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  About Pain       Pain Mechanisms      
 
 
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Pain Transmission
  • How do we feel pain ? - let's take an example like cutting your finger with a sharp knife - what are the pain pathways that allow the pain messages to travel from your finger to your brain, and what are the mechanisms of pain processing along the way?
  • Pain receptors in the skin are stimulated by the injury, due to the release of various chemicals by the damaged cells including histamine, substance P, serotonin (5HT), bradykinin and prostaglandins - see Multiple action drugs e.g. tramadol (Zamadol), tapentadol (Palexia) block morphine, noradrenaline and serotonin receptors
  • Pain signals are generated by these receptors which then are transmitted via the sensory nerves to the spinal cord . The cell bodies of these sensory nerves are grouped together in a small swelling called the dorsal root ganglion.
  • In the spinal cord the pain impulses are processed by a "computer" called the dorsal horn.
  • Messages come out of the spinal cord and travel via motor nerves to the arm muscles, causing the arm to withdraw quickly. This is an automatic reflex that does not involve the brain or conscious thought.
  • Depending on the settings in the dorsal horn computer (see Gate Control Theory and Dorsal Horn Sensitisation below), pain signals are also sent upwards in the spinal cord via the Spinothalamic tract (amongst others) to an area in the brain stem (base of the brain) called the thalamus.
  • Further processing occurs in the thalamus with signals being sent to areas controlling blood pressure, heart rate, breathing, and emotions. An acute pain event often causes a rise in heart rate, blood pressure, and breathing rate, as well as a change in emotions and behaviour e.g. shouting "ouch", contorted facial expressions, and behavioural displays such as waving the arm in the air.
  • Pain signals are also sent upwards from the thalamus to the primary sensory cortex (part of the outer surface of the brain dealing with sensory input). It is thought that some crude perception of pain and sensation occurs at the thalamic level, with much finer discrimination occurring in the primary sensory cortex.
  • There is initially a sharp fast onset short lived pain transmitted from the injured area to the spinal cord dorsal horn by large diameter high velocity sensory nerves (A-delta fibre nerves). This is followed by a dull slower onset longer lasting pain transmitted from the injured area to the spinal cord dorsal horn by smaller diameter low velocity sensory nerves (C fibre nerves).
  • Pain Mechanisms is a short PowerPoint presentation (.pps) about acute and chronic pain mechanisms. Please click on the "Slideshow" icon if the presentation doesn't start automatically. You can either "Open" it or "Save" it to your hard drive. You may need to download the PowerPoint Viewer if you're having problems..
  • The following publication (PDF) is available from the British Pain Society (UK):-
    • Understanding and Managing Pain: Information for Patients
Gate Theory
  • Rubbing an injured area often helps to relieve the pain. Rubbing stimulates vibration receptors, sending signals to the dorsal horn via large diameter A-beta fibres (L in the diagram ).
  • These vibration signals enter the dorsal horn computer at the same time as the small diameter C fibre pain signals from the injured area (S in the diagram ).
  • If the vibration signals are of the correct magnitude, they prevent further onward transmission (Projection neuron (P) and Spinothalamic tract in the diagram ) i.e. closing the gate on pain.
  • Pain relieving treatment modalities like TENS, Pain Gone Pen, acupuncture and heat produce pain control by a similar mechanism. TENS stimulates the A-beta fibres, and acupuncture stimulates the A-delta fibres.
Dorsal Horn Sensitisation
  • Within hours of an injury, changes take place in the dorsal horn of the spinal cord which alter the way that sensory impulses are processed. When these changes have occurred the dorsal horn is said to have become sensitised. This means that sensory and painful signals are more likely to be transmitted up the spinal cord to the brain, rather than being blocked at the dorsal horn level. Sensitisation is said to be dependant on N-methyl-D-aspartate (NMDA) receptor activation. NMDA receptor antagonists (blockers) like ketamine can help prevent sensitisation occurring.
  • Clinically dorsal horn sensitisation can be measured as changes in pain and sensory thresholds e.g. for temperature sensation the normal comfortable range of 4 - 60 deg C is reduced to 30 - 40 deg C in the area of skin supplied by the sensitised dorsal horn.
