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. Author manuscript; available in PMC: 2026 Jul 1.
Published in final edited form as: Clin Neurophysiol. 2025 May 19;175:2110750. doi: 10.1016/j.clinph.2025.2110750

Reliability of Short Interval Cortical Inhibition in children with Attention Deficit Hyperactivity Disorder

Donald L Gilbert 1,2, David A Huddleston 1, Karlee Y Migneault 1, Deana Crocetti 3, Paul S Horn 1,2, Steve W Wu 1,2, Stewart H Mostofsky 3,4
PMCID: PMC12250949  NIHMSID: NIHMS2084855  PMID: 40411982

Abstract

Objective:

Utility of biomarkers depends on test-retest consistency. Reduced Short Interval Cortical Inhibition (SICI) has emerged as a consistent finding in children with Attention Deficit Hyperactivity Disorder (ADHD); however, test-retest has not been established for children with ADHD. This is particularly crucial given that Transcranial Magnetic Stimulation (TMS) motor evoked potential (MEP)-based measures have high intra-subject variability and susceptibility to state-effects on motor cortex excitability. Addressing this, the objective of this study was to estimate test-retest reliability of paired-pulse SICI and of Intracortical Facilitation (ICF) in children, including those with ADHD and typically developing (TD) controls.

Methods:

Sixty-four 8-to-12-year-old children (28 female; 35 ADHD; 29 TD) were recruited at two sites and SICI and ICF measured across two visits separated by 21 days. Intraclass correlations (ICCs) were calculated.

Results:

Good reliability for SICI (ICC>0.75), but only moderate for ICF (ICC>0.50), was found among ADHD children, after accounting for test-pulse MEP amplitudes (>0.25 mV) and outliers.

Conclusions:

These findings support the potential for SICI as a biomarker of ADHD in school-aged children.

Significance:

This research addresses a critical gap in clinical neurophysiology, as estimating the reliability of SICI is needed to utilize it in interventional or longitudinal studies.

Keywords: Transcranial Magnetic Stimulation, Children, Attention Deficit Hyperactivity Disorder, Biomarker, Motor Cortex, Short Interval Cortical Inhibition

1. Introduction

Transcranial magnetic stimulation (TMS) administered over motor cortex can generate evoked potentials reflecting inhibitory and excitatory properties of cortical interneurons, as quantitative, brain-based, physiological biomarkers reflecting symptoms of attention deficit hyperactivity disorder (ADHD) and anomalous motor development (Gilbert et al., 2011, Moll et al., 2001). One informative application for TMS-based biomarkers would be to complement subjective, clinician- and parent-rated behavioral scales in pediatric diagnostic (Chen et al., 2023) and treatment studies (Lewis et al., 2024, Sahin et al., 2018).

To this end, we and others have used common paired pulse TMS paradigms (Kujirai et al., 1993) to quantify short interval cortical inhibition (SICI) and intracortical facilitation (ICF) in motor cortex in healthy children and children with ADHD (Buchmann et al., 2007, Moll et al., 2001, Pedapati et al., 2019, Wu et al., 2012). Various forms of imaging (Kahl et al., 2022) and pharmacodynamic (Ziemann et al., 2015) evidence link SICI to GABA and ICF to glutamate signaling. A majority of studies show reduced SICI in ADHD, (Chen et al., 2023) reporting stronger associations with reduced cortical inhibition among the more hyperactive/impulsive children (Gilbert et al., 2011, Hoegl et al., 2012). ICF findings have been statistically non-significant, and inconsistent (Gilbert et al., 2011, Moll et al., 2001). However, a critical requirement for extending these biomarker findings into interventional or longitudinal studies is high reliability (Brown et al., 2017). Unfortunately, while several studies have examined reliability of TMS measures in adults (Brown et al., 2017, Nielsen et al., 2021, Osnabruegge et al., 2023), in children, minimal research supports this use.

