Whiplash and the Central Nervous System

A tremendous amount of research has been done in the last few years on the etiology of whiplash pain. The bulk of the study has focused on the problem of abnormal spinal mechanics, with particular attention on the facet joints. This current study examines another aspect of the whiplash problem, by examining the role of the central nervous system in the development of chronic muscular pain from whiplash.

The researchers began with eleven whiplash patients with a mean duration of whiplash pain of 4 years and 5 months. These patients were matched with eleven pain-free control subjects.

All of the study participants were subjected to three tests:

1. A pressure sensibility test that measured pain threshold levels in the infraspinatus, brachioradialis, and anterior tibial muscles.
2. A pin-prick and touch sensibility test in the same areas of the body.
3. Saline injections in the infraspinatus and anterior tibialis muscles. After the injections, the subjects were instructed to draw the areas of pain on an anatomical map.

The whiplash group exhibited more pain in the pressure sensibility tests, as is expected. There were no differences between patients and control subjects in the pin-prick and touch sensibility tests. The most dramatic results, however, were found in the saline injection tests:

• The patient group experienced significantly more intense pain than the control subjects.
• The whiplash patients reported pain that lasted much longer than the controls.
• "Infusion of hypertonic saline into the infraspinatus muscle caused pain areas that were significantly larger in patients than in control subjects. Similar results were obtained after infusion into the anterior tibial muscle."
• Most dramatically, "The distribution of pain was different between control subjects and patients. In control subjects, the spread of pain was typically distal to the infusion site, whereas whiplash patients experienced both distally and proximally referred pain. This was seen after infusion of hypertonic saline into the infraspinatus and anterior tibial muscle."

This is shown in the following illustration. In the control group, the pain is localized in a small area around the injection area; in the patient group, the referred pain spreads much further from the injection site, and both proximally and distally.
The authors conclude:

"In the present study, we have found muscular hyperalgesia to painful muscle stimulation not only in the should region, but also in distant areas in which the patient does not normally experience pain. This finding could be a manifestation of a generalized central hyperexcitability and support the hypothesis that central pathogenic mechanisms are involved in the whiplash syndrome. Widespread hyperalgesia to intramuscular electrical stimulation has been found in fibromyalgia and myofascial pain patients with significantly lower pain thresholds in painful and non-painful areas."

"It is not clear how central hyperexcitability is maintained and eventually causes chronicity, but most likely an ongoing nociceptive afferent barrage is needed. A possible explanation could be that the whiplash trauma causes tissue injury and possibly in some cases minor nerve injuries not easily detectable. These injuries could lead to an increased neural activity at the site of the injury resulting in the release of excitatory amino acids and neuropeptides, which can lead to hyperexcitability in dorsal horn cells."