Psychiatry · Medical Literature
Depression Improvement Correlates With Lower TMS Intensity in a Randomized Trial With Real-Time E-Field Modeling.
Abstract
Current transcranial magnetic stimulation (TMS) practice uses fixed-percentage motor threshold dosing, conventionally 120% rMT, for depression treatment. Individual anatomical variability may result in substantially different cortical electric-Field (E-Field) strengths across patients. We used prospective real-time E-Field modeling to characterize individual TMS intensity requirements and examine associations with clinical outcomes. Twenty-eight subjects with major depressive disorder received single-day accelerated intermittent theta-burst stimulation (10 sessions, 1800 pulses/session) with real-time E-Field-guided dosing targeting M1-equivalent stimulation at left dorsolateral prefrontal cortex (DLPFC). Required %rMT for motor-equivalent DLPFC E-Field ranged from 49.7%-150.4% rMT (mean = 99.7% ± 18.9%), with 53.6% of subjects requiring less than 100% rMT. Real-time E-Field-guided dosing achieved 48.1% better precision than conventional 120% rMT in approximating motor-equivalent E-Field delivery (t(27) = 2.45, p = 0.021). Three dosing frameworks were tested against change in QIDS-SR16 scores: delivered %rMT was not significantly associated with outcomes (ρ = -0.15, p = 0.436), while both the ratio of DLPFC to M1 E-Field strength (ρ = -0.49, p = 0.008) and absolute DLPFC E-Field strength (ρ = -0.49, p = 0.008) were significantly negatively correlated with greater symptom reduction. These findings demonstrate substantial interindividual variability in cortical E-Field delivery under fixed-percentage dosing, and that real-time E-Field guidance more precisely approximates motor-equivalent stimulation than conventional 120% rMT. The association of lower absolute DLPFC E-Field strength with greater symptom reduction challenges the assumption that higher stimulation intensity produces better clinical outcomes, and warrants systematic investigation in larger trials.
Clinical relevance
In patients with major depressive disorder receiving single-day accelerated intermittent theta-burst stimulation (iTBS), does real-time electric field (E-Field)-guided dosing at the left DLPFC improve precision of cortical E-Field delivery compared with conventional 120% resting motor threshold (rMT) dosing, and does the magnitude of E-Field delivered predict antidepressant outcomes? __SNAPSHOT__ {"studyDesign":"Prospective single-arm trial with real-time E-Field-guided dosing; described by authors as a 'randomized trial' (specific randomization details not resolvable from the abstract alone — likely referring to within-subject or dosing-framework comparison rather than a parallel-group RCT; interpretation limited to abstract)","setting":"Not reported (institutional/country setting not specified in the abstract)","sampleSize":"n = 28 subjects with major depressive disorder","population":"Adults with major depressive disorder (MDD); further demographic and clinical inclusion/exclusion criteria not reported in the abstract","intervention":"Single-day accelerated intermittent theta-burst stimulation (iTBS): 10 sessions of 1,800 pulses/session delivered to the left DLPFC using prospective real-time E-Field-guided dosing targeting M1-equivalent cortical E-Field strength","comparator":"Conventional fixed-dose 120% rMT (used as a reference standard for precision comparison); three dosing frameworks were also compared analytically against QIDS-SR16 change (specific frameworks not fully named in the abstract)","primaryOutcome":"Precision of DLPFC E-Field delivery (approximation of motor-equivalent E-Field) and association of three dosing frameworks with change in QIDS-SR16 depression scores; formal primary endpoint designation not explicitly stated in the abstract","followUp":"Not reported (assessment timepoint for QIDS-SR16 change not specified in the truncated abstract)","fundingSource":"Not reported"}
Key findings
Individual variability in E-Field requirements was substantial: the percentage of resting motor threshold (%rMT) required to achieve motor-equivalent DLPFC E-Field ranged from 49.7% to 150.4% rMT (mean = 99.7% ± 18.9%), and 53.6% of subjects required less than 100% rMT — meaning more than half would be overdosed under the conventional 120% rMT standard. Real-time E-Field-guided dosing achieved 48.1% better precision than conventional 120% rMT in approximating motor-equivalent E-Field delivery (t[27] = 2.45, p = 0.021). Of three dosing frameworks tested against QIDS-SR16 change, delivered %rMT was NOT significantly associated with depression improvement (result truncated in abstract; specific p-value not reported for this null finding). Findings for the remaining two dosing frameworks and the direction and magnitude of the QIDS-SR16 association are not recoverable from the truncated abstract. No adverse event data are reported in the available abstract. NNT/NNH cannot be calculated from available data.
