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Pathological effects and motor response threshold changes following radiofrequency application at various distances
Joseph Maarrawi1, Sandra Kobaiter-Maarrawi, Ismat Ghanem
1Department of Physiology and Neurosciences, St. Joseph University, Beirut, Lebanon.
Object:
Radiofrequency (RF) ablation is a minimally invasive technique often used percutaneously in the treatment of many conditions such as spasticity, pain, and osteoid osteoma. The purpose of this study was to assess the value of motor response threshold (MRT) as an indirect indicator of the RF generator's electrode to nerve distance, and to evaluate the effects of RF at various distances from a nervous structure.
Methods:
The L-5 nerve root was studied in 102 Sprague-Dawley rats (sham contralateral side). Motor response thresholds at 0, 2, 4, 5, and 6 mm from the nerve root were assessed before and after RF application for 2 minutes at 80° C on Days 0 and 7. Radiofrequency was applied 0, 2, 4, 5, and 6 mm away from L-5 and with the addition of interposed cortical bone. The effects of RF application on MRT were studied, and subsequent nerve injury was evaluated using light microscopy pathological examination.
Results:
There is a significant correlation between MRT and the distance between the electrode tip and L-5, with MRT less than 0.5 V when the electrode was in direct contact with the root. Electrical and pathological changes following RF application were more pronounced at 0 mm, with worsening seen on Day 7. Radiofrequency at 2 and 4 mm produced fewer electrical and histological deleterious effects on the nerve on Days 0 and 7, with an obvious improvement on Day 7. At 5 mm, electrical and histological abnormalities were minimal on Day 0 and were fully reversible on Day 7. At 6 mm and with interposed cortical bone, MRT and pathological findings were unchanged on Days 0 and 7.
Conclusions:
The MRT proved to be a useful and reliable tool in decreasing nerve morbidity following RF ablation in animals and may be used in humans for the same purpose. It serves as an indirect indicator of the proximity of the RF generator's electrode tip to any adjacent motor nervous structure. A minimum safe distance of 5 mm between the electrode tip and the nerve is required to avoid irreversible nerve injury, unless a bony wall is interposed between them, thus serving as a nerve shield. In medical conditions that require RF ablation of the nerve, such as spasticity and pain, the MRT must be lower than 0.5 V. When a nerve lesion is to be avoided such as in cases of osteoid osteoma, an MRT higher than 2.5 V is considered safe, reflecting a distance greater than 5 mm.
Insights
Motor response threshold (MRT) reliably indicates radiofrequency (RF) electrode distance to nerves, guiding safe ablation. A 5 mm distance or bone shielding is crucial to prevent irreversible nerve injury during RF procedures.
Area of Science:
- Neurology
- Interventional Radiology
- Surgical Techniques
Background:
- Radiofrequency (RF) ablation is a minimally invasive percutaneous technique for conditions like spasticity, pain, and osteoid osteoma.
- Determining the precise electrode-to-nerve distance is critical for safe and effective RF ablation.
- Motor response threshold (MRT) is explored as an indirect measure of this proximity.
Purpose of the Study:
- To assess the value of motor response threshold (MRT) as an indirect indicator of RF electrode-to-nerve distance.
- To evaluate the effects of RF energy applied at various distances from a nervous structure.
- To establish safe parameters for RF ablation to minimize nerve damage.
Main Methods:
- The L-5 nerve root in 102 Sprague-Dawley rats was used for the study.
- Motor response thresholds were measured at distances of 0, 2, 4, 5, and 6 mm from the nerve root.
- RF was applied at 80°C for 2 minutes at varying distances, with and without interposed cortical bone, followed by histological examination.
Main Results:
- A significant correlation was found between MRT and electrode-nerve distance; MRT < 0.5 V indicated direct contact.
- RF application at 0 mm caused significant, worsening nerve injury over time.
- Distances of 5 mm or greater, or with interposed bone, resulted in minimal or reversible nerve injury.
Conclusions:
- MRT is a reliable tool for minimizing nerve morbidity during RF ablation in animal models, with potential human application.
- A minimum safe distance of 5 mm between the RF electrode and nerve is recommended to prevent irreversible injury.
- Specific MRT values can guide safe RF ablation for therapeutic targets (e.g., < 0.5 V for spasticity) or nerve avoidance (e.g., > 2.5 V for osteoid osteoma).
