Does the FDA‑cleared NeuroStar protocol for obsessive‑compulsive disorder use a figure‑8 coil instead of an H7 coil, and is the figure‑8 coil sufficiently effective despite its lower penetration depth compared to the H7 coil?

NeuroStar FDA Clearance and Coil Technology Discrepancy

You are correct that the NeuroStar system uses a figure-8 coil, not an H7 coil, and this represents a critical distinction in both the physical characteristics and likely clinical efficacy for OCD treatment. The FDA clearance was indeed granted through a "substantial equivalence" pathway despite fundamental differences in coil penetration depth and field distribution.

Coil Technology Differences

Physical Characteristics

• Figure-8 coils induce suprathreshold electrical fields at depths of only up to 1.5 cm beneath the cortical surface, providing highly focal but superficial stimulation 1.
• H7 coils achieve effective stimulation at depths of 2-6 cm depending on the specific brain region targeted, with the medial prefrontal cortex (mPFC) and dorsal anterior cingulate cortex (dACC) being reached at 4-5 cm depth 1.
• The H7 coil's deeper penetration comes at the cost of a wider, less focal electrical field distribution in the brain, enabling simultaneous stimulation of multiple interconnected regions 1.

Clinical Implications for OCD

• The Deep TMS H7 Coil studies provide clear evidence for efficacy targeting the mPFC-dACC circuit, which are deep midline structures critical to OCD pathophysiology 2.
• Studies using figure-8 coils for OCD show small sample sizes, heterogeneous targeting approaches, and no clear consensus on optimal stimulation sites 2.
• Real-world data from 22 clinical sites using the H7 coil demonstrated 57.9% response rates after 29 sessions in 167 OCD patients, with 72.6% achieving first response 3.

The Substantial Equivalence Problem

The FDA clearance pathway allowed NeuroStar's figure-8 coil system to gain approval based on equivalence to H7 coil studies, despite the figure-8 coil's inability to reach the same deep brain structures that were the actual therapeutic targets in the pivotal trials. This regulatory decision creates a significant gap between the evidence base and the cleared device's physical capabilities.

Evidence of Differential Efficacy

• A 2024 direct comparison study found that when targeting the supplementary motor area (SMA) for OCD with comorbid depression, the double-cone deep coil (DB80) produced significantly lesser clinical improvements compared to the figure-8 coil, despite achieving higher stimulation intensities 4.
• However, this finding may reflect off-target stimulation effects rather than validating figure-8 superiority, as the DB80 coil affected broader regions beyond the intended SMA target 4.

Critical Caveats

The fundamental issue is that figure-8 coils cannot effectively stimulate the mPFC and dACC—the validated therapeutic targets for OCD—due to their deep midline location. These structures lie 3-5 cm from the cortical surface, well beyond the 1.5 cm effective depth of figure-8 coils 1.

Any clinical benefit observed with figure-8 coil protocols for OCD likely derives from stimulation of alternative, more superficial cortical targets (such as dorsolateral prefrontal cortex or SMA), which have less robust evidence supporting their role in OCD pathophysiology 2.

Regulatory vs. Clinical Reality

• The H7 coil received FDA clearance specifically for OCD based on multicenter sham-controlled trials demonstrating efficacy 3, 5.
• Figure-8 coil systems like NeuroStar gained clearance through substantial equivalence claims without generating comparable evidence for deep structure stimulation in OCD 2.

Bottom Line for Clinical Practice

If treating OCD with rTMS, the H7 coil targeting mPFC-dACC represents the evidence-based approach with demonstrated efficacy in controlled trials and real-world settings 3, 5. Figure-8 coil protocols lack the physical capability to replicate this mechanism of action and should be considered investigational for OCD despite FDA clearance 2, 1.