Medicine
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MRI-guided Focused Ultrasound Thalamotomy for Patients with Medically-refractory Essential Tremor
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Summary December 13th, 2017
High-intensity MRI guided focused ultrasound is an emerging noninvasive technique to precisely ablate brain tissue. It has been shown to be safe and effective in treating medically-refractory essential tremor. This article describes the protocol for thalamotomy from patient selection to equipment setup to post-treatment follow-up.
Transcript
The overall goal of this magnetic resonance guided focused ultrasound is to treat patients with medically refractory essential tremor by thermal ablation of the ventral intermediate nucleus of the thalamus. This method will help us answer key questions in the field of neuroscience about less invasive treatments for treatment refractory essential tremor. The main advantages of this technique are that it's less invasive, feedback is immediate and possibly more cost-effective than other open neurosurgical approaches.
Implications of this technique extends to treatment of other movement disorders such as Parkinson's disease. Our experiences with MR guided focus ultrasound can provide insight about motor circuits inside the brain. Multidisciplinary teamwork is important for a successful treatment.
The patient should be seen by a movement disorder specialist to determine the suitability for this procedure. Before beginning the procedure, completely shave the head of the patient and check for scalp lesions. Have a neurosurgeon put the stereotactic frame in place and inject local anesthetic at the four pin sites.
Place a rubber diaphragm on the patient's head to prevent leakage of the degassed water that circulates between the ultrasound transducers and the scalp and check the patient for metal pieces. Have the patient lie head first in the ultrasound helmet in the supine position and cover the patient with a warming blanket. Give the patient a stop button to allow pausing of the procedure at any time.
Then, after performing a preliminary 3D localizer magnetic resonance scan and a t2 weighted sequence, contour any lesions on the scalp, calcifications in the brain or sinuses and air volumes as no-pass zones, so the transducers can avoid these specific areas. You have to customize the lesion to the patient. The width of the third ventricle may vary widely in elderly patients, and you have to be mindful of the internal capsule as you extend the lesion laterally.
To select the target, start the anterior posterior commissure line at 25 percent of the intercommissural distance anterior to the posterior commissure, locating the target midway between points 14 millimeters lateral to the midline and 11.5 millimeters lateral to the lateral edge of the ventricle. To perform a test sonication, raise the transducer temperature in the target region to 45 degrees Celsius and verify the alignment of the heated volume to the expected target. Correct the alignment in all three dimensions and raise the temperature to approximately 50 degrees Celsius.
Adjust the target according to the patient's responses and increase the energy in 0 to 2 degree increments to make a permanent lesion at the target around 55 to 60 degrees Celsius. You have to adjust the target according to the patient's real-time responses. A knowledge of neuroanatomy is critical.
Obtain a second t2 weighted MRI to assess the lesion size and any evidence of adverse events such as hemorrhage. If the results are satisfactory, help the patient out of the scanner and remove the frame. After treatment, assess the patient's hand tremor.
Admit the patient to a post surgical unit overnight for observation and perform a third MRI the next morning to confirm the presence of the lesion and to rule out any adverse events followed by a full neurological exam as outlined in the table. The success of the treatment can be immediately evaluated after sonication, through the radiologic observation of the lesion at the Vim as well as the patient's performance on clinical measures such as the Clinical Rating Scale for Tremor or CRST. The Archimedean spiral is a part of the CRST used to measure the severity of the essential tremor and to follow patients'improvement after treatment.
The CRST also requires the patient to complete several line drawings which should also demonstrate significant improvement after treatment. With experience this procedure can be performed in between three to four hours. While performing this procedure it's important to remain in constant communication with both the patient and the rest of the team.
After watching this video you should have a good understanding of how a team works together to perform a MR guided focus ultrasound thalamotomy for essential tremor. Focus ultrasound thalamotomy is an effective less invasive surgical procedure for patients with refractory essential tremor, that reduces the risks associated with open neurosurgical procedures. Researchers are also investigating the use of focus ultrasound in other neurologic and psychiatric disorders such as Parkinson's disease and obsessive compulsive disorder.
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