Time for magnetoencephalography to optimise epileptic neurosurgery

Professor Stefano Seri, consultant clinical neurophysiologist of the Epilepsy Surgery programme at Birmingham Children’s Hospital provides expert insight into Magnetoencephalography’s (MEG) ability to optimise diagnosis and improve paediatric and adult post-surgery outcomes. 

Magnetoencephalography (MEG) has been available to support the diagnosis and pre-surgical evaluation of epilepsy patients for more than 30 years, but in spite of a very successful track record, its uptake has developed at a relatively slow pace. Results from a survey of European clinical research centres as well as a similar study conducted in the U.S., confirmed the inconsistent uptake by clinicians (De Tiege et al., 2017). However, the speed of adoption may be accelerating as the weight of published evidence demonstrates the modality’s utility in the pre-surgical assessment of patients with drug-resistant epilepsy (Foley et al., 2019; Hall et al., 2018). 

Time to reassess outdated assumptions about MEG diagnostic efficiency

• Healthcare’s slow pace in adopting MEG into presurgical evaluation protocols is attributable to outdated assumptions of the technology and its therapeutic value to clinicians as an essential diagnostic tool:
• MEG is more expensive and no more effective than other modalities capable of detecting and localising brain activity.
• Data processing requires an army of in-house or outsourced physicists and engineers, limiting the “ownership” of the test by the clinical team at the point of care.
• The lack of an internationally accepted analysis pipeline and robust standards holding MEG back from becoming a widely available, economically efficient clinical tool.
• Lack of dedicated EEG technologist training programs and certification of and clinical neurophysiologists.
• Complicated healthcare reimbursement bureaucracies that make financial planning difficult for healthcare providers to manage.

For clinicians with a superficial understanding of MEG, it is perceived as an interesting technology, with vague benefits and an unclear return on investment in clinical application.

A Canadian study in 2011 conducted a health technology assessment of MEG in the presurgical assessment of children and adolescents with drug-resistant epilepsy (Hall et al., 2018). The study concluded that using MEG prior to the initial multidisciplinary seizure conference resulted in a shorter time-to-surgical-candidacy decision compared to introducing MEG later in assessment, emphasising the importance of early referral. Although compelling scientific evidence of MEG’s efficacy is widely available, it is not yet formally recommended. That is concerning especially when comparing the clear advantages MEG offers compared to currently approved modalities.

MEG v. EEG: Contending for the gold standard in epileptic pre-surgical evaluation

When weighing the relative diagnostic utility and efficacy of MEG versus an electroencephalogram (EEG), practical aspects of the procedure highlight critical distinctions between the modalities and help elevate the clinical and therapeutic value of MEG scans above scanning alternatives. (Murakami, et al., 2016)

Although high-density EEG recordings (128-256 channels) are less expensive in terms of initial equipment investment, cost per test is not significantly different. For example, electrode caps and the consumables like helium used to record high-density EEGs are expensive, and their performance tends to degrade rapidly after each study and the devices require specific attention in terms of infection control. 

Applying many electrodes to a patient can be challenging, particularly among those with learning or behavioural difficulties. Valuable brain activity can be easily recorded using MEG in patients that categorically refuse to sit for a standard EEG. One of the primary advantages of MEG is that it requires less a-priori assumptions in modelling how brain currents flow inside the brain. Further, over time leading MEG system suppliers have continuously improved scanner designs, engineering in features that simplify the procedure and improve scan results.

MEG in comparison to fMRI

Functional magnetic resonance imaging or functional MRI (fMRI) measures brain activity by detecting changes associated with blood flow. The modality relies on the fact that cerebral blood flow and neuronal activity are closely related. When an area of the brain is activated, blood flow to that region increases proportionally more than the underlying need; fMRI capitalises on this mismatch by measuring the associated Blood Oxygen Level Dependant signal.

In contrast, MEG detects direct changes in neural activity and not as a proxy measure of the BOLD signal. Therefore it is less sensitive to distortion due to altered microvasculature, something not uncommon adjacent to highly vascular lesions in the brain (Wellmer et al., 2009). Even though both modalities have high spatial accuracy, the added time-resolution of MEG offers a distinct advantage in refining diagnosis and determining their prognosis for treatment.

MEG scans offer easier patient access with a much less restricted and limiting patient/scanner interface. This feature again, is particularly relevant for patients in paediatric age – the demographic most likely to benefit from epilepsy surgery. Young patients (as well as many older ones) tend not to tolerate well the enclosed, often claustrophobic MRI scanning environment. 

Evidence of MEG’s efficacy continues to mount

Experience applying MEG imaging to eloquent cortex mapping continues to mount. Researchers are now able to reliably define hemispheric dominance for language with MEG, something that only invasive studies like the Wada test (which establishes cerebral language and memory representation of each hemisphere) or more recently fMRI could accomplish. Accordingly, MEG studies have proven capable of correctly identifying the location of sensory and motor cortices as well. 

With more than 40 years of application experience and thousands of patients investigated, research has unequivocally demonstrated that regardless of the patient group, MEG studies reliably support epileptologists and surgeons in deciding treatment options and in doing so help improve surgical outcomes. 

Time for attending physicians to accelerate MEG’s clinical application

Study results continue to clearly offer clinicians and healthcare providers a firm basis to consider moving MEG up higher in their diagnostic pipeline. Regardless, whether studies are conducted with current superconducting quantum interference device (SQUID) technology or with new and emerging technical advancements (many currently in their infancy), it is up to clinicians and attending physicians to accelerate its clinical application. The procedure’s broader uptake is also heavily dependent on all clinical stakeholders taking ownership of their role in supporting and guiding the design of analysis pipelines, as well influencing the functional aspects of MEG technology to improve scan integrity and simplify the procedure to the benefit of both clinicians and patients.