Surgical Techniques for Resection of Vascular Malformations within the Spinal Cord

Spinal vascular malformations (SVMs), including arteriovenous fistulas (AVFs) and arteriovenous malformations (AVMs), are complex and challenging to treat. Advances in microsurgery, endovascular techniques, and neuroimaging have enhanced our ability to treat SVMs safely and effectively. A thorough understanding of spinal cord anatomy, spinal vascular anatomy, and the pathophysiology of these lesions is needed for their proper management.

25.2 Patient Selection

The SVMs represent a group of lesions with various clinical manifestations, anatomical considerations, pathophysiology, and management strategies; therefore, patient selection for treatment is often based on lesion type. To simplify this task, the senior author (R.F.S.) of this chapter, developed a classification system to group these lesions based on their anatomy and pathophysiology and to improve on previously confusing classification systems. ¹ A brief review of this classification system helps guide patient selection.

SVMs can be separated into AVFs and AVMs. AVFs are separated into extradural and intradural types. Intradural fistulas can be either dorsal or ventral. AVMs are divided into extradural–intradural or intradural. Intradural AVMs are further subdivided into intramedullary, which can be compact or diffuse, and conus medullaris AVMs.

25.2.1 Spinal Arteriovenous Fistulas

Extradural Arteriovenous Fistulas

Extradural AVFs involve an extradural radicular artery branch in connection with the epidural venous plexus ( ▶ Fig. 25.1 a). They are rare lesions and are typically treated endovascularly.

Fig. 25.1 Classification of spinal vascular malformations. (a) Extradural arteriovenous fistula (AVF). (b) Intradural dorsal AVF. (c) Intradural ventral AVF showing the fistulous connection between the anterior spinal artery and coronal venous plexus (arrow). (d) Combined extradural–intradural arteriovenous malformation. Illustrations on the left show posterior views; images on the right show the axial plane. (AVM) (e) Compact and (f) diffuse intramedullary AVMs. (g) Conus medullaris AVM. Illustrations on the left show posterior views; images on the right show the axial plane. (Used with permission from Barrow Neurological Institute, Phoenix, Arizona.)

Intradural Dorsal Arteriovenous Fistulas

Intradural dorsal AVFs are the most common type of SVMs. They result from a connection between a radicular artery and the spinal cord venous system ( ▶ Fig. 25.1 b). Patients typically present with progressive myelopathy, which portends a poor natural history. ² These lesions can be treated either endovascularly or surgically. Endovascular treatment is contraindicated if the arterial feeder to the fistula supplies the anterior spinal artery, posterior spinal artery, or a radiculomedullary artery. If the proximal medullary draining vein cannot be accessed, endovascular treatment should not be attempted. Recurrence is possible, and follow-up is important even if the draining vein is penetrated. ³ Advances in endovascular techniques have improved obliteration rates, but even in modern series, failure to achieve complete obliteration and lesion recurrence are challenges. ⁴^, ⁵^, ⁶^, ⁷^, ⁸^, ⁹ Open microsurgery is highly safe and effective and remains the gold standard for complete long-term obliteration. ⁹^, ¹⁰^, ¹¹^, ¹²

Intradural Ventral Arteriovenous Fistulas

Intradural ventral AVFs involve a connection between the anterior spinal artery and an enlarged venous network in the ventral midline of the subarachnoid space ( ▶ Fig. 25.1 c). They can be further divided into three types. Type A involves a single feeder. Type B is intermediate in size. Type C is large and complex with multiple feeders. Patients develop worsening symptoms as the lesion grows due to progression of vascular steal and spinal cord compression. Type A lesions can be obliterated surgically with excellent results. ¹² Although ventrolateral lesions may be accessible through a posterolateral approach, these lesions often require anterior or anterolateral approaches with a team adept at complex spinal approaches and reconstruction. Large lesions are best managed endovascularly. The crucial aspect of managing intradural ventral AVFs is preservation of the anterior spinal artery.

25.2.2 Spinal Arteriovenous Malformations

Extradural–Intradural Arteriovenous Malformations

Combined extradural–intradural AVMs have previously been known as metameric, juvenile, or type 3 AVMs. They are complex lesions with both extradural and intradural components, typically involving the corresponding bone, muscle, skin, spinal canal, spinal cord, and nerve root of a single somite level ( ▶ Fig. 25.1 d). They are formidable lesions in which cure is rarely possible and treatment goals are to optimize neurologic function by reducing mass effect, venous hypertension, and vascular steal. Treatment is primarily by endovascular techniques, and surgery is reserved for decompression of mass effect on the spinal cord and nerves.

