INTRODUCTION
The petrosal artery (Figs. 1, 2), also known as the petrous branch of the middle meningeal artery (MMA) or the superficial petrosal artery, is an important supplier to the intrapetrous portion of the facial nerve (CN VII) and contributes to supplying several other structures near the floor of the middle cranial fossa [6]. Arising from the third branch of the first portion of the maxillary artery, the MMA is the dominant supplier to the meningeal arterial network in the cranial dura [32]. As it enters the foramen spinosum, its proximal intracranial segment gives off the petrosal and cavernous branches prior to its bifurcation into anterior and posterior divisions [3, 6, 28, 29, 32]. The textbook anatomy of the petrosal artery involves it entering the hiatus for the greater petrosal nerve to supply the facial nerve, geniculate ganglion, and tympanic cavity; however, it has more commonly been described as reaching this point by passing through the bone surrounding the tympanic segment of the facial nerve and the geniculate ganglion [6, 32, 33]. Together with the stylomastoid branch of the posterior auricular or occipital artery, the petrosal artery forms an anastomotic arterial arch supplying the geniculate ganglion, known as the facial nerve arterial arcade, the disturbance of which has several clinical consequences. Interruption of the petrosal artery during invasive extradural subtemporal approaches to the middle cranial fossa, such as elevation of the dura or MMA embolisation, risks iatrogenic facial nerve palsy [6].
This paper describes an investigation of the artery through cadaveric dissection, and reviews the anatomical structure, its variations, and clinical significance of this important MMA branch. To facilitate effective results of endovascular treatment and to preclude iatrogenic injury, it is important during approaches to the middle cranial fossa to appreciate the variable anatomy of the petrosal artery and its relationship with the facial nerve and other neurovascular structures.
MATERIALS AND METHODS
Twenty latex injected cadaveric heads (40 sides) underwent microsurgical dissection of the petrosal artery. A surgical microscope (Zeiss, Germany) was used for all dissections. The specimens were 68–88 years old at death with a mean age at death of 71.5 years. Twelve specimens were male and eight were female. Documentation of the course of the artery and its branches were made. Measurements of the petrosal artery’s length and diameter were performed using microcallipers (Mitutoyo, Japan). The diameters were measured at the take-off of the vessel from the MMA. Morphometric data were compared based on sides and sex. Descriptive and inferential statistics were calculated using SPSS version XX (Armonk, NY).
RESULTS
A petrosal artery was identified on all sides (Figs. 3–8). The mean length of the artery within the middle cranial fossa was 2.4 cm (range 1.6–2.6 cm). The diameter of the petrosal artery ranged from 0.23 mm to 0.66 mm (mean 0.38 mm). Branches included the following: dural, ganglionic, V3 branches, branches extending through the foramen ovale, branches directly to the greater petrosal and lesser petrosal nerves, branches to the floor of the hiatus of the greater and lesser petrosal nerves, branch to the arcuate eminence, and superior tympanic artery. Some of these branches, especially branches to V3 were duplicated. The petrosal artery arose from below the foramen spinosum on two left and three right sides (12.5%). On 1 (2.5%) of these, the artery did not enter the middle cranial fossa but entered the skull through a more medial and unnamed aperture to then travel under a bony bridge (Fig. 7). Bony bridges covering the petrosal artery were found on 3 (7.5%) sides and these were found within 1.5 cm from the artery exit from the foramen spinosum. A superior tympanic artery arose from the petrosal artery on 3 (7.5%) sides. One left sided petrosal artery (2.5%) was found to form an arterial circle that gave off dural branches and also branches to V3. No statistically significant differences were noted between male and female specimens, but right-sided petrosal arteries were in general, larger in diameter than left sides (p > 0.05).
DISCUSSION
We identified a petrosal artery on all sides. This artery commonly supplied the mandibular nerve and continued posteriorly parallel to the greater petrosal nerve. The mean length of the artery within the middle cranial fossa was 2.4 cm (range 1.6–2.6 cm). The diameter of the petrosal artery ranged from 0.23 mm to 0.66 mm (mean 0.38 mm). Branches included the following: dural, ganglionic, V3 branches, branches extending through the foramen ovale, branches directly to the greater petrosal and lesser petrosal nerves, branches to the floor of the hiatus of the greater and lesser petrosal nerves, branch to the arcuate eminence, and superior tympanic artery.