TMS research in children poses numerous challenges. First, compared to adults, children have higher resting motor thresholds (RMTs) (Garvey and Mall, 2008). Consequently, measures indexed to RMT, such as paired pulse short interval cortical inhibition (SICI) or intracortical facilitation (ICF), where the single, test pulse intensity is set at a suprathreshold intensity, may be unobtainable because the pulse intensity required exceeds 100% of maximum stimulator output, or because children do not tolerate the loud sound and strong sensations. Second, children have smaller MEP amplitudes (Nguyen et al., 2023). Therefore, the standard approach, to compare inhibitory and excitatory paired-pulse effects to a single pulse MEP amplitude of approximately one millivolt (1 mV) (Kujirai et al., 1993), may not be achievable without exceeding the maximum stimulator output. Finally, while TMS measures of motor cortical excitability demonstrate trait-dependent properties (Chen et al., 1998), they also reflect state, especially movement preparation (Chen et al., 1998). Thus, at sequential visits, a child’s varying levels of distractibility, hyperactivity, or anxiety might create excessive artifact or measurement variability. Taken together, these observations support the importance of systematic study of test-retest reliability of TMS measures in children.

We previously evaluated test-retest reliability using Intraclass Correlation Coefficients (ICCs) in a cohort of 34 children and adults with Tourette Syndrome (TS) and variable ADHD symptoms (Gilbert et al., 2005). We found, across visits separated by median one month, an ICC for SICI of 0.76. To our knowledge, this finding has not been replicated, nor is there published TMS data focusing more specifically on reliability in younger children, including those with ADHD. As the broad impact of neurobehavioral symptoms becomes apparent in childhood, obtaining estimates of ICC in younger children is essential if TMS measures are to be utilized as biomarkers in this age group (Grabb and Brady, 2024). Therefore, our objective in the present study was to estimate ICC for the two most common paired pulse measures, SICI and ICF, in children with ADHD and typically developing (TD) controls. In our prior work involving children (Gilbert et al., 2011, Gilbert et al., 2005), given their smaller MEP amplitudes, we have tolerated average single pulse amplitudes down to 0.5 mV, resulting in some loss of data for those with even smaller MEPs. Therefore, in hopes of facilitating future studies including more children as participants, we also sought to determine whether reliability differs among children with smaller, potentially insufficient MEP amplitudes.

2. Methods

2.1. Participants

We recruited 64 8-to-12-year-old right-handed children (35 ADHD, 29 TD; 32 Cincinnati, 32 Baltimore). Demographics and relevant clinical characteristics are shown in Table 1. These children are participants in the United States, National Institute of Health-funded, “Anomalous Motor Physiology in ADHD” (AMPA) study. A key aim of AMPA has been identification of diagnostically relevant measures in the motor system in children with ADHD that might reflect clinical symptom severity (Gilbert et al., 2011), identify heterogeneity within domains of impaired function (Detrick et al., 2021, Gilbert et al., 2025, Gilbert et al., 2019), reflect treatment results (Chen et al., 2014), and elucidate pathophysiology (Harris et al., 2021).

Table 1.

Participant Characteristics

Characteristic Full Cohort ADHD TD p*
n (%) n (%) n (%)
City/Site
Cincinnati/CCHMC 32 (50%) 18 (51%) 14 (48%) >0.9
Baltimore/KKI 32 (50%) 17 (49%) 15 (52%)
Sex
Female 28 (44%) 16 (46%) 12 (41%) >0.9
Male 36 (56%) 19 (54%) 17 (59%)
Race
Asian 2 (3.1%) 1 (2.9%) 1 (3.4%) 0.8
Black or African American 7 (11%) 5 (14%) 2 (6.9%)
More Than One Race 5 (7.8%) 3 (8.6%) 2 (6.9%)
White 50 (78%) 26 (74%) 24 (83%)
Ethnicity
Hispanic or Latino 4 (6.3%) 4 (11%) 0 (0%) 0.1
Not Hispanic or Latino 60 (94%) 31 (89%) 29 (100%)
Test-pulse MEP
> 0.25 mV 39 (61%) 21 (60%) 18 (62%) 0.9
< 0.25 mV 25 (39%) 14 (40%) 11 (38%)
mean (SD) mean (SD) mean (SD)
Age 9.97 (1.34) 10.00 (1.51) 9.93 (1.13) 0.8
ADHD-RS
Total 18 (14) 30 (8) 6 (7) 0.001
Inattentive 10 (8) 17 (4) 4 (4) 0.001
Hyper/Impulsive 8 (7) 13 (6) 2 (3) 0.001
Days between visits 21 (13) 19 (9) 23 (16) 0.3
RMT
Visit 1 61.9 (10.4) 62.3 (10.2) 61.3 (10.7) 0.7
Visit 2 60.7 (9.7) 60.3 (8.8) 61.2 (10.7) 0.7
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*

Chi Square, Fisher’s Exact test, Welch’s T test as appropriate, comparing ADHD and TD groups.

