Page 134 - The clinical aspects and management of chronic migraine Judith Anne Pijpers

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Chapter 7
Chronic migraine and other chronic pain conditions
Patients with chronic migraine also experience more non-cephalalgic pain than episodic migraine patients,25 presumably due to more prevalent and more severe central sensitization.26 After all, central sensitization is not unique for migraine, but is an important manifestation in many chronic pain conditions, such as low back pain, fibromyalgia, non-headache neuropathic pain, post-surgery pain.27 In such pain disorders, allodynia is also a predictor for pain chronification, although the definition of chronic is usually defined in periods of time (presence of pain for ≥ 3 months) instead of frequency (presence of headache on ≥ 15 days).27 For most chronic pain conditions, central sensitization can be reversed upon removal of the peripheral pain generator, or blockage of specific central receptors.27 In headache disorders, the peripheral pain generator will end by itself due to the paroxysmal character of the disorder, and might result in a more dynamic character of central sensitization.
Despite similarities in shared mechanisms for chronicity, there is also an important difference: medication overuse is the most important risk factor for migraine chronification, but pain medication is generally not associated with aggravation of other pain conditions, and withdrawal of pain medication is not advocated. However, awareness of opioid-induced hyperalgesia is rising, and in some patients, opioid reduction or cessation improves pain scores. Whether opioid-induced hyperalgesia is the consequence of opioid therapy, or caused by pre-existing risk factors, such as dysfunction of endogenous opioid systems, potentially located in the brainstem, is not clear yet.28
The mechanism of hyperalgesia due to pre-existing risk factors would again be a link between chronic migraine and other chronic pain conditions. Clinical data on headache disorders also suggest an essential role for pre-existing risk factors. Patients with a primary headache disorder overusing pain medication for another pain condition are at risk for chronification of headache but this increased risk seems absent for patients without a tendency for headaches.29–31 Alternatively, non-headache patients who do develop headaches de novo upon use of acute pain medication, consider this as a side-effect and will stop the medication. However, the latter seems less likely, and there is no evidence to support this hypothesis. Hence the limited available data suggest that there has to be an intrinsic increased sensitivity (i.e. pre-existing primary headache disorder; pre-existing dysfunctional endogenous opioid system) in order for medication overuse to cause or aggravate pain. In animal models, the overuse