Anatomy
Normally, as the MMA enters through the foramen spinosum, it immediately gives off two branches prior to its split into anterior and posterior divisions: the petrosal branch laterally, and the cavernous branch to the trigeminal ganglion medially. The petrosal artery is the first intracranial branch of the MMA and can originate slightly below or above the level of the foramen spinosum [6]. Craniotomy of the overlying temporal bone reveals that it usually arises from the first 10-mm intracranial segment of the MMA, or in some cases 15–20 mm above this level [6, 13]. It is commonly described as coursing posteriorly along the greater petrosal nerve, but it has been observed to run closer to the lesser petrosal nerve as seen in some of our cases (Fig. 5) [6]. The petrosal artery first enters the floor of the middle cranial fossa floor, where its distal portion is largely obscured, either partially or wholly, by the overlying bone of the middle cranial fossa floor [6]. Most commonly, it reaches the facial canal, also known as the fallopian canal, to supply the facial nerve by directly piercing the adjacent petrous part of the temporal bone surrounding the geniculate ganglion and the tympanic segment of the facial nerve. However, variations of this course will be detailed later [6]. Tiny branches to the neighbouring dura, as seen in our study, such as the dura over the posteromedial floor of the middle cranial fossa, and the petrous apex region, can also be given off before the petrosal artery enters the facial canal (Fig. 5) [17]. While travelling through this canal, it supplies the greater petrosal nerve [2].
The petrosal artery is usually the main supplier to the facial nerve crossing the middle cranial fossa floor and it provides other anastomosing branches in the region [6]. After entering the petrous part of the temporal bone laterally, it supplies the geniculate ganglion via small branches and subsequently supplies the tympanic segment of the facial nerve and the walls of the tympanic cavity [2]. At what can be considered an arterial crossroads between the external carotid, internal carotid, and vertebrobasilar systems, the petrosal artery is in position to participate in anastomoses with the anterior and posterior MMA divisions, accessory meningeal artery, carotid branch of the ascending pharyngeal artery, inferolateral trunk, and medial and lateral tentorial arteries [3, 6, 17]. Moreover, the stylomastoid branch of the posterior auricular artery anastomoses with the petrosal artery near the tympanic segment of the facial nerve via a tiny branch that enters the facial canal [9, 20]. Although no single artery supplies the entire facial nerve, the petrosal artery together with the stylomastoid branch is an important contributor to the facial nerve arterial arcade, an anastomotic loop supplying the intratemporal facial nerve and geniculate ganglion (Fig. 1) [30]. The petrosal artery alone supplies the pregeniculate part of the facial nerve [13]. Potentially, it can also participate in anastomoses with the meningohypophysial trunk, the recurrent meningeal artery of the ophthalmic artery, the mastoid branch of the occipital artery, and the subarcuate artery [3, 17, 33]. Additionally, the stylomastoid branch, along with other collateral arteries including the inferolateral trunk of the cavernous internal carotid artery and labyrinthine arteries, can supply the facial nerve [30].
Among other anastomosing branches, the initial portion of the petrosal artery can give off a branch entering the middle ear. This is the superior tympanic artery, the petrosal remnant of the stapedial artery. The superior tympanic artery generally arises from the middle meningeal system but has been more specifically described as arising from the petrosal artery itself. Entering the middle ear adjacent to the lesser superficial petrosal nerve, it supplies the superior portion of the tympanic cavity and participates in a middle ear anastomosis with the caroticotympanic branch of the internal carotid artery, the inferior tympanic artery from the ascending pharyngeal artery, and the posterior tympanic artery from the occipital artery (Fig. 1) [3, 4, 14].
