• A Brief Overview of Acute versus Chronic Pain by Dr Jack Lewis

    I wrote this blog in response to a request from one of my Twitter followers, so this is dedicated to her. If you would like to receive a pearl of brain wisdom each day you too can “follow” my NeuroTweets.

    Chronic pain is a very broad subject so this is NOT a comprehensive review. Rather I’ve tried to create a brief and accessible overview that might help people understand some of the basic principles of acute and chronic pain to arm them during further exploration of the subject.

    If you, or someone you know, suffers from chronic pain you may find a blog written by a consultant in pain management at St Thomas’s Hospital London. It describes the powerful psychological aspects of chronic pain which greatly compliments my explanation of the strictly physical/neurobiological aspects.

    A firm belief in any specific pain relieving therapy, even the esoteric ones, can harness the brain’s natural ability to suppress pain signals according to a strong expectation that the pain will subside. This is the famous “placebo effect” and my personal belief is that placebo is one of our best weapons against chronic pain. If an individual is utterly convinced that crystal healing, or alignment of the shakras, or a colourful sugar pill will make the pain go away these are all perfectly acceptable methods of inducing the brain to reduce the sensation of pain.

    PAIN IS GOOD #1 – helping us to avoid damaging ourselves

    Believe it or not, under normal circumstances, pain is a good thing. It alerts you to the location of objects in your environment that pose a threat to the health of your body. Pain provides an invaluable service by informing you that you are, for example, treading on a drawing pin or touching a scalding hot pan, thus giving you a chance to avoid damaging yourself. In fact, pain-sensing neurons are wired straight into motoneurons of the spinal cord to produce an automatic withdrawal reflex. This means that you can find yourself recoiling from the painful stimulus i.e. pulling your foot up away from the sharp object, or moving your hand away from the heat, before you are even aware of the pain. This reduces the time that the body part is in contact with the sharp or hot object to minimise the damage caused.

    PAIN IS GOOD #2 – helping us to protect damaged body parts

    If you are not quick enough and do end up hurting yourself, then the pain also serves a second purpose – reminding you to tread carefully / use your other hand, to avoid worsening the wound so that it can heal as quickly and thoroughly as possible. Damaged regions become hypersensitive i.e. produce a painful response even to weak stimulation, as the inflammatory agents automatically released in response to tissue damage cause molecular changes within the nociceptors (pain receptors) to make them more sensitive. This may sound like a terrible idea, but it is actually very useful and highly adaptive as it encourages behaviours that protect the damaged body part from further injury.

    PAIN AND THE UNCONSCIOUS BRAIN

    Everything I’ve described so far is acute pain where the sensation of pain lasts only as long as the threat of bodily damage is still present, or whilst the damaged body part is still healing. Every night acute pain helps to ensure that you wake up with your limbs in full working order. This is because at some point in the night most of us inevitably end up rolling over into an awkward sleeping position in which our body weight cuts off the blood supply to a limb. At first this results in the sensation of pins and needles, which after a while develops into the sensation of pain. The pain may wake us up so that we become consciously aware of our predicament, or more often simply causes us to roll over into a new position in our sleep to allow blood to flow back into the limb in question before any permanent damage is caused.

    WHY SMACK ADDICTS WALK WITH A LIMP

    Heroin addicts often walk with a limp. The reason for this boils down to the fact that the substance they are addicted to is a highly purified form of the best pain killer known to man – the opiates. They inject heroin to get the desired high but also render themselves insensitive to pain. Like the rest of us, when they fall asleep, they may end up assuming an awkward sleeping position, cutting off the blood supply to an arm or leg. Unlike the rest of us, the sensation of pain is not generated in the brain, and so there is no signal compelling them to move into a different sleeping position. With no supply of oxygen / glucose and no removal of waste products of metabolism for several hours the tissue within literally starts to die.

    WHAT IS CHRONIC PAIN?

    Chronic pain differs from acute pain in that it persists beyond the period of time that an injury usually needs to heal. It can be caused by a wide variety of different mechanisms. For instance, chronic pain can be caused when inflammatory agents that make nociceptors near to the damaged tissue more sensitive continue to be produced long after the injury in question has already healed. In this case anti-inflammatory agents can often do the trick.

    WHAT IS NEUROPATHIC PAIN?

    Neuropathic pain syndromes are a different matter and the treatment is not so straightforward. They are chronic pain disorders caused as a direct consequence of damage to the brain or by a disease of any part of the nociceptive system that normally a) detects painful stimuli (nociceptors), b) transport it into the spinal cord and/or c) conveys this message up the spinal cord to the brain.

    MEDICAL DETECTIVE WORK – looking for clues to the causes of chronic pain

    It is often very difficult to tell what the precise cause of the chronic pain is but the first part of the detective work involves establishing what kind of stimuli causes the pain and if the pain is constant then what makes it worse. A medical physician who specialises in pain management may assess this by gently pressing on the painful body area with a variety of instruments: stroking the area with a ball of cotton wool, a series of single tooth brush bristles that each bend after a certain amount of pressure has been applied, deep pressure to reach nociceptors deep under the skin’s surface, hot (45oC) and cold rollers (20oC) etc. This helps them to establish whether the pain is generated by stimuli that are usually not painful in healthy individuals – a condition known as allodynia, or whether it is heightened pain to stimuli that would normally cause some pain even in healthy individuals – hyperalgesia.

