Effect of Anatomical Variability on Electric Field Characteristics of Electroconvulsive Therapy and Magnetic Seizure Therapy: A Parametric Modeling Study

Abstract

Electroconvulsive therapy (ECT) and magnetic seizure therapy (MST) are conventionally applied with a fixed stimulus current amplitude, which may result in differences in the neural stimulation strength and focality across patients due to interindividual anatomical variability. The objective of this study is to quantify the effect of head anatomical variability associated with age, sex, and individual differences on the induced electric field characteristics in ECT and MST. Six stimulation modalities were modeled including bilateral and right unilateral ECT, focal electrically administered seizure therapy (FEAST), and MST with circular, cap, and double-cone coils. The electric field was computed using the finite element method in a parameterized spherical head model representing the variability in the general population. Head tissue layer thicknesses and conductivities were varied to examine the impact of interindividual anatomical differences on the stimulation strength, depth, and focality. Skull conductivity most strongly affects the ECT electric field, whereas the MST electric field is independent of tissue conductivity variation in this model but is markedly affected by differences in head diameter. Focal ECT electrode configurations such as FEAST is more sensitive to anatomical variability than that of less focal paradigms such as BL ECT. In MST, anatomical variability has stronger influence on the electric field of the cap and circular coils compared to the double-cone coil, possibly due to the more superficial field of the former. The variability of the ECT and MST electric field due to anatomical differences should be considered in the interpretation of existing studies and in efforts to improve dosing approaches for better control of stimulation strength and focality across patients, such as individualization of the current amplitude. The conventional approach to individualizing dosage by titrating the number of pulses cannot compensate for differences in the spatial extent of stimulation that result from anatomical variability.

Predictors of Seizure Threshold in Right Unilateral Ultrabrief Electroconvulsive Therapy: Role of Concomitant Medications and Anaesthesia Used

“…This is consistent with prior studies in brief pulse ECT that reported explained variances ranging from 15 to 50% [8,12,17,48]. Importantly, it has been proposed that the unexplained variance in ST might be largely accounted by head anatomical differences, such as skull thickness [49] and conductivity [50] and, possibly, cerebrospinal fluid volume [51].…”

Individualized Low-Amplitude Seizure Therapy: Minimizing Current for Electroconvulsive Therapy and Magnetic Seizure Therapy

“…This observation is consistent with our computational modeling studies and is likely explained by the higher sensitivity to most head anatomical parameters of the electric field induced in the brain by ECT compared with MST (Deng et al, 2014).…”

“…This observation is consistent with the understanding that individual anatomical differences (eg, head shape and size, scalp and skull thickness and conductivity) affect the electric field strength in the brain (Deng et al, 2014;Edwards et al, 2013;van Waarde et al, 2013a, b), and that pulse amplitude individualization can normalize it across individuals, as the electric field strength is directly proportional to the pulse amplitude (Deng et al, 2013;Peterchev et al, 2012).…”

“…Even when ST is titrated and the dose is set relative to ST, this is done by adjusting the duration and frequency of the stimulus train but not the current amplitude, which does not compensate for the individual variation of the electric field strength in the brain (Peterchev et al, 2010b). Consequently, individual variation in anatomy results in variable strength and focality of the induced electric field, which could account for some of the differences in side effects and therapeutic efficacy across patients (Deng et al, 2014). Therefore, individualization of the pulse amplitude in ECT, as well as in MST, could help maintain a consistent electricity exposure of the brain to compensate for individual anatomical differences (Deng et al, 2013;Lee et al, 2013b).…”

Recent Developments in Noninvasive Neuromodulation for Mood and Anxiety Disorders

“…MST enables more precise control of the spatial extent of the induced electric field and resultant seizure than conventional ECT, presenting the opportunity to reduce involvement of medial temporal and frontal structures implicated in cognitive side effects (51). Computational modeling demonstrates that MST is less susceptible to variability introduced by anatomical differences across individuals than ECT (52).…”