Effects of Linearly Polarized Near-Infrared Irradiation in Patients With Sympathetically Maintained Neuropathic Pain Syndrome

Sympathetically maintained pain (SMP) is defined as an aspect of pain which is maintained or mediated by sympathetic efferent activity including the action of circulating catecholamine and is often linked to the complex regional pain syndrome (CRPS). SMP has been categorized as a subset of neuropathic pain mediated by the underlying mechanism of peripheral sensitization referring to noradrenergic sensitization of peripheral afferent nociceptive fibers and a pathologically sympathetic-sensory coupling interaction between the efferent sympathetic and the afferent system. Besides, SMP is associated with vasomotor and sudomotor dysfunction through either peripheral integerized by an adrenoreceptor stimulated or central organized by sensitization. Therefore, modulation of sympathetic activity either by means of pharmacological analgesics or local sympathetic ganglion blockade may influence the pain course in patients with chronic pain and hyperalgesia that were suspected to be sympathetically maintained.

The pain associated with sympathetic activity can be relieved by blockage of efferent sympathetic nerves to the affected area. Stellate ganglion block (SGB), a local anesthetic blockade of sympathetic ganglia, has been advocated as an early intervention to achieve sympatholysis and its strong evidence had been identified. However, a success of SGB depends on the skilled invasive technique in application and several potential complications may be elucidated. Alternatively, the light irradiation near the stellate ganglion region (SGI) using either low-reactive level laser or linear-polarized near-infrared light irradiation had been used as a noninvasive method for local anesthetic sympathetic blockade. Furthermore, SGI can be safely and conveniently performed in clinical practice even when an anesthetic physician is not available, had been well tolerated by patients, and no thermal injury or side effects were reported by literatures.

SGI had been identified to offer similar effects of SGB for patients with neuropathic pain, including improving blood flow by dilating vessels and reducing pain by directly blocking afferent nociceptive signal traveling via sympathetic pathways. The irradiation effect on heart rate variability (HRV), which is an objectively non-invasive marker of the autonomic nervous system, had not yet been well established. If the pathogenesis of SMP is closely referred to some abnormality in the function of autonomic nerve system (ANS), it is more necessary to measure autonomic activity for validation rather than pain status, local temperature or peripheral blood flow. Whether the relieved pain level after SGI is associated to HRV changes remains unclear. Identifying the relationship between HRV and pain status following SGI allows the clinicians to set a prompt protocol of phototherapy using SGI, and to accurately follow the outcome of autonomic dysfunction in patients with SMP.

The purpose of this study was to demonstrate the sympatholytic and clinical effects of linear polarized near-infrared irradiation applied on stellate ganglion in patients with SMP.

An experimental design with two groups will be used and a double blind, prospective randomized control trial will be conducted at the chronic pain medicine center in the physical medicine and rehabilitation department. Potential patients will be recruited. The enrolled patients will be randomly assigned to the experimental group or the control group (CG) using a table of random numbers. Patients in the experimental group will receive a 6-week standard SGI treatment protocol (12 irradiation sessions), while those in the control group being undergo a sham irradiation during the same period. Both the investigators and the patients will be blinded to the group assignments. All outcome measures will be collected by a trained research assistant at the baseline admission prior to SGI therapy, before and after SGI at each irradiation session, and at the follow up admission immediately after the end of 6-week treatment period.