Radiofrequency treatment
Sluijter, Menno E., Prof.em.dr.med., Schweizer Paraplegiker Zentrum, Guido A. Zäch-Straße,
CH-6207 Nottwil, Switzerland
Stokke, Trond M., Dr.med.habil., Klinikk for radiofrekvensdenervering as, Valkyriegaten 8,
NO-0366 Oslo, Norway
Introduction
There are few fields in medicine where opinions are as diametrically opposed as Whiplash
Associated Disorders (WAD). This is for a number of reasons. A whiplash trauma cannot be
seen on an MRI-scan, nor is there another investigation that can reliably prove or disprove the
condition. Pre-existing psychological factors play an important role, and this is true both for
the clinical presentation and for the chances of recovery. And finally there are considerable
financial interests involved.
It is not surprising then that the role of Radiofrequency (RF) in the treatment of WAD has its
limitations. Yet encouraging results have been reported and the possibilities have widened
since pulsed RF has become available. Since the reader may not be familiar with the subject,
this chapter will give a brief overview of the principles of RF treatment before discussing the
applications in WAD patients.
Radiofrequency
There are two types of RF: continuous RF (CRF) and pulsed RF (PRF). CRF has been with us
for a number of decades as a tool to treat pain of spinal origin. It is the objective of CRF to
burn a nerve that conducts nociceptive stimuli.
Physical principles of CRF
RF is applied through a needle that is insulated except for a 4 – 10 mm so-called active tip.
The temperature is monitored with a thermocouple in the tip of the needle. The RF electric
fields, with a frequency of 400 – 500 kHz, cause friction of ions in the tissue, and this
produces heat. The resulting tip temperature depends on the power deposition on one hand
and on the heat washout on the other. The power deposition (P) depends on the voltage (V),
the time (t) and on the impedance (R):
P = ½ tV2/R
The heat washout, being the sum of conductive heat loss and of the effect of circulation, may
vary considerably. Therefore the power output of the generator that is needed to reach a
predetermined temperature, is variable too.
A CRF lesion typically has an initial phase of about 10 seconds’ duration, to build up the
temperature to the desired level, and a maintenance phase of 60 – 90 seconds, during which
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the temperature is kept constant. In a typical case the output voltage of the generator would be
30 V during the initial phase and 16 V during the maintenance phase.
Mode of action of CRF
Initially the main indication was lesioning the medial branch of the posterior ramus of the
spinal nerve, which innervates the zygapophyseal or facet joints, and the principle was simple.
If one burns a nerve that conducts nociceptive stimuli from a nociceptive focus to the spinal
cord, the pain should go away. Since the principle was so simple, and since the method
seemed effective in a proportion of patients, nobody ever questioned the mode of action.
Questions like why this is one of the very few ablative procedures that is effective, refuting
what we know about changes in the more central parts of the nervous system in chronic pain
patients, were – and are – just ignored.
The development of PRF
What could not be ignored was the finding that RF may also be effective if the geometry is
less convenient than in the case of the facet joints. For example, it was found that an RF
lesion may also effectively treat the pain that is caused by an acute herniated disc. In this case
the lesion is made distal to the site of nociception. Another example is the treatment of cluster
headache by making a lesion in the sphenopalatine ganglion. It is for these reasons that the
role of heat and destruction in producing the clinical results of RF lesions came under
discussion, and this ultimately led to the development of PRF [16]. It is important to note that
PRF did not have the pretence of being an invention de novo. Its sole purpose was to prove or
disprove the role of heat in conventional RF lesions, the parameters were arbitrary and there
was no concept of how RF without heat could be effective.
Physical parameters of PRF
In PRF RF fields are applied to the electrode in 2 pulses/sec of 10-20 msec each. The voltage
that is most commonly applied is 45 V. Therefore the overall power deposition during PRF is
much smaller than during CRF (a smaller value of t in the above formula) but during the
active pulse the voltage – and therefore the electric fields - are higher.
Like CRF, PRF has temperature effects; they are twofold. During the active pulse the heat
formation causes a sharp but brief rise of the tip temperature. Since the duration of the pulse is
short, the magnitude of these so-called heat spikes depends exclusively on the power
deposition, not on the heat washout. It can be calculated that the heat spikes will be in the
order of magnitude of 5 – 100 °C when PRF is applied at 45 V, and this has been confirmed
by measurements with an ultrafast thermocouple [4]. It can also be calculated that the
temperature falls off very rapidly away from the electrode, so that the effect is virtually nil at
a distance of > 0.2 mm.