CCHMC Cincinnati Children’s Hospital Medical Center. KKI Kennedy Krieger Institute. ADHD children with attention deficit hyperactivity disorder. RS rating scale (I=inattentive; H/I = hyper/impulsive) TD Typically Developing Children (controls). MEP motor evoked potential. SD Standard Deviation. RMT Resting Motor Threshold.

The Institutional Review Boards at both sites approved the TMS protocol as a minimal risk study. Informed consent was obtained from parents and assent from children aged 11 and older.

2.2. Clinical Assessments

In the present study, participants were assessed at the same time of day, at two visits separated by a mean of 21 days. All evaluations were obtained and reviewed by physician investigators and their research teams. ADHD versus TD diagnoses were confirmed by child neurologists using DSM-5-based structured diagnostic interviews (Kaufman et al., 2013) and clinical rating scales (DuPaul et al., 1998). Exclusion criteria included standard TMS safety reasons (Rossi et al., 2021), full scale IQ (Wechsler, 2014) < 80, other neurological or psychiatric disorders, and medications, other than psychostimulants for ADHD, which were held the day before and day of the TMS study session.

2.3. Transcranial Magnetic Stimulation

Both sites used Magstim 200 TMS (Magstim, New York, NY) systems with a Bistim module and a round 90-mm coil, as well as identical amplifiers, filter settings, and Signal software (Signal v6, Cambridge, UK).

Resting motor threshold (RMT) was obtained using standard pediatric procedures (Gilbert et al., 2011) with the TMS coil placed flat at the vertex with the handle directly posterior and counterclockwise current. Single TMS pulses were administered starting at 20% maximal stimulator output, increasing by 10% until a consistent MEP was observed from surface EMG recording in the right first dorsal interosseous muscle, then decreasing the intensity until 50% (3 of 6) of pulses evoked a minimal EMG response of approximately 0.05 mV (Boroojerdi et al., 2000, Kujirai et al., 1993). Consistent with our prior studies (Gilbert et al., 2004, Gilbert et al., 2011, Harris et al., 2021), to reduce the burden of participation on restless young children, we used 3 of 6 rather than the standard 5 of 10 pulses employed in most adult research. To measure SICI and ICF, 60 trials were administered at intervals of 6 (±10%) seconds, 20 each under one of 3 conditions, in randomized order: 1) single (test) pulse (SP); 2) paired pulse (PP) at a 3-ms interval; and 3) PP at a 10-ms interval. The test pulse intensity was set at 1.2*RMT, the conditioning pulses at 0.6*RMT (Boroojerdi et al., 2000, Kujirai et al., 1993). Each trial tracing was 400 ms in duration. Pre-TMS background rectified area-under-the-curve EMG from a 50 ms epoch (65 to 15 ms prior to the single, test TMS pulse) and post-TMS EMG, peak-to-peak MEPs, from a 20 ms epoch (15 to 35 ms after TMS pulse) were extracted automatically from each trial for each participant, blinded to all clinical data. This was included as the “artifact” covariate in the regression analysis.

2.4. Statistical Analyses

The primary TMS measures for this study are SICI and ICF, represented as ratios with numerators as means of conditioned 3-ms (SICI) and 10-ms (ICF) PP evoked potentials and denominators as means of test, SP evoked MEPs. As our primary interest was to support or caution use of TMS biomarkers in ADHD, and further we were concerned that reliability might be lower among children with hyperkinetic disorders, we analyzed children with ADHD and TD controls separately.

Intraclass correlations Intraclass correlations (ICCs) were calculated using the R v.4.4.0 psych package, ICC1 function for each individual’s mean of MEP amplitudes at 3-ms PP, 10-ms PP, and SP, 20 trials each and, separately, for SICI and ICF.

