Chronic Pain & Central Sensitization
Patients who suffer from chronic pain instinctively know that slight touches and bumps shouldn’t cause significant discomfort. They may wonder if they are experiencing phantom pain that is somehow mentally driven. Friends and family may think the same thing, seeing the patient grimacing at next to nothing.
But these patients aren’t delusional. In fact, they may be experiencing central sensitization, a condition of the nervous system associated with the development and maintenance of chronic pain. This condition, also known as central pain syndrome or centralized pain, is best-defined as a state in which the nervous system amplifies sensory input, resulting in the perception of pain from non-painful stimuli (allodynia) and greater pain than would typically be expected from painful stimuli (hyperalgesia).
When central sensitization occurs, the nervous system goes through a process known as a “wind-up” and devolves into a constant state of high reactivity. This heightened state lowers the threshold for what causes pain and subsequently comes to maintain pain even after its source, whether an injury or illness, is no longer present. The result is it takes almost nothing for these patients to experience severe pain.
Hyper-Reactivity Can Affect More Than Touch
Heightened sensitivity can overlap across all senses, not just the sense of touch or feel. Patients may report sensitivity to light, sounds or odors. For example, normal levels of light may seem too bright or the perfume section in a department store may produce a headache.
Cognitive deficits, such as poor concentration and poor short-term memory, as well as emotional distress, are also associated with central sensitization. It makes sense, as the nervous system is responsible for not only sensations, like pain, but also emotion. With the nervous system trapped in a persistent state of reactivity, patients are apt to feel uncomfortable and anxious.
What Causes Central Sensitization?
Research links central sensitization to damage within the nervous system, particularly the dorsal horn of the spinal cord and the brain. Changes occur at the cellular level at receptor sites, which become hypersensitive to touch. Strokes and spinal cord injuries have long been recognized as possible sources of central sensitization, as both can damage the nervous system – the brain in the case of strokes, the spinal cord in the case of spinal cord injuries.
However, it has become increasingly clear that a variety of chronic pain disorders may also be responsible. Chronic low back and neck pain, whiplash, tension headaches, migraine headaches, rheumatoid arthritis, osteoarthritis and traumatic injuries, may all be predicates of central sensitization.
For healthcare providers, the causes that contribute to the development of centralized pain are generally divided into two groups – those occurring prior to the original injury and those occurring after the original injury.
Predisposing Factors for Heightened Sensitivity
Research has yet to establish a link between pre-existing conditions and the subsequent development of central sensitization, but it is widely believed that there are a variety of underlying factors.
Influences such as the stress-response may play a role. In fact, experimental evidence points to a relationship between stress and lower pain thresholds. Pre-existing anxiety, as it relates to pain, may also foretell higher pain sensitivities.
A history of physical and psychological trauma, and/or depression has the potential to lead to chronic pain later as well. Predictably, the common denominator between chronic pain, anxiety, trauma and depression is the nervous system.
It’s important to note that pre-existing conditions do not make individuals more prone to injury or illness. They simply make people prone to the development of chronic pain after an injury or illness. Researchers submit that a dysregulated nervous system may interfere with the sense of recovery, thereby extending pain beyond the time when the damage related to trauma or illness has healed.
Factors Following the Onset of Pain
For factors that can contribute to centralized pain after an original injury, a top one is poor sleep. This is a common consequence of living with chronic pain, and it’s also associated with increased sensitivity to pain. A review of studies indicates that sleep loss both increases the likelihood that pain-free individuals will develop chronic pain and worsens the long-term prognosis of existing chronic musculoskeletal pain.
The onset of pain is also often associated with subsequent development of conditions such as depression, fear-avoidance and anxiety. The stress of these responses can, in turn, further exacerbate the reactivity of the nervous system, leading to central sensitization.
Making a Diagnosis
While science has not yet yielded a specific test to diagnose central sensitization, healthcare providers often start by stimulating the affected areas and measuring the patient’s response. Once responses are evaluated, additional tests may be ordered to determine if there is nerve damage in the area. Imaging tests such as a CT scan and MRI may be used to check for physical causes that could cause pain. If tests are negative, the provider may conclude that the patient is suffering from centralized pain.
“If you brush the person’s skin with your hand, instead of feeling a light touch, they may feel as if needles are touching the skin,” says Reginald Kapteyn, MD, medical director of the Pain Relief Center at the University Hospital in Cincinnati. “That sensation still sends a signal to the brain, but because the circuits aren’t working normally, a touch may trigger even more pain. The pain pathways are not regulated the way they used to be.”
A patient’s pain history is important, too. That includes pain location and whether the pain is patchy or extensive. If a patient has had a left-sided stroke, for example, do they have symptoms on the right side, as well? Providers can test for those symptoms with light touch and stimulate all the pathways that may be affected with cold or heat, or a pin prick. The idea is to rule out alternative diagnoses before concluding that a patient has centralized pain.
Treatment of Central Sensitization
Treatments for centralized pain often include a combination of strategies, each targeting a different “desensitizing” mechanism, as opposed to singular therapies. Combined therapy may include both top-down strategies (opioids and related approaches, including activation of endogenous opiates) and bottom-up strategies (topically applied analgesics have shown potential).
Additionally, studies suggest that targeting metabolic and neurotrophic factors offers promising new avenues for diminishing symptoms in central sensitization patients. More conservative treatments – including pain neuroscience education, cognitive behavioral therapy and exercise therapy – are also emerging as possible treatments for centrally maintained pain.
Opioids, whether synthetic or endogenous, help block the transmission of pain signals by filling the pain receptors, thereby reducing their ability to transmit pain signals. However, from concerns about addiction to questions about effectiveness, it’s clear that synthetic drugs are no longer considered the go-to treatment for chronic pain conditions.
Elevating levels of endogenous opiates, though, may provide an effective and non-habit forming solution to aid centralized pain patients. One method of activating the body’s internal pain-relieving mechanism is high frequency interferential electrotherapy. This treatment may provide an effective alternative to synthetic opioid therapy for patients who are able to tolerate electrotherapy treatments.
Additional Resources on Centralized Pain:
Those wanting to further investigate the Central Sensitization may wish to review the following articles:
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Blog: Exploring the Gate Theory of Pain
This blog provides general information and discussion about medicine, health and related subjects. The words and other content provided in this blog, and in any linked materials, are not intended and should not be construed as medical advice. If the reader or any other person has a medical concern, he or she should consult with an appropriately-licensed physician or other healthcare professional.