According to Lasjaunias et al. (1987) [14], the superior tympanic artery belongs to the petrosal artery, which divides initially into two branches, medial and lateral. The medial branch courses towards the cavernous sinus area. It provides some arterial contribution to the dura over the inferior wall of the cavernous sinus and anastomoses with the ascending pharyngeal branches in the temporal bone and with branches of the internal carotid artery [17, 28]. However, a superior tympanic artery arising from the petrosal artery was found in only 7.5% of sides in our cadaveric study. Sant’Anna et al. (2021) [26] report that this cavernous branch contributes to supplying the trigeminal ganglion, the mandibular nerve, and the dura surrounding the foramen ovale. The lateral branch also provides two additional smaller branches. One of these, the so-called basal tentorial branch, reaches the basal tentorial margin to distribute blood to the upper part of the cerebellopontine angle above the internal acoustic meatus [13–15]. The petrosal artery thus contributes to supplying blood to the medial portion of the tentorium cerebelli at its insertion along the petrous ridge, in addition to the walls of the superior petrosal sinus [3, 17]. At this level, it participates in the basal arcade of the tentorium cerebelli, anastomosing medially with the lateral branch of the clivus and laterally with the basal tentorial branch of the petrosquamosal branch [13–15]. The other smaller branch of its lateral division, extending more posteriorly and superiorly, accompanies the greater petrosal nerve, often giving off small branches to supply it [14, 15]. It is via this nervous branch that the petrosal artery reaches the facial canal, contributing blood to the geniculate ganglion and facial nerve, anastomosing with the stylomastoid artery, and giving off the superior tympanic artery [14, 15]. While acknowledging it as a branch of the petrosal artery, some sources have stated that the superior tympanic artery is responsible for supplying the facial nerve and geniculate ganglion [4, 10].
Other descriptions of the branches of the intracranial MMA distinguish the petrosal and superior tympanic arteries as two distinct entities and this is supported by our study with the superior tympanic artery originating from the petrosal artery found only as a variation. According to one such description, the petrosal artery, upon giving off an anastomotic branch to the superior tympanic artery, divides into two primary branches [27]. The first of these supplies the geniculate ganglion and further divides into two smaller vessels, passing within the facial nerve territory to course either in the direction of the internal acoustic meatus or towards the periphery. The second main branch bypasses the geniculate ganglion and continues as a descending branch in an area between the facial nerve trunk and the facial canal. The petrosal artery then anastomoses with the stylomastoid artery in the upper third of the vertical segment of the facial canal [27]. The superior tympanic artery originates from the MMA separately from the petrosal artery and can either anastomose with the petrosal artery or travel into the facial canal accompanying the lesser petrosal nerve [3, 13].
Regardless of the inconsistencies in the literature, the evidence can be generally summarised as follows. The petrosal artery courses along the petrous apex posterolaterally, traverses within the facial canal, pierces the petrous bone, participates in several potential anastomoses, and gives off branches (varying in size and number) that provide arterial arborisation to several structures: the proximal tympanic (horizontal) segment of the facial nerve, the geniculate ganglion via the facial nerve arterial arcade, the maxillary and mandibular divisions of the trigeminal nerve, the medial portion of the tentorium cerebelli, and the superior aspect of the tympanic cavity just distal to the geniculate ganglion via its superior tympanic branch [3, 6, 17].
Variations
The branch of the petrosal artery that traverses to supply the facial nerve is anatomically variable in respect of its dominance, course, branches, and origin. In nearly all cases (90%) studied by Lasjaunias et al. [15], the petrosal artery was consistently the dominant supply to the intrapetrous facial nerve territory. In the remaining 10%, the branch of the petrosal artery that reached the facial nerve supplied only the geniculate ganglion; the remaining portion of the facial nerve was supplied by the stylomastoid artery which arises from the posterior auricular artery. Paullus et al. (1977) [24] reported that the branch with the largest diameter of the petrosal artery travelled with the greater petrosal nerve in 92% of cases and with the lesser petrosal nerve in 8%. In the same study, the petrosal artery was noted in 46% to send branches to the trigeminal ganglion.