Winsorizing outliers To evaluate the influence of the positive outliers, we winsorized the MEP data at 5% and 10%, then recalculated ICC for MEPs and for SICI and ICF ratios.

Stratifying by SP amplitude MEPs in children tend to be smaller than those in adults. As most adult studies target 1 mV for SP MEPs, and in our prior work in children we have tolerated ≥ 0.5 mV (Gilbert et al., 2011, Harris et al., 2021), we wished to determine whether reliability changes among children with small MEPs. Therefore, we selected 0.25 mv, half our standard 0.5 mV value, and stratified participants based on mean SP MEPs at the first TMS session. There were 39 children with mean MEP amplitudes >0.25 mV and 25 with mean MEP amplitudes ≤0.25 mV. We recalculated the ICCs and correlations for these two groups.

Pearson correlations across visits were also calculated from mean values.

Between group comparisons of ICC and Pearson r Reliability metrics were compared between the two groups using Fisher’s r-to-z transformation.

Optimizing/evaluating number of trials To assess the effect of number of trials on SICI and ICF, we calculated ICC based on 3-ms PP, 10-ms PP, and SP, starting from the first 5 trials each, within subject and session, and increasing up to the full 20 trials each (Biabani et al., 2018). Results for the larger SP MEP group, no winsorizing, were plotted.

Session (visit) effects As an additional check of cross-visit consistency, we evaluated the TMS EMG data using repeated measures, mixed models with participant as a random effect and log-transformed MEP amplitude as the dependent variable. This method accounts for non-normality and intrasubject variability of MEPs (Gilbert et al., 2019). We estimated session effects separately for SICI and ICF, using SAS® statistical software version 9.4 (SAS Institute Inc, Cary NC). City, sex, age, and pre-TMS artifact (50 ms EMG) were included in all models.

3. Results

3.1. Reliability

No stratification by mean SP MEP amplitude (“All”)

SICI SICI ICC ranged from 0.63 to 0.66 in ADHD, and 0.59 to 0.68 in TD, improving with winsorizing. Pearson correlations between visit 1 and visit 2 were significantly positive (higher at visit 1, higher at visit 2; p = 0.001), ranging from 0.62 to 0.69. There were no diagnostic-group differences (Table 2).

Table 2.

Intra Class Correlations and Pearson Correlations

SP MEP TMS Winsor ADHD
 TD
 ADHD vs TD
ICC 95% CI r p ICC 95 % CI r p ICCp* rp*
n = 21 n = 18
>0.25 mV SICI none 0.76 (0.51−0.90) 0.76 0.001 0.67 (0.32−0.86) 0.73 0.001 0.82 0.93
5% 0.79 (0.56−0.91) 0.79 0.001 0.77 (0.49−0.91) 0.78 0.001 0.95 0.97
 n = 39 10% 0.81 (0.59−0.92) 0.81 0.001 0.85 (0.64−0.94) 0.85 0.001 0.87 0.87
ICF none 0.55 (0.17−0.79) 0.59 0.005 0.15 (−0.32−0.56) 0.15 0.561 0.56 0.51
5% 0.55 (0.18−0.79) 0.63 0.002 0.03 (−0.42−0.48) 0.01 0.977 0.46 0.36
10% 0.55 (0.17−0.79) 0.64 0.002 0 (−0.45−0.45) −0.04 0.862 0.44 0.32
n = 14 n = 11
<0.25 mV SICI none 0.38 (−0.15−0.74) 0.36 0.21 0.42 (−0.19−0.8) 0.45 0.161 0.96 0.92
5% 0.33 (−0.20−0.72) 0.32 0.27 0.42 (−0.18−0.8) 0.48 0.138 0.92 0.86
 n = 25 10% 0.35 (−0.18−0.73) 0.35 0.22 0.43 (−0.17−0.8) 0.47 0.146 0.93 0.89
ICF none −0.15 (−0.60−0.39) −0.07 0.82 0.19 (−0.41−0.69) 0.18 0.597 0.74 0.81
5% −0.13 (−0.60−0.40) −0.07 0.80 0.16 (−0.44−0.67) 0.14 0.689 0.78 0.84
10% −0.13 (−0.59−0.41) −0.06 0.84 0.11 (−0.48−0.64) 0.09 0.797 0.82 0.89
n = 35 n = 29
All SICI none 0.63 (0.38−0.79) 0.62 0.001 0.59 (0.29−0.78) 0.65 0.001 0.92 0.93
5% 0.64 (0.40−0.80) 0.65 0.001 0.63 (0.36−0.81) 0.66 0.001 0.98 0.98
 n = 64 10% 0.66 (0.42−0.81) 0.67 0.001 0.68 (0.42−0.83) 0.69 0.001 0.95 0.95
ICF none 0.22 (−0.11−0.51) 0.24 0.171 0.16 (−0.21−0.49) 0.14 0.46 0.85 0.86
5% 0.20 (−0.13−0.50) 0.21 0.218 0.09 (−0.27−0.44) 0.07 0.7 0.79 0.81
10% 0.21 (−0.13–0.50) 0.22 0.202 0.05 (−0.31−0.4) 0.03 0.861 0.79 0.75
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ADHD = attention deficit hyperactivity disorder. TD = typically developing children. SP = single pulse. MEP = motor evoked potential. SICI = short interval cortical inhibition ratio. ICF = intracortical facilitation ratio. ICC = intraclass correlation coefficient. R = Pearson correlation coefficient. p = p value for correlation. CI = 95 % confidence interval. p

