An infratentorial dural arteriovenous fistula mimicking cervical myelopathy: A case report

Dural arteriovenous fistulas may have insidious clinical presentations and are often challenging to diagnose. A small number of cases have been associated with perimedullary venous congestion and cord oedema, mimicking common pathologies, such as cervical myelopathy. We describe a case report of a patient presenting with a constellation of symptoms and radiological signs mimicking C5/6 cervical myelopathy secondary to disc herniation. The patient was managed with anterior cervical discectomy and fusion, with postoperative neurological deterioration unresponsive to steroid therapy. This prompted further investigation of other pathologies. An infratentorial Cognard 5 and Borden type 3 dural arteriovenous fistula was diagnosed on 6-vessel DSA and managed with onyx embolization. Marked improvement of neurological symptoms, notably bilateral lower limb weakness, was achieved postoperatively. In summary, this case demonstrates the importance of considering alternative, less common pathologies that involve the cervical spinal cord when neurological improvement is not achieved following decompressive surgery for cervical myelopathy.


Introduction
Dural arteriovenous fistulas (dAVFs) are rare vascular abnormalities in which posterior and/or anterior circulation arteries fistulate and drain directly into the dural leaflets of veins.They represent 15% of cerebrovascular malformations and are often idiopathic in nature.dAVFs are most found in patients aged between 50 and 60 years old.They can present with a heterogenous range of symptoms including headaches, seizures, pulsatile tinnitus, cognitive decline, and cranial nerve palsies [9].One of the primary concerns with dAVFs is their association with venous hypertension and acute hemorrhage, which may cause significant neurological morbidity or mortality.Higher rates of haemorrhage are seen in the male sex, with studies reporting 60-80% male predominance.
Because dAVFs are often heterogeneous in presentation, they are often difficult to diagnose and differentiate from other pathologies.Of particular relevance to this study, a limited number of dAVFs have been reported to cause cervical cord oedema and mimic cervical myelopathy on presentation [3,8].In these case series, subtle perimedullary venous congestion on MRI was not recognised, and lesions were often treated with steroids for myelitis, with subsequent neurological deterioration.The majority of patients in these case series have involved male patients with supratentorial dAVFs, more commonly involving the transverse-sigmoid junction.
After obtaining patient consent and local ethics approval, we describe a case of a female patient who presented with cervical myelopathy and an MRI demonstrating a large C5/6 disc herniation with associated cervical cord oedema.During the treatment for the presumed lesion, a Borden type 3 dAVF was identified.

Case report
The patient is a 48-year-old female who presented semi-electively for a C5/6 anterior cervical discectomy and fusion (ACDF) operation, indicated for cervical myelopathy.She initially presented with three months of episodic but progressive symptoms, including: neck pain, upper limb paraethesia and fine motor motricity deficits, ataxia requiring a walking stick or wheelchair to mobilise, falls, and intermittent episodes of sphincter dysfunction including urinary urgency and incontinence.
On examination she had an ataxic myelopathic gait, a positive Romberg's sign, inability to squat due to ataxia, 4 beats of clonus bilaterally, hypertonia and hyperreflexia in upper limbs, and a positive Hoffman's sign bilaterally.A preoperative MRI demonstrated a disc protrusion at C5/6 and spinal cord compression and extensive cord oedema extending from the craniocervical junction to C5/C6 [Fig.1.a.].The patient did not have preoperative contrasted MRI of the head.
She underwent an uncomplicated C5/6 discectomy and spinal cord decompression via a Smith-Robinson approach with insertion of a titanium coated PEEK implant (Shoreline ® SeaSpine Carlsbad, California, USA) with cadaveric bone graft (NovaBone, Alachua, Florida, USA).Throughout her surgery, routine spinal cord protective measures were followed which included: checking degree of the patient's tolerable neck extension awake, maintaining a MAP over 85 and 12 mg of dexamethasone at induction.Following extubation, she was examined in recovery with no neurological deficits found.Two hours later, she had an acute neurological deterioration with bilateral lower limb paraparesis with 0/5 power in all lower limb movements, except for 2/5 power in ankle plantarflexion bilaterally.The patient underwent an urgent MRI of her cervical spine to exclude a surgical complication, such as an epidural haematoma.This scan demonstrated satisfactory spinal cord decompression and no unexpected findings [Fig.1.b.].The patient was commenced on high dose steroids (8 mg BD oral dexamethasone) however had minimal improvement in her neurology in the subsequent 48 hrs.The patient remained euvolaemic postoperatively, with no clinical signs of fluid overload and no past medical history of liver, kidney or heart failure, contributing to her postoperative deterioration.
Her preoperative and postoperative imaging was reviewed and discussed at a neurosurgical multidisciplinary meeting.Given the extensive cord oedema present from C5/6 up to the craniocervical junction, the possibility of a dAVF as an additional contributing pathology was raised and discussed.This pathology was also supported by the presence of dilated perimedullary veins at her craniocervical junction and dilated veins in her posterior fossa.An MRI head was subsequently arranged and demonstrated additional features of a posterior fossa dAVF [Fig.She subsequently underwent treatment of her dAVF with transarterial embolisation (Onyx-18 -EV3, Irvine, California, USA) with complete occlusion of the fistula achieved [Fig.2.c. and 2.d.].The posterior fossa dAVF was supplied by the right posterior meningeal artery and left ascending pharyngeal artery.The interventionalist was not convinced that there was supply from the posterior cerebral or superior cerebellar arteries on either side.Heparin was infused at time of embolization.There were no immediate complications from the endovascular procedure, which was performed 9 days following the initial ACDF surgery.Following this treatment, the patient's neurology significantly improved to 4/5 and 5/5 power throughout her lower limbs in the days immediately following embolisation.She was able to return to assisted mobility and was subsequently transferred to an inpatient rehabilitation unit including a period of spinal rehabilitation, with a focus on further improving mobility as well as bowel and bladder retraining.