The heat spikes cannot be observed on the temperature readings of commercially available
lesion generators, because the conventional thermocouples are slow, only indicating the mean
tip temperature. The mean tip temperature is affected by the heat formation too, and it will
rise to a value that is determined by the power deposition and by the heat washout, as in CRF.
A normal value would be 41 °C.
During the active pulse, PRF also creates an electric field (E) [4], which depends on the shape
of the electrode and on the applied voltage. The magnitude can again be predicted from a
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computer model. The strongest field is found just ahead of the electrode tip, but it then falls
off very rapidly. The field along the cylindrical part of the electrode tip is lower initially, but
it falls off less rapidly with distance.
Mode of action of PRF
The efficacy of PRF has now been proven in two double blind, sham controlled studies [15,
20]. The search for the mode of action of PRF has centred around the primum agens, and on
the way in which the primum agens exerts its effect. The first part has been fairly well
established, the answer to the second question has not yet been given.
As for the primum agens, there are four options: an ablative effect, a temperature effect, an
effect of the electric field or an effect of the current density. PRF does have a very mild
destructive effect [5]. This may be caused by various factors. The heat spikes may play a role.
The effect of these short lasting elevations of temperature on tissue is not known. The electric
fields ahead of the tip are strong enough to kill a cell, certainly when they are maintained for
20 msec, which is very long in cell biological terms. A third factor is the fact that we are here
dealing with an alternating current. The resulting shaking up of molecules has an additional
destructive effect, which has more to do with duration than with voltage [2]. Ablation then
cannot be excluded completely, but since the ablative effects only occur over a very short
distance from the electrode, it is an unlikely explanation.
As for the temperature, it has been found that there is no relationship between the mean tip
temperature and the clinical result [17]. Since the effect of the heat spikes is only noticeable
over a very small distance, it may be confidently assumed that temperature effects are not
instrumental in causing the clinical effect of PRF.
The relationship with current density has been investigated during a study of the relationship
between impedance and clinical result. It turned out that there is an inverse correlation: the
lower the impedance, the higher is the chance on a good result. In another group of patients
the current was kept constant, and the voltage was adjusted as needed. If current density were
important, this should have favoured the high impedance cases. It did not, the high impedance
cases did even worse. It is therefore unlikely that current density per se has an effect.
By exclusion of other options it can therefore be said that the electric field probably causes
the effect of PRF. Since the range of E is so extensive during PRF, it is of interest to know
which part of the range is likely to be responsible for causing the clinical effect. We then have
to look at a standard situation of the electrode-target geometry in a procedure with a proven
effect, such as the treatment of the dorsal root ganglion (DRG). During that procedure the
target is approached by the electrode, until a response is obtained during 50 Hz stimulation at
a voltage of 0.2 – 0.3 V. The distance to the target could be measured during procedures that
were performed under CT monitoring. It was about 1.5 - 2 mm. It can be calculated that E
must then have been in the 1500 to 2500 V/m range. Surprisingly then these low strength
electric fields must be the biologically active factor.
How then do these low-strength electric fields have an effect? From the early days of PRF
there has been a suspicion that PRF might somehow effectuate a neuromodulating effect on
more central structures in the nervous system. And indeed, it was found in experimental work
that PRF application to the DRG provokes expression of c-fos in the dorsal horn [6, 19].
Another ingenious theory [4] implicates that the active pulse causes a partial depolarisation of
the cell membrane, and that this in turn causes subliminal stimulation. Subliminal stimulation
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may cause long term potentiation or long term depression (LTD), depending on the used
frequency. In the case of PRF, with its frequency of 2 pulses/sec, LTD of the first synapse
would result.
Both these propositions – neuromodulation and LTD – are attractive, but unfortunately the
facts are ill at ease with the theories. Both neuromodulation and LTD presume an immediate
effect. Indeed there is such an immediate effect in many patients undergoing PRF treatment
without the use of local anaesthesia. They are immediately free of pain, and this has been
named the stunning phase. However, this stunning effect does not last. After a few days it
may be followed by postoperative discomfort, lasting for about 2 weeks during which the
original pain may be worse than before treatment.