In 2008, El-Khouly et al. [6] examined the course of the petrosal artery as it travelled along the middle cranial fossa floor in 25 cadaveric specimens and described three distinct types of variation in its supply to the facial nerve. Of the 25 middle cranial fossae studied, the MMA was absent in one specimen; in four others, the branch of the petrosal artery to the facial nerve was absent although the MMA was intact. In patients lacking a branch of the petrosal artery to the facial nerve, the accessory meningeal artery or the stylomastoid artery from the posterior auricular artery, or a combination of the two, possibly contributes to supplying this nerve [29]. The labyrinthine artery has also been cited as a potential contributor to this arterial supply, joining the nerve at the cerebellopontine angle or the internal acoustic meatus [6]. The classification system of the petrosal artery supplying the facial nerve in the remaining middle cranial fossae (n = 20) is as follows:
- type A (65%): the petrosal artery travels through the bone near the geniculate ganglion and/or tympanic part of the facial nerve;
- type B (20%): the petrosal artery travels via the hiatus of the greater petrosal nerve to supply the facial nerve near the geniculate ganglion;
- type C (15%): two petrosal arteries, arising from either the MMA or a single petrosal artery, reach the facial nerve. One of them pierces the bone adjacent to the geniculate ganglion, as in type A; the other enters the hiatus of the greater petrosal nerve, as in type B.
In type A, the petrosal artery was either exposed as it traversed the floor of the middle cranial fossa floor, or hidden, partially or completely, in the bone below this surface. Upon reaching the geniculate ganglion, the type A petrosal artery either sent a branch only to the ganglion and the tympanic part of the facial nerve, or divided into two arteries distal to its origin: a medial branch following along the ganglion and the greater petrosal nerve, and a lateral branch running along the tympanic segment of the facial nerve. The type B petrosal artery split into two branches near its origin, one artery traveling to the facial nerve and the other to the trigeminal nerve. El-Khouly et al. [6] thus reported the petrosal artery frequently giving rise to trigeminal ganglion branches, as mentioned previously. It is worth noting that the trigeminal branch is more likely to arise when the petrosal branches pass via the hiatus of the greater petrosal nerve, as opposed to passing through the bone surrounding the geniculate ganglion [6]. With type C arteries, in which the MMA gave off two petrosal arteries, one branch reaches the geniculate ganglion and the other the tympanic part of the facial nerve. In the alternative type C pattern, one petrosal artery divides into two arteries, one which reaches the Gasserian ganglion and the other reaches the tympanic part of the facial nerve.
Moreover, El-Khouly et al. (2008) [6] observed variations in the origin of the petrosal artery. In 17 (81%) of the specimens studied, the artery originated from the MMA above the foramen spinosum, an average of 5 mm distal to its intracranial end. In the remaining 19% it arose an average of 4 mm below this level, traversing the middle cranial fossa through the foramen spinosum or through a small adjacent opening. Jittapiromsak et al. (2009) [12] observed that the petrosal artery originated superior to the foramen spinosum in 50% of their specimens. Paullus et al. (1977) [24] described the petrosal artery arising distal to the foramen spinosum in 42% of specimens and proximal to it in 58%. Rarely, it arises from the internal carotid artery to give branches to the dura mater and subarcuate fossa [13]. It is duplicated in a small number of cases [13].
In our cadaveric study of the petrosal artery, variations were common. The petrosal artery arose from below the foramen spinosum on 12.5% of sides. On 1 (2.5%) of these, the artery did not enter the middle cranial fossa but entered the skull through a more medial and unnamed aperture to then travel under a bony bridge. Bony bridges covering the petrosal artery were found on 7.5% of sides. A superior tympanic artery arose from the petrosal artery on three sides. One left sided petrosal artery (2.5%) was found to form an arterial circle that gave off dural branches and also branches to V3.
