Electrochemical Messaging
A simple model of nerves as wires that transmitted electrical impulses held sway until 1952 when Dr Glen Player 1 suggested that the signals could be "restrained".
The major neurotransmitter known as acetylcholine was soon understood to be only one of many chemical messengers that mediate communications through the nervous system.
Gate Theory
In 1965 Melzack and Wall2 proposed that a "gate" could be closed to block onward relaying of electrical pain signals.
They used the analogy of throwing a track switch to give precedence to an express train over a slower pain train on converging rail-tracks.
With further investigation, it became clear that pain could be magnified as well as attenuated.
Neurophysiological research has revealed some of the immensely complex chemistry whereby this modulation of pain is effected by neurotransmitters in the dorsal horn of the spinal cord3.
Anatomists and physiologists have now demonstrated specialised nerve pathways from the brain to the dorsal root ganglion of the spinal cord that can not only dampen but also amplify pain signals.
Here then is an empirically- confirmed mechanism whereby higher brain activities - emotions, attitudes and beliefs - can modify the experience of acute and chronic pain.
Neuroplasticity
The changes that cause chronic pain are in the spinal cord, not in the injured tissue or the psyche.
In 1990 McQuay and Dickenson4 suggested that the spinal cord was not only highly sophisticated but also plastic.
Neuroplasticity means that spinal cord and brain can be physiologically and anatomically altered long-term or even permanently by a single pain event5.
Severe acute pain during Personal Injury can thereby trigger chronic changes in the functioning of the spinal cord that amplify or generate pain.
Practice PointIn susceptible people, an acute pain event can Cause permanent changes in the spinal cord that result in chronic pain |
Empirical support for this revolutionary theory is increasing3: there are demonstrable changes in the structure and function of the dorsal horn of the spinal cord.
Physicians and lawyers alike had previously been looking for the mechanisms of chronic benign pain in the injured tissues, the local nerves or the psyche.
MEDICAL APPLICATIONS
Gate Theory
Blocking of impulses by competing electrical interference is the basis of the pain-relieving device known as a Transdermal Electrical Nerve Stimulation (TENS).
The user wears an instrument that generates a continuous low voltage electrical flow through the skin adjacent to an area of chronic pain.
The dorsal column stimulator is a more effective and permanent implanted medical device based on the same principle.
Pain modulation
Empirical confirmation and refinement of the Gate Theory has improved understanding and management 6 of pain, for instance that arising from multiple nerves7 [Full Text] and chronic abdominal pain8 [Full Text].
It provides a model for the conundrum of pain where there is no evidence of tissue damage.
Pain modulation explains why sensations that most would experience as pleasant or neutral - light touch or pressure - are painful to a few ( allodynia).
Neuroplasticity
Even though general anesthesia makes a patient unconscious and therefore completely unaware of pain during major surgery, adding complete blocking of pain signals in the form of epidural analgesia materially reduces postoperative pain8 [Full Text].
Such pre-emptive analgesia is an important clinical application of neuroplasticity, for severity of postoperative pain is a major determinant of subsequent chronic pain9 [Full Text].
Patients with widespread chronic pain (FibroMyalgia Syndrome) not only experience allodynia at tender points but also have a low but fluctuating threshold for pain.
It appears that both inherited predisposition and early experiences - particularly various forms of childhood abuse - can permanently set the pain thermostat lower than usual.
For FMS sufferers, subsequent traumatic events - physical, psychological and emotional - can temporarily reset the pain threshold too low for comfort.
MEDICOLEGAL APPLICATIONS
Gate Theory
Although cervical whiplash may result in immediate facet joint and intervertebral disc damage, the onset of neck pain can be delayed until other injured parts heal and competing pain signals disappear.
Pain modulation
Chronic pain following soft tissue injury may be amplified by Depressive Disorder or Post-Traumatic Stress Disorder that were caused by the same Personal Injury.
Until those psychological injuries heal or are relieved, the chronic pain will not abate.
Pain modulation refines the prediction of disability in low back pain10 [Full Text].
Neuroplasticity
Neuroplasticity is central to litigation of chronic pain after soft tissue Personal Injury.
The connection between tissue damage and nociception (awareness of tissue injury) is already broken by 3 months after soft tissue injury, but in a minority of victims the pain-suffering-behaviour persists as chronic benign pain.
Practice PointChronic pain specialists are the experts on spinal cord changes that Cause chronic pain after soft tissue injury |
Because the soft tissue damage is long healed, expert testimony by the traditional "tissue injury" specialists is largely irrelevant.
In their stead, chronic pain specialists (anesthetists and psychiatrists) should be asked explicitly to explain to the Court this current understanding of pain modulation and neuroplasticity.
Related Articles
| Chronic Pain 1994 | Volume 1, Issue 7 |
| Chronic Pain 1995-6 | Volume 2, Issue 10 |
| Chronic Pain 1997 | Volume 3, Issue 9 |
| Chronic Pain 1998 | Volume 5, Issue 1 |
| Chronic Pain 1999 | Volume 6, Issue 3 |
| Chronic Widespread Pain | Volume 5, Issue 5 |
Copyright © 2008 Electronic Handbook of Legal Medicine
Copyright © 2008 Electronic Handbook of Legal Medicine