anomaly is sometimes difficult to define. Especially common are hypoplastic A1 segments, from mildly narrow to nonfunctionally threadlike, with both distal ACAs filling from the larger A1 segment. ^, In one study, 7% of brains had a stringlike A1 segment, and 6% had a hypoplastic ACoA. In another study, 22% of brains had A1 segment hypoplasia, which was severe in 8% of cases and was associated with additional anomalies of the ACA or the posterior cerebral, posterior communicating, or basilar arteries in 82%. Such anomalies are associated with a greater frequency of saccular aneurysms, and ACA occlusion secondary to cardiac embolism is often accompanied by proximal hypoplasia of the contralateral ACA. ^,

A smaller ACA often occurs on the same side as a smaller ICA, and a hypoplastic A1 segment tends to be associated with an ACoA of larger diameter than usual. ^, Small A1 segments are several times more common among patients with symptomatic cerebrovascular disease than in the general population. Cerebral angiography in one young man with episodic vertigo, loss of consciousness, and left leg weakness showed absence of the ACAs; the MCAs and one intracavernous carotid artery provided collateral vessels to the patient’s medial cerebral hemispheres.

In 50 adult autopsy specimens, 60% had one ACoA, 30% had two, and 10% three ; other investigators have also found doubling and tripling of this vessel, and some have found absence of the ACoA. ^{, ,} A1 segment duplication also occurs, as well as a third or median ACA arising from the ACoA (arteria termatica), which is sometimes as large as the two other ACAs and may be the major supplier to the posterior medial hemispheres. ^{, ,}

The recurrent artery of Heubner rarely arises from the ICA at its bifurcation, from the MCA, from the ACoA, or from the orbitofrontal or frontopolar branches of the ACA. ^, Such anomalies increase the risk of infarction in the territory of the recurrent artery of Heubner during surgical treatment of aneurysms arising from the anterior circle of Willis. Absence or doubling of the Heubner artery occurs. ^, Embryologically, this artery is a remnant of the primitive olfactory artery; thus a patient with a persistent primitive olfactory artery has no Heubner artery.

Another well-recognized anomaly is a supernumerary vessel arising from the ICA at the level of the ophthalmic artery, coursing below the optic nerve, ascending in front of the optic chiasm, and terminating on the ipsilateral ACA near the ACoA. The A1 segment may be normal, hypoplastic, or absent ; in one instance, both ACAs were absent. Such an anomaly, which may be bilateral, is commonly associated with ACA saccular aneurysm ^{, , ,} and with other anomalies such as duplication of the MCA, median corpus callosum artery, distal moyamoya, aortic coarctation, facial congenital defects, cerebral lipoma, and absence of the ICA (in which the remaining carotid artery gives off a branch that passes beneath the optic nerve and divides into two ACAs while the other MCA arises from the PCA). The anomalous vessel itself can cause visual symptoms from compression of the optic nerve or chiasm.

The infraoptic ACA has been considered a remnant of the embryonic primitive maxillary artery, present in 3- to 4-mm embryos as an ICA branch and normally becoming a cavernous carotid branch, the inferior hypophyseal artery. ^{, ,} (The ACA normally arises from the primitive olfactory artery, eventually becoming the dominant vessel.)

Other anomalies, reported in an autopsied infant, include unilateral absence of the proximal MCA, ACA, and anterior choroidal artery, with much of the ipsilateral inferior frontal lobe supplied by branches from the opposite ACA, and secondary porencephaly of the orbital frontal lobe. Autopsy of a neurologically healthy man showed a plexiform anterior communicating system connected to the left ICA by an anomalous vessel arising from the ICA near the ophthalmic artery, a single distal ACA, marked right A1 segment hypoplasia, and right plexiform vessels in the area of the Heubner artery, along with other anomalies of the posterior circulation. Such anomalies, rare in combination, are not unusual individually. For example, in a series of 1250 consecutive autopsies, a plexiform anterior communicating system was found in 15% of subjects, hypoplastic ACAs were found in 4%, and fused distal ACAs were found in 4%; a plexiform Heubner artery was much less common. An ophthalmic artery arising from the ACA has also been reported, ^, as has an accessory MCA arising from the A2 segment of the ACA.