Embryology
The foundations for cerebral blood flow are laid at the 1.3 mm embryonic stage by the pharyngeal arch arteries. The internal carotid artery pair begin their development at the 3 mm stage (day 24), formed by fusion of the third pharyngeal arch arteries [19, 23]. At the 4 mm stage, the internal carotid artery splits into two parts, the anterior providing major contributions to the circle of Willis, olfactory region, and optic region; the posterior persists as the posterior communicating artery [19, 22]. From 11 to 12 mm, the middle cerebral artery can be seen sprouting from the anterior communicating artery off the anterior part of the internal carotid artery. It is the major source of blood for the cerebral cortex, becoming more prominent at the 16–18 mm stage [22, 23].
After the early groundwork stages, intercarotid anastomosis formation peaks between the 20- and 40-mm stages, the 1st and 2nd pharyngeal arches forming the keystone stapedial artery [18]. The stapedial artery also gives off anterior (maxillofacial) and posterior (supraorbital) divisions, the former becoming the maxillary and mandibular arteries, the latter becoming the intracranial segment of the MMA while also carrying anastomosing branches to the ophthalmic artery (supraorbital branch of the MMA) [5]. The segment of the MMA outside of the skull is formed by fusion of the external carotid artery and maxillary artery. The stapedial artery begins to involute, leaving the remnant tympanic branch of the MMA. In some instances, the lacrimal artery can remain attached to the retroorbital branches of the stapedial artery. If these fail to regress, a lacrimal-MMA connection can form in addition to many other aberrant variations owing to the number of sprouting and regression events involved in forming the supra- and infra-orbital anastomoses with the MMA, and in anastomoses between the carotids [5].
Imaging
When the middle cranial fossa vasculature is examined, the petrosal artery (Fig. 9) should not be confused with the superior tympanic artery, which is either (a) occasionally another branch of the MMA closely related to the petrosal artery, or (b) a branch of the petrosal artery itself, as mentioned above [1, 8, 13, 20]. However, the petrosquamosal branch of the MMA is most frequently mistaken for the petrosal artery on digital subtraction angiography imaging, especially in lateral view. These two arterial structures are difficult to distinguish by this technique owing to the superimposed projection of the petrosquamosal branch and the overlying dense petrous bone almost completely obscuring the course of the petrosal artery [29]. Frontal projections are far more useful for the purpose because the petrosal artery projects more superiorly in this view [29].
Clinical implications
The petrosal artery can be injured during invasive procedures such as extradural approaches to the middle cranial fossa floor, drilling of the middle cranial fossa, or MMA embolization, risking facial nerve palsy [6]. Elevation of the dura and drilling of the middle cranial fossa floor are most common during approaches to the internal acoustic meatus through the middle fossa and anterior petrosectomy. The risk of petrosal artery injury is greater with the former procedure since the latter is performed posterior to the greater petrosal nerve and medial to the internal acoustic meatus [6]. Drilling away bone anterosuperior to the geniculate ganglion and tympanic segment can pose a greater risk of petrosal artery injury since it most commonly traverses through that area. El-Khouly et al. [6] recommend intradural dissection around the foramen spinosum, splitting the dura mater into two parts, to avoid injury to the petrosal artery during elevation of the dura. However, Shibao et al. [30] contend on the basis of histological data that such a precaution could lead instead to venous plexus haemorrhage and make it far more difficult to identify the greater petrosal nerve.
In the literature, the origin of the petrosal artery varies. In some cases, it originates inferior to the level of the foramen spinosum, so interruption of the MMA at this level would not result in facial nerve ischaemia [6]. However, since it most often arises from the MMA distal to the foramen spinosum, certain procedures such as intraspinosal MMA ligation could pose a greater risk of facial nerve ischaemia, though a sufficient collateral arterial supply could prevent it [30]. Regardless, interruption of the MMA greater than 10 mm beyond the foramen spinosum would unlikely result in ischaemia to the facial nerve as the petrosal artery would already have originated [6]. Other risk factors of facial nerve palsy after extradural subtemporal procedures include direct intradural dissection and tension on the greater petrosal nerve [6, 30]. Although these latter cases could also be due to disruption of blood flow to the facial nerve, e.g. petrosal artery.
