Peripheral nerve blocks are used frequently in the management of the chronic pain patient with varying success. Nerve blocks may be used as a diagnostic tool, but can be employed to provide symptomatic relief on a temporary or more permanent basis. Injection of local anesthetic around a nerve can often provide relief in chronic pain patients that is beyond the normal expected duration of the blockade. The blocks may be repeated over the course of weeks or months. If good results are obtained, more destructive methods can be used to provide more lasting results. However, a good result with local anesthetic does not always mean a successful block by more permanent means.
Phenol and absolute alcohol are both used to provide more permanent neural blockade. Phenol is available as a 5-6% solution in water, as a hyperbaric solution of 5-10% in glycerol, or of 5-10% in non-ionic X-ray contrast medium. It causes temporary nerve degeneration by coagulating proteins in the nerve sheath. It is used for lumbar sympathectomy, para-vertebral and peripheral nerve blocks and for epidural and intrathecal blocks. Phenol is toxic at high doses, with hepatic and cardiac complications. It can cause painful neuralgias, well described in the ileo-inguinal neuralgia that may follow lumbar sympathetic blockade. Absolute alcohol produces a more lasting block than phenol by causing axonal degeneration. It is associated with a higher incidence of neuralgia. Alcohol is used for celiac plexus blocks (50% alcohol), cranial neural blockade (trigeminal nerve and branches) and intrathecal neurolysis. Absolute alcohol is hypobaric with respect to CSF and thus is useful for intrathecal neurolysis at the thoracic level.
Radio Frequency Lesioning Radio frequency (RF) lesioning uses heat to destroy nervous tissue. The nerve to be lesioned is identified using appropriate surface markings and radiological guidance. The RF needle is placed at the appropriate site. The RF generator has modes that allow testing of temperature, impedance and response to physiological stimuli to be undertaken. Confirmation of position of the needle tip is done by stimulating at tetanic rates (about 50 Hz) and at low twitch rates (about 2 Hz). The higher rates give good appreciation of the spread of the paresthesiae, which should be in the same distribution of the target nerve, whilst the lower rates give visual evidence of motor nerve involvement. When correct placement is shown, by appropriate paresthesiae at low voltage (e.g. less than 0.5 V for trigeminal lesioning), a radiofrequency current (of 300 kHz) is then passed through the thermocouple probe for 60 seconds at a power to raise the tip temperature, as measured by thermocouple or thermistor, to 60-80°C, depending upon the nerve to be treated. Repeat lesions may be required, depending on the outcome of post-lesion re-testing. Radiofre-quency lesioning is used for facet joint dener-vation, ganglion blockade and blockade of spinal dorsal roots .
Cryotherapy produces axonal degeneration (Wallerian) that is not permanent. Regeneration usually occurs. The cryoprobe consists of two tubes, one inside the other. CO2 or N2O is passed under pressure through the outer tube, which has a tapered end. As the gas emerges from the tapered end, it expands, causing a drop in temperature and an ice ball to form on the end of the probe. This temperature drop (to about -60°C) causes freezing of the immediate surrounding tissues. Nerves to be blocked are again identified using surface markings with the aid
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