*

Fisher’s r-to-z transformation comparing ICC and Pearson r between groups.

ICF ICF ICC ranged from 0.20–0.22 for ADHD and 0.09 to 0.16 in TD, with all confidence intervals crossing into the negative range. All Pearson correlations were non-significant. There were no diagnostic group differences.

With stratification by first-visit SP MEP amplitude (>0.25 mV; ≤0.25 mV)

At visit 1, there were 39 children with mean SP MEP amplitudes > 0.25 mV. All 39 also had mean SP MEP amplitudes > 0.25 mV at visit 2. At visit 1, there were 25 children with mean SP MEP amplitudes ≤ 0.25 mV. Among these, at visit 2, 13 had mean SP MEP amplitudes > 0.25. Only first-visit SP MEP amplitude was used for stratification.

SICI For both children with ADHD and TD controls, at every level of winsorizing, ICCs were higher in the larger MEP stratum. SICI ICC ranged from 0.76 to 0.81 in ADHD, and 0.67 to 0.85 in TD. Winsorizing increased ICCs and correlations only in this stratum. (Table 2; Figure 1). In contrast, among children with SP MEP <0.25 mV, ICCs were < 0.5, Pearson correlations were positive but not significant (Table 2, Supplemental figure 1).

Figure 1.

Figure 1

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SICI ICF Test-retest Visit 1 and Visit 2

SICI (short interval cortical inhibition) and ICF (intracortical facilitation) ratios in 8-to-12-year-old children by diagnosis of ADHD (attention deficit hyperactivity disorder) versus TD (typically developing) controls. Data from 5% Winsorizing. Group with first-visit single pulse MEP>0.25 mV. 1A-1D Intraclass Correlations for SICI and ICF ratios for ADHD (red) and TD (blue). ICC = intraclass correlation coefficient. 1E-1H Pearson r correlations with best-fit regression line and 95% confidence interval.

Comparing children with ADHD to TD controls, we found no evidence of a difference in either ICC or correlations across visits.

ICF ICF ICC in the larger MEP stratum was estimated at 0.55 for children with ADHD and ranged from 0 to 0.15 in TD control children. Confidence intervals were large, and the diagnostic group differences were not significant. In the smaller MEP stratum, all ICCs and Pearson correlations were less than 0.20.

Comparing children with ADHD to TD controls, we found no evidence of a difference in either ICC or correlations across visits (Table 2).

Motor Evoked Potential Analyses

To better understand reliability of SICI and ICF, for the larger SP MEP stratum, ICC and correlation analyses were performed at the level of the mean MEP amplitudes for each participant (Figure 2).

Figure 2.

