Minimally Invasive Treatments for Leg Pain

Therapeutic approaches to leg pain are closely related to their underlying mechanism. Leg pain arising for low back pathology can be either inflammatory, or neuropathic.

Therapeutic Epidural Injections

Epidural steroid injection is probably the most frequent procedure performed to treat radicular pain. Technique is simple, and safe. Complications occur and may be related to the needle placement or to the drug administered. They include infections, dural tap and very rarely neurological damage (Nelson et al., 2001). Manchikanti et al. evaluated the effects of neuraxial steroids and found no significant effect of epidural steroids on weight and bone mass density (Manchikanti et al., 2000). Moreover, the commonly available steroid preparation can be safely used in the epidural space (Dunbar et al., 2002).

Three approaches to the epidural space are possible: Transforaminal, Inter-laminar or Caudal. The efficacy of epidural steroids injections has been questioned in many studies, most of them supporting the use of the technique (McQuay et al., 1998; Devulder, 1999). However, many studies either prospective or retrospective mixed the results of these three different techniques and did not consider the possible differences in the spread of the medication in the epidural space. Since, this problem has been addressed and in each of the three approaches differences have been shown (Price et al., 2000).

The most effective technique is probably the transforaminal approach (Karppinen et al., 2001). It is however associated with the highest complication rate and has been recently questioned. The most worrying complication is related to inadvertent injection of steroid solution into the Adamkie-wicz artery (Houten et al., 2002). The entry of the artery into the foramen is subject to a high anatomical variability and enters between L2 to T9 in 85% of patients but may arise from the lower lumbar spine and even from as low as S1. To reduce the incidence of such complications, it is advisable to not only aspirate on the syringe but also to inject dye before injecting a solution with potential aggregates prone to induce small vessels occlusions.

The combined evidence of caudal epidural steroid injections with randomised trials and prospective and retrospective trails is strong for short-term relief and moderate for long-term relief. It is a safe technique and should always be performed under fluoroscopy. Two studies have specifi cally addressed the problem of FBSS. Revel in a study including 60 patients showed significant improvement of symptoms in 49% of patients against 19% in the control group (Revel et al., 1996). However, another multicenter randomized study including 47 patients reported no short or long term benefit in this group of patients (Meadeb et al., 2001).

Trans-laminar epidural injections show moderate evidence for short term relief and no evidence for long-term relief. This may be due to the repartition of the solution in the epidural space, probably remaining in the posterior epidural compartment. It may also be related to the fact that most inter-laminar procedure are performed without fluoroscopic guidance.

Percutaneous Epidural Neuroplasty (Racz Procedure) and Epiduroscopy

If the effect of epidural steroid injections is local, i.e. a direct effect on the injured nerve root or on the ''leaky disc'', it is essential that the steroid reach the site of injury. Historically, epidural steroid injections have been performed ''blindly'', without any radiological guidance, however many factors may prohibit steroids from reaching the intended nerve root, such as scarring, adhesions, adipose tissue and septa, which may be present in the operated and non-operated backs. Thus theoretically drugs injected into a scarred epidural space will follow the path of least resistance, away from the painful site.

Percutaneous epidural neuroplasty (Racz procedure): It seems rational to assume that mobilization or dissolution of fibrosis may remove barriers that prevent application of drugs. Epidural neuroplasty (also known as Racz procedure) consists of accessing the epidural space in a caudal or transforaminal approach, injecting non-ionic contrast material (thus performing an epidurogram) in order to detect ''filling defects'' in the epidural space. This is followed by gentle manipulation of a metal reinforced catheter in order to liberate adhesions (''filling the defects''), and then injecting the targeted medication (Heavner et al., 1999). This procedure, which allows prolonged pain relief in refractory cases, has the advantage of targeted drug delivery, but has the disadvantage of an indirect, two dimensional vision of the presumed pathology.

The epidurographic diagnosis of spine pathology may be followed by neurolysis with the injection of corticosteroids, hypertonic saline and/or hyaluronidase. Two RCT and 3 retrospective evaluations showed pain relief up to a year, with cost effectiveness gains of up to 8,127 US$ per year per patient. When performed by appropriately skilled personnel this procedure has a low complication rate, however dural puncture, spinal cord compression, catheter shearing, hypertonic saline toxicity, infection and bleeding remain worrisome (Manchikanti et al., 1999 b).

Spinal Endoscopy: Even when injection is done under fluoroscopy, the image obtained is two-dimensional and can be misleading. Thus, epi-dural endoscopy provides us a three-dimensional, real-time, color view of anatomy-pathology in the epidural space.

