Transcranial ultrasound for arteriovenous malformations: something old is new again.

The concept of ionizing radiation to treat arteriovenous malformations was devised in 1914 (3). At that time, Vilhelm Magnus, a neurosurgeon from Norway, used radium therapy to treat a patient with a large arteriovenous malformation (AVM) located in the motor cortex. Two years later, Magnus found that the patient’s seizures ceased. He remarked that “In this case radiotherapy was more lenient than the knife” (3). Since then, thousands of patients with cerebral arteriovenous malformations (AVM and dural arteriovenous fistulae [dAVF]) have undergone stereotactic radiosurgery. Most of the vascular malformations obliterate within 3 to 4 years after radiosurgery. In the interim, patients are typically followed with serial magnetic resonance imaging (MRI) performed at 6-month intervals. Once the vascular malformation seems to have disappeared on the MRI, a diagnostic angiogram is usually performed to confirm obliteration. The interval between radiosurgery and angiographically confirmed obliteration is filled with some degree of uncertainty and anxiety for patients and clinicians alike. The risk of hemorrhage appears unchanged until complete obliteration is achieved. However, from a clinical perspective, we know that hemodynamic changes are occurring during the timeframe between treatment and obliteration. Favorable hemodynamic changes may be accompanied by improvements in symptoms such as tinnitus for patients with dAVF or seizures in patients with AVM. Unfavorable hemodynamic changes may be accompanied by clinical worsening. Dr. Jo and colleagues are to be commended for their research using transcranial Doppler (TCD) evaluations to track hemodynamic changes in AVMs treated with Gamma Knife radiosurgery. They demonstrated favorable hemodynamic changes and eventual obliteration of unruptured AVMs fed by the middle cerebral artery or its major branches. Standard TCD technology would likely confine this approach to middle cerebral artery fed AVMs and, thus, prevent its use for AVMs with anterior cerebral artery, posterior cerebral artery, or posterior circulation feeding vessels. In those AVMs studied, they observed a decrease in the mean blood flow velocity and an increase in the mean pulsatility index correlated with diminished flow and, later, complete obliteration. Although TCD evaluation should not be used in place of MRI or cerebral angiography, it can be used as an adjunct to the postoperative radiographic follow-up of selected patients with AVM. It may also provide valuable data for modeling of AVM and dAVF hemodynamics before and after stereotactic radiosurgery. Such hemodynamic measurements could be useful for development of predictive modeling related to the treatment of patients with AVMs and dAVFs undergoing radiosurgery, with or without embolization. Ultrasound has long played a role in neurosurgery. In fact, Lars Leksell was introduced to high frequency ultrasound by a neurosurgical colleague, Peter Aron Lindstrom. In 1954, Lindstrom explored the use of high frequency ultrasound for lesioning as an alternative to the then popular lobotomy procedure (2). Leksell went so far as to develop a specifically adapted frame to mount an ultrasound transducer (2). Unfortunately, Leksell could not circumvent the intact cranium and sufficiently focus the ultrasound as a therapeutic tool. He abandoned this energy source and eventually turned to ionizing radiation from the Gamma Knife to treat intracranial pathology including AVMs. He did continue to use ultrasound to perform evaluations of patients in the emergency department in an attempt to identify a mass lesion or intracranial hematoma (2).