However strange it may sound, there are clear upsides to our ability to experience pain: some forms of pain provide a powerful protective mechanism for the body and are an effective part of nature’s strategy to support survival (in a supposedly dangerous environment).
The International Association for the Study of Pain defines pain as ‘an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage’. This qualifies pain indirectly as a symptom, which in turn makes the tacit assumption of an underlying and identifiable physical pathological condition. When looking a little closer into the slightly mysterious multitude of conditions called pain, one can see that this definition could do with some slightly more imaginative and encompassing expansion. But one can also see that it is hard to define ‘this thing called pain’.
It is currently not possible to measure or otherwise quantify pain by any diagnostic tests or measurements (with some possible exceptions emerging from recent pre-clinical MRI studies of brain function and a range of further studies ongoing worldwide), yet everybody will agree that intense pain is an all-consuming experience and requires urgent intervention. Verbal communication (where possible) about type and intensity of pain is at the centre of any attempts to curb pain.
Worldwide, analgesics (drugs to treat pain) are the most widely used medications (with a close second being antibiotic agents for the treatment of infections), yet for the treatment of some forms of pain we currently only have rather blunt instruments with limited efficiency and often a severe burden of unwanted effects. On the other hand, anaesthesia has been developed to a very high level of control and is the major enabling mechanism for any surgery.
Broadly, two different types of pain need to be distinguished: acute pain and chronic pain. Acute pain is part of the body’s alert and protection system, with a high and adaptive threshold for activation. An intense sudden cause (such as heat, cold, mechanical injury, irritating chemicals) triggers this immediate nociceptive pain sensation which typically overrides all other activities and initiates a withdrawal reflex. This nociceptive pain is essential in protecting the body from harm. There is a further variety of adaptive and protective pain experience which functions over a slightly more extended time scale and is sometimes referred to as inflammatory pain. Its protective role kicks in during the healing phase after tissue injury or infection; essentially a heightened sensitivity to stimuli reminds the body not to overuse a body part that is currently being repaired / healing and thus prevents further tissue damage and can promote healing. Both of the expressions of acute pain are transient, albeit on different time scales of action.
The second category of pain is chronic, not adaptive, and essentially results from some form of malfunction of the nervous system. The malfunction can be in the peripheral nervous system (where pain signal receptors are located), in the transmitting nerve fibres, or in the central processing of pain signals in the brain. This type of chronic pain is commonly referred to as neuropathic or pathological pain. It has no initiation threshold similar to that of acute pain as it is usually not triggered by an external signal (with a few noticeable exceptions), it has no protective function and typically is a lasting condition. One could thus argue that neuropathic pain should not be considered solely as a symptom but as a disease.
Clearly, these different types of pain require different control strategies and treatments, with the currently most unmet needs in the area of chronic pathological / neuropathic pain.
First, we will have a closer look at the mechanisms of pain sensation, the mechanisms of local recording, transmission and central (brain) processing of pain signals and sensations. Equipped with this insight, we can discuss the different types of analgesics and their working for different types of pain, including the many different options for delivery of medications. In addition, we consider a range of other options for the management of pain conditions, including placebo effect, cognitive behavioural therapy, acupuncture, nerve stimulation by electrodes.
Next section: Pain