Radiofrequency treatment

in

Whiplash Associated

Disorders

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 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 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 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].

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 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 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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