Neuropathic Pain Demystified

The following situation is no doubt occurring on a daily basis somewhere in the UK or equally in a stable near you .................


Horse owner - “ My instructor told me that my horse had a problem with its back and that I should have it looked at and if necessary treated “.


So being one who chose to exercise his / her democratic rights, the owner did some investigating to see what their options were.

Now it appeared as though there were plenty of choices, chiropractor, acupuncturist, physiotherapist, osteopath, electrotherapist, hydrotherapist, laser therapist, massage therapist, magnetic field therapist, bone-joint manipulator ( long lever or short), trigger point therapist, touch therapist and ‘natural’ horse person.....  all seemingly qualified and claiming to treat back problems or one of its aliases i.e. trigger points, acupressure-points, vertebral subluxations, muscle spasm, muscle strains, muscle imbalances , joint strain, bone inflammation, tendons strain, ligament strain etc, ............

and all with their own methodologies and terminology's or languages.

And then to further confuse the issue his / her vet suggested that “ the problem was probably in the hock and that they would be wasting  their time and money if they were to pursue the back problem angle” ......... has left them


                                                                                    “ SLIGHTLY CONFUSED! “


.......... and I can certainly concur with those emotions as would most other veterinarians, horse riders, owners and trainers. Fortunately though after 20 years of research and with the assistance of numerous qualified persons  practising in these seemingly different fields ( some world leaders) I am confident that at least the basic principles are now quite clear. ** (footnote)

** N.B.  as with any medical subject, research into the many causes, mechanisms of and treatments for these conditions and the debate that accompanies this research is ongoing.


In this article we will discuss some of the more relevant information about which we now have a more realistic understanding.


We will tackle the subject using a question and answer format . These will consist of the most common questions put to me in discussing this subject with veterinarians, owners, riders and trainers in numerous countries about the globe.


Q. Are all these people treating the same problems?


Ans. Generally speaking, Yes? As much as approaches to treatment and terminology's vary greatly they are in the main treating problems of the back or more correctly of the spine and associated structures. The spine consists of a series of joints the first of which is behind the ears ( atlanto-occipital ) and extends the full length of the ‘back’ to the last tail joint (coccygeal). Spinal joints are similar to other joints in the body in that their primary function is to facilitate movement. In addition these joints provide protection for the spinal cord and its neural offshoots ( nerve roots).


What is the true nature of the problem being treated? Are they spasms, misalignments, slipped discs...?


Ans. The presenting problem has two major elements or symptoms. Both symptoms are usually present however the relative severity or intensity of each can vary greatly. They are ..

        (i) stiffness -which is more correctly referred to as reduced ‘range of movement’ (R.O.M.). e.g. Stiff to the left , stiff to the right , reluctant to collect , head up wont bend, stiff shoulders, stiff in the hind quarters and doesn’t follow through... etc. Basically any reduction in your horses ability to bend or stretch amounts to a loss of R.O.M. Where spinal ( back) problems are implicated the loss of ROM can be either a direct or indirect consequence of spinal stiffness.

                        (a) direct: a stiff upper neck can mean a horse is unable to bring its head under properly during a

                                dressage test.

                        (b) indirect: a stiff lower neck will affect shoulder ROM and this will then most often inhibit forelimb

                                extension movements.

        (ii) Neuropathic Pain (NP) - otherwise referred to as spinal, nerve or back pain -  Neuropathic pain varies significantly from the normal pains that we as veterinarians deal with in equine sports medicine. The more common aches and pains are derived from joint, bone, ligament, tendon or muscle damage or degeneration. The majority of the pain in these situations is the product of a process called inflammation. Chemicals produced  as part of this inflammatory process stimulate local pain receptors. This pain has two important features which distinguish it from NP. Firstly this normal pain is felt or perceived locally within the traumatised tissue and secondly it can be reduced or eliminated by using anti-inflammatory medication. The most common anti-inflammatory medication used in horses is Phenylbutazone (butazolidin), alias ‘bute’.