  • Sensory thresholds can be altered for all the sensory modalities including vibration, heat, cold, light touch.
  • Thresholds for pain can also be altered in two ways:-
    • A stimulus which was not painful before is now perceived as painful.
    • What would have produced a little pain, now causes a great deal of pain.
  • Normally after an injury dorsal horn sensitisation reduces in line with tissue healing. However, in some people the sensitisation seems to go on for much longer, and may explain why some go on to develop chronic pain. In some of these people there is a continuing focus of pain in the periphery which continues to keep the dorsal horn sensitised, and in others the exact cause is unknown.
  • There is also a connection between emotions and dorsal horn sensitisation. In severe anxiety and depression states, lack of descending inhibition is enough to maintain the dorsal horn in its sensitised state.
  • Chronic pain management techniques can therefore be divided into three broad areas:-
    • Reducing the magnitude of pain signals coming from the periphery by either blocking the nerves that carry the pain or by doing something to the tissue that is generating the painful signal e.g. steroid injections reducing peripheral tissue  inflammation.
    • Reducing the degree of dorsal horn sensitisation by using analgesic drugs, TENS, Acupuncture, and spinal manipulation.
    • Improving descending inhibition by examining patient beliefs, improving education, treating anxiety and depression, and by providing reassurance that there is nothing terrible going on.
Emotions
  • Emotions can also affect the gate in the dorsal horn computer. The normal state of affairs is that there are continuous descending signals from the brain to all the dorsal horn computers in the body.
  • These descending signals (descending inhibition) keep nearly all of the gates in a closed state, preventing unnecessary sensory information reaching the brain i.e. preventing sensory overload.
  • Emotions like anger and excitement tend to increase the degree of descending inhibition, making it harder for pain signals to gain access to the spinal cord and brain e.g. a footballer injures himself on the pitch but doesn't notice the injury until he stops playing. Distraction therapy also works by a similar mechanism.
  • Emotions like anxiety and depression tend to reduce descending inhibition, making it easier for pain signals to gain access to the brain and spinal cord e.g. patients with anxiety and depression have increased pain perceptions compared to normal people.
  • There aren't too many adults in this world that do not carry some form of emotional baggage around with them. Carrying this baggage around on a daily basis can seriously impair your ability to deal with many things in life, including relationships, work, and coping with pain.
  • Examples of baggage are:-
    • Guilt about things that they should or should not have done or said in relation to partners, spouses, children or parents.
    • Emotional turmoil caused by bereavement, separation, divorce, a bad relationship, or even marrying the wrong person.
    • Socioeconomic Distress - caused by the effects of sudden loss of income due to redundancy, battling with government departments about income support and disability living allowance etc.
    • Childhood Abuse - emotional, physical and sexual abuse in childhood / early teens can have catastrophic effects on the ability to cope with life in general. Some adults have already come to terms with the past and no longer require any attention, whereas others are desperate for help and guidance, but are afraid to talk because of feelings of guilt or shame. Some abused children receive support afterwards from trusted family members, helping them to weather the storm. The most pernicious situation seems to be where the child was blamed by an adult (usually the mother) for leading the perpetrator astray. As adults this latter group seem to have the greatest disruption to their coping abilities, and require the most care and support.
  • Assessing and understanding people's emotional baggage is therefore very important when trying to understand their pain.
  • There is a very close link between our emotions, beliefs and our behaviour. Please read the next section to learn more.
Beliefs
  • Most of our behaviour in life revolves around our own individual set of beliefs - for example:-
    • You believe that brushing your teeth is good for your gums and teeth, and therefore you do it twice a day
    • I believe that sitting in front of this computer is going to make me rich one day, and therefore I sit here for hours on end typing away !!
  • We develop our beliefs from our own life experiences - for example:-
    • What we have witnessed with our own eyes (personal interactions and interpretations)
    • What we have been told by others (parents, media, education, health professionals)
  • Generally speaking if you believe something is good for you, you keep doing it, and if you believe something is bad for you, you stop doing it (avoid it).
