Abstract
Both pain and inflammation are mediated by unmyelinated peripheral nerve terminals of small diameter Aδ and C fibers, the nociceptors, which lie a few microns below the surface of the skin in the epidermis. 1,2 The superficial nature of these fibers means that these important somatosensory nerves are within the penetration depths of all laser and LED wavelengths. These afferent nerve endings connect the outside world via second order neurons to the midbrain, sub-cortical, and cortical centers of the brain. By transducing noxious heat, cold, and other stimuli to sensory information that is processed into sensation and emotions, the skin is directly connected to the brain. Deeper nerve fascicles and trunks in the dermis and below are also within the penetration depths of infrared laser wavelengths as are the nervi vasorum, the nerves supplying blood vessels. Neural inflammation, which is the phenomenon of inflammation arising from the release of bioactive substances, such as bradykinin, from activated peripheral sensory nerve endings, is an important but often unrecognized component of any acute or chronic pain condition. 3 This too, in addition to pain, can be inhibited by suppression of activity of peripheral nerve endings. The absorption of photons by this complex neural network of the somatosensory nervous system is one of the mechanisms underlying modulation of pain and inflammation, and is one of the most important applications of phototherapy.
In acute pain, such as that arising from accidental injury or intentional injury (surgery), direct effects of laser irradiation to the site can offer both immediate and long-term benefits. Although acute musculoskeletal pain is acknowledged as one of the main clinical indications for phototherapy in medicine, there are less well-known but potentially important applications to be actively researched. Kevin Moore's seminal work showing a dramatic reduction in postoperative pain and drug intake following immediate postoperative laser irradiation to a surgical incision, illustrates the benefits of the antinociceptive and anti-inflammatory effects of a single application of phototherapy. 4 The benefits to the individual and to reduction in costs by such a simple technique for reducing postoperative pain, have yet to be realized, and this study needs to be replicated on a large scale. The capacity of phototherapy to relieve acute pain, whatever the cause, by suppression of neural activity at the site of acute injury, is also very important in modulating downstream spinal cord response. In acute pain, changes such as sprouting of neurites in the dorsal horn of the spinal cord, which set up conditions for acute pain to become chronic, occur within a few hours of injury. Long-term suppression of pain by phototherapy applied in acute conditions is seen in a neck pain study where treatment of acute pain suppressed recurrence 6 months later. 5 It is the long-term, “preventive” benefits of treatment of acute pain that are yet to be fully appreciated and systematically evaluated.
Chronic pain also presents an opportunity for phototherapy to achieve recognition as a unique modality. The predicted “epidemic” of chronic pain is recognized as one of the major medical challenges for governments and individuals. The cost of work-related injury alone, in the United States, the United Kingdom, and elsewhere in Europe is counted in billions, 6 –8 and the costs of increasing pain in the aging populations is of the same magnitude. Drugs for pain are expensive, have multiple serious side effects, must be taken long term, and are not particularly effective. The setting is right for phototherapy to gain a foothold as a mainstream treatment. The World Health Organization, Consensus Committee of the Decade of the Bone and Joint, concluded that there is strong evidence for low-level laser therapy for chronic neck pain, 9 and our review of neck pain has added “platinum” evidence for this. 10 Again, it is the capacity of phototherapy to modulate chronic pain by initiating a cascade of effects from peripheral nerve endings to the central nervous system, which is unique. Phenomena such as “windup”, central sensitization, and long-term potentiation associated with persistent pain, can be suppressed, and long-term depression of persistent pain 11 be achieved by repetitive application of phototherapy, through modulation of the “pain matrix”. 12 Diseases such as fibromyalgia and diabetic neuropathy, which present some of the most difficult of pain challenges, can be addressed by understanding how phototherapy affects the PNS and, importantly what are the optimal parameters and techniques of application.
How phototherapy modulates neural activity remains the subject of continuing research. Findings from our research group has found that both visible and infrared wavelengths inhibit electrophysiological activity in both sensory and motor fibers, although motor fibers appear to be less affected. 13 This supports a large body of evidence that demonstrates that laser irradiation has predominantly inhibitory effects on many aspects of nerve function and specifically on nociceptors. 14 The unique morphology of peripheral nerves, in which an axon, which is an extension of a neuronal cell body located in the dorsal root ganglion, can be up to 1 m long, might make them particularly sensitive to the effects of phototherapy. Given that ‘stimulation” is the mantra of phototherapy, the notion that inhibition has positive aspects, such as pain relief and anti-inflammatory activity, is greeted with anxiety and apprehension. Nevertheless, our findings are consistent both clinically and experimentally.
Although my reductionist discussion focuses on the specific effects of laser irradiation on nerves, effects on Schwann cells, which form myelin; fibroblasts, which provide a scaffold for nerve fascicles; and mast cells, which lie within nerves, undoubtedly modulate neural function in as yet unknown ways. Clinically, phototherapy will affect all the cells and tissues in the irradiated area in addition to having effects on the nerve alone, and this multiplicity of concurrent effects contributes to the overall recovery of the patient. Understanding how laser irradiation modulates the peripheral nervous system, however, will allow us to optimize treatment protocols for an expanding range of very difficult to treat clinical conditions, and bring it into mainstream medicine as “laser medicine” alongside its cousin “laser surgery.”