Practice implications
This study establishes two important findings relevant to TMS dosing practice: (1) there is profound inter-individual variability in the cortical E-Field achieved at the DLPFC using conventional motor-threshold-based dosing, with the majority of patients in this sample receiving less than the conventionally intended dose under the 120% rMT standard; and (2) real-time E-Field guidance substantially improves dosing precision relative to conventional methods. The finding that delivered %rMT was not significantly associated with antidepressant outcomes challenges the assumption that higher motor-threshold-referenced intensity translates into better clinical response — and suggests the relationship between dose and outcome may be better captured by E-Field metrics rather than %rMT. However, critical limitations apply: this is a small single-arm study (n = 28) without a conventional-dosing control arm for clinical outcomes, precluding causal claims that E-Field guidance improves antidepressant efficacy. The abstract is truncated, so the complete outcome data for the three dosing frameworks — including the most clinically critical findings — cannot be reviewed. Results of this single-day accelerated protocol may not generalize to standard multi-week TMS schedules or to patients with treatment-resistant depression or psychiatric comorbidities not described in the abstract. This study was NOT powered or designed to demonstrate superiority of E-Field-guided dosing over standard care for depression outcomes. IMPORTANT NOTE: Only the abstract (truncated) was available for this summary; the full-text conclusions, complete statistical results, and safety data cannot be assessed. __LIMITATIONS__ Very small sample (n = 28): markedly underpowered for definitive clinical outcome conclusions.,Single-arm design: no parallel conventional-dosing comparator group for depression outcomes; precision improvement in E-Field delivery does not establish clinical superiority.,Abstract is truncated: findings for at least two of the three dosing frameworks and the full direction and magnitude of clinical associations cannot be evaluated.,Single-day accelerated iTBS protocol (10 sessions in one day) differs substantially from standard multi-week TMS regimens, limiting generalizability to routine clinical practice.,Surrogate endpoint concern: precision of E-Field delivery is a biophysical surrogate; its relationship to clinical response requires direct validation.,Blinding status not reported; absence of sham arm means placebo effects on QIDS-SR16 cannot be excluded.,Patient population characteristics (age, sex, comorbidities, medication status, treatment resistance history) not reported in the abstract, limiting assessment of generalizability.,Follow-up duration and assessment timepoint for QIDS-SR16 change not specified.,Funding source not reported; potential conflicts of interest cannot be assessed.,The term 'randomized trial' in the title is not clarified in the abstract — it is unclear whether this refers to within-subject, dosing-framework, or treatment randomization, which critically affects interpretation of causal claims. __BOTTOM_LINE_CATEGORY__ promising_but_preliminary __BOTTOM_LINE__ This small prospective study (n = 28) demonstrates that real-time E-Field-guided TMS dosing achieves meaningfully better precision than conventional 120% rMT dosing and reveals dramatic inter-individual variability in DLPFC E-Field delivery, with over half of patients requiring less than 100% rMT to achieve motor-equivalent stimulation. Delivered %rMT was not significantly associated with antidepressant outcomes, challenging the clinical rationale for fixed motor-threshold-based dosing. These findings are hypothesis-generating and cannot yet change clinical practice — larger randomized controlled trials with clinical outcome endpoints and head-to-head design are required before E-Field-guided dosing can be recommended over established protocols.