The matter is therefore unresolved and this frustrates attempts to improve the method. As
said, the parameters of PRF have been arbitrarily chosen. But there are six variable
parameters: pulse duration, pulse frequency, the RF frequency during the pulse, the voltage,
the timing of the pulse (regular vs. irregular) and the total exposure time. It is therefore not
feasible to determine the optimal parameters with clinical studies. A biological model is
needed to solve this problem.
Algorhythms in RF treatment
The spine is a complicated structure and spinal pain may therefore be multifactorial. For
example, after successful treatment of sciatica due to a herniated disc, there is often residual
back pain, that has to be treated separately. This is true for surgical treatment, and it is true for
RF treatment as well.
RF treatment is therefore often performed in a number of steps, and there are algorhythms for
the sequence of these steps [17]. The indication for a next step is often confirmed by
diagnostic nerve blocks, unless there is an anatomical abnormality with concordant pain.
Conditions following trauma to the cervical spine
The cervical part of the spine is probably the region that is most exposed to trauma. The
actual trauma may have seemed insignificant, or it may have been forgotten, but pain may
later develop as cervicogenic headache or as the sequence of a solitary disc lesion in the C3 to
C5 region in an otherwise unaffected spine. These patients have no specific psychological or
other problems, and they generally respond well to RF treatment. WAD patients have a
number of characteristics that are not commonly found in non-WAD posttraumatic patients.
Psychological problems in WAD-patients
Many WAD-patients are involved in litigation. Opinions vary widely on the consequences.
Some investigators find a high incidence of malingering [14]; others report no correlation
between litigation and recovery following medial branch neurotomy [13].
Depression has a high prevalence in WAD-patients. Persistence of symptoms following
whiplash injury correlates strongly with depression before the accident [10, 1]. On the other
hand, resolution of psychological symptoms after successful neurotomy has been reported
[21].
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The contradiction may not be as large as it seems, because patients who have been selected
for a neurotomy obviously have focal pathology. They have therefore been preselected and
they need not be representative for WAD-patients as a group. It may be concluded that
patients with proven focal pathology are candidates for RF treatment, even if they are
depressed and even if they are involved in litigation. It is the patient with a more global
symptomatology, as it is regularly found in WAD-patients, who may need psychological
evaluation and treatment in the first place.
Symptomatology of WAD-patients
The symptomatology of WAD patients is not confined to neck pain. Involvement of the upper
cervical joints may lead to headache, balance problems, vertigo, dizziness, eye problems,
tinnitus, poor concentration and sensitivity to light [8]. Central damage may cause
hypersensitivity and generalized muscular hyperalgesia [3] and loss of attention and
concentration [9].
Brachialgia has a high prevalence in WAD patients [7]. It is often diagnosed as a thoracic
outlet syndrome, possibly associated with damage to the scalenus muscles. The
symptomatology is typically at the ulnar aspect of the arm. Diagnostic segmental nerve blocks
at the levels C5 and C8 are often positive, rather then C6 and C7, which are more involved in
degenerative disease.
RF treatment in WAD-patients
RF treatment is not a suitable option for the acute whiplash patient. There is a consensus that
since there is a tendency to spontaneous recovery during the first 3 months after the accident,
RF should only be offered if there is no improvement at the end of this period.
WAD is a condition with many interacting aspects. It is therefore not surprising that RF
treatment is not a panacea that is suitable for every patient. RF only comes into focus if a
preliminary diagnosis is made of pain that is emanating from a focal source. Since the role of
imaging is mostly limited to the exclusion of fractures and dislocations, such a diagnosis must
be made on the basis of the history and of physical examination. The source may be one or
more facet joints, it may be a spinal segmental level, or it may be one of the joints in the
upper cervical region. But the findings must not be ubiquitous in the entire cervical region.
RF treatment of the medial branch
There are two approaches to the medial branch, both with their advantages and disadvantages.
First, there is the approach from posterior with the patient in the prone position. In this
technique the procedure is preceded by accurate, multiple diagnostic blocks. The procedure
consists of burning the medial branch completely with several lesions in a procedure that lasts
several hours. The efficacy has been proven in a double blind study [11]. The result was
positive in 60% of cases. This sounds modest considering the elaborate selection of patients,
but the sample was small and the study was not designed to determine the efficacy of the
procedure.
