Feasibility study of greater occipital nerve blocks by focused ultrasound – an animal study
Jiun-Yi Chiou, Tamer Abd-Elrehim, Chou-Ching Lin, Gin-Shin Chen
Journal of Neural Engineering 2020 · 10.1088/1741-2552/abb14d
Abstract
Objective Greater occipital nerve (GON) block may provide substantial relief for headache in the occipital location. This study tested the feasibility of focused ultrasound (FUS) to induce the conduction block of GONs in rats. Approach For in vitro experiments, the nerve was dissected and cut from C2 to the site near the ear of the rats and preserved in Ringer's solution. Pulsed FUS was used for the block, and sensory action potentials were recorded in the GON. For in vivo experiments, the GONs of the rats were surgically exposed for precise ultrasonic treatment. All data are expressed as the mean ± the standard deviation. Main results A single ultrasonic treatment temporarily suppressed the amplitude of action potentials of the in vitro nerves to 42 ± 14% of the baseline values, and the time to recovery was 55 min. The in vivo results showed that FUS acutely inhibited the amplitude of action potentials to 41 ± 8% of the baseline value in rat GONs, and the time to recovery was 67 min. Histological examination revealed no appreciable changes in the nerve morphology caused by FUS. Therefore, FUS reversibly blocked the conduction of the rat GON when the sonication parameters were appropriate. Significance Noninvasive FUS may be a novel treatment paradigm for occipital headache by blocking the occipital nerve, and the procedure is repeatable if indicated.
Abstract via europepmc.
Exposures
Exposure 1: In vitro FUS block of excised greater occipital nerve
Target: peripheral nerve — “greater occipital nerve (GON)”
Device: Sonic Concepts · SONIC Concepts · 2.68 MHz, 6 cm aperture diameter, 5 cm focal length transducer ✓
| Waveform | pulsed | |
|---|---|---|
| Fundamental frequency (kHz) | 2,680 | ✓✓✓ |
| Pulse duration (ms) | 10 | ✓✓✓ |
| Pulse repetition frequency (Hz) | 10 | ✓✓✓ |
| Duty cycle (%) | 10pulse duration × PRF gives 10% | ✓✓✓ |
| Sonication duration (s) | 20 | ✓✓✓ |
| Free-field pressure (kPa) | 680, 860, 970, 1,070swept | ✓✓✓ |
|---|---|---|
| Free-field Isppa (W/cm²) | not reported | |
| Free-field Ispta (W/cm²) | 0.131, 0.167, 0.208, 0.24swept | ✓✓✓ |
| In-situ estimate | not applicable | |
| In-situ pressure (kPa) | not applicable | |
| In-situ Isppa (W/cm²) | not applicable | |
| In-situ Ispta (W/cm²) | not applicable |
One nerve was exposed to ultrasound at one intensity for the first in vitro experiment to identify the blocking dose (four intensities tested); additional in vitro nerves were then treated with the selected intensity (131 mW/cm2) and sonication time (20 s) to confirm the effect.
Exposure 2: In vivo FUS block of surgically exposed greater occipital nerve
Target: peripheral nerve — “greater occipital nerve (GON)”
Device: custom-built · 2 MHz, aperture diameter 2.0 cm, focal length 1.6 cm transducer ✓
| Waveform | pulsed | |
|---|---|---|
| Fundamental frequency (kHz) | 2,000 | ✓✓✓ |
| Pulse duration (ms) | 10 | ✓✓✓⚑ |
| Pulse repetition frequency (Hz) | 10 | ✓✓✓ |
| Duty cycle (%) | 10pulse duration × PRF gives 10% | ✓✓✓ |
| Sonication duration (s) | 20 | ✓✓✓ |
| Free-field pressure (kPa) | 680 | ✓✓✓⚑ |
|---|---|---|
| Free-field Isppa (W/cm²) | not reported | |
| Free-field Ispta (W/cm²) | 0.131 | ✓✓✓ |
| In-situ estimate | not reported | |
| In-situ pressure (kPa) | not reported | |
| In-situ Isppa (W/cm²) | not reported | |
| In-situ Ispta (W/cm²) | not reported |
The location order of stimulating/recording electrodes, equipment, stimulus parameters, and sampling rate matched those in the in vitro experiments. The SAPs of the GONs were recorded before and after sonication over a 2-h total recording period, with the experimental group (n=7) receiving FUS and the sham group (n=7) as comparison.
Flags from extraction
n_subjects— Total animals actually exposed to real FUS reconstructed from group statements (5 rats/10 nerves in vitro dose-finding and confirmation, which is internally consistent with the 4+6 nerve counts reported; 7 rats in the in vivo experimental group); the 7 in vivo sham rats and any rats beyond these 12 are not counted as 'exposed'. Only n=5 of 7 in vivo experimental nerves yielded usable recordings due to technical failures.sham_type— Paper states an in vivo sham group (n=7) was compared to the experimental group but never describes what was done to the sham animals (e.g., inactive transducer, no sonication) beyond 'the sham group was almost constant'; classified as undescribed.exposures[1].device.manufacturer— The 2 MHz in vivo transducer is described as 'developed' for this study with no manufacturer named, unlike the in vitro Sonic Concepts transducer; classified as custom_or_research.exposures[1].timing.pulse_duration_ms— In vivo timing parameters are not independently restated; the paper says stimulus parameters 'matched those in the in vitro experiments', so the in vitro values (10 ms, 10 Hz, 10% duty cycle, 20 s) are carried over by that statement rather than being independently quoted.exposures[1].free_field.pressure_kpa— Domain classification (free_field vs in_situ) is judgement-based: the GON was surgically exposed and gel-coupled with no intervening skull, so the measured pressure 'acting on the in vivo GONs' was treated analogously to a bath/no-tissue-path measurement (free_field) rather than in_situ.