The persistent primitive olfactory artery (PPOA) is a rare variant of the anterior cerebral artery (ACA) that was first described by Yamaura et al. in 1979 (1). Its incidence is 0.14% in Japan (2011) and 0.29% in Korea (2012) (2). It is mostly unilateral and rarely bilateral; moreover, it has no laterality or sex predilection (3). According to Kwon et al. (2), the left side has a higher frequency in Korea. Five types of PPOA have been reported, and type 6 has been recently suggested (4, 5). We report a case of PPOA type 4 with a fusiform aneurysm in the hairpin turn segment.

A 46-year-old female presented with occasional headaches for 1 month. She had no specific medical history other than hypertension, diabetes mellitus. Physical examination revealed no neurological deficits, including anosmia or visual disturbances.

The patient underwent brain MRI, which revealed no abnormal findings in the brain parenchyma. Time-of-flight MR angiography revealed a saccular ophthalmic aneurysm in the left internal carotid artery (ICA). The A1 segment, an unusual branch arising from the left ACA, was also observed (Fig. 1A).

Fig. 1
Persistent primitive olfactory artery type 4 with aneurysm.
A. Time-of-flight MR angiography. Maximal intensity projection superoinferior view (left image) and lateral view (right image). There is a paraolfactory hairpin curve of the left PPOA (arrows). Paraophthalmic aneurysm is also noted in the left internal carotid artery (empty arrows).

B. Digital subtraction angiography (upper images) and matched three-dimensional CT angiography (lower images) of the left ICA. Lateral view (left images) and oblique views (right images). The left PPOA arises from left A1, makes a hairpin turn, and finally continues to the left accessory middle cerebral artery (arrows). There is fusiform dilatation at the hairpin segment (arrowheads). A paraophthalmic aneurysm is also noted in the left ICA (empty arrows).

C. Illustration of five types of PPOA in the left lateral view. It is divided into five types depending on the presence or absence of the following blood vessels and hairpin turns (left image). Illustration of PPOA type 4 with hairpin-turn aneurysm (arrow) (right image).

ACA = anterior cerebral artery, aMCA = accessory middle cerebral artery, EA = ethmoidal artery, ICA = internal carotid artery, MCA = middle cerebral artery, PPOA = persistent primitive olfactory artery

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Transfemoral cerebral angiography revealed that this branch moved anteromedially along the ipsilateral olfactory tract, with a hairpin turn posterior to the olfactory bulb. Without anastomosis to the distal ACA, it continued to the left accessory middle cerebral artery (MCA). Moreover, a fusiform aneurysmal dilatation involving a hairpin curve was observed (Fig. 1B).

Therefore, left PPOA type 4 with fusiform dilatation was diagnosed, and no surgical or endovascular treatment was performed. Follow-up imaging is advised to monitor aneurysms.

This retrospective study was approved by the Institutional Review Board of the Inje University Busan Paik Hospital, which waived the requirement for informed consent (IRB No. 2022-10-010).

Embryologically, at 4 weeks of gestation, the rostral division from the primitive ICA becomes the primitive olfactory artery (POA) with three branches. During normal development, the medial branch becomes the ACA proper, and the lateral branch becomes the recurrent artery of the Heubner, anterior choroidal artery, and MCA, but the terminal portion of the POA in the nasal fossa degenerates at around 7 weeks of gestation. However, if the regression of the terminal portion of the POA does not occur, the remnant artery PPOA persists (6).

Until recently, the PPOA was classified into five types depending on the artery that continued and whether the hairpin turn was present (Fig. 1C). Most cases reported that PPOAs are type 1, which is considered to have the most typical appearance, with other types rarely reported (5).

Type 1 arises from the proximal ACA and continues to the distal ACA, making a hairpin turn along the olfactory bulb (7).

Type 2 does not have a hairpin turn and continues into the ethmoidal artery of the nasal cavity (7).

Type 3 makes a hairpin turn and continues to the distal ACA, like type 1, with branching continuing to the ethmoidal artery, similar to type 2 (4).

After an abrupt hairpin turn, type 4 continues into the accessory MCA (4).

Type 5 does not have hairpin turns and continues to the A3 segment of the ACA (4).

Clinically, PPOA is strongly associated with the development of aneurysms. Several cases of subarachnoid hemorrhage (SAH) with ruptured aneurysms in the hairpin turn segments have been reported (3, 7, 8). Although the exact pathogenesis has not yet been elucidated, hemodynamic characteristics induced by the curvature of the abrupt hairpin turns are thought to play a role in this phenomenon (Fig. 1C). Two of the 29 patients with PPOA and one of the 14 patients in the studies by Kwon et al. (2) and Uchino et al. (3) had hairpin-turn segment aneurysms.

Another clinical presentation of PPOA is anosmia. There have been several case reports of anosmia, some of which were unilateral or bilateral, and some patients recovered postoperatively (2, 7, 8). According to Kwon et al. (2), it was reported that two of the 28 (7.1%) patients with PPOA had anosmia. This was thought to be related to olfactory nerve perfusion.

PPOA is mostly found incidentally but can also be found as a neurologic defect or SAH, as described above. Therefore, even if a clinically asymptomatic PPOA is noted, regular examinations, including computed tomography angiography, are required to monitor the aneurysm. Moreover, these arterial variants are important in intracranial surgeries of the skull base to prevent vascular complications.

In conclusion, we found a rare variant of PPOA linked to the accessory MCA rather than to the distal ACA. Type 4 PPOA has been classified relatively recently and has been reported in only a few cases compared to the classification of other types of PPOA (9). It is important to note the different types of PPOA and identify the accompanying symptoms that may occur.