In contrast NP is not a product of inflammation and therefore will not respond to anti-inflammatory medication and in addition the pain is rarely  felt or perceived within the damaged or degenerate tissues ( nerves). NP is most often ‘referred’ and we thus speak of it as being ‘referred pain’ . Another example of referred pain is heart or cardiac pain where pain perception is often referred to an area inside our left arm or to the skin and muscles in the chest wall over the area of the heart. It is not felt within the heart itself. A very common example of referred NP is ‘sciatica’ where pain can be felt or referred as far south as the soles of our feet. The sites of pain ( referred) are areas where normal  pressure / stretch ( mechanoreceptors) and temperature ( thermoreceptors) nerve endings have undergone a physical change and as a direct result of these changes become hypersensitive and therefore  hyperresponsive. This means that light touch may be perceived by the animal as a prod ( greater pressure) and the reaction or more correctly overreaction of the animal will often indicate this. A change in temperature perception could lead to premature sweating.


The basic distinguishing features between these two types of pain are:

Normal inflammatory pain ( somatic) results from an abnormal stimulus ( chemical) which then triggers a normal response from a normal pain receptor.

Neuropathic Pain (NP) results from a more often normal stimulus ( touch, pressure, temperature) which triggers an abnormal or exaggerated response from an abnormal or altered receptor.


Q. What causes NP and reduced ROM ?


Ans. TRAUMA!!!! to spinal joints and associated neural or nerve elements. Trauma that occured when your horse flipped over backwards, pulled back when tied, fell, was cast ..... etc and unfortunately these incidents often happen when we are not there to witness them. If enough abnormal pressure is applied to a spinal joint, as with any other joint in the body, tissues can be traumatised. Unfortunately as the spinal joints have such a good covering it is often impossible to discern the heat and swelling that would be obvious if the trauma were to a fetlock or hock joint.

They are NOT caused by...  poor riding, ill-fitting saddles, or aggressive handling by prior owners , unless one of these triggered one of the above traumatic situations. Poor riding, ill-fitting saddles etc quite often aggravate  pre existing NP states but are rarely the primary cause. The fact that correcting these situations often leads to some degree of clinical improvement can be misleading in that we may then consider these to be the primary causative agent. My experience with some 7000 cases tells me otherwise.


Q. Why does NP occur ?


Ans. Pain of any description has a purpose. Firstly pain will draw our attention to a functional problem or site of injury . Secondly it will reduce the likelihood of ourselves or an animal doing itself further injury. Stomach pain says ‘stop eating’, wrist pain says ‘don’t pick up that bucket of sand’, whilst NP says stop doing whatever causes undue tension about an injured area of the spine. The referred nature of NP is quite clever in that it often has multiple targets. For example sciatic pain may be perceived in the thigh, calf muscle and sole of foot. The net result is the desired one though, that is , we move very slowly and carefully and thus reduce the chance of further injury!  


Q. Why does NP persist? Shouldn’t it subside in time as tissue healing occurs?


Ans. If as a result of tissue healing full ROM of spinal joints and associated nerve elements occurs, then NP will subside and disappear.

But if full ROM  is not restored or does not return then NP will probably persist. In other words if any significant degree of stiffness remains then elements of NP will also. In human sports medicine post operative and post injury programs of mobilisation therapy ( manual or machine) are now the norm for this very reason.


Q. How do I know if my horse is experiencing NP?


Ans. NP occurs when normal nerve receptors such as those that sense alterations in pressure / stretch or temperature undergo changes and as a result of these changes begin to exaggerate their responses to seemingly normal stimuli. e.g. When you  place a saddle on a horse’s back the pressure receptors under the saddle send a message to the brain that a 4 kg weight is being applied. Response from the horse? None! as it is of course a big strong animal. But what if the message read a ‘400 kg’ weight ? This would mean that the nerve receptors had sent an exaggerated message to the brain. These nerve receptors or endings are referred to neurologically as being ‘hypersensitised’. The exaggerated response from the animal is referred to as a hyperaesthetic response. Hyperaesthesia is this state of increased sensitivity . Have you ever seen a horse with symptoms of hyperaesthesia, or have you made the same mistake as many and referred to this animal as being ‘touchy’, sensitive, cold backed, pole shy, sore shouldered, girthy...... etc? Another tissue which often becomes hyperaesthetic when horses have reduced lower cervical (neck) ROM  is the laminar corium of the fore feet. These horses often don’t enjoy trotting, particularly on hard ground. This is because trotting is one of the most concussive gaits. After hoof trimming  they are often sore for 4-5 days despite all precautions by the farrier. Horses can be more reluctant when asked to jump the higher obstacles or more correctly are concerned about the fore limb concussion that occurs when landing on the other side. This condition is referred to as ‘laminar corial hyperaesthesia’.