Figure 2

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Motor evoked Potentials Visit 1 and Visit 2

Motor evoked potentials from single pulse TMS (single), 3 ms interval paired pulse TMS (3 ms pp) and 10 ms interval paired pulse TMS (10 ms pp) in 8-to-12-year-old children by diagnosis of ADHD (attention deficit hyperactivity disorder) versus TD (typically developing) controls. Data from 5% Winsorizing. Group with first-visit single pulse MEP>0.25 mV. ICC single pulse, 3 ms, and 10 ms for ADHD (red) (2A-2C) and TD (blue) (2G-2I). ICC = intraclass correlation coefficient. 2D-2F; 2J-2L Pearson r correlations with best-fit regression line and 95% confidence interval.

Single pulse MEPs ICCs were in the moderate range, 0.74 for ADHD and 0.65 for TD, and correlations from visit 1 to visit 2 were positive and significant. The means of the single pulses are the denominator for both SICI and ICF ratios (see methods).

3 ms paired pulse MEPs ICC was moderate for ADHD at 0.62 and excellent for TD at 0.93. Pearson correlations were positive and significant. The means of 3 ms pulses are the numerator for ICF ratio.

10 ms paired pulse MEPs ICCs were in the moderate range, 0.69 for ADHD and 0.71 for TD. The means of 10 ms pulses are the numerator for ICF ratio.

Trial number analysis

As expected, ICC generally improves, for both SICI and ICF, as the number of pulses increases from 5 to 20. Overall, for SICI, ICC reaches the moderate reliability range (between 0.5 and 0.75) between 5 and 10 pulses but then plateaus. ICF is in the poor reliability range throughout (Figure 3).

Figure 3.

Figure 3

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Pulse number and ICC

ICC = intraclass correlation coefficient. SICI = short interval cortical inhibition. ICF = intracortical facilitation. ADHD = attention deficit hyperactivity disorder. TD = typically developing controls.

3.2. Effects of Session – regression analysis

Within the larger SP MEP group, for regression models of SICI and ICF, there was no effect of session (visit), supporting the significance of ICCs in Table 2. There was also no effect of city, supporting generalizability/scalability of measures across labs. Finally, there was no effect of diagnosis. This supports use of SICI in typically developing controls and children with ADHD. However, with the smaller SP MEP group, the effect of session was significant for both SICI and ICF, supporting lower test-retest reliability among these children (Supplement: Regression output).

4. Discussion

Despite challenges inherent to pediatric TMS studies (Gilbert, 2016), we found, across two visits separated by three weeks, moderate reliability in both ADHD and TD children for TMS-evoked SICI. We also found evidence that SICI reliability was higher and correlations more robust among children whose average single pulse MEP amplitudes were greater than 0.25 mV. Winsorizing 5% to 10% of these trials improved SICI ICC to >0.75, which is considered good reliability. The ICC curve for SICI tested across multiple pulses peaked quickly and then leveled off, with the final value less than that achieved by 10% winsorizing. This emphasizes the benefit of removing outliers and suggests that further increase in pulse number per condition would not improve ICC. The consistency and scalability of SICI was corroborated by the absence of a statistical effect of session or city in a regression model. These findings also compare favorably to recent studies of other potential electrophysiological measures in children (Beker et al., 2021, Cremone-Caira et al., 2020, Démas et al., 2024). Taken together, these observations support consideration of SICI as a biomarker for repeated measurements in children with ADHD.

In contrast to SICI, reliability of ICF was less robust. As they are measured at the same time and share a “common denominator” (single pulse MEPs), one might expect the explanation to have been higher ICC for 3 ms than for 10 ms. It has been suggested that the cortical activity underlying ICF might be more complex, involving a more variable summation of both inhibitory and excitatory interneurons, whereas at 3 ms the balance may be more consistently inhibitory (Siebner et al., 2022). Mechanistic underpinnings in children may differ in other unknown ways, possibly related to recruiting within less completely myelinated cortical layers or contributions of Indirect/Intracortical I-waves versus Direct D-waves (Di Lazzaro et al., 1998). We conclude that further research is needed to understand this difference, but at present ICF is not a strong biomarker candidate for longitudinal pediatric research. This finding that ICF is less reliable than SICI is consistent with prior findings in 15 healthy adults (Biabani et al., 2018).