    CAUSES OF CHRONIC PAIN #1 – up-regulation of sodium channels in peripheral nerves

    Chronic pain may be caused by damage to the nerve fibres that carry pain messages from a certain body area into the spinal cord. These C-fibres are unusual as they DO NOT have the fatty myelin sheath that other nerve fibres in the body and brain are endowed with in order to make the electrical messages travel at a faster speed. Injuries to nerves in the periphery (i.e. in a certain body part rather than in the spinal cord or brain) can cause neuropathic pain resulting from up-regulation of voltage-gated sodium channels in the wire-like axons of the C-fibres that carry the pain message to the spinal cord. Voltage-gated sodium channels are special proteins embedded in the outer wall of ALL nerve fibres because they enable electrical signals to move along the axon toward the synapse. Every time one sodium channel opens, positively-charged sodium ions rush into the axon, changing the voltage, triggering the next sodium channel to open and so on. This domino effect continues until the electrical message reaches the synapse, where special communication molecules (neurotransmitters) cross the gap between brain cells, triggering a second neuron within the spinal cord to send electrical signals to pain-processing areas of the brain. Chronic pain can therefore result because the up-regulation of sodium channels causes spontaneous activity (i.e. electrical impulses not triggered by the pain receptors in body parts to which they are attached) which means the sensation of shooting pains is artificially generated by a fault on the line. It feels like the pain originates from the body area to which the damaged nerve is connected, whereas in actual fact that body part is in perfect health. There are various types of selective sodium channel blockers that may help to treat this type of chronic pain, however as with all drugs that influence the nervous system, side effects are likely as sodium channels of each type are distributed throughout the body and brain and all of them will be affected by the drug, not just those that are causing the painful sensations.

    For more info: http://www.touchneurology.com/articles/emerging-treatments-chronic-pain

    CAUSES OF CHRONIC PAIN #2 – physical pressure on a nerve

    Trigeminal neuralgia is a type of chronic pain affecting the head, face and neck that can sometimes be attributed to a blood vessel sitting tight right up against the trigeminal nerve. As the blood pulses through the vessel it can physically squeeze the adjacent trigeminal nerve triggering strange and often painful sensations in the head, face and neck.  In a sense, the impingement of the blood vessel is another example of pain being caused by “a fault on the line”. The nociceptors in the head, face and neck are functioning perfectly well, but as the hundreds of pain-relaying neuronal cables that connect the nociceptors converge en route to the brain, compression of this nerve results in feelings of pain in all these different body parts. If no other therapy has worked and brain scans reveal that a blood vessel touching the nerve IS the likely cause, then an operation to remove the offending artery can improve matters greatly. Complications are always possible during any operation, so surgery should always be sought only as a last resort.

    For a full explanation of treatment options, click here.

    CAUSES OF CHRONIC PAIN #3 – constriction of nerves caused by spinal misalignment

    The concept of a “Fault on the Line” being responsible for pain and discomfort in a certain body area is also fundamental to the philosophy of chiropractic care, but in this instance the location of the fault is the site at which nerve fibres enter the spinal cord. The spine is made up of many spinal bones, or vertebrae, all stacked one on top of the other separated by a cushioning disc. Hundreds of nerves, which are bundles of brain cell cabling, enter the left and right sides of the spinal cord through the gap between pairs of vertebrae to carry neuronal commands from the brain to the body and vice versa. The basic principle of chiropractic is that your posture reflects the alignment of these vertebrae and that misalignment of the vertebrae can place physical pressure on the nerves ferrying information in and out of the spinal cord. In particular, a postural examination can establish which vertebrae are neatly stacked one on top of the other and which are stacked at an angle. If they are all neatly stacked then it is likely that there is a large gap between each vertebrae through which the nerves can ferry their vital messages between body and brain without being impeded. If a person’s posture has been pushed out of line by a fall, an accident or endless hours stooping over a desk, then the gap through which the nerves pass into the spinal cord may have narrowed by misalignment of the vertebrae, impinging upon the nerve and causing feelings of discomfort and pain, muscle weakness and a variety of other possible symptoms. Chiropractors correct these postural abnormalities with a variety of methods including orthopaedic supports in the shoes to get the hips level (most of us have one leg slightly longer than the other), spinal adjustments to loosen up and realign the vertebrae and exercises for patients to work on at home. Chiropractic treatment can be a costly business, and there is debate in some circles about the long term safety of high velocity spinal adjustments, but I for one have benefitted enormously after a terrible rugby injury to my back in my late teens which over 10 years later is still a source of chronic pain and limits me in my ability to participate in sports. However, with weekly maintenance at the chiropractor I play 5-a-side every Monday night, go to the gym twice a week and surf as often as possible.

    For more on chiropractic, click here.

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