Intraoperative Imaging Techniques in the Surgical Management of Spinal AV Fistulas

Introduction, diagnosis and treatment: Spinal dural arteriovenous fistula (SDAVF) is a rare disease with an estimated annual incidence of 5–10 per million population mostly affecting middle age and elderly men. The etiology of SDAVF is still unknown, as they are identified as part of heterogenous group of spinal vascular malformations. SDAVF diagnosis can be challenging. The presenting symptoms are often non-specific and consist of sensory and/or motor deficits up to, in a late stage, severe myelopathy with bladder incontinence and sexual dysfunction. This is mostly due to venous congestion and spinal cord edema. The variable disease course and misleading findings can lead to a delayed or missed diagnosis. The definitive diagnostic technique of SDAVF is spinal digital subtraction angiography (DSA); however, magnetic resonance (MR) imaging is the first tool in the diagnostic disease workup. Spontaneous occlusion of a SDAVF is very rare; therefore, treatment is almost always required and consists by disconnecting the draining radicular vein of the fistula at the point of dural attachment (AKA point of fistula, PoF). There are two main options: endovascular treatment and open surgery, alone or in combination. The aim of this chapter is to present the “state of the art” of the intraoperative imaging techniques in the surgical management of SDAVF. Intraoperative imaging techniques for surgical management of SDAVF: Surgical occlusion of the SDAVF is simple and safe. It involves hemilaminectomy/laminotomy, followed by dural opening and identification of the arterialized radicular vein under the microscopic view; then, following the vein to its dural attachment, the PoF should be coagulated and cut or clipped. Effective occlusion of the SDAVF is indicated by an immediate change in color of the arterialized intradural vein(s). However, exact identification of the PoF can be sometimes challenging; moreover, it is crucial to obtain confirmation of the complete fistula occlusion and restoration of normal blood flow to avoid ischemic complications. Intraoperative or postoperative DSA are the “gold standard” to achieve these goals, but they are invasive, expensive and time consuming. For these reasons, in recent times, several intraoperative imaging techniques have been introduced to improve surgical results of SDVAF treatment; these include the following: • Intraoperative indocyanine green videoangiography (ICGV) In the last two decades, intraoperative indocyanine green (ICG) videoangiography (ICGVA) has widely spread as a safe and simple adjunctive tool able to provide real-time information about the precise location of spinal and cranial DAVFs and confirmation of their complete occlusion. The operative field is illuminated by a light source of a wavelength that covers part of the ICG absorption band (700 ± 850 nm, maximum 805 nm). ICG is then injected into a peripheral/central vein line as a bolus. The fluorescence light (780–950 nm, maximum 835 nm) is then recorded by a non-intensified video camera, with optical filtering. Thus arterial, capillary and venous angiography can be observed on the video screen in real-time; semiquantitative measurement of blood flow is possible too. ICGVA can be used several times throughout the procedure. A dedicated microscope is necessary for this technique. • Ultrasound Intraoperative ultrasound (ioUS) associated with contrast enhanced ultrasound (CEUS) have been successfully introduced into oncological and vascular brain and spinal surgery. In literature, the experience on the application of ioUS in spinal SDAVF is extremely limited. The association of standard B-mode imaging, color-doppler US and CEUS represent an effective intraoperative tool to define vascular features of the SDAVF and their anatomical relationship with the spinal cord, allowing to recognize vessels deeper than the only pial surface or in any case, not directly illuminated by the microscope. • Sodium fluorescein More recently, sodium fluorescein video angiography (SFVA) seemed to be a valuable alternative in vascular neurosurgery. SFVA has been explored in aneurysm and cerebral arterio-venous malformation (AVM) surgery, but there are few studies reporting the use of fluorescein in SDAVF. Sodium fluorescein (SF) fluoresces with wavelengths of 520–530 nm in response to excitation with light at 465–490 nm. In practice, this results in yellowish-green fluorescence that can be detected by video angiography. SFVA allows a better visualization of the operative field than ICG, especially concerning very small vessels, and it is a low-cost technique. The main drawback of SFVA is the extravascular leakage of fluorescein, so the best visualization can be obtained with the fist injection and an interval of at least 20 min is necessary between two SF injections. A dedicated microscope is necessary for this technique.