Whiplash, Referred Pain and the Central Nervous System
The most frequent symptoms after whiplash injury are headache and neck pain, which are understandable, since the neck is directly injured during a rear-end collision. Many auto injury patients, however, experience a much wider range of symptoms. Arm pain, shoulder pain, back pain, TMJ pain, and dizziness are also very common symptoms, and these are difficult to ascribe to direct trauma. A new study shows us that these other symptoms are actually related to the initial neck trauma, through the process of referred pain.
Referred pain is pain that is perceived in one area, but that originates in another. Researchers believe that referred pain occurs when pain signals from injured tissue activate adjacent nerves in the spinal cord or brain. These adjacent nerves then cause the brain to perceive pain in the area of the body innervated with those nerves.
Referred pain can be induced, even in pain-free people. In a new study from Sweden, researchers set out to determine whether whiplash patients had a different referred pain response to stimulus than non-whiplash subjects.
The authors started with 12 patients and 12 pain-free control subjects. Referred pain was created by electrical stimulation of the shoulder and the upper arm. The electrical stimulation level was set by the test subjects, and was adjusted to create five progressive levels of pain – innocuous, pain threshold, 2 out of 10, 4 out of 10, 6 out of 10 (on a scale of 1 to 10).
The results showed a dramatic difference between patients and controls:
- Pain-free subjects required almost three times more stimulation to reach the designated pain levels than did the whiplash subjects.
- While the controls reported limited areas of referred pain, the whiplash patients reported a much wider area of referred pain.
- Whiplash patients showed a unique type of referred pain. In healthy individuals, induced referred pain was always distal to the stimulation (or further from the center of the body), but in the whiplash patients, half of the patients had referred pain that was proximal to the stimulation.
These findings are important in understanding the nature of whiplash injuries. As previous studies have shown, some whiplash patients demonstrate “central hypersensitivity,” a phenomenon where the central nervous system becomes over-stimulated from the injury to the spine.
According to the literature, a certain percentage of patients are susceptible to chronic pain after an auto injury. In these patients, central nervous system changes may develop in reaction to pain, which can, in turn, increase the patient’s sensitivity to pain. It is critical to identify these patients early on after injury, in order to break this cycle. Sensory tests, such as pressure pain threshold testing, are an important diagnostic tool, since they can help us identify those patients with a posttraumatic stress reaction.
Kosek E, Januszewska A. Mechanisms of pain referral in patients with whiplash associated disorder. European Journal of Pain 2008;12:650-660.