Dural arteriovenous fistula (dAVF) is defined as an abnormal connection between an artery and vein within or around the dural mater [8]. It is generally recognized that if there is cortical venous reflux (CVR) or venous ectasia, the risk of hemorrhage is high, thus requiring treatment [2,8]. Among them, middle meningeal arteriovenous fistulas (MMAVFs) are a condition occurring in locations such as the middle cranial fossa, induced by an abnormal connection between the middle meningeal artery (MMA) and surrounding veins, most commonly resulting from head trauma [4]. Additionally, MMA aneurysm is also known to be associated [11]. Due to its high risk of bleeding and potential manifestations such as intraparenchymal hemorrhage, epidural hematoma, and some cases of non-aneurysmal subarachnoid hemorrhage, treatment is usually necessary, with embolization via trans-arterial approach being feasible in most cases.1) Complete obliteration can be achieved with polyvinyl alcohol, coil, n-butyl-2-cyanoacrylate (NBCA), or combinations of them [1,3,10,20,22]. Spontaneous MMAVFs without head trauma history are extremely rare, with only 13 cases reported in English-language literature [1,3,12,14,15,17,19,21]. Although associations with MMA aneurysm or segmental arterial mediolysis have been suggested, the exact mechanism remains unclear [11,15,19]. Here, we present a case of spontaneous and simultaneous bilateral MMAVFs, its radiologic features, treatment process, and hypotheses regarding its etiology.

A 37-year-old female patient was admitted with persistent pulsatile tinnitus. She had been experiencing dizziness for the past year, along with headache responsive to medication. There was no history of head trauma, and physical examination showed no exophthalmos, bruit, chemosis, or cranial nerve palsy. On Time-of-flight (TOF) source images, abnormally dilated vessels with high signal intensity were identified along both MMAs, extending from the foramen spinosum (Fig. 1). Fortunately, T2-weighted magnetic resonance imaging (MRI) demonstrated no evidence of abnormal cerebral venous congestion. Similarly, TOF-MRA revealed abnormal vascular signals along both MMAs (Fig. 2), and gadolinium-enhanced MRA demonstrated consistent findings (Fig. 3A, B).

Subsequently, performed brain computed tomographic angiography (CTA) demonstrated significant thinning of the skull along the meningeal grooves above the foramen spinosum bilaterally (Fig. 4). Digital subtraction angiography (DSA) confirmed abnormal connections between the MMA and the middle meningeal vein (MMV) on both sides, and the fistulous sacs measured 10×21 mm on the right and 7×22 mm on the left (Fig. 5A, B, C, D). A distinct venous drainage pattern through the pterygoid venous plexus was observed on the left, while no definite outflow route was recognized on the right (Fig. 5C, D). No contribution from the internal carotid artery (ICA) was observed.

Trans-arterial coil embolization of the bilateral MMAVFs was planned. Upon readmission six days later, the patient reported marked improvement of tinnitus, and angiography revealed spontaneous closure of the right-sided fistula, which likely explained the symptomatic improvement. Endovascular treatment was subsequently performed for the left-sided lesion. Under general anesthesia, a 6-Fr Chaperon guiding catheter (MicroVention Terumo, Tustin, CA, USA) was advanced into the left external carotid artery (ECA) and superselective catheterization of the MMA was achieved using a Headway 17 microcatheter (MicroVention Terumo, Tustin, CA, USA) (Fig. 6A, B). The fistula sac was packed beyond the damaged MMA wall, followed by partial embolization of the MMA to achieve complete obstruction of abnormal connection (Fig. 7A, B). Post-procedural angiogram showed complete obliteration without evidence of CVR (Fig. 8). The patient discharged in good condition, without residual symptoms or neurological deficits.

According to the studies conducted by Freckmann et al., the incidence of traumatic MMAVF is estimated to be approximately 1.8% [4]. However, spontaneous MMAVFs occurring without head trauma are known to be very rare, with only 13 cases reported worldwide [1,3,12,14,15,17,19,21]. Among these cases, simultaneous bilateral occurrences and even detection before inducing bleeding, as seen in this case, appear to be the first case ever. The simultaneous occurrence without trauma suggests the possibility of a more substantial influence of genetic factors. However, the patient reported no significant family history related to cerebrovascular diseases or connective tissue disorders. Thrombosis, a common mechanism of dAVF development, is also speculated to have contributed in part [6]. On CTA, significant thinning of skull along the meningeal groove above the foramen spinosum was noted, and this could serve as indirect evidence suggesting that the onset of the lesion likely occurred several months to years prior, with symptoms possibly manifesting later as shunt flow increased over time.

An intriguing aspect of this case is the disappearance of the MMAVF on one side without any additional interventions such as medication or external manual carotid compression. Some dAVFs demonstrate a natural course of spontaneous closure. This spontaneous closure tendency is more common in cases where there is a single or limited number of feeding arteries, atypical venous drainage routes, occurrence of sinus thrombosis, occlusion following diagnostic angiography, absence of CVR, or when intrinsic pressure is exerted on the fistula within the dural sinus [3,9,13,16]. Spontaneous closure also tends to occur more frequently within three months from the initial diagnosis [9]. In this case, performing a selective ECA angiogram may have induced some degree of thrombosis, potentially contributing to the observed closure. Recent studies have also shown the association between segmental arterial mediolysis and MMAVF, prompting concurrent intraperitoneal vascular evaluation during cerebral angiography, although no specific findings were noted [19].

Similar to cerebral arteries, the medial defect observed in the intracranial MMA would significantly contribute to the development of MMA aneurysms, regardless of trauma [7]. If an MMA aneurysm resulting from this defect ruptures into surrounding veins or sinuses, MMAVF can occur without trauma. In such cases, it is important to consider non-invasive tests like TOF-MRA to easily detect MMAVF in patients presenting with persistent pulsatile tinnitus [18].

Idiopathic MMAVFs themselves are rare, with an even lower probability of being discovered without concomitant intracranial hemorrhage. Only three cases have been reported without hemorrhage, while the rest were associated with bleeding [12,19]. Therefore, in cases of incidentally discovered MMAVFs in patients complaining of tinnitus, considering treatment is generally warranted, and if cortical venous reflux is present, treatment is deemed necessary for safety [1]. Treatment of MMAVFs can be safe and effective in most cases through a trans-arterial approach, irrespective of the underlying etiology [1,12,15,17,18]. While a trans-arterial approach is feasible in the majority of cases, instances where the MMA is tortuous and difficult to access may benefit from using a dilute mixture of NBCA to increase the likelihood of achieving complete obliteration [1]. Embolization can be attempted using coils, liquid embolic agents like Onyx, Phil, NBCA, or a combination of them, and it is generally considered safe. Given the challenges associated with advancing devices through the pterygoid venous plexus, trans-arterial embolization is typically prioritized over a trans-venous approach. Surgical or radiation therapy may also be considered in certain cases [5].

It is imperative to bear in mind that MMAVFs are a rare condition and one of the potential causes of persistent pulsatile tinnitus. TOF-MRA is simple, helpful, and non-invasive test for detecting MMAVF, which can also cause cerebral hemorrhage. Moreover, treatment can be safely and effectively achieved through a trans-arterial approach. While spontaneous closure is occasionally observed, it is not easily predictable, and considering treatment, especially in cases accompanied by cortical venous reflux, looks safe.