Intramedullary and Conus Medullaris Arteriovenous Malformations

Intramedullary AVMs penetrate the spinal cord parenchyma. They can have single or multiple feeders, and the nidus can be either compact ( ▶ Fig. 25.1 e) or diffuse ( ▶ Fig. 25.1 f). Conus medullaris AVMs are considered in a category of their own because of their common location, unique angioarchitecture, and clinical findings. They can have multiple feeding arteries, multiple niduses, and complex venous drainage. They can also have multiple direct arteriovenous connections from the anterior and posterior spinal arteries as well as a nidus. The nidus is often extramedullary and pial based, but an intramedullary component may be present ( ▶ Fig. 25.1 g). Intramedullary and conus medullaris AVMs are best managed with a combination of endovascular and open surgical techniques.

25.3 Preoperative Preparation

A thorough clinical and radiographic evaluation is necessary with any patient known or suspected to have an SVM. These lesions are a cause of reversible myelopathy; therefore, a high index of suspicion for SVMs must be kept, the diagnosis exhaustively investigated, and appropriate treatment rendered. Preoperative signs and symptoms may include spinal or radicular pain and signs and symptoms of myelopathy or radiculopathy. The myelopathy is typically progressive but can be acute if a hemorrhage occurs. The initial diagnostic test is typically magnetic resonance imaging, which may suggest an SVM if edema is present in the spinal cord and flow voids are seen on T2-weighted imaging. Spinal magnetic resonance angiography is becoming an increasingly helpful tool, but digital subtraction angiography remains the gold standard and should be performed in all cases of known or suspected SVM. Digital subtraction angiography not only provides detailed information on the angioarchitecture of the lesion but also allows for possible treatment in the same setting.

Intraoperative neuromonitoring with somatosensory and motor evoked potentials is routinely used. Fluoroscopy is used to help localize the spinal level of the lesion. Preoperative antibiotics and dexamethasone are routinely given. Hypotension and severe hypertension are strictly avoided to ensure adequate perfusion of the spinal cord and avoid hemorrhagic complications. A microscope equipped with indocyanine green technology is optimal for intraoperative imaging.

25.4 Operative Procedure

Most lesions can be exposed from a posterior or posterolateral approach, with extension more laterally and with additional bony removal as needed depending on the lesion. Anterior approaches are occasionally required, and expertise in both the approaches and spinal reconstruction is needed. We prefer to have the exposure include one level above and below the limits of the malformation. For a posterior or posterolateral approach, the patient is positioned prone, the operative level is confirmed with fluoroscopy, and a midline incision is made ( ▶ Fig. 25.2 a). Subperiosteal dissection is performed to expose the posterior elements of the spine as widely as needed for the given lesion and planned approach. Once this exposure is obtained, we prefer to use fishhooks for retraction, because they offer good retraction and a low-profile field, allowing optimal surgical dexterity ( ▶ Fig. 25.2 b). We prefer to perform a laminoplasty rather than a laminectomy, which can be achieved by making troughs on either side of the laminae with either a high-speed drill, the foot attachment of a high-speed drill, or an ultrasonic bone instrument. The laminae are then elevated in one piece for future replacement ( ▶ Fig. 25.2 c). The laminoplasty offers the advantages of preventing thick scarring over the dura, facilitating future operations, protecting the neural elements, and reducing the development of future deformity ( ▶ Fig. 25.2 d).

Fig. 25.2 Artist’s illustration of operative procedures. (a) For posterior and posterolateral approaches, the patient is positioned prone. For cervical lesions, the head is fixed to the bed, and the patient rests on gel rolls. (b) Retraction with fishhooks offers a low-profile field. Troughs are made on either side of the laminae. As shown, the foot attachment of the high-speed drill is used. (c) The laminae are elevated in one piece to allow for future laminoplasty. (d) At the end of the procedure, the laminae are replaced using miniplates and screws. (e) Before dural opening, excellent hemostasis is essential, and a microsuction device is placed to help keep blood and cerebrospinal fluid out of the field. (f) The pial resection technique depicts an intramedullary spinal cord arteriovenous malformation, where the malformation vessels are truncated at the pial surface rather than being chased into the spinal cord parenchyma. (a,f, Reproduced with permission from Barrow Neurological Institute, Phoenix, Arizona. b–e, Reproduced with permission from Spetzler RF and Koos WT (eds). Color Atlas of Microneurosurgery Vol. 3. Intra- and Extracranial Revascularization and Intraspinal Pathology. 2nd ed. New York: Thieme; 2000.)