Position and anatomical landmarks
The entry point is determined in the same way as for diagnostic blocks, by projecting a metal ruler over the caudal and posterior part of the intervertebral foramen. The cannula (22G SMK-C5 needle 51 mm with a 4-mm active tip) is introduced parallel to the beam and, if necessary, the direction is corrected while the cannula is still in the uppermost subcutaneous layers. The correct position is reached when the cannula is projected as a point on the screen (Figure 1).
Figure 1. Antero-posterior view after injection with contrast dye. The needle and spread of the contrast dye can be seen.
This point must lie just above the dorsal part of the intervertebral foramen. This is the transition between the middle and most caudal third of the neural foramen. This position is chosen in order to avoid possible damage to the vertebral artery that runs anterior to the intervertebral foramen. The direction of the beam is then changed to the anteroposterior position and the cannula is moved further up until the tip is projected over the middle of the facet column (Figures 2 and 3).
Figure 2. Cervical DRG procedure: the needle is in the centre of the facet column in AP view.
When the segmental nerve is identified using 0.4 ml iohexol contrast dye, 0.5 to 1.0 ml lidocaine is slowly injected around the nerve. Overflow into the epidural space is avoided by 'real time' observation of the radio-opaque mixture (Figure 3).
Figure 3. Antero-posterior view after injection with contrast dye. The needle and spread of the contrast dye can be seen.
The stylet is then exchanged for an RF electrode. The impedance is measured in order to check if a closed electrical circuit is present. Then stimulation is started at 50 Hz in order to determine the sensory stimulation threshold. The patient must feel a tingling at less than 0.5 V. This indicates that the tip is in close proximity to the DRG.
Pulsed Radiofrequency Treatment.
The PRF current is delivered in small bursts at 45 V; this output can always be adjusted if the temperature rises above 42°C. Forty-two degrees is the maximum temperature, but not the obligatory temperature to be reached. The pulsed current is delivered for 120 seconds.
Complications are rare, but it should be noted that, if the needle ends up too far in the foramen (especially on the anterior side), there is a risk that the vertebral artery will have been punctured. To prevent injection of local anaesthetic into the intrathecal space, verification of the position of the tip of the needle in AP fluoroscopy must be performed. Measurement of saturation and the presence of resuscitation equipment are mandatory. Infections have been described, but the incidence is unknown and probably very low.
- Up until now, no complications have been reported in association with PRF.