Pain
What is pain?
Pain is highly complex with many interactive dimensions, including physiological, sensory, affective, cognitive, behavioural and psychosocial. The evidence base related to pain is often confusing for the novice therapist because of the different terminology used in different areas of speciality and a lack of clarification between types of pain, the physiological processes involved and the signs and symptoms presented. This section will attempt to summarize the issues surrounding pain, related to the neurologically impaired patient. For more in depth reading, the therapist is referred to the International Association for the Study of Pain (IASP) website (www.iasp-pain.org).
Types of pain
Nociceptive
This type of pain is physiological and arises as a consequence of the activation of nociceptors (pain receptors) following a chemical, thermal or mechanical event. The activation of primary nociceptive afferents by actual or potentially tissue-damaging stimuli is then processed within the nociceptive system (Treede et al. 2008). This type of pain may involve the musculoskeletal system but can also be from a visceral origin. It is important to note that not all nociceptive activation is perceived by the individual as ‘pain’. The perception of an unpleasant experience (pain) is highly subjective. Dystonic pain is associated with abnormal sustained muscle contraction (S3.18), which mediates the activation of nociceptive afferents in the muscles.
Psychogenic
This is pain that is caused, increased, or prolonged by cognitive, emotional, or behavioural factors. The IASP (2007) describe psychogenic pain as:
Neuropathic
This type of pain arises by activity generated within the nociceptive system without adequate stimulation of its peripheral sensory endings (nociceptors) and is caused by a primary lesion or dysfunction in the nervous system (IASP 2007). Some authors have further narrowed the dysfunction to a lesion of the somatosensory system (Treede et al. 2008). This includes the afferent neuron, the ascending and descending pathways, medulla, thalamus, or cerebral cortex. Therefore, neuropathic pain is sub-divided in relation to the anatomical site of the lesion (Dworkin et al. 2003).
Physiology of pain
The experience of pain has a protective role which warns us of imminent or actual tissue damage and elicits responses, via signals within the nervous system, which keep such damage to a minimum. This brings about temporary pain hypersensitivity in the inflamed and adjacent tissue (peripheral sensitization). This process assists healing as contact and movement will be avoided. However, persistent pain offers no benefits and can be extremely debilitating. This maladaptive pain often occurs following damage to the peripheral nerve, the spinal cord or the CNS and is termed neuropathic pain (Woolfe and Mannion 1999). Figure 29.1 shows a simple flowchart outlining the normal pain pathway and the processes involved in pathological pain.
Figure 29.1 Pathophysiology of pain.
Symptoms associated with pain
Central neuropathic pain (CNP)
The symptoms of CNP occur as a result of CNS damage particularly to the somatosensory system which may lead to disinhibition and central sensitization (Fig. 29.1). With the pain pathway now inappropriately active, activity dependent neural plastic changes occur at the synapses within the whole pathway and may lead to an increase in the cortical representation (sensory homunculus) of the painful part. The timescales within which these neural changes take place and in which the symptoms develop is not clear with different studies stating from 1 month to 5–6 years.
The symptoms produced as a result of disinhibition and central sensitization include:
Allodynia
This is pain perceived following a non-noxious stimulus which does not normally provoke pain (e.g. touch, heat or non-noxious cold). This is common (90%) in central neuropathic pain (Widar et al. 2002).
Dysaesthesia
This is an unpleasant abnormal sensation. For example, burning, wetness, itching, electric shock, pins and needles. This is common in CNP (80%) (Attal et al. 2008).
Distribution of neuropathic pain
Peripheral neuropathic pain: In PNS lesions the pain distribution will conform to the cutaneous innervation of the peripheral nerve, the branches of the brachial or lumbar plexus, or spinal roots (dermatomes).
