Persistent primitive olfactory artery connected with middle cerebral artery: case report

A persistent primitive olfactory artery (PPOA) is an extremely rare variation of the proximal anterior cerebral artery (ACA). The PPOA is connected to the distal ACA, ethmoidal artery, or both. I describe one patient with a PPOA connected to the middle cerebral artery (MCA). I analyzed the radiological characteristics of this anomalous vessel in this patient, who presented with headache. Computed tomography–angiography revealed an abnormal vessel in the patient, which originated from the distal ACA and ran anteromedially along the olfactory tract; it then made an abrupt turn and became the MCA. A PPOA connected to the MCA has been described in only two patients, including my own, in the English-language literature.


Introduction
Persistent primitive olfactory artery (PPOA) is extremely rare. The PPOA variant 1 originates at the terminal portion of the internal carotid artery (ICA) and runs anteriorly along the olfactory tract toward the crista galli. The distal portion of the PPOA makes an acute angle to the distal anterior cerebral artery (ACA) and supplies the normal ACA territory [6]. Whereas PPOA variant 1 involves the ACA, variant 2 passes through the cribriform plate to supply the nasal cavity as the ethmoidal artery. The PPOA variant 2 does not involve the ACA. The PPOA is classified into these two variant forms according to its vascular territory [6]. Horie's group [2] reported another variant of PPOA that constitutes a transitional type between variants 1 and 2 that partially involves the ACA.
I experienced a patient with a PPOA connected to the distal middle cerebral artery (MCA). This type of PPOA has been reported in only one other patient [8]. Here, I describe the computed tomography (CT)-angiographic findings for this type of PPOA.

Case report
A 78-year-old woman presented with headache. CT-angiography revealed a right PPOA. The vessel originated from the right distal A1, ran along the right olfactory tract toward the olfactory bulb, and made an abrupt posterior turn, ultimately becoming the distal MCA. In this patient, I identified the normal A1 and a short anterior communicating artery (Fig. 1). Her headache was relieved with medication.

Discussion
PPOA is extremely rare. Nozaki's group [6] classified these arteries into two variants and provided an explanation of the embryological development of PPOA. Horie's group [2] reported PPOA variant 3, which is a transitional type between variants 1 and 2. I encountered yet another variant of PPOA, so I reviewed the literature and analyzed the radiological characteristics of PPOA. In PPOA variant 1, the anomalous artery arises from the ICA, runs along the olfactory tract, and makes a hairpin turn to supply the distal ACA territory. The normal A1 is absent. A long or absent anterior communicating artery is observed because of the large distance between the contralateral A1 and the PPOA. However, one patient reported to have PPOA variant 1 had a normal A1 and a normal anterior communicating artery [4].
In PPOA variant 2, the artery arises from the normal A1 and passes through the cribriform plate to supply the nasal cavity as the ethmoidal artery. Three cases of PPOA variant 2 have been reported, in all of whom a normal A1 was present [1,5,7].
Horie's group [2] reported PPOA variant 3, which is a transitional type between variants 1 and 2. The anomalous artery of PPOA variant 3 has two branches: the superior branch forming the callosomarginal branch of the ACA, and the anterior branch extending toward the cribriform plate, with an anastomosis with the ethmoidal artery.
The type of PPOA observed in my patient had some interesting characteristics. The patient had a normal A1, from which the PPOA originated before running along the olfactory tract. The distal portion of the PPOA then made an abrupt acute angle and ran into the Sylvian fissure. This PPOA supplied the distal MCA territory. This variant is very similar to case 5 in Uchino's group [8] who described 14 cases of PPOA after a retrospective review of 3,626 magnetic resonance angiographies.
Embryologically, the primitive olfactory artery (POA) is the rostral division of the primitive ICA [3]. The POA terminates in the nasal fossa and the secondary artery constitutes the medial olfactory artery, which supplies the olfactory bulb. The medial olfactory artery becomes the ACA proper, whereas the terminal portion of the POA usually regresses. Lateral olfactory branches of the POA include the recurrent artery of the Heubner, anterior choroidal artery, lateral striate artery, and later MCA. Recurrent artery of Heubner is not the remnant of the POA. When the POA maintains its embryological course along the olfactory tract, it is called a 'PPOA'. The most important point in the definition of PPOA is the embryological course of the anomalous artery along the olfactory tract [3]. The PPOA vessel can be divided into four variants, according to the distal vascular territories it serves. Variant 1 supplies the distal ACA territory, variant 2 forms an anastomosis with the ethmoidal artery, and variant 3 connects with the callosomarginal and ethmoidal arteries. The new variant seen in my patient supplies the MCA territory, as an accessory MCA (Fig. 2).
Aneurysm is rarely seen at the hairpin turn of the PPOA, probably because of hemodynamic stress [8]. A follow-up study is required in patients with PPOA to determine the risk of an aneurysm at the hairpin turn of the PPOA, especially in young patients.