The Pharyngo-Tympano-Stapedial Variant of the Middle Meningeal Artery

Summary

Atypical origin of the middle meningeal artery from the cervical internal carotid artery is a rare angiographic finding. We describe a case of the pharyngo-tympano-stapedial variant of the middle meningeal artery in a young patient. In this vascular variation the proximal segment of the middle meningeal artery, corresponding to an annexed inferior tympanic artery, originates from the cervical carotid artery. Then over the promontory it unites with the superior tympanic artery and continues with its intracranial course via the petrous branch.

Introduction

The middle meningeal artery (MMA) usually arises from the proximal maxillary branch of the external carotid artery and enters the skull through the foramen spinosum. Several cases have been reported of the anomalous origin of the MMA from the internal carotid artery. We describe such a case with the MMA arising from the cervical portion of the internal carotid artery, revealed incidentally by cerebral angiography in a patient with hemophilia and spontaneous subdural hematoma (SSDH).

Case Report

A 16-year-old male patient presented with a SSDH. He was a known patient of hemophilia B. The patient underwent a diagnostic catheter angiogram to investigate a potential bleeding lesion. No such lesion was found. However, an interesting variation of the vascular anatomy was noted. The MMA was seen as a continuation of a vessel originating from the cervical segment of the internal carotid artery (Figure 1A,B). This vessel did not follow the normal course of the MMA via the foramen spinosum. A CT scan confirmed atresia of the foramen spinosum (Figure 2). This was accompanied by the absence of opacification of the Inferior tympanic branch in the ascending pharyngeal artery injection. Interestingly, in the same injection, the known anastomosis between the superior pharyngeal branches of the ascending pharyngeal with the mandibular artery was simultaneously and bilaterally opacified (Figure 3).

Figure 1.

A,B) Selective left ICA injection. The pharyngo-tympano-stapedial variant arising from the medial aspect of the cervical ICA and coursing postero-laterally through the middle ear to the floor of the middle cranial fossa.

Figure 2.

Absence of the foramen spinosum on the left side.

Figure 3.

Selective injection of the left ascending pharyngeal artery. The inferior tympanic branch is not opacified.

Discussion

In its usual disposition, the middle meningeal artery (MMA) originates from the maxillary artery, travels cephalad and enters the intracranial cavity via the foramen spinosum. It then turns laterally and ramifies over the surface of the dura for its territory of supply.

Several variations in the origin of the MMA have been described in the literature. In the absence of foramen spinosum the MMA may partially (i.e. only the anterior branch) or completely arise from the ophthalmic artery. Under such circumstances it passes through the lateral end of the SOF or the meningo-orbital foramen¹. The MMA may arise as a branch of the persistent stapedial artery². Newton and Potts described the MMA originating from the lateral aspect of the prepetrous internal carotid artery (ICA) and then running directly anterior along the floor of the middle cranial fossa³. Anomalous origin of MMA from the basilar artery and a rare instance of MMA arising from the PICA has been also reported^4,5.

In the present case, the MMA was not seen to be originating from the external carotid artery, but it had its origin from the medial aspect of the cervical ICA. It was also noted to enter intracranially via a different route, i.e. not via the foramen spinosum, which was absent. This set of variations correspond to the pharyngo-tympano-stapedial variant of the MMA described for the first time in 1977 by Lasjaunias et al. ⁶. In this variant the tympanic branch of the ascending pharyngeal artery has been annexed by the ICA and is directly connected to the superior tympanic artery, a twig of the petrosal branch of the middle meningeal artery.

Except for the original publication ⁶ and subsequent inclusion of the same case(s) in Surgical Neuroangiography ⁷, there has been no other report of such a variation to the best of our knowledge. In 1974 McLennan et al. ¹ reported a case with an angiographic appearance identical to the intrapetrosal MMA but its origin was not clearly distinguished (proximal petrosal or distal cervical ICA) and they considered it a stapedial-middle meningeal artery variation.

