Percutaneous Nerve Block: Determining the optimal duration of phenol in contact with the nervous tissue
Date Issued
2005
Date
2005
Author(s)
Lin, Jau-Jia
DOI
zh-TW
Abstract
Percutaneous phenol block continues to be among the most frequently used local injection methods for reducing spasticity in adults. It can produce a prolonged but not permanent motor block through nonselective axonal degeneration, a mechanism that interrupts components of the stretch reflex arc. Its effects are immediate and dramatic. Although the procedure is relatively easy to perform and low cost, many physicians in the world are reluctant to use it because they think the blocking effects are not easily titrated. The degrees of nerve destruction and the body’s ability to regenerate after damage by phenol block have been poorly evaluated. Many variables can influence the effect of phenol block, including the concentration and volume of phenol injected, the duration of phenol in contact with the nervous tissue, localization techniques, selection of block sites, and application techniques. In the current practice, nerve localization is determined if maximal target muscle contraction by minimal electrical current is achieved by using an electrical stimulator. Five percents of aqueous phenol solution is continuously injected until visible muscle contractions disappear. The optimal duration needed for adequate neural damage by injected phenol has not yet been well understood, and the totally used phenol volume may be significantly larger than actually needed. Larger dose of phenol injection can cause greater nearby tissue damage and make further localization more difficult. In this study, we try to determine the optimal duration of phenol in contact with the nervous tissue in Wistar rat, and investigate the dose-response relationship of 5% aqueous phenol solution in peripheral nerve block.
Materials and Methods
The optimal duration of phenol in contact with the nervous tissue. After surgical exposure of the sciatic nerve, stimulation electrodes were placed on the main trunk and recording electrodes were placed on the midbelly of the gastrocnemius muscle. We adjusted the stimulation parameters to obtain the steady amplitude of compound motor action potential (CMAP) under continuous stimulation. The filter setting was 10 to 100,000Hz. The CMAP of the gastrocnemius muscle were recorded after 8 to 12 supramaximal stimuli (square wave stimulus of 100-μs pulse width) of the sciatic nerve. Peak to peak amplitude was measured in the response of the maximal amplitude. A small trough that held approximately 100 μL of solution was fabricated around the sciatic nerve. The trough was made of a polyethylene tube that was incised longitudinally. It was spread out with a curved mosquito forceps to transform its tubular shape into a flat sheet. Then it was inserted beneath the tibial nerve. When the mosquito forceps were removed, it returned to its original tubular form that then encircled the tibial nerve. The lower end of the trough was packed by wax. 0.02μL of 5% aqueous phenol solution was dropped into the trough, and then recorded the duration for the muscle CMAP amplitude attenuated by 10%( start to decline), 25%, 50%, 75% and less than 0.1 mV.
Dose-response relationship. Sciatic nerves were located percutaneously under guidance by the electrical stimulation. After maximal muscle contraction was achieved by using minimal electrical current, a 27 G Teflon-coated needle（BOTOX Injection Needle, Allergan）was used to inject phenol solution continuously until the muscle CMAP amplitude disappeared on the monitor. The injected volume of 8 samples was averaged. Twenty percent, 40%, 60% and 80% of the above averaged volume of 5% phenol solution was injected percutaneously at a rapid rate after electrical localization of the sciatic nerves. Then the electrical stimulation was turned off immediately. After the adequate duration which was determined by the previous experiment, the electrical stimulator was turned on and continuously recorded CMAP amplitude of the gastrocnemius muscle to determine the dose-response relationship. Finally, surgical exposure of the sciatic nerve was made and observed if the muscle CMAP amplitude still presented with electrical stimulation.
Result
After phenol in contact with the exposed sciatic nerve, the muscle CMAP
amplitude disappeared at an average duration of 74 seconds (range: 48 to 90). So the
optimal duration for complete phenol effect was set as 90 seconds. After this period of time, full reaction with phenol at all the tested nerves could be expected. The average injected volume of phenol solution to achieve complete absence of the muscle CMAP amplitude by percutaneously continuous injection was 0.8 μL. Ninety seconds after injecting 0.16 μL (20%) of 5% aqueous phenol solution rapidly, the muscle CMAP amplitudes were totally absent in 8 tested sciatic nerves. So the injected volume was adjusted to 5%, 10% and 20% of the average volume which was 0.8μL for comparison. Using 5% or 10% of the average volume, complete absence of the CMAP amplitude was only seen in some tested nerves after the duration of 90 seconds.
Conclusion
The continuous injection model for percutaneous phenol block indeed uses significantly more phenol than actually needed. Clinically, we suggest progressive injection model which inject small amount initially, and determine if there are any need for further injection after drug effects for an optimal duration. This study can not show the dose-response relationship clearly, however, dose-dependent effect is evident before the threshold volume is given.
Subjects
痙攣張力
酚
神經阻斷術
上運動神經元疾病
spasticity
phenol
nerve block
upper motor neuron disease
Type
text
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