To differentiate NP sensitivity from normal excitable behaviour you should ask your veterinarian to carry out a neurological examination to determine whether responses to in particular touch and pressure are normal or exaggerated. This examination should include forelimb corial compression tests (hoof compression).

 

Q. Is there any medication or medical treatment for NP?


Ans. Presently there are none available. Human research using a laboratory model referred to as CCI ( Chronic Compressive Injury) is revealing that there are medications that have significant effects on NP. However these need to undergo a lot more research and testing before they will be available for clinical use.


Q. So how is NP treated today?


Ans. Much of the treatment for NP is based on the premise that for NP to exist there must be some degree of reduction in the normal ROM of nerves or neural tissue. With sciatica a prolapsed disc traps and compresses the adjacent segment of sciatic nerve which can then no longer slip and slide to facilitate limb or leg movements such as those required to ambulate or walk! In this case a primary nerve injury with loss of neural ROM will result in secondary reduced ROM of the limb ( hip joint movement). A person with sciatica will often struggle to perform a raised leg manoeuvre and will often walk with a stiffened gait.

To illustrate this situation take a 20 cm piece of elastic ( nerves are elastic structures, that can stretch and recoil), lay it on a piece of board and then fix one end to the board with a pin. Stretch the other end and mark the point of maximum stretch ( this is referred to as the elastic limit) with a pencil. Now let the elastic recoil to its relaxed position and mark this point. Next take a second pin and fix the elastic at the mid point (10 cm). Then stretch the free end and again mark the point of maximum stretch. The difference between the original and this new point of maximum stretch constitutes a loss of ROM. The second pin is then the equivalent of the point of nerve fixation or compression. The sciatic nerve runs the full length of our leg or hind limb.

Similarly if a joint or series of joints ( spinal joints are in series) and / or their associated joint support structures are injured, resulting in stiffness or loss of ROM, then the nerves which course through these tissues also loose ROM .

NP can also occur  where a  single nerve courses through injured and then normal tissues. Think of that elastic band again. The fixed segment between the pins represents the section of nerve within the injured tissues with reduced ROM . The free end is the segment of nerve coursing through normal tissues. Imagine your horse has lower neck stiffness ( left sided) and that a nerve is coursing along the entire left side of your horses neck. The first segment of the nerve is fixed amongst the stiffened tissues of the lower half of the neck. What happens when we ask for movement to the right. Through the initial less severe range there are no problems. As we ask for the limits or near limits of range the horse suddenly stiffens up and returns its head neck to the midline. Why? The elastic band ( nerve) can’t stretch that far and attempting to do this has led to the perception of NP which in this case we refer to as being positional.


The answer to the above question is that NP is either reduced or eliminated by restoring normal neural or nerve ROM. Treatments can be direct, indirect or symptomatic. In some cases multiple approaches to treatment are utilised.


Q. How then is normal neural ROM restored?


Ans. In one of four ways.

     (i)  Direct: This  method may involve the removal of any structure or tissues which are creating the

           compression or fixation. An example of this method of treatment is where prolapsed disc material is

           surgically removed thus freeing the entrapped nerve. Medical treatments which reduce the inflammation

           and swelling of tissues about a nerve is another.

     (ii) Indirect: The primary aim here is to restore the ROM  of stiffened joints and or joint structures with which the

           nerves are associated. If we achieve this then the nerves coursing through these structures should also

           have their normal ROM and thence neural function restored. If an area of the spine is stiff, for example the

           neck, then this stiffness must be removed. That is full cervical vertebral ( neck ) ROM must be restored.

           Mobilisation and certain forms of manipulative therapy are examples of this approach to treatment. The first

           principle of equestrian training ‘ to relax your horse and then stretch’ is another.

    (iii) Symptomatic: In general this involves the treatment of the pain (NP) associated with the injury or loss of

            ROM. Most often these treatments are applied on or near the sites of pain referral. For example the whither,

            brisket, long muscles of the back and various areas over the neck, rump and limbs. Acupuncture,

           acupressure, trigger point therapy, massage, muscle manipulation, laser therapy, magnetic field therapy

           and electrotherapy have all  been employed by therapists to reduce or eliminate this referred NP.

