Placing of the electrode
In spinal cord stimulation (SCS), the placing of the electrode must be performed under local anaesthesia. The cooperation of the patient is vital since, during the procedure, he has to inform the treating pain specialist whether the paraesthesia provoked by the lead covers the area of his pain.
Rather extensive local anaesthesia is necessary as otherwise an anxious patient, who experiences the procedure as painful, cannot cooperate in an adequate way.
It is advisable to administer a minimal dose of fast acting opioids, such as alfentanyl (0.25 mg). Moreover, the position of the patient is of the utmost importance.
For the mid-thoracic placement of electrodes, the most common technique is with the patient in the prone position, legs slightly flexed and a pillow under the abdomen to decrease the lumbar lordosis. The moment the lead is in the correct position, the position of the patient also enables sedation with, for example, a propofol drip or midazolam. This sedation is only necessary during fixating of the electrode and tunnelling of the extension test lead. The entry level of the lead depends on the indication for SCS.
In patients with lumbar radiculopathy and a history of lower back surgery, the entry level is preferably at Th12/L1. This entry level is sufficiently far from the operated level (less chance of epidural fibrosis). At the same time, this entry level is also located above the lumbar lordosis, making the approach to the epidural space easier, with the needle at a large angle via the para-median route.
For patients with radiculopathy and lower back pain, the tip of the electrode must end around level Th8/Th9. For patients with radiculopathy only, the tip of the electrode should end around level Th9/h10.
The exact position of the electrode can be achieved using the so-called 'trolling technique'. In this way, the electrode is carefully and slowly withdrawn under continuous stimulation until the so-called 'sweet spot' is reached. Of course, this technique demands an experienced and mutually well-organised team at the operating table. In experienced hands, placement of the electrodes can also be performed in a sitting position. This sitting position may be necessary in patients who are medically unable to lie in a prone position (obesity and COPD), or who experience too much pain in the prone position. The disadvantage of the sitting position is the chance of vagal reflexes that need careful monitoring of the heart rhythm, and possibly preventive administration of atropine and ephedrine. Moreover, the sitting position involves the risk of the specialist receiving a higher dose of radiation.
Choice of electrode
An important aspect of the placement of the electrodes is screening during the operation. Quadripolar electrodes have a possible 50 combinations, octopolar 6050 combinations. So, it is not a matter of 'trial and error', but there is a certain rationale to the choice of an electrode that demands knowledge of neuro-anatomy and neurophysiology. If lower back pain co-exists, the electrical currents have to be sent deep into the dorsal columns to the nerve fibres responsible for the lower back pain.
The level has to be found where the layer of cerebral spinal fluid (CSF) is the narrowest. Electrical resistance to CSF is high and will conduct the electrical currents towards the nerve roots. The electrical threshold of the nerve roots is lower than the threshold of the dorsal columns. As a consequence, painful stimulation of the nerve roots can be experienced as cramp in the thorax or abdomen. In this case, the 'guarded cathode' technique is preferred. In this technique, a cathode is used, guarded by two anodes (+ - +) positioned nearby. The cathode is the stimulating pole and is decisive for the direction of the stimulation. The pulse range is decisive for the extent of the stimulated zone.
For patients with failed back surgery, electrodes are selected with a short distance between the various poles (the shorter the distance, the deeper the current is directed). For more extent areas, such as an extremity, electrodes with more interspaced poles can be used. In this way, an anode that is further away from the cathode can be activated.
Figure 1: Octopolar lead at level Th10/Th12: AP view
Octopolar electrodes have more possibilities than quadripolar because they cover larger areas and provide a broader choice of programmes for the patient.
Figure 2: Octopolar lead at level Th10/Th12: lateral view
Disadvantages include the cost of the hardware. However, due to a lower impendence, power consumption is significantly lower. The positioning of octopolar electrodes is far more difficult and demands greater experience. With respect to the power consumption, it must be realised that high amplitudes, large pulse ranges and high frequencies will raise the rate of consumption. The 'trolling technique' is helpful in finding the level with the lowest power consumption.
The danger of infection is substantial with all implants. Because the system's temporary extension lead tunnels to the exterior, the chance of infection is greater. A strictly aseptic surgical procedure, good antibiotic policy, and rigorous protocol for wound care help to prevent infection.
Choice of generator
Due to compatibility and linkage, the choice of the most adapted generator and of electrode is strongly mutually dependent. Moreover, the cost of treatment must be considered, and the cost of the generator will weigh heavily. More recent generators are able to connect 16 poles. For example, two octopolar or one octopolar with two quadripolar electrodes can be connected together with the generator. The software for these generators is able to programme various strategies for the patient. However, experience has shown that only few patients make use of all these possibilities, and this should be considered against the extra costs.
The complications of SCS range from minor and easily correctable, such as insufficient paraesthesia, and cover the painful area to paralysis, nerve damage and mortality. Therefore, it is of the utmost importance that the patient is adequately informed about the potential complications of SCS, so that he is able to identify the problems and can subsequently contact the multidisciplinary pain team.
Other complications include infections (3-5%), which present as superficial skin infections, wound infections and epidural abscesses. The most frequent infections are seen around the implanted generator (54%), the connection (17%), and at the level of the incision in the back (8%). Should the infection reach the implant, the whole system will have to be removed. Migration of the lead (5-14%) and electrode breakage (0-23%) are the most frequent complications leading to re-operation. The use of silicon anchoring can prevent migration of the lead.
In order to prevent the connection wearing out, due to repeated bending and pressure on it, it is preferable to press the tip of the connection through the deep fascia. In addition, it is advisable to retain a loop between the connection and the generator. The best site for the generator to be implanted is the abdomen. In this way, tension on the electrode is reduced.