Since the MMA is a common pathway for endovascular embolisation to treat dural arteriovenous fistulas, meningiomas, and chronic subdural haematomas, embolisation of the intracranial MMA near the foramen spinosum carries an inherent risk of facial nerve palsy due to the petrosal artery [6, 8, 29]. MMA embolisation at a proximal position near the origin of the petrosal artery is particularly dangerous. The resulting facial nerve palsy is generally temporary, with symptoms resolving following the development of a collateral supply and steroid therapy [8]. In any case, certain precautions have been recommended when proximal embolisation of the MMA is necessary. The microcatheter should be advanced to a position beyond the point where the petrosal artery branches [29, 33]. As found in our study, bony bridges, might inhibit such microcatheter advancement. Embolic materials should be injected cautiously to avoid regurgitation into anastomosis points of the petrosal artery [33]. The consensus is that particles (depending on size) carry less risk of penetrating smaller anastomotic channels than liquid agents and should be used for precluding embolic complications involving cranial nerves [8, 29, 33]; certain calibrated particles used as embolic agents are too large in diameter to penetrate the vasa nervorum branches of the petrosal artery [31]. Non-adhesive liquid embolic agents such as the copolymer ethylene-vinyl alcohol can be difficult to control owing to their viscosity and lack of adhesion to vessel walls [16]. There are reports of complications with Onyx migration following endovascular treatment of dural arteriovenous fistulas, resulting in facial nerve palsy [16, 31]. Ischaemic injury related to the facial nerve can result from migration of Onyx or cyanoacrylate derivatives into the vasa nervorum branches of the petrosal artery and/or compression by the embolic agent [16]. For example, Onyx can reflux into the petrosal artery, which as previously mentioned is an important contributor to the facial nerve arcade, potentially leading to dysfunction of the facial and greater petrosal nerves. Thus, the severity and outcome of such injury depends largely on the permanence of the embolic agent used [16]. Onyx is a permanent embolic agent with a low recanalisation rate, so any facial nerve injuries it causes have a poorer prognosis than when temporary agents are used [7]. Recommendations in the literature to minimise complications include a slower rate of Onyx injection, using detachable tip microcatheters, distal injection with wedged microcatheters, and using temporary embolic agents [2, 8, 21, 31].
Vascular disturbance to the facial nerve has been associated with Bell’s palsy. Additionally, ischaemic events can result from compression of the arterial supply by oedema of the nerve in the facial canal. El-Khouly et al. [6] suggest that the arterial supplies to both the trigeminal nerves and facial nerves might be compromised. As mentioned in multiple reports, the petrosal artery sends branches to the trigeminal nerve. This could explain trigeminal nerve involvement in some cases of Bell’s palsies if the petrosal artery suffers any disturbance. Pecket and Schattner (1982) [25] proposed that Bell’s palsy is a diabetic mononeuropathy. Diabetic small vessel disease could affect the peripheral blood supply to the facial nerve through the stylomastoid artery and branches of the petrosal artery, embolisation of both arteries producing clinical outcomes comparable to Bell’s palsy.
CONCLUSIONS
The petrosal artery is an important branch of the MMA and supplies the facial nerve. Along with cadaveric and angiographic data, this paper provides a comprehensive review of the anatomy of the petrosal artery and variations in its origin, dominance, anastomoses, and course. Particular emphasis has been placed on correlating its anatomical structure clinically. Recommendations in the literature to minimise iatrogenic injury to it have also been provided. Awareness of the petrosal artery is crucial for interventionalists and neurosurgeons when they consider the risk of interrupting it during invasive extradural subtemporal approaches to the floor of the middle cranial fossa and postoperative facial nerve palsy. A thorough anatomical knowledge of the petrosal artery and to its relationship to the facial nerve and other neurovascular structures is necessary to facilitate effective endovascular treatment and to preclude facial nerve complications.
Acknowledgements
The authors sincerely thank those who donated their bodies to science so that anatomical research could be performed. Results from such research can potentially increase mankind’s overall knowledge that can then improve patient care. Therefore, these donors and their families deserve our highest gratitude [11].