    (iv) Combinations of treatments: Physiotherapists mobilising joints and associated tissues following spinal

            surgical  procedures is in many human hospitals a standard practise. Recommending rider training ‘relax

            and stretch’ therapy following either standing or under anaesthetic mobilisation treatments is also the norm

            with some therapists. Other therapists combine pain treatments like acupuncture with manipulative or

            mobilisation techniques.


 

NP as we have already stated is expressed through hypersensitised nerve endings. Aggregations of large numbers of these altered nerve endings (receptors) can be present at various ‘points’ on, or over ‘broader areas’ of the horses body.  Different therapists may refer to these as either areas of muscle spasm, trigger points, muscle strains, acupoints, touch points or displacements and pinched nerves etc .

Symptomatic treatments which reduce or eliminate NP appear to in many cases instigate a state of fatigue in these sensitised nerve endings. This occurs as a result of using different forms of constant, intermittent or pulsed stimulation. To illustrate this, press two of your finger tips together and hold them there. Initially you are aware of the pressure but if you keep going ( constant stimulation) eventually the nerve receptors will begin to run out of transmitter agent ( messenger chemical) and will fatigue. As less impulses are transmitted along the nerves the tips of your fingers will begin to go numb i.e. no message -no feeling. Massage, acupressure, trigger point therapy, electrostimulation, magnetic field therapy , point injection therapy.. etc are some of the therapies which attempt to fatigue or confuse the hypersensitised receptors. If successful, pain will be reduced or eliminated at least in the short term. If joint and neural ROM are restored as a consequence of these treatments then pain relief should be permanent. If not, one can expect a resumption of pain symptoms days, weeks or months later.


Q. Are some areas of  the horses back / spine more important than others ?


Ans. Quite obviously all the spinal joints are important. If however we are looking at performance from an equestrian view point then the level of importance is directly related to the innate range of movement of individual  and specific groups of joints. Anatomically much of the horse’s spine is quite ridged. Riders tell me that roughly 90% of their horse’s spinal movement (athleticism) is derived from the cervical spine ( neck). Much of the remainder is concentrated about the lower lumber region of the spine. That equates quite well with their innate ROM.


Q. Which areas of the spine should I be more concerned about?


Ans. Quite obviously those that play the biggest part in performance. Cervical (neck) and lower lumber (lower back).


Q. If there is a hypersensitised area for example under the saddle ( cold backed) does this NP come from a loss of ROM of the joints in that area ( thoracic vertebral joints) ?


Ans. Not necessarily! Remember NP is almost always referred. In some instances pain may be referred to tissues close to the source of the problem but more often the site of referral is quite distant. Cold back syndrome is usually caused by lower neck reduced ROM. Sensitivity in the whither, chest, girth and shoulders are also most often a consequence of trauma and loss of lower cervical ROM.


Q. Who should I get to treat my horse?


Ans. Hopefully a qualified ( trained ) therapist . If the horse has multiple problems it might be best to have your vet and therapist discuss the case with each other first. The effectiveness of different forms of therapy will in  general be dependant on three factors,

            (i) the experience and skills of the individual therapist. This relates to both diagnostic and treatment skills.  

           (ii) the nature of the injury itself. The severity of the initial trauma, the degree of subsequent tissue repair  or

                 the lack of, and the longevity ( time since initial injury) all play a part in the decision as to which treatment

                 protocol would be more appropriate.

            (iii) the position of the injury. Some joint complexes and associated structures are better treated in the

              standing unsedated animal whilst others are better treated whilst the horse is recumbent ( anaesthetised).

Discuss the different options with your veterinarian and / or therapist.

 

Q. How do I know if the treatment is working?


Ans. Two changes should be apparent.

              (i) Firstly there should be an improvement in the horses spinal ROM. This means your horse should

                   become more supple and flexible.

              (ii) Secondly there should be a significant and maintained reduction in, or elimination of, any previously

                    apparent hyperaesthestic states or areas. Cold back, pole shy, girth sensitivity ( above normal

                    sensitivity), hypersensitivity to brushing or mane pulling should also in time subside.

