Explaining chronic pain following soft tissue injury

At a recent conference with counsel regarding a case of ongoing chronic pain of four years duration arising from an allegedly heavy-handed physiotherapy treatment, counsel posited, 'The defence are, of course, going to assert that a soft tissue injury normally heals within three months, so why does the claimant still suffer from pain?'

Among the issues that arise during medico-legal work, ongoing chronic pain following soft tissue injury must surely be one of the most controversial. While this is an issue that the majority of clinicians will readily accept in their day-to-day work (e.g. that 20 per cent of ankle sprains or whiplash injuries will experience long-term problems with pain and disability), it can, of course, present considerable problems when it comes to explaining in a medico-legal context precisely how and why a claimant continues to report ongoing pain and disability following a soft tissue strain.

This short article aims to present a summary
in simple terms of the extant literature on the development of chronic pain following soft tissue trauma. It is clearly not intended to be an exhaustive review but offers the legal practitioner a snapshot of current thinking in this burgeoning clinical field of study, which has revealed fascinating relationships between trauma, pain, and the mind and the cascade of events that not infrequently follow soft tissue injuries such as whiplash or low back strain.

Pain science

The intensity of pain a person describes following
an injury and the resulting disability rarely correlate with the degree of soft tissue trauma. Indeed, in early laboratory studies where subjects received precisely the same pain stimulus (usually an unpleasant electrical stimulus), it was found that some subjects would rate this as only a very mild discomfort - some not even registering the stimulus as pain - while others would rate exactly the same pain stimulus as the 'worst pain imaginable'.

Of course, this has entered common parlance and in day-to-day life we now talk about people having high or low pain thresholds, but what causes such wide variations in pain experience?

What appears to be coming out of the research literature is that pain thresholds, even among healthy individuals in the laboratory setting, vary according to that person's fear of injury, their previous experiences of pain, and their current psychological state (if they are stressed or depressed, for example). This will be discussed further below, but one thing we can take from this research is that clearly we can no longer regard the intensity of pain experienced as a marker per se of the extent of the soft tissue damage.

The stress response

A person experiencing a stress response will describe irritability, increased sweating, and 'jumpiness'. It has long been known that stress delays wound healing, but there are now a plethora of human studies demonstrating that stress responses have a direct influence on the way that pain is processed, and researchers have found several important relationships between post-traumatic stress disorder (PTSD) and pain - for example, PTSD sufferers appear to have lower pain thresholds and lower levels of the body's natural pain relieving substances (endorphins).

Indeed, a common neurobiological pathway is shared between stress responses, negative mood, and pain in the neurotransmitter systems of the brain, and when we are stressed hormones are released that make our pain nerve-endings more sensitive. In fact, it is thought that pain can be present - via these stress-related pathways -
even in the complete absence of physical trauma.

Interestingly, there is also evidence that a one-off brief stressful event can make our skin and muscles more sensitive and tender, as if the brain has turned up the body's sensitivity to the environment to screen for further threats to its survival (for a review of how this phenomenon can be used to predict prognosis following whiplash, see 'When range of motion is not enough: Towards an evidence-based approach to medico-legal reporting in whiplash injury', Journal of Forensic and Legal Medicine, July 2014). Of course, experiencing greater levels of pain can lead to the avoidance of activity, be it occupation, hobbies, or socialising, thus leading to greater disability.

One interesting study has highlighted the important role of stress-related genes in recovery from soft tissue injury. Researchers followed up 90 individuals in an A&E department, hours after experiencing a whiplash injury. Those with a ‘pain susceptible’ stress-related gene were more than twice as likely to complain of moderate to severe pain, headache, and dizziness in A&E, and this group also estimated a longer time for physical and emotional recovery.

There are now calls for larger clinical studies looking at the role of genetic factors and their influence on stress response pathways. Contemporary approaches to chronic pain management – pioneered by Jon Kabat-Zinn’s meditation/mindfulness approach to pain – often include stress reduction strategies such as meditation or clinical hypnosis. By virtue of this emergent evidence base, physiotherapists are now using stress reduction techniques in the early days following soft tissue injury to expedite recovery.

Central sensitisation

Soft tissue injuries may also produce chronic ongoing pain in those claimants whose nerves and spinal cord are vulnerable to what has been termed 'sensitisation'. The term 'central sensitisation' refers to hypersensitivity of the spinal cord, and once again both stress-related responses and genetic factors have been implicated as a cause. Interestingly, this is usually measured in the laboratory by inflicting pain on a person and measuring how intense the pain stimulus has to
be to produce a reflex movement away from the pain: the less painful the stimulus required to produce a reflex movement, the greater the
central sensitisation.

A number of non-traumatic conditions have
been associated with central sensitisation,
including fibromyalgia and tension-type headache. Management of sensitisation often includes medication that targets the nervous system and stress-reduction techniques, as mentioned above, such as meditation, clinical hypnosis, or relaxation training.

Catastrophisation

As we have seen, there is increasing evidence that psychological factors have a critical role to play in the magnification of pain following soft tissue injury, and pain catastrophising is perhaps one of the most important measures. Heightened vigilance, rumination on pain, and pessimistic beliefs regarding the long-term outcome of a soft tissue injury - so-called 'catastrophisation' - can all increase the intensity of pain, most probably by inhibiting the body's natural pain-relieving endorphin system. So, those who catastrophise will feel more pain and distress and display more overt pain behaviours such as grimacing and moaning. We also know
that claimants who catastrophise will be at much greater risk of poor response to treatment.

Closely related to catastrophisation is fear of re-injury or harm. Claimants who catastrophise may believe that physical activity will harm or injure them, and this can lead to reduced activity levels, deconditioning, and an increased vigilance with regard to performing the simplest of day-to-day tasks. Movements become what has been termed 'maladaptive', with some claimants displaying a phobia to movement ('kinesio-phobia'), which is defined as an excessive, irrational, and debilitating fear of movement
(for example, the low back pain patient who uses
a wheelchair).

Risk of poor recovery

There are now robust and valid questionnaires
that can be used to screen for stress responses, catastrophisation, and fear of movement, and these are being increasingly utilised by physiotherapists working with soft tissue injuries
in the NHS and private sector to identify and guide management in those at risk of chronic pain. Indeed, more recently physiotherapy researchers at the University of Keele have pioneered a simple generic screening tool that can identify these factors early following soft tissue injury, with a view to stratifying care (the STarT Back screening tool, published in The Lancet, 29 October 2011).

This may have utility in the medico-legal context: factors such as fear of injury, worry, lack of enjoyment, poor expectations for recovery, catastrophic thoughts, and 'bothersomeness' all indicate heightened risk of poor or non-recovery following soft tissue injury. Those individuals scoring highly on this screening tool are treated with 'psychologically informed' physiotherapy integrating both physical and psychological approaches.

As it has become clearer that recovery following soft tissue injury bears little or no relationship
to the degree of physical soft tissue injury itself,
the limitations of the traditional medical model's capacity to explain and predict chronic pain in an individual claimant is being exposed. Thus clinical objective measures such as range of motion, x-rays, and MRI scans are found to be weak predictors
of ongoing pain. However, a claimant's stress
response, the presence of central sensitisation, catastrophising, and fear of movement and re-injury have been found to be strong predictors of poor outcome, and are proving to be critical factors to measure and address through psychologically informed physiotherapy management of soft tissue injuries, regardless of the severity of the pain or disability experienced by the claimant. SJ

Chris Worsfold is a neck pain specialist and expert witness based in Blackheath, London, and Tonbridge, Kent

tonbridgeclinic.co.uk

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