Laser Therapy of Epilepsy
Robert Watson1

1Mayo Clinic, United States

Synopsis

Epilepsy cannot be controlled by medications in 30-40% of patients, and in these patients, resection of the epileptogenic focus may be the only hope for a cure. Traditionally, epilepsy surgeries involve craniotomies with resection of substantial portions of the brain. MR-guided laser interstitial thermal therapy (MRgLITT) provides a minimally invasive alternative. MRgLITT is based on stereotaxic image-guided placement of a small laser fiber into the target and subsequent monitored heating and ablation of the epileptogenic lesion. The technique and advantages, as well as concerns and ways to mitigate them, will be discussed.

Target audience

Physicians, physicists, and others interested in minimally invasive MR guided therapy for treatment of medically refractory epilepsy.

Outcome / Objectives

Epilepsy cannot be controlled by medications in 30-40% of patients, and in these patients, resection of the epileptogenic focus may be the only hope for a cure. Traditionally, epilepsy surgeries have often included sizable craniotomies with resection of substantial portions of the brain, with significant associated morbidity.

MR-guided laser ablation techniques may have distinct advantages over traditional resections, particularly in regard to the discreteness of the lesions, and the ability to spare adjacent non-epileptogenic tissues.

Purpose

The presentation will provide background in clinical epilepsy, and identify the brain lesions that most commonly are epileptogenic. Conventional surgical treatments, including temporal lobectomy will be described. These approaches will be contrasted with MR-guided laser interstitial thermal therapy (MRgLITT), with its MRI basis described. How the procedure is performed will be reviewed. Several issues and complications that have been associated with MRgLITT for treatment of epileptogenic lesions will also be described, as will the responses to these concerns and efforts to mitigate them.

Methods

MRgLITT is based on stereotaxic image-guided placement of a small caliber, < 2mm, laser fiber into the target. The fiber contains a circulating fluid-based cooling catheter. Proton resonance frequency mapping, in which there is phase shift associated with temperature, is the basis by which the temperatures resulting from laser heating can be monitored, and the anatomical extent of the temperature spread assessed. Current commercially available laser ablation platforms permit “stops” to be placed on nearby structures that are intended to be spared. Target temperatures when structures are being ablated range from 44-59 degrees Celsius, with the completion of tissue death being time dependent. At 60 degrees and above, there is instant denaturation and coagulation of cell components. At and below 43 degrees, thermal damage does not occur regardless of the time of exposure.

Results

The most common epileptogenic target using laser ablation techniques is a sclerotic hippocampus and adjacent amygdala in the context of mesial temporal sclerosis. Other epileptogenic lesions, including malformations of cortical development and hypothalamic hamartomas, also lend themselves to laser ablation. Seizure freedom following amygdalohippocampal MRgLITT lesions in the setting of mesial temporal sclerosis are slightly less than for surgical temporal lobectomy. Data is being accumulated whether MRgLITT has benefits versus conventional temporal lobectomy in terms of memory and cognitive preservation. Visual field cuts are a relatively common outcome of MRgLITT for mesial temporal sclerosis presumed to be due to affecting components of Meyer’s loop.

In early 2018 the FDA released a letter to providers cautioning about possible tissue overheating due to inaccurate MR thermography displays on commercial platforms. Additional education and training materials were made available to mitigate issues related to magnetic field changes or “drift” over time. Cautions were raised also about inaccuracies resulting from relying on thermography data in proximity to unstable pixels or when images are subject to motion. To mitigate against the possibility of an underestimated thermal damage estimate, lower laser power and slower tissue heating is viewed as advantageous. Limiting the number of imaging planes, if possible, is also advantageous as this permits more real time assessment of heating.

Discussion

Due to its minimally invasive nature, particularly when compared to conventional neurosurgery, MRgLITT has provided another means to ablate epileptogenic lesions in the setting of medically intractable epilepsy. With increasing experience, the degree of seizure freedom achieved by these therapies vs more conventional open surgical procedures can be assessed. Also, whether there is better preservation of cognitive, memory, and visual function with MRgLITT remains a topic of great interest for best patient care.

Conclusion

Minimally invasive MRgLITT holds great promise for care of patients with medically intractable epilepsy. Patients are typically released from the hospital the next day after observation. In addition, there may be advantages to sparing of cognitive, memory, and visual function with these approaches. It is anticipated that with further experience, additional clinical scenarios will be found to be amenable to MRgLITT treatment. This will reinforce the central role of the radiologist in these MR-guided procedures.

Acknowledgements

No acknowledgement found.

References

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2. Grewal SS, Zimmerman RS, Worrell G, et al. Laser ablation for mesial temporal epilepsy: a multi-site, single institutional series. J Neurosurg. 2018:1-8.

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6. Youngerman BE, Oh JY, Anbarasan D, et al. Laser ablation is effective for temporal lobe epilepsy with and without mesial temporal sclerosis if hippocampal seizure onsets are localized by stereoelectroencephalography. Epilepsia. 2018;59(3):595-606.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)