Low-Flow Vascular Malformations of the Orbit: A New Approach to a Therapeutic Dilemma
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* Jonathan S. Lewin

Low-flow vascular malformations of the head and neck present many challenges for the treating physician. Surgical intervention is often fraught with difficulty, with a high potential for bleeding complications, difficult anatomic dissection, and ultimately a high recurrence rate. These drawbacks have restricted the use of a surgical approach alone to a limited subset of small and well-defined lesions. The highest degree of success has been found when low-flow vascular malformations are treated in a multidisciplinary setting. A key element of this collaborative approach has been image-guided sclerotherapy through the percutaneous injection of ethanol or other sclerosing agents. Image-guided sclerotherapy has proved highly effective, with good to excellent results possible in 75–90% of patients (1). As experts in imaging and percutaneous needle placement, radiologists have taken a central role in the multidisciplinary teams at many institutions, and often drive decisions regarding the type of image guidance, sclerosing agent, and staging of therapy. As the range of modalities within the imaging armamentarium has increased, successful sclerotherapy has been performed with fluoroscopy, duplex sonography, CT, and MR image guidance, with the choice of technique based on the location of the malformation, experience of the radiologist, and availability of technology.

Within the spectrum of head and neck low-vascular malformations, which represent a challenging entity at best, treatment of orbital venolymphatic malformations, as described by Ernemann et al in this issue of the *AJNR*, presents a distinct challenge due to the severity of potential complications. In particular, the consequences of inadvertent ophthalmic vein thrombosis may be catastrophic and can lead to orbital compartment syndrome, cavernous sinus thrombosis, and loss of vision. While accurate needle insertion and careful monitoring of sclerosant injection is always important, the margin for error in treatment of orbital and periorbital disease is small, and meticulous care is necessary.

Treatment with sclerotherapy can be broken down into several discrete steps, and each may be best performed with a specific imaging technique. One of the most important stages of the sclerotherapy procedure is the preprocedural evaluation of the patient, requiring careful planning of the safest percutaneous approach to the malformation, definition of the anatomic extent of the lesion, identification of critical adjacent neurovascular structures, and when possible, delineation of venous drainage pathways. MR imaging has become the primary technique for therapy planning. Numerous authors have demonstrated the ability of MR imaging to characterize and delineate vascular malformations of the head and neck (2), and specific aspects of the MR imaging appearance have been shown to have prognostic value with regard to percutaneous sclerotherapy (3). Percutaneous puncture of the malformation can be successfully performed with X-ray fluoroscopy, CT, duplex sonography, or MR imaging. The best puncture-guidance technique for an individual lesion depends on the complexity, depth, and size of the malformation, along with the availability of an acoustic window. The next step in sclerotherapy is estimation of the volume of sclerosing agent required for effective treatment, often performed through the monitored injection of contrast agent until the malformation is filled. The final and most critical step in the sclerotherapy procedure is real-time monitoring of the distribution of the sclerosing agent during injection. In particular, in orbital and periorbital malformations such as those described by Dr. Ernemann et al, drainage via the ophthalmic vein must be carefully assessed, and the treating physician must be ready to stop injection quickly should this avenue for venous egress of sclerosant be identified.

The puncture-guidance phase of the procedure can be difficult for orbital lesions, and the use of image fusion and frameless stereotactic guidance for needle placement along with X-ray fluoroscopic monitoring of the injection procedure, as described in the article in this issue of the *AJNR*, represents a novel solution to the particular challenges of orbital low-flow malformations. Duplex sonography combined with fluoroscopy, a highly successful combination for many head and neck malformations, can be particularly challenging with the complexity, location, and adjacency to bone noted with orbital and periorbital lesions. Dr. Ernemann et al have taken a relatively straightforward technical solution from the operating room and have applied it to one of the more challenging steps in the sclerotherapy procedure in this anatomic location. The spatial accuracy of frameless stereotactic systems, typically around one to two millimeters, is also well suited to the size of the lesion treated. It is possible that the procedure could have been further simplified with the use of MR image data alone, since the size of the lateral orbital wall defect was sufficient to allow easy visualization on the MR images, possibly obviating the need for CT fusion. However, the CT information would likely contribute to the safety and ease of needle placement in lesions with smaller bony defects. The ability of the authors to bring the advantages of MR and CT into the X-ray fluoroscopic suite allowed full advantage of the temporal and spatial resolution of X-ray fluoroscopy for monitoring of potential ophthalmic vein filling, the most critical step in the treatment session.

In contrast to the combination of modalities demonstrated in this current article, recent advances in sclerotherapy have included the modification of a single technique to provide each of these procedural steps. Most notably has been the recent description of MR imaging as a technique for, not only the diagnosis, but also the treatment phase of the sclerotherapy procedure. MR imaging developments have allowed the accurate imaging characteristics typically used for diagnosis and characterization of low-flow vascular malformations to be directly applied for needle puncture, sclerosant volume determination, and sclerosing agent injection monitoring (4), and more recently has been shown to document that a sufficient concentration of sclerosing agent has been attained within the treated malformation during the therapeutic procedure (5). MR imaging has also been shown to monitor temperature within the lesion during treatment, a feature that can be useful when thermal methods of therapy are used for low-flow vascular malformations as an alternative to chemical sclerotherapy (6). However, although real time imaging has steadily been improving with MR imaging and may make a “single-technique” approach feasible for many anatomic sites, the temporal and spatial resolution of these real time MR techniques is still limited as compared with that of X-ray fluoroscopy, and the size and critical nature of ophthalmic vein filling makes this technology less applicable to orbital malformations. The lack of general availability of open MR imaging systems equipped with interventional accessories also provides a barrier to the use of this technology as a stand alone treatment-guidance technique.

In summary, the necessary steps for safe performance of sclerotherapy include precise preprocedural lesion visualization and characterization, accurate needle placement, determination of the correct volume of sclerosing agent for injection, and real time monitoring of venous egress during the injection procedure. In this issue of the *AJNR*, Dr. Ernemann et al have described a novel method that combines the advantages of MR imaging and CT for accurate needle placement with the unsurpassed temporal and spatial resolution of X-ray fluoroscopy for the monitoring of sclerosant injection, and this strategy should be considered for vascular malformation at various anatomic locations that are difficult to approach with ultrasonography or MR imaging guidance alone. Image fusion, frameless stereotaxy, and computer-assisted device guidance are central to the future of image-guided minimally invasive therapy, and the authors are to be commended for bringing this combination into the realm of low-flow vascular malformation therapy.

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