Relevant surgical anatomy

Initial investigation with CT-angiography and MRI revealed a vascular lesion ventral to the C6 spinal cord level (Figs. 1 and 2). Further preoperative characterization of the lesion using spinal angiography confirmed the presence of an arteriovenous fistula, which was confined within the dural root sleeve and supplied by a radiculomeningeal artery with no anterior spinal artery (ASA) involvement (Fig. 3). Given this latter characteristic, the lesion most resembled a type I AVM as an intradural fistula fed by a radiculomeningeal branch of the right vertebral artery. However, it consisted of a drainage into the ventral venous plexus rather than dorsal (Fig. 4). Traditionally, the presence of this ventral drainage would have classified the fistula as type IV AVM, but this was a notable exception since there was no involvement of the spinal artery (Table 1) [3, 4]. Prompt recognition of this unique vascular anatomy was important in deciding the best treatment approach for this patient.

Fig. 1
figure 1

Sagittal (a) and coronal (b) reconstructions of admission CT-Angiography displaying dilated vascular structure ventral to the spinal cord with an intranidal aneurysm versus varix

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Fig. 2
figure 2

Sagittal projections of admission MRI. (a) T1-weighted image displaying abnormal flow voids ventral to the spinal cord. (b) T2-weighted image displaying ventral flow voids and signal change in the spinal cord

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Fig. 3
figure 3

Diagnostic cerebral angiogram. Right vertebral artery injection revealing a dominant feeding artery to the AVF. A radicular artery contributing to the ASA (white arrow) is seen one cervical level above the fistula

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Fig. 4
figure 4

Spinal AVM consistent with a dural AVF with ventral venous drainage and no involvement of ASA

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Table 1 Spetzler [5] classification of spinal vascular malformations with corresponding ABF classification based on description of spinal AVM in parentheses
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Description of technique

Because the lesion was located within the dural root sleeve, a posterior approach with ipsilateral C5–C7 laminectomy, partial C5–C6 facetectomy, and a durotomy was performed to expose the C7 nerve root. Once the arterialized vein was identified ventral to the spinal cord, Doppler ultrasound assisted in confirming the arterialized flow. A temporary clip was initially placed and the Doppler ultrasound showed absence of arterial flow distal to the clip, again verifying the fistula. After no changes in neuromonitoring were observed with the clip in place for 20 min, a permanent clip was used to occlude the fistula. On postoperative day 1, CT-angiography was obtained demonstrating complete obliteration (Fig. 5). At 6-month follow-up, the patient showed signs of functional recovery.

Fig. 5
figure 5

Postoperative sagittal (a) and coronal (b) CT-angiography showing no further filling of the fistula

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Indication

The closer resemblance of the fistula to type I AVM argued for a simpler management with surgical disconnection [3, 7]. For intradural AVFs, microsurgical technique offers a more reliable visualization and direct access to the fistula point compared to an endovascular approach, and successful obliteration can be achieved in 94–100% of cases. In comparison, endovascular obliteration is associated with higher risks of treatment failure and recurrence, as well as major complications afterwards [1, 4, 5, 8, 9]. As this and previously described cases have demonstrated, a posterior cervical approach with lateral extension allows for adequate exposure and access to lesions located on the ventral/ventrolateral spinal cord [2, 6, 8].

Limitations

Surgical treatment is not without risk, including perioperative blood loss, pseudomeningocele, spinal instability, or new or worsened postoperative neurological deficits. In contrast, although endovascular embolization might confer better short-term outcomes, it is associated with higher recanalization rates and sufficient data on its efficacy is still lacking [3, 4, 7, 8].

How to avoid complications

During the procedure, care should be taken to preserve the branches of ASA when occluding the fistula. In addition, there is a possibility of cerebrospinal fluid leak due to the interruption of the dural sleeve, which can be controlled, for instance, using fat graft over the resected area. Each of these intraoperative measures helps to ensure the success of the procedure and minimize the risk of complications [4, 9].

Perioperative considerations

Spinal angiography remains the gold standard for diagnosis of an AVF, which should be obtained preoperatively to identify and accurately delineate its complex vascular anatomy. Postoperative spinal angiogram is important in confirming complete occlusion of the fistula [4, 5]. At subsequent follow-ups, MRI can be useful in assessing signs of recovery, generally seen within 6 months of treatment [9].

Information for patients

It is important for patients to be aware that despite the high rates of complete obliteration of the AVF with surgical intervention, functional outcomes are more variable, in particular, depending on the severity of the initial neurologic impairment. Nevertheless, improved long-term outcomes can be achieved with either stabilization of preoperative symptoms or recovery of previous neurologic dysfunction [4, 8].

Summary

  • Dural arteriovenous fistulas represent the vast majority of cases of spinal vascular lesions.

  • Spinal AVFs form abnormal communications between the arterial and venous systems and are associated with severe neurological dysfunction.

  • The American/British/French and Spetzler classification systems are widely accepted to characterize spinal AVMs.

  • Anatomic variants of spinal AV fistulas still exist, adding to the complexity in identifying and managing this condition.

  • Magnetic resonance imaging or angiography and computed tomography angiography are initial studies that can help in identifying the lesion.

  • Spinal angiography remains to be the diagnostic test of choice for spinal vascular lesions.

  • For spinal AVFs, surgical repair allows for better obliteration rates than endovascular treatment.

  • The success of surgical intervention depends on the location and precise anatomy of the lesion, surgeon’s expertise, and preoperative neurological status of the patient.

  • Prompt identification and recognition of the anatomic anomalies of spinal AVFs impact the surgical technique and clinical outcome for patients.

  • Modification to the traditional classification scheme should be considered given that a thorough understanding of the anatomy of spinal vascular lesions is paramount in determining approach to treatment.

Patient consent was obtained for use of patient’s clinical information, including patient consent to submission of the case report.