Access of the epidural space with a flexible fibre-optic catheter via the sacral hiatus appears to be safe and efficient (Geurts et al., 2002). The procedure is done under local anaesthesia while continuously monitoring intra-epidural pressures, and patient's response. Normal nerve roots when touched cause paraesthesia, diseased ones pain, so patient report is essential while gently performing adhesiolysis. The technique allows examination of the epidural space and its contents, targeted injection of medication, lysis of scar tissue (adhesiolysis) and (potentially) retrieval of foreign bodies (Kitahata, 2002). As technology grows new possibilities such as minimally invasive surgery, intraoperative nerve stimulation and immuno-biological interference evolve, promising an important role of spinal endos-copy in the treatment of spinal pain.

In a prospective case series all patients undergoing epiduroscopy suffered from adhesions between nerve roots, dura and ligamentum flavum, 41% very dense, associated with previous surgery. If fibrosis is a result of chronic radiculitis, neurogenic inflammation and impaired fibrinolysis, repeat surgery will probably aggravate the situation and is thus ill advised. The authors hypothesize that adhesions obstruct radicular veins and interfere with the nervi vasorum, creating intra-neural edema and abnormal pain transmission. Dilution or ''washing out'' phospholipase A2 and synovial cytokines may also contribute to symptom improvement (Richardson et al., 2001).

In another recent study Igarashi et al. showed that epiduroscopy reduces back and leg pain among 58 elderly patients suffering spinal stenosis. Pain relief lasted more than a year after the procedure without any neurological complications, especially in patients suffering from abundant adhesions (Igarashi et al., 2004). This is of importance since persistent pain among patients suffering from FBSS is thought to be due to epidural scar. Furthermore, reservations about using this technique in patients suffering from a ''restrictive'' epidural space and thus fear from elevated intra-epidural pressures during the procedure, have been founded to be clinically debatable.

Spinal Cord Stimulation

Electrical stimulation has been used in the treatment of a variety of disease since the ancient Greeks. From the torpedo fish or ''narke'' inducing narcosis to the Faradization in the 18 th century, electricity has been regarded as a therapeutic tool. In the end of the 19th and the early 20th century electricity has been disregarded in favor to emergent new pharmaceutical agents and it is not until the early 60's that electrotherapy reappeared. The publication by Wall and Merzack in 1965 of the ''Gate control theory'' of pain gave birth to the contemporary spinal cord stimulation (Melzack and Wall, 1965). The argument that electrical stimulation of large fibers would close the gate to input from the smaller diameter and unmyelinated A-delta and C fibers mediating pain was determinant to the success of SCS. Since, this hypothesis has been subject to criticism and we know now that it is not the only mechanism involve in pain control (Linderoth et al., 1999).

Spinal cord stimulation is achieved using a voltage-controlled pulse generator. It creates a potential difference between two outputs. The injected current is distributed in a 3-dimentional space made up of electrically conducting anatomical structures. The resulting 3-dimentional electric field can be represented by its potential distribution and by its current density distribution. These distributions can be visualized by isopotential line and isocurrent lines, respectively, as shown in the transverse section of spinal cord stimulation model. The stimulation induces mainly a depolarization of the nerve large myelinated fibers, both orthodromically and antidromi-cally (Oakley et al., 2002).

The principle is to stimulate the dorsal column and interfere with the sensory information coming from the painful area. The analgesic mechanisms of SCS are however not clear. It is universally accepted that pares-thesia coverage of the painful area, indicating the activation of the dorsal column, is necessary to obtain pain relief, it may however not be the mechanism of SCS. A possible stimulation target may be the dorsolateral funi-culus which is known to contain descending pain controlling pathways. There is no convincing evidence for the involvement of opioid mechanism in the effect of SCS. Endorphins levels are not influenced by SCS and naloxone does not reverse pain relief induced by SCS (Meyerson et al., 1977). A possible role of GABA and adenosine in the analgesic action of SCS is suggested by animal and human studies indicating that GABA antagonists reverse partially the effect of SCS (Cui et al., 1996) and that a synergic effect of adenosine with SCS was observed (Cui et al., 1997).

The first spinal cord stimulator was placed in 1967 by Shealy by a D2-D3 laminectomy (Shealy et al., 1967). The first indication was cancer pain. Rapidly, it became clear that not all ''pains'' were sensible to SCS. Mainly, neuropathic pain was, nociceptive pain was not. Thanks to numerous publications on SCS, we now know that intermediate clinical states and other sympathically maintained pain may be responsive to SCS which has progressively gained acceptance in a number of clinical pain syndromes including FBSS (Krames, 1999).

Implantable devices have a place in treatment of FBSS patients when all other conservative and minimally invasive tests and therapies have failed to diagnose or treat a particular condition. This includes an important number of patients (North et al., 2002).

A proper patient selection is essential to achieve adequate pain relief with SCS. History is essential for the appropriate selection of the candidates to SCS. Pain characteristics and neuropathic features, must be searched, psychological screening may be useful. The evaluation of these crucial elements may lead to a shorter trial period, resulting in less infection rate and therapeutic failures.

In FBSS patients, leg pain responds better than axial back pain to SCS and neuropathic better than nociceptive, mechanical pain, the later almost non responsive to SCS.

