Discussion

There have been no documented reports of a TCR-mediated vagal response during DMSO injection via a percutaneously placed needle. In both our cases, the TCR was activated during the percutaneous injection of DMSO into a JNA tumor, before the embolization procedure with Onyx. Previous reports have shown that TCR occurs during physical manipulation of structures surrounding the face, orbit, cranial base, and brain. Examples include craniomaxillofacial operations following trauma, periorbital surgery, strabismus surgery, endoscopic sinus surgery, pituitary surgery, tumor resections in the cerebellopontine angle or falx cerebri, and percutaneous procedures on the trigeminal ganglion.2 5–8 More recently, asystole has been described during the trans-arterial embolization of a dural arteriovenous fistula with Onyx. In that report, Lv et al hypothesized that the direct compression of the nervus spinosus in the foramen spinosum secondary to physical distension of the middle meningeal artery after Onyx injection triggered the TCR pathway.3 In our cases, the percutaneously placed needle was in the proximity of the foramen ovale, where the mandibular division of the trigeminal nerve exits the skull base. Thus, we hypothesize that the injection of DMSO directly stimulated this nerve, triggering the TCR. All vital signs preceding DMSO injection were stable.

We feel that the TCR was triggered by the injection of DMSO, and was not due to the barometric stress from the injection since this has not been seen in our experience with the percutaneous injection of contrast during the parenchymography or the injection of Onyx. According to Newton's equation the resulting shear of a fluid is directly proportional to the force applied and inversely proportional to its viscosity. In the two cases described DMSO, contrast, and Onyx were injected through a 1 ml syringe. Thus applying a similar force during each injection. Furthermore, the three fluids have a similar viscosity. Therefore the shear of the fluid should be roughly the same for the percutaneous injection of each material.

DMSO is an organic solvent used to prevent premature solidification of Onyx during embolization procedures. The neurotoxicity of DMSO has been an area of controversy, and it is conceivable the TCR in our cases occurred owing to a neurotoxic effect of DMSO. However, one study has shown that the use of DMSO with Onyx during the endovascular embolization of intracranial aneurysms failed to produce arrhythmias or decreases in heart rate and blood pressure in 39 patients.9 Thus, in our cases, the TCR was probably triggered by the forces generated during the process of DMSO injection.

As described by Schaller, the afferent pathway of the reflex arc is initiated by the stimulation of a sensory branch of the trigeminal nerve, which then transmits the signal to the sensory nucleus of the trigeminal nerve via the Gassarian ganglion.10 The afferent path then continues through the short internuncial nerve fibers of the reticular formation to synapse in the dorsal motor nucleus of the vagus nerve. The branches of the vagus nerve, which supply parasympathetic innervation to the heart, blood vessels, and abdominal organs, comprise the efferent pathway of the reflex arc.

While the TCR is unlikely to be lethal, the ensuing hemodynamic compromise warrants quick recognition by the anesthesiologist so that proper management can take place. The onset of the TCR is acute in nature and should resolve once surgical manipulation ceases. The use of local anesthetics before the procedure may prevent activation of the reflex, but once the reflex is encountered, the procedure should be halted temporarily while vagolytic drugs (eg, atropine, glycopyrrolate) or sympathomimetics are administered.11 Risk factors for developing TCR may include hypercapnia, hypoxemia, light general anesthesia, young age, prolonged and/or vigorous manipulation of the surgical site, potent narcotics such as sufentanil and alfentanil, and the use of certain β blockers and calcium channel blockers.12