The original publication(s) clearly stated that the inferior tympanic artery fills the MMA via its connection to the superior tympanic, which is the distal remnant of the stapedial artery. It is also described that the artery (pharyngo-tympano-stapedial) runs in the tympanic cavity, follows the major deep petrosal nerve away from the stapes and the facial canal and continues as the petrosal branch of the middle meningeal artery. It also appears that the tympanic branch of the ascending pharyngeal artery joins the stapedial system at the level of the superficial and deep petrous nerve ⁷. The same reference quotes that the medial group of anastomoses of the inferior tympanic artery accompanies the major and minor deep petrosal nerves and subsequently anastomoses with the petrosal branches of the MMA. It is also mentioned that the petrosal branch of the MMA runs posteriorly and superiorly, retrograde along the major superficial petrosal nerve ⁷.

Rodesch et al., in another publication on the complete persistence of the hyoido-stapedial artery (HSA) reported that the HSA penetrates the middle cranial fossa through the petrous n. hiatus ⁸. Lasjaunias et al., in another publication, described the same variation by stating that the “petrous artery will give rise to the superior tympanic artery after reaching the facial canal” ⁹.

We think that some ambiguity with regard to the exact anatomy emerges from the above-cited descriptions, enhanced by the preexisting confusion in anatomical terminology. The description below would be our approach to this variation and the related vascular anatomy (Figure 4).

The HSA regresses proximally together with its maxillo-mandibular branch. The flow into the MMA (supraorbital branch of the stapedial) is provided by the inferior tympanic branch of the ascending pharyngeal artery ¹⁰. The inferior tympanic artery (ITA) enters the anterior part of the floor of the middle ear, accompanied by Jacobson's nerve, by passing through the inferior tympanic canaliculus. The artery then ascends over the promontory, anterior to the round window region. In the promontorial part of its course, the caroticotympanic artery or arteries join the vessel. Still in close relationship with Jacobson's nerve the ITA unites with the superior tympanic artery (STyA) in the vicinity of the oval window ¹¹.

On the other side, the STyA arises from the middle meningeal artery – petrosal branch, but typically slightly more proximal and lateral to the origin of the superficial petrosal artery (SPA) above the foramen spinosum. It courses posteriorly and laterally to pass through the canaliculus tympanicus superior (or hiatus of the petrous nerve minor, otherwise named lesser superficial petrosal nerve) into the middle ear, accompanying the homonymous nerve. In the upper part of its canalicular portion it regularly gives origin to an anastomotic branch to the superior branch of the anterior tympanic artery. Near the geniculate ganglion the STyA anastomoses, by a very short branch, with the SPA or with one of its descending branches. The stem of the artery, still closely associated with the lesser superficial petrosal nerve and the contribution of it from the Jacobson's nerve, then descends in the promontorial sulcus and joins the stem of the ITA ¹¹.

Moreover, the SPA arises from the distal petrosal branch of the MMA, courses posteriorly and laterally with the greater superficial petrosal nerve in a groove (sulcus of the superficial greater nerve) to the hiatus of the facial canal. In the sulcus several small branches are given off to the adjacent dura and one larger vessel frequently branches off and crosses over the petrous crest in front of the superior semicircular canal, to enter the posterior fossa and anastomose with the subarcuate artery. In the lateral portion of the sulcus it sends an anastomosis to the neighboring STyA and then divides into two main branches. One enters the facial nerve at the ganglion geniculi and divides into two, one centrally to the internal auditory meatus and the other peripherally following the facial nerve which in turn may anastomose with the stylomastoid artery. The other main branch by-passes the geniculate ganglion, not infrequently sending a short anastomosis to the STyA, then continues down along the facial canal and anastomoses with the stylomastoid artery.

Figure 4.

Simplified drawing showing the described vascular anatomical disposition, topography, relation with adjacent nerves and other structures. ITA-inferior tympanic artery, STyA-superior tympanic artery, SPA-superficial petrosal artery, GSPN-greater superficial petrosal nerve, LPN-lesser petrosal nerve, MMA-middle meningeal artery, ICA-internal carotid artery.

Conclusion

The pharyngo-tympano-stapedial variant, as mentioned in the present case report, is probably the rarest of the variants of MMA origin, where the vessel originates from the cervical ICA. Knowledge of the presence of this variation is important for planning and executing interventions in this region, either open surgical or endovascular due to the relation of the vessel with Jacobson's and facial nerves.