  • Now let's take an example of two patients with acute low back pain seeing different doctors about their problem:-
    • Patient A consults Doctor A who says, the pain is due to an acute soft tissue sprain, the body has tremendous powers of healing, it's a self limiting problem, no real harm has been done, keep as active as you can within the pain, and then you will have a 90% chance of it all settling down on its own without treatment in 2 weeks.
    • Patient B consults Doctor B who says, you've damaged your back whilst lifting at work, the X-ray you had yesterday shows early osteoarthritis, rest if it hurts too much, your pain is a warning that you've overdone things, it can only can worse with age, there is no cure for spinal arthritis.
  • You can see that these two patients will come out of the surgery with completely differing ideas about their back pain. Their doctors have instilled different beliefs into their minds. From now on their behaviours in relation to their back pain are going to completely different:-
    • Patient A will have a positive beliefs around his back pain, expecting that the pain will go away on its own, and that maintaining normal activities will be good for his back. It is quite likely that this patient will recover fully and go on to have a normal lifestyle.
    • Patient B will have a negative set of beliefs around his back pain, expecting that he is doomed for ever, that it can only get worse, that rest is the only cure, and that activity will cause more pain and therefore more damage. Because he's "been told by his doctor", he will now modify his behaviour, become less active, develop more back pain because of his inactivity, and slowly spiral downwards into disability, chronic pain and dependency.
  • If over a period of time patient B is repeatedly told by his doctor and other health professionals (nurses, physiotherapists) that his back pain is due to spinal arthritis (spondylosis), the message becomes reinforced and more entrenched in the patient's mind.
  • Every time he modifies his behaviour (does less) in response to the pain, he becomes more unfit and more prone to having back pain, reinforcing his own beliefs. These beliefs can also be inadvertently reinforced by loved ones and colleagues at work by being over concerned about the pain and telling them to do less / take it easy.
  • When the pain comes on with every movement, and the pain in the patient's mind means that his back has been damaged further, several things then happen:-
    • The patient becomes frightened to do anything that may cause the pain - this is called Pain Avoidance Behaviour or fear of the pain.
    • He anticipates the pain before he moves, causing him to hold his breath and guard his back, whilst tightening his back muscles - this is called Guarded Movements. Guarding only serves to increase the pain during movement, as most of the pain is muscular in the first place.
    • Anxiety and depression develop over time with a tendency to catastrophise about the pain, its cause, and its consequences (to make it seem worse than it actually is, to make the pain into a catastophe). Anxiety and depression may also cause the patient to misinterpret the severity of the pain leading to a vicious spiral downwards.
  • When patient B eventually presents to a chronic pain clinic for help, he has firmly entrenched views about the pain, it's cause, and how he should manage it. The clinic will examine his beliefs about the pain in order to try to help him, but the longer the abnormal beliefs have been held, the harder it is to change them, and the stronger the emotional reaction during the process of trying to change.
  • The technical word for a belief is a "cognition". Psychological treatment to try to re-educate the patient about his beliefs is called Cognitive Behavioural Therapy (CBT), and is administered by someone properly trained in clinical psychology.
  • Many chronic pain clinics have multi-disciplinary teams (pain doctor, clinical psychologists and physiotherapists) who will try to use psychology treatments like Cognitive Behavioural Therapy (CBT) to try to modify the patient's set of beliefs about the pain, in order for him to begin the long road towards physical and psychological rehabilitation. They often operate within a Pain Management Program. If his beliefs cannot be changed,  then he will not modify his behaviour (not get fit), and not win the battle against chronic pain.
  • Some patient can manage their pain by combining CBT plus specialised physiotherapy, whereas others need some form of pain relieving procedure before embarking down this road.
  • Whatever technique is used, the messages are the same:-
    • You must learn as much about your pain as possible through education.
    • You must stop thinking that pain equals more damage.
    • You must learn how to control the fear of the pain, and stop anticipating it by guarding your muscles.
    • You have to stop catastrophising about the pain, instead trying to minimise it in your mind (e.g. telling yourself it's only muscle spasm). 
    • You must be as active as you possibly can be, in order to prevent the negative consequences of inactivity.
  • A vital message to all health professionals is therefore:-

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