Monorail snare technique for the retrieval of an adherent microcatheter from an onyx cast: technical case report. 
Microcatheters retained after Onyx (eV3 Neurovascular, Inc., Irvine, CA) embolization represent a potential source of thromboembolic complications.  Catheter retention depends on the degree of Onyx reflux and vessel tortuosity.  To overcome this problem, we have adapted a previously described monorail snare technique for stretched coils to remove an adherent microcatheter from the occipital artery during Onyx embolization of a dural arteriovenous fistula.  We used this technique successfully in a 62-year-old man with a posterior fossa dural arteriovenous fistula.  An Echelon-10 microcatheter (eV3 Neurovascular, Inc.) system became adherent in the right occipital artery because of reflux and vessel tortuosity.  Significant stretching of the microcatheter was observed during attempted removal.  A 2-mm Amplatz Goose Neck microsnare (Microvena Corp., White Bear Lake, MN) was placed through a Rapid Transit microcatheter (Cordis Corp., Miami, FL).  The hub of the indwelling Echelon microcatheter was cut off and the snare advanced over the outside of the microcatheter.  The snare and Rapid Transit microcatheter were then advanced into the guiding catheter (6-French) as a unit over the indwelling Echelon microcatheter.  Using the adherent Echelon as a "monorail" guide, the snare and Rapid Transit microcatheter were advanced distally into the occipital artery and the snare was retracted to engage the microcatheter.  The microcatheters and snare were then easily removed because of the second vector of force placed by the snare system on the adherent microcatheter very close to the point of adherence.  The monorail snare technique represents a simple and safe way to remove an adherent microcatheter from an Onyx cast during the embolization of dural arteriovenous fistulas.  Prospective knowledge of this technique will facilitate more aggressive embolization without the reservation that a retained microcatheter could require surgical removal or anticoagulation.