Introduction

The precommunicating segment of the anterior cerebral artery (A1) runs above the optic nerve on its course from the internal carotid bifurcation to the anterior communicating artery. An infraoptic course of the A1 segment is a rare occurrence.10)16) Various authors have suggested different terminologies such as infraoptic anterior cerebral artery (ACA)16) and carotid–ACA anastomosis.11) Robinson12) made the first report of an infraoptic course of the A1 in 1959. Since then, 56 cases have been reported.1)3-5)7)8)13)15)17)18) In about 39% (11/28) of patients with an infraoptic course of an ACA, there is an association with an anterior communicating artery aneurysm.5) Thus, it is important to recognize this cerebrovascular variation when planning for treatment.

Here we describe an infraoptic A1 associated with a middle cerebral artery aneurysm. The possible genesis of this anomaly is discussed and practical considerations in treating an associated cerebral aneurysm is considered.

Case Report

A 52-year-old man with a history of hypertension and diabetes mellitus presented to the emergency room with a severe headache that had started one day before. Neurological examination revealed no abnormalities except neck stiffness but computed tomography (CT) of the brain revealed a subarachnoid hemorrhage (Fig. 1). Three- dimensional CT angiography demonstrated an anomalous right A1 vessel arising at the ophthalmic level of the right internal carotid artery. This anomalous artery followed an infraoptic and medial course. The left A1 followed a supraoptic course (Fig. 2). Left carotid angiography revealed an aneurysm of the left middle cerebral artery (Fig. 3). Injection of the right carotid artery demonstrated that an anomalous A1 vessel arose at the ophthalmic level of the right internal carotid artery and followed a horizontal- medial course (Fig. 4). The aneurysm was embolized with placement of Guglielmi detachable coils. The patient did not experience any postoperative neurological complications and fully recovered. At a one month follow-up in the outpatient department, he demonstrated no neurological deficits.

Discussion

Embryological pathogenesis of infraoptic A1

The infraoptic course of the A1 segment has a characteristic appearance on angiography.2)7)14)16) The first characteristic is an apparent low bifurcation of the internal carotid artery at the level of or just above the ophthalmic artery. Second is a horizontal-medial course of the proximal ACA as it passes under the ipsilateral optic nerve before turning superiorly to the anterior communicating artery. Finally, the infraoptic ACA joins the distal A1 segment of the normal ACA or the anterior communicating artery. In our patient, the infraoptic A1 had a typical appearance.

The exact embryogenesis of this vascular anomaly is unclear. Several alternative explanations have been proffered. First, the anomalous vessel may arise from an anastomotic loop of the primitive ventral and dorsal ophthalmic arteries around the optic nerve. Second, this vessel may develop from an anastomosis of the primitive maxillary artery and the ACA. A third possibility is that this vessel can arise from the primitive prechiasmal anastomosis comprising minor branches of the ACA, internal carotid artery, and ophthalmic arteries.6) This faulty embryogenesis most likely occurs during the early stage of cranial artery development at the perioptic arterial plexus.6)18)

This infraoptic A1 is frequently associated with other intracranial vascular anomalies secondary to the embryogenic disorder, such as the carotid–basilar artery anastomosis, a fused pericallosal artery, or a plexiform anterior com- municating artery. In our patient, we found a plexiform anterior communicating artery.

Clinical significance

This anomaly may be completely asymptomatic and have little clinical significance. In our patient, the infraoptic A1 also was discovered incidentally. When seen together with an anterior communicating artery aneurysm, these infraoptic ACA do not alter the surgical approach when treating the aneurysm.9) However, an awareness of the infraoptic course of A1 is especially important in patients with a proximal aneurysm, which may be obscured by the optic nerve during dissection and clipping.18) Aneurysms that have a more proximal origin from the infraoptic ACA are more difficult to treat surgically because they are embedded within the cavernous sinus and are very close to several cranial nerves.2)

If a single infraoptic A1 gives rise to bilateral distal ACA, the time for temporary clipping is more limited. And in this condition, more cerebral protection methods should be considered in temporary clipping. In addition, inadvertent occlusion of the single A1 will result in detrimental effects.18)

Sakai, et al.13) reported one case of temporary mild deterioration of visual acuity after clipping of an infraoptic A1 aneurysm. The aneurysm was located on the curved midportion of the infraoptic A1 and extended underneath the right optic nerve. Retraction of the optic nerve during surgery was probably the cause. Removal of the anterior clinoid process and unroofing of the optic canal might facilitate manipulation of the optic nerve.

McLaughlin, et al.10) reported the first case of an abnormal gyrus segmentation in association with an infraoptic course of an ACA. They reported that no interhemispheric fissure was observed in the surgical field and they insisted that awareness of the possibility of an abnormal gyral segmentation may prevent erroneous dissection during exposure. In their experience, dissection could be continued through the fissure lying between the median gyrus and the left gyrus rectus. No removal of gyrus or subpial dissection was required for adequate exposure and clip placement.

Recognition of this anomaly is vitally important in planning surgery for aneurysms of the ACA-anterior communicating artery complex because an alternative surgical approach may be required to gain control of the aneurysm.14) Failure to take into account this anomaly might result in unnecessary dissection along with possible damage to the optic apparatus during aneurysm repair.

The presence of an infraoptic A1 together with a supraoptic A1 may actually facilitate endovascular treatment of an aneurysm. For example, Al-Qahtani, et al.2) reported a patient with a supraoptic A1 and the infraoptic A1 was embolized in order to eliminate an aneurysm because of a straighter configuration of infraoptic A1. Hillard, et al.6) reported a patient where the infraoptic A1 actually provided an easier route for coiling an anterior com- municating artery aneurysm because of a straighter con- figuration and a larger size of infraoptic A1 compared to the supraoptic A1.

Because infraoptic A1 is frequently associated with hypoplasia or aplasia of the supraoptic A1, this vessel can be used as a more direct route for insertion of a microcatheter into the anterior communicating artery.

Conclusion

An infraoptic course of the ACA is an extremely rare anomaly and is frequently associated with another intracranial vascular anomaly. Recognition of an infraoptic ACA anomaly is important to allow for optimal planning of surgical or endovascular aneurysm treatments.

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