Pain: An Overview 1

By Dr. Ranjith Pallegama

 

 History and evolution

     Conceptual models have been developed from the history to explain pain and related suffering. In several historical models of pain, the word ‘pain’ was related to suffering and punishment and was defined as a consequence of sin by different religious groups. In the ancient Greek the word ‘poini’ meant penalty and the Latin word ‘poena’ meant pain, punishment or again penalty. Pain has been a phenomenon addressed in almost all religions and some relate it to a punishment coming from gods. This could be due to the fact that pain was related to illness or injury that had no cure during those periods. Over the generations man has attempted to find a cure and has come across many ways of overcoming pain. These methods have later evolved into modern analgesic mechanisms such as the use of morphine. From the time of Aristotle (384 to 322 B.C.), man has attempted to find the source of pain and till l8th century there has been argument on whether the heart or the brain is the location of perception of pain. In the 17th century, Descartes postulated that the brain was the center of sensation. Since then there were many advances to explain ‘pain pathway’ and a huge development in the science of pain was brought about since the historical revelations by Wall and Melzack in 1960s, including the Gate Control Theory.

     Now pain has been understood as a complex emotional experience but not a simple physical sensation. Pain has been defined by the International Association for the Study of Pain (IASP) as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, and described in terms of such damage". As definition indicates, pain isn't always associated with actual stimulus and tissue damage. It may be caused by potential tissue damage, or even just feel like tissue damage.

 

Nociception

     Free nerve endings act as pain receptors (nociceptor) and can be activated by multiple types of stimuli such as mechanical, thermal, chemical and artificial ones like electricity. Stimulation and activation of nociceptors and consequent transmission of nerve impulses along the nociceptive pathways (ascending pain pathway) to the brain is called nociception.

     A dual transmission of pain signals to the central nervous system has been described. Fast pain, which is acute, sharp and pricking in nature, is carried by thick, fast and myelinated A-delta fibers and the slow pain, which is chronic and can be nauseous, throbbing, burning and boring, is carried by thin, slow and unmyelinated C fibers. Fast pain, through a faster tract, is transmitted directly to the sensory cortex as it has a protective role that will let the person to avoid more injury by escaping from the source of pain. On the contrary, the slow pain signal, which is usually inflammatory in origin, terminates widely in the brain stem and a few signals will reach higher areas of the brain. The exact role of the slow inflammatory pain or chronic pain, perhaps with its continuous and wide distribution of signals to the reticular formation at the brain stem, is to keep the person alert on the ongoing tissue injury in the body and will make the patient to suffer. Although patients can consciously feel pain with the signals reaching the brainstem reticular formation and thalamus, for determination of specific attributes of pain such as the intensity, quality, nature, location, duration and so on, the signals should reach the sensory cortex.

     Several descending pain inhibitory systems (analgesia systems) exist in the central nervous system for modulation of upward nociceptive signal and thereby achieving natural control of pain. If not for these mechanisms humans and all other animals would feel severe pain even with minute stimuli. The Gate Control Theory proposed by Melzack and Wall in 1965 with later modifications by authors themselves and several others explains another possibility of pain control mechanism. This can also be considered as one possible mechanism that is used to explain the effectiveness of acupuncture and Transcutaneous Electrical Nerve Stimulation (TENS) in pain relief. Readers are referred to the chapter on physiology of pain.

     But transmission of nociceptive signals to the brain may not always culminate in perception of pain in every instance. The experience of pain, that is pain perception, is always much more than this physiological state of nociception. There are instances that one may experience pain without having obvious nociception related to a physical cause. And hence, the experience of pain (pain perception) is always subjective and uniquely individual experience and lacks objective evidence. But recently, scientists have been able to detect a "molecular memory for nociceptive pain stimulation" and a brief account on this is available under pain genetics.

     Yet even with further developments, such advance techniques may not be available for clinical application in the near future. Hence, in the clinical practice, the most reliable indicator for the existence of pain and its intensity is the patient’s description. If a patient complains of pain the clinician has no right to deny the patient’s claim of the experience of pain although the clinician is unable to detect objective evidence, obvious related pathology or any related signs. Among the many, phantom limb pain is a good example for this.  Even though they are in pain, some patients may be reluctant to report the presence of pain, due to number of reasons. Among those may be fear of diagnostic tests, fear of medications, fear of physicians and nurses, fear that complaining may affect quality of care, and belief that nothing can be done to alleviate pain.