With SCS, the active electrode, the cathode or negative electrode must be located near the level of the spinal cord dorsal columns that anatomically represents the level to be stimulated. The electrode is therefore placed in the epidural space under fluoroscopy guidance and with a patient awake and anesthetized locally at the needle entry point. The Tuohy needle is inserted into the epidural space using the loss of resistance technique and advanced rostrally up to the desired level. At this point, the external stimulator is connected and the patient is asked whether the stimulation, the paresthesia felt is covering the painful area or not. When the pain is unilateral, the electrode is placed on the side of the patient's pain lateral to the midline on the homolateral dorsal column. If the pain is bilateral or axial, single or multiple electrodes must be placed on the midline or close to it.

SCS includes 3 components: The epidural electrode the connection between the epidural electrode and the battery and the Implantable-pulse-generator (IPG). A wide range of electrodes may be used. Two main categories are percutaneous leads and surgical leads, the later requiring laminotomy.

In failed-back patients, the implantation of the SCS is divided in three steps. The test electrode implantation performed under local anesthesia, the trial period ranging from one to four weeks and the IPG implantation commonly achieved under general anesthesia.

We think that placing a test lead without patient's collaboration leads to a higher failure rate. The only indications for a direct implant of a surgical electrode are recurrent displacement of percutaneous leads or of if a predicted target is in the area of prior surgery.

Trial period duration is debatable. Most authors recognize a one week test is minimal to obtain reasonable information to proceed to a definitive implantation. According to local practice the period extends from one to four weeks test. Criteria for a positive test are listed (Table 1).

The definitive implant requires connecting the implanted epidural lead to the connection, tunneled under the skin to the hypochondria where the IPG is placed.

P. Mavrocordatos and A. Cahana Table 1. SCS Screening Trial Criteria

1. Minimum of 50% pain reduction in VAS score with test-lead implant

2. The area of induced paraesthesia must cover the area of pain

3. Paraesthesia well tolerated

4. Mood, sleep, activity improvement

Once the patient is implanted, treatment really begins. The surgical and trial periods are the easy part of the work. The follow-up of these patients is a dynamic process and may require long hours and programming is not always easy. Numerous consults may be needed and the willingness and patience of the physician and his team are essential.

Complications may be divided in 3 groups: surgical complications, device related and stimulation related complications.

Potential surgical complications include infection, spinal fluid leakage, hemorrhage and neurological injury. In 1995, Turner reviewed 31 studies referring between 0 to 12% infection rates, mean 5% (Turner et al., 1995).

In over 20 years, North's group reported no major morbidity defined as neurological injury, meningitis or life-threatening infection (North et al., 1993). Electrode migration is the most common complication occurring 24% of the time (Turner et al., 1995). For this reason multichannel devices have been shown to be more reliable in this regard. It has also been advocated that paddle electrodes are more stable (North et al., 1997). Although no randomized studies have been published, it seems that paddle electrodes are associated with improved long term effectiveness, particularly for low back pain. This region needs high voltage stimulation and the design of the paddle leads with the stimulating electrode directed towards the dura unlike the percutaneous electrodes which directs all the usable current towards the medulla. This problem is of utmost importance for the development of new technology: What we really needed is a percutaneous paddle-like electrode.

Other problems like discomfort due to inadequate IPG position in the abdomen needing repositioning are uncommon.

Stimulation related discomfort is rare as it usually precludes definitive implant. If stimulation is painful or bothers the patient during the trial period, it is usually not a successful test and the electrode is removed. Patients usual complaint is related to posture induced changes in the intensity of stimulation. Important reprogramming sessions are mostly related to electrode displacement.

Most studies on SCS for FBSS are retrospective. Turner et al. reviewed 41 articles reporting approximately 50-60% of patients with FBSS describing a >50% pain reduction from the use of SCS (Turner et al., 1995). Hieu et al., showed a long term efficacy in 63% of patients and fair in 22% after 42 months follow-up (Hieu et al., 1994).

Although no controlled studies have been conducted on SCS, recent prospective series reinforced the role of SCS in FBSS. North conducted a randomized comparison of SCS with re-operation with a 6 months crossover arm in the study. 51 patients with FBSS consented to randomization. This study demonstrated a significant difference between patients who opted for cross over from SCS to re-operation but not visa versa and concluded that SCS is a viable alternative to re-operation (North et al., 1995).

Cost effectiveness can be evaluated comparing the estimated cost of therapy per year in groups treated by SCS versus alternative treatment. Bell et al. compared SCS versus surgeries and other alternative treatment over 5 years. The reduced demand for medical care of successfully SCS treated patients leads to the observation that SCS pays for itself in an average of 2.1 years (Bell et al., 1997).

Considering that SCS is an end stage technique used in patients in whom everything has failed, SCS is an effective treatment, particularly considering the low complication rate. However, new technology developments are needed to allow percutaneous placement of more efficient electrodes in terms of energy sparing and precision of current distribution (Deer et al., 2001).

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