In the second method diagnostic blocks are only made if there is doubt about the diagnosis.
This stems from the argument that in the neck a diagnosis of facet pain can confidently be
made by palpation, and that the number of procedures should be kept to a minimum. This is
because it is common in a WAD-patient to treat a DRG in a later stage in order to get an
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optimal result. The technique then is approaching the medial branch from lateral with the
patient in a supine position in a procedure that takes up to 30 minutes. The treatment is most
commonly done with CRF.
RF treatment of the dorsal root ganglion
PRF is used for the treatment of the DRG. It is always preceded by a diagnostic segmental
nerve block, because differentiation between the various segmental levels by physical
examination alone may be difficult. For the DRG procedure the electrode is aimed, under an
oblique projection and using tunnel vision, at a target point that is somewhat caudal on the
posterior aspect of the foramen. The ganglion is then approached until a 50 Hz stimulation
response is obtained at < 0.5 V.
The technique is different for the upper two cervical levels. For C2 the approach is from
straight lateral. Treatment at the C1 level [18] has been useful in patients with painful upper
joints. At this level a preceding diagnostic block is usually omitted because the area around
the ganglion/nerve is very vascular. The approach is from straight lateral like for C2, but the
electrode should not be inserted too deep to avoid puncturing the vertebral artery.
Results of RF treatment
The efficacy of CRF treatment of the medial branch has convincingly been proven [11]. Good
results were also reported in another study [12], where the lateral approach was used, but in
this case a number of patients had additional RF treatment. Controlled studies of PRF
treatment of the medial branch are lacking so far.
The efficacy of PRF treatment of the DRG in WAD patients has also been proven in a double
blind placebo controlled study (15). In this study WAD patients were included who had not
responded to facet treatment and who had predominantly brachialgia. 19 patients were
actively treated and 12 patients received sham treatment. There was a significantly better
effect on pain and disability scores in the actively treated group, 1 and 6 months after
treatment.
New developments
A new application of PRF is still in its infancy, and it is mentioned here with the caution that
the results of controlled clinical studies have not become available yet. It has been found that
PRF may be used to treat pain that is emanating from joints, by placing an electrode
intraarticularly.
The principle is based on the special electrical environment within a joint. Since bone acts as
an insulator, the current that is generated during the active pulse is (partially) deflected by the
bone in the direction of the periphery of the joint, where the capsule is. This increases the
current density in the capsule, and therefore the electric field is larger than would be expected
at larger distances from the electrode. It can be calculated that the strength of the electric field
in the joint capsule can easily reach the values that are thought to cause the clinical effect of
PRF.
This method has been tried out both in large and in small joints. In WAD patients with
symptomatology suggesting involvement of the AA joint it has been particularly useful. The
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advantage of the method is that the approach is not difficult, that no injection of fluid into the
joint is required and that so far the results seem to be durable (maximal follow up: 9 months).
The joint is approached using a slightly oblique projection, directing the needle to the anterior
portion of the joint from a somewhat caudal approach. Final insertion into the joint is then
done under AP projection. PRF is then delivered at 40 V, using 10 msec pulse width for a
total duration of 10 minutes.
Pain emanating from one single facet joint has also been treated with good results, but here
the approach may at times be more difficult, due to a variable anatomy or because arthrotic
changes block the entry into the joint. Performing the procedure under CT-guidance is a
distinct advantage.
Conclusions
PRF was developed as a method to prove or disprove the role of heat in the mode of action of
CRF. Now that it has been proven that it is clinically effective, the search for the mode of
action has not been completed. The action is probably initiated by low strength electric fields.
WAD is a complicated condition with many interacting factors. The application of RF should
only be considered if there is a preliminary diagnosis of focal pain, which has not improved
over three months following the accident.
WAD may be caused by focal pain from one or two facet joints, and this can be successfully
treated by RF treatment of the medial branch. In a majority of cases however the
symptomatology is more complex, involving brachialgia and/or headache, and treatment of
one or more DRG’s may be required. The efficacy of the medial branch procedure with CRF
and of the DRG procedure with PRF has both been proven in double blind placebo-controlled
studies.
Intraarticular application of PRF may offer a solution for WAD patients in conditions that
have so far been difficult to treat.
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