Discussion
Dural AVFs can occur at various locations and their clinical presentation is heterogeneous, depending on the fistula location and its associated oedema secondary to venous congestion.Posterior fossa dAVFs have a greater tendency to present with an insidious onset.When dAVFs drain into spinal perimedullary veins, the clinical presentation typically involves fluctuating and progressive cervical myelopathy due to progressive cervical cord oedema.Evidence of cord oedema and dilated perimedullary veins is often subtle.Expert neuroradiological review of MRI sequences is therefore valuable for early diagnosis.
As mentioned previously, misdiagnosis is common, and therefore so are delays in treatment of dAVFs.The standard care for alternative diagnoses, such as cervical myelitis, often involves corticosteroid therapy, and there are case reports associating the use of corticosteroids with acute neurological deterioration in patients with a posterior fossa dAVF and spinal cord oedema.This may be due to transient fluid retention and increased venous hydrostatic pressure or procoagulant properties associated with steroid use, as theorised in some case series [4][5][6][7].
Dural AVFs are treatable vascular lesions.The current gold standard treatments include: endovascular approaches, surgery, or stereotactic radiosurgery which is usually considered a salvage therapy, with the treatment being individualized to the anatomy of the lesion [1,2].Onyx™ (Medtronic, USA) embolisation involves the use of a non-adhesive viscous substance that allows for deeper vessel penetration and occlusion, thus achieving embolisation of complex multiple feeders [1].This technique has been proven to be very successful in higher grade dAVFs.In this case, Onyx-18 transarterial embolisation was the chosen approach to treat this high grade dAVF, resulting in complete occlusion and marked neurological recovery despite a delayed diagnosis.Follow up imaging to confirm dAVF obliteration is planned.

Conclusion
This study illustrates a rare case with a patient presenting with neurological symptoms secondary to dual pathology including discogenic cervical myelopathy and cervical spinal cord oedema secondary to a posterior cranial fossa dAVF.It highlights the importance of careful consideration of the specifics of the patient's history, examination, and imaging.In retrospect, the pattern of episodic neurological weakness in this case combined with the atypical distribution of cord oedema beyond the level of stenosis should raise suspicion of alternative pathologies such as the insidious presentation of a posterior fossa dAVF.Patients who undergo successful decompressive surgery are usually expected to demonstrate either neurological stability or early signs of neurological improvement post operatively [1].Although this patient had an acute neurological deterioration which prompted further investigations, alternative diagnoses should also be considered in patients who deteriorate following decompressive surgery.

Fig. 1 .
Fig. 1.Pre-and Post-Operative MRI.a. Pre-operative cervical spine MRI -T2 weighted sagittal and axial images demonstrating a large C5/6 central disc herniation with cord compression and signal change.b.Post-operative MRI of the cervical spine -T2 weighted sagittal, SWI axial and STIR sagittal images demonstrating decompression and no new compressive haematoma.c. Posterior fossa dAVF demonstrated on MRI T1 post gad MPRAGE axial and sagittal images.
1.c.].To further characterize the lesion, the patient underwent a 6-vessel cerebral digital subtraction angiogram that demonstrated a dAVF supplied by the right posterior meningeal artery and left ascending pharyngeal artery [Fig.2.a. and 2.b.].The fistula site was found to lie adjacent to the inferior margin of the tentorium cerebelli with rapid shunting into the inferior vermis vein.The fistula also congested the anterior and posterior spinal veins, with dilation of these veins into the thoracic segment.It was consistent with a Cognard 5 and Borden type 3 arteriovenous fistula [Fig.2.a. and b.].

Fig. 2 .
Fig. 2. Six Vessel Digital Subtraction Angiogram and Complete Occlusion of dAVF following Onyx Embolisation.a. Left image.Lateral projection right vertebral injection demonstrating supply of the dAVF through right posterior meningeal artery.Right image.Lateral projection right vertebral injection demonstrating supply via the left ascending pharyngeal artery.b.Left image.Lateral projection right vertebral injection.Right image.Anterior projection.Both images demonstrating the venous outflow into the inferior vermis vein with an associated aneurysm.In addition, there is congestion of the perimedullary spinal veins.c.Left image.Lateral projection right vertebral injection.Right image.Anterior projection right vertebral injection.Both images demonstrating no early venous drainage into the previously demonstrated inferior vermis vein.d.Left image.Lateral projection right vertebral injection.Right image.Anterior projection right vertebral injection.The previously demonstrated venous congestion has resolved.