Comparing our findings to the adult literature, it is notable that we showed SICI reliability with test pulse MEP amplitudes of > 0.25 mV, much lower than the standard 0.5 to 1.5 mV (Boroojerdi et al., 2000, Kujirai et al., 1993, Nielsen et al., 2021). This lower threshold means that SICI as a biomarker may be useful in more children. The threshold of 0.25 mV is arbitrary: a more precise “floor” for reliability would require a sensitivity analysis in a larger sample of participants. The gain of reliability in individuals with larger MEPs versus smaller MEPs merits further study but may partly relate to coefficients of variation within children with smaller MEPs.

We also note that, unlike prior studies by our group and others using larger SP MEP amplitudes (Moll et al., 2001), we failed to show consistently less SICI (ratios closer to 1.0) in children with ADHD. We believe the most likely explanation is that this cohort was less hyperactive than our prior cohorts, who had much higher ADHD T scores (Gilbert et al., 2019, Gilbert et al., 2011).

4.1. Limits and future directions

Although our total cohort of 64 children is larger than most other TMS reliability studies in adults, a sample size of 35 children with ADHD should be interpreted with caution. As is typically the case, these represent convenience samples – families who agreed to and had sufficient time to participate in multiple study visits. More severely affected children with higher hyper/impulsive rating scale scores were underrepresented, possibly because our study protocol required pausing stimulant medications for multiple days. In addition, despite the temporary discontinuation of stimulants, some SICI enhancement (Moll et al., 2000) may have persisted in children with ADHD. Finally, although our diagnostic process is rigorous, ADHD remains a clinical diagnosis, and thus misclassification is possible.

Our choice of conditioning pulse intensities, based on published conditioning pulse “dosing” curves (Orth et al., 2003), may also have influenced our findings. As noted by others, there has been substantial heterogeneity in many components of TMS protocols (Osnabruegge et al., 2023). We have set our conditioning pule at 60% of RMT intensity consistently for over 15 years (Chen et al., 2014, Gilbert et al., 2019, Gilbert et al., 2011, Harris et al., 2021), similar to protocols used by others (Hoegl et al., 2012, Mainberger et al., 2013).

To enhance reliability, future studies using paired pulse TMS measures as markers of pharmacological or behavioral interventions in ADHD may utilize winsorizing, log transformation, or other techniques to manage outliers. In addition, as we found lower reliability when evoked amplitudes were small, it may be reasonable for future studies to increase the test pulse intensities to 1.3–1.5 times RMT, in children who will tolerate it. This might not increase the sample size, however. For example, at 1.2*RMT, children with RMT greater than 83% of maximum stimulator output are excluded; at 1.3*RMT, children with RMT > 77% of maximum stimulator output are excluded, etc. Finally, reliability studies using other paired pulse intervals or conditioning pulse intensities, particularly for ICF, may be helpful.

5. Conclusions

In conclusion, our study provides estimates of test-retest reliability over 4 weeks for SICI and ICF in typically developing children and children with ADHD, ages 8–12 years, as well as methods for improving reliability helpful for SICI.

Supplementary Material

1

Text, table, figure, regression output in Supplementary file.

Highlights.

  • A cohort of 64 8-to-12-year-old children with ADHD and typically developing controls participated in a study to estimate reliability of paired pulse transcranial magnetic stimulation TMS measures short interval cortical inhibition (SICI) and intracortical facilitation (ICF) at visits 21 days apart.

  • Removing outlier motor evoked potentials (winsorizing) and eliminating individuals with the smallest single pulse motor evoked potentials (MEPs less than 0.25 mV) improved SICI reliability.

  • Test-retest and cross-site reliability supports consideration of SICI as an ADHD biomarker in children.

Acknowledgements

The authors gratefully acknowledge the time and effort devoted to participation in this study by the children and their families. The authors also thank the research coordinators and technicians.

This research was funded by R01 MH095014, R01 MH085328, and the NICHD: P50 HD103538.

Footnotes

Potential Conflicts of Interest

The Authors declare no competing interests.

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Data Availability

Per policy of the National Institute of Health, all data will be available after publication through the NIMH Data Archive (https://nda.nih.gov).

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Associated Data

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Supplementary Materials

1
NIHMS2084855-supplement-1.docx (181.4KB, docx)

Data Availability Statement

Per policy of the National Institute of Health, all data will be available after publication through the NIMH Data Archive (https://nda.nih.gov).

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