Neurophysiology of Fibromyalgia

Fibromyalgia is a syndrome associated with fatigue and chronic pain, leading to significant physical limitations and impaired quality of life. Fibromyalgia affects 1.7% of Canadians, with a higher prevalence in females compared to males at 9:1 [1]. There are several challenges that complicate the diagnosis and management of fibromyalgia. The etiology is not well defined, as there are several proposed factors that may trigger the genesis of pain in fibromyalgia. Chronic pain in fibromyalgia arises in the absence of tissue pathology, and consequently a lack of consensus on reliable diagnostic criteria. Understanding the pathophysiology of fibromyalgia would aid in the identification of objective biomarkers that could be used for diagnosis.

Multiple theories have been posited to explain the genesis of chronic pain. The gate control theory describes the attenuation of pain signals in the spinal cord prior to cortical processing, and it has been hypothesized that loss of this gate control leads to the genesis of chronic pain [2]. Gate control can be observed by reduction of afferent signals during active muscle contraction. For example, the amplitude of the somatosensory-evoked potential (SEP) is attenuated during active contraction [3]. To our knowledge, it is unknown whether such gate control is observed in fibromyalgia. The lack of gate control may contribute to chronic pain in this population.

The sensorimotor theory suggests that incongruency between motor intention and sensory feedback underlies chronic pain where there is an absence of tissue pathology [4]. This may align with the genesis of fibromyalgia, given the findings that those with fibromyalgia have altered tactile and proprioceptive functioning [5]. Corticomuscular coherence (CMC) is a useful tool that uses electroencephalography (EEG) and electromyography (EMG) to probe the synchrony of neural firing between the brain and muscle [6]. To our knowledge, it is unknown how the magnitude of CMC varies in fibromyalgia compared to healthy controls.

Non-invasive brain stimulation in the form of Transcranial Magnetic Stimulation (TMS) has been used to probe the activity of corticospinal and cortical networks in fibromyalgia. When TMS pulses are delivered in a repetitive train, a protocol known as repetitive TMS (rTMS), short-term neuroplasticity can be induced (i.e., a change in the activity of neurons in the brain). In fibromyalgia, Mhalla et al. [7] found that 5 days of 10 Hz rTMS reduced pain intensity and improved quality of life metrics. Controlled pulse parameter transcranial magnetic stimulation (cTMS) is a novel technique that may produce stronger analgesic effects by delivering monophasic pulses that could enhance neuroplasticity compared with conventional rTMS. It is unknown whether a longer intervention period or the use of cTMS could lead to greater analgesic effects.

Finally, central sensitization may explain the widespread chronic pain experienced in fibromyalgia. There are several neuromodulators that contribute to the neurobiology of central sensitization and may be implicated in this condition including serotonin, dopamine, and brain-derived neurotrophic factor (BDNF). Serotonin is linked to pain modulation, such that increased levels of 5-HT are associated with hyperalgesia [8]. BDNF has been implicated in the genesis of neuropathic pain [9]. In fibromyalgia compared to healthy controls, serum BDNF levels have been reported to be higher [10]. Abnormal dopamine function may also be associated with fibromyalgia [11]. Positron-emission tomography (PET) studies show lower cortical dopamine D2/D3 binding availability in fibromyalgia compared to healthy controls [12].

Ultimately, a combination of events may lead to widespread chronic pain in fibromyalgia. Understanding the neurophysiology of fibromyalgia would aid in the discovery of objective biomarkers for diagnosis. Therefore, the goals of this study are to:

• Compare the neurophysiological responses in fibromyalgia compared to healthy controls.
• Determine whether a two-week cTMS protocol will alter pain in individuals with fibromyalgia.

Inclusion criteria

• 18-65 years old

Exclusion criteria

• contraindications to TMS
• chronic pain associated with diagnoses other than fibromyalgia