In a study of 381 brains, distal ACA anomalies were found in 25%. Such anomalies include pericallosal artery triplication, absence of ACA pairing, branches from one ACA to the other hemisphere, and bihemispheric branches ( Fig. 23.3 ). ^b Triplicate ACAs with a variably developed midline accessory artery arising from the ACoA and supplying little, much, or most of either or both hemispheres have been observed in up to 22% of autopsy specimens. ^{, , ,} Also, a long callosal artery (medial artery of the corpus callosum, anterior MCA) can arise from the pericallosal artery and pass parallel to it, giving off callosal perforating branches. ^, At angiography, these anomalies, such as a hypoplastic A1 segment, produce apparent bilateral ACA filling after unilateral injection of a carotid artery. ^{, ,} Bihemispheric ACAs, with either ACA taking over the supply of part or all of the other hemisphere, have been reported in up to 64% of brains. ^{, ,} (The highest value comes from a study in which any contralateral supply, however small, was included; brains in which most of both hemispheres are supplied by one of two ACAs are less common. )

In the fetus, there is gradual embryonic transition from one to two ACAs. ^, An unpaired or azygous ACA, arising through proximal union of the ACAs without an ACoA, occurs in 5% or less of adult brains. ^c Sometimes the ACAs fuse for up to 3.9 cm, and an ACoA is absent. Azygous ACAs are associated with a variety of other anomalies, including hydranencephaly, septum pellucidum defects, meningomyelocele, hydroencephalodysplasia, and vascular malformations and, like other ACA anomalies, with a higher frequency of saccular aneurysm. ^, In holoprosencephaly (fusion of the frontal neocortex and absence of the interhemispheric fissure), an azygous ACA courses just beneath the inner table of the skull.

b References 41, 44, 83, 97, 99, 126, 127.

c References 42, 97, 98, 128–131, 137, 138.

As noted, ACA anomalies are associated with an increased frequency of saccular aneurysms, especially at the ACoA but also on distal or anomalous branches. ^{, ,} The embryonic prominence of the interhemispheric arterial plexus that develops into the ACoA is the most common site for the development of intracranial aneurysms. Of 206 patients with ACoA aneurysms in one study, 44 (21.4%) had ACA anomalies, especially a median artery of the corpus callosum and duplication of the ACoA. Ruptured fusiform aneurysm of the A1 segment of the ACA has been reported. Giant aneurysms have been found on azygous ACAs. ^, After subarachnoid hemorrhage, a congenitally narrow A1 segment may be mistaken for vasospasm. Furthermore, proximal ACA ligation in patients with surgically unclippable ACoA aneurysms is not a valid option if one A1 segment fills both distal ACAs or if, in the absence of cross compression, the aneurysms fill well from either side. ^,

Dolichoectasia—pathologic elongation, tortuosity, and dilatation of an artery—most often affects the vertebrobasilar system and rarely the ACA. The cause can be either acquired (e.g., trauma or atherosclerosis) or congenital. Headache, seizure, visual field defects, dementia with hydrocephalus, subarachnoid hemorrhage, and mania are described in patients with dolichoectatic ACAs.

ACA fenestration, a common anomaly, has no clinical significance except when it is mistaken for an aneurysm on angiography. Vestibulocochlear symptoms developed in a 22-year-old patient with fenestration and ectasia of the left ACA and persistence of the right trigeminal artery.

Species differences in the anatomy of the ACA (and other cerebral vessels) must be kept in mind when one is interpreting animal studies of cerebral ischemia and stroke. For example, birds, amphibians, and anteaters have paired arteries without an ACoA or other left-to-right anastomoses. In most mammals, the two ACAs join to form a single pericallosal (azygous) artery, which may or may not bifurcate distally, and there is no ACoA. In subhuman primates, several recurrent medial striate arteries (the equivalent of the Heubner artery in humans) supplying the anterior caudate, putamen, and globus pallidus have rich preparenchymal anastomoses with lateral lenticulostriate arteries from the MCA; the orbitofrontal artery, supplying most of the orbital surface of the frontal lobe, arises from the MCA and anastomoses with branches of the ACA, and extensive anastomoses exist between the ACA and the proximal MCA in the sylvian fissure. ^, In cats, the presence of an ACoA has been both claimed ^, and denied. The feline ACA supplies the medial hemispheric cortex containing hindlimb motor representation, but cerebral arterial occlusion tends to cause smaller and deeper infarcts than in higher primates. In rats, the rostral caudatoputamen is supplied by penetrating ACA branches and a vessel running alongside the lateral olfactory tract; this area accounts for 25% of strokes in stroke-prone spontaneously hypertensive rats. ^,

Symptoms and Signs