 

Pain: impact on the quality of life

     Understanding the fact that pain, particularly chronic pain, is a major  threat to the quality of life of patients worldwide IASP launching the first ‘Global Day Against Pain’ had  a summit in Geneva, Switzerland on 11^th October 2004, and declared that chronic pain as a major healthcare problem and disease in its own right. People who suffer from pain should have access to adequate pain treatment and it is considered as a basic right. As pain is highly personal experience, the extent of suffering differs among individuals. Numerous studies have convincingly revealed that pain reduces the quality of life of patients remarkably. In addition to experience of pain, it is found to disable individuals limiting their movements, ability to enjoy food, ability to getting to work and social involvements having a huge impact on the daily life. In chronic pain conditions these limitations often translate to considerable socioeconomic constrains of the patients.

 

Specific physiological and clinical states of pain

     When pain due to a stimulus which does not normally provoke pain it is defined as allodynia and when there is an increased response to a stimulus which is normally painful, the condition is called hyperalgesia. The state of absence of pain in response to stimulation which would normally be painful is called analgesia.

     Deep somatic pain, visceral pain, headaches and facial pain, neuropathic pain and cancer pain are among many others described as specific clinical states of pain. The reader is referred to other chapters for more information on these clinical states.

     With regard to acute pain, usually the underlying pathology is detectable and easily diagnosed. It poses little challenge to the clinician. Usually with correct diagnosis and treatment the pain should disappear. However, chronic pain is not only the long-lasting nociceptive inflammatory pain, but there are number of other conditions and states that would give rise to chronic pain in patients. Clinically, to diagnose a pain condition as chronic we expect it to be there for a longer period (conventionally more than 3 months). But the reader should understand the fact that physiologically there is no difference in between two inflammatory pain conditions one that has been there for 2 months and the other 3 months, and hence 3 months’ duration is arbitrary.

     However, in most chronic pain conditions, the pathology is masked and difficult to diagnose. Usually, those are of multifactorial in origin and psychosocial elements are involved. Classification of Chronic pain by Merskey and Bogduk describes hundreds of chronic pain conditions including orofacial pain conditions, headaches, back pains, psychogenic pain and so on. Neuropathic pain is also a very common chronic pain condition occurring in various parts of the body including orofacial region. For example, trigeminal neuralgia is one of the commonly found neuropathic pain conditions in the orofacial region. In the latest literature, neuropathic pain is defined as a pain arising as direct consequences of a lesion or disease affecting the somatosensory system. It is important to remember that, currently chronic pain is not considered only as a symptom or a sign of another disorder or a condition. Chronic pain itself is a condition that should be adequately managed to ensure the quality of life of a person.

 

Multidimensional nature of pain

     Pain is a complex subjective experience and has intense affective, cognitive, behavioral and sensory components. Whether or not a particular stimulus will be perceived as painful depends not only on the nature of the stimulus, but also on the context within which it is experienced, previous memories and emotions etc. For example, a player who is actively engaged in a game might not feel the pain of an injury till the game is over; a mother might forget her own pain when her child is in danger.

 

Psychosocial aspects of chronic pain

     As it was briefly mentioned above psychosocial models of pain explain that pain experienced following nociception is influenced by the overall context in which the person get the nociceptive stimulation. Their beliefs and thoughts, mood, pain behaviors, previous experiences and environmental interactions (e.g., in the home or workplace) are some of the factors that will influence the ultimate perception, i.e., pain. The nociception is overlaid by all these factors and eventual sum is felt as pain.

     Involvement of psychosocial conditions with chronic pain can be viewed mainly from three aspects. In certain patients suffering from chronic pain conditions with an organic cause for pain, there can be a considerable amount of psychosocial modulation such as attention seeking, anticipation, mood changes, anxiety and catastrophizing. There can also be psychological reactions to chronic pain conditions with an underlying organic cause. Frustration, suffering and eventually development of more serious states such as depression can be considered as such psychological reactions. In some patients, chronic pain conditions with psychological origin (psychogenic pain) can be found, but it is important to keep in mind that the prevalence of these conditions is very low.