These  should be long term changes. If problems keep recurring ( days weeks or months later) then you should consider using a different approach to treatment or therapy. Recurrence of problems often indicates that NP is being reduced at least temporarily but normal or full ROM is not being restored. It may also be that the problem is not neuropathic but orthopaedic meaning that the diagnosis is incorrect. Another possibility is that dual problems exist. Consult both your vet and therapist unless of course they are one and the same!

All techniques have their limitations and in some cases none will be effective. Fortunately these non responsive cases are in the minority.


Q. If I give my horse loads of time off will the problem fix itself by way of ‘natural’ or normal healing processes?


Ans. Yes if this ‘time off’ results in full spinal joint and / or neural ROM being restored. However quite often the problem appears to have subsided with rest, and then when the more demanding stages of training are resumed the symptoms recur. We often refer to this as a recurrence of that ‘brick wall’ . ‘Brick walling’, and you know what I mean by that, may occur to the left at a trot, at a canter, to the right., at attempts at flying changes , higher obstacles... etc


Q. If my horse has this problem is he always in pain?


Ans. No! NP is often positional and it is more likely to be perceived at times of pressure. The horse will only be aware of the discomfort when those altered receptors are being stimulated in sufficient numbers. Pain is then often associated with particular situations such as increasing flexion demands and also with increasing solar (hoof) concussive forces that occur on hard work surfaces when trotting, jumping heights and at fast gallops.  Heavy handling can also initiate NP. In contrast whilst alone and relaxed in the paddock it is unlikely that any discomfort would be experienced.


Q. Does NP cause behavioural problems?


Ans. EMPHATICALLY ...YES!!!!!!

Humans who suffer from chronic NP can be described in one situation as being touchy and sensitive, in another as aggressive and in the next as being depressive and despairing.

I like to refer to this as the ‘Jekyll & Hyde’ syndrome. To ride they can be like a nervous plank and yet on the ground they are soft lambs. In the field they can be solitary, inactive and just plain sad. Another at home may be relaxed and work well whilst at a show will be OK one minute and then go off like a gun the next.

They are as we have stated ‘hyperaesthetic’ ( NOT TOUCHY) or sensitive.  Being constantly prone to overreaction must in itself be wearing and eventually depressing.


Q. Why is it important that NP be both recognised and treated?


Ans. Aside from the opportunity to improve the athletic performance of your horse there is a much more important issue.

Overreactive or hyperaesthetic horses can be of danger to both themselves and also those who handle and ride them. They are much more likely to overreact in an aggressive or defensive manner than a normal horse. And on the subject of reduced ROM or stiffness, which horse would you like to be aboard at Aintree or Badminton. One described as a relaxed ‘elastic band’ or the animal referred to as a sensitive, grumpy ‘lump of wood’ ?


IN SUMMARY -

If we are to ‘begin’ to understand this form of pain and the manner in which it effects both our equine companion directly and ourselves indirectly then we should first agree on the basics.

So let us agree to

1) drop that rather unprofessional term ‘touchy’ and replace it where relevant with the neurological term ‘hyperaesthetic’.

2) stop referring to these problems as ‘back problems’ as this too often omits one of the most important areas of the horses spine (the neck) and instead refer to them as ‘spinal problems’.

N.B. There are other problems involving the muscles and other structures about the spine that do not involve NP. ‘Tying up’ is one of these. Other orthopaedic injuries like sacroiliac ligament tears may or may not have associated NP.

3) Instead of my horse being ‘stiff’ to the left refer to it as having reduced range of movement (R.O.M.) to the left , right, during ventral flexion ( collection) ..........etc

4) Remember that nerve pain (NP), being that pain commonly associated with problems of the spine, is unlike normal pain in two very important ways.

    (i) Normal pain will most often respond to anti-inflammatory medications like ‘bute’ whilst NP will not.

   (ii) Normal pain is felt or perceived within the damaged tissue itself. NP is most often ‘referred’. Thus regarding the site of pain ( hyperaesthetic or sensitive areas) as also being the injury site may be and is very often incorrect. Likewise selecting the nearest spinal segments to these pain sites and then referring to them as the site of trauma can also be ‘well off the mark’!


Author: Tom Ahern BVSc                ahernvet@hotmail.com