Treatment of Postherpetic Neuralgia With Low Level Laser Therapy
Herpes zoster, also known as shingles, produces a painful vesicular rash that results from the reactivation of the varicella zoster virus (VZV), the virus that originally causes chickenpox. Although the rash typically resolves over the course of 4 to 5 weeks, the pain may persist for months, or even years, after the rash has disappeared. Known as PHN, this phenomenon can often be debilitating to the patient. A number of different treatment options are available to clinicians, including pharmaceuticals, nerve block injections, and physiotherapy, but unfortunately no single therapy is 100% effective at reducing PHN.1,2 LLLT is a noninvasive, pain-free, light-based therapy that uses red and infrared light to resolve the inflammatory process and eliminate pain in patients with PHN. We present a number of PHN cases that have responded favorably to LLLT and review the mechanisms of action of the technology.
The VZV is a linear, double-stranded DNA virus that causes two pathological manifestations. Initial infection with the virus, usually during childhood, may result in chickenpox. The virus then migrates to a dorsal root ganglion, where it can remain latent indefinitely (Figure 1). Later in life, immune system depression or stress can trigger a reactivation of the herpes zoster virus. The lifetime risk of developing herpes zoster is approximately 30%.3,4 An estimated 1 million new cases of herpes zoster occur each year in the United States,5 resulting in 50,000 to 60,000 hospitalizations. On occasion, the disease can lead to loss of bladder or bowel control and may even be fatal.
The incidence and severity of herpes zoster has been shown to increase with advancing age.5 Individuals over the age of 60 years make up approximately 40% to 50% of the 1 million new cases that occur each year. The increased risk of herpes zoster with advancing age is thought to be caused in part by immunosenescence, the gradual deterioration of the immune system that is part of the natural aging process. In addition, patients who are immunocompromised have a greater chance of manifesting the disease, with increased severity of symptoms.6
Herpes zoster is characterized by a unilateral vesicular eruption in a dermatomal distribution, most commonly over the trunk and cranial regions. Initial symptoms prior to the appearance of a rash include headache, pain, malaise, and acute photophobia. The rash itself may be accompanied by pruritus, altered sensitivity to touch, or allodynia. Initially, the maculopapular rash is characterized by varying degrees of inflammation, and it progresses to take the form of coalescing clusters of clear vesicles containing high concentrations of VZV.4
PHN is a common complication of herpes zoster that affects 50% of individuals over the age of 50 years who develop shingles. PHN is characterized by ongoing pain subsequent to the resolution of the rash associated with this disease. The pain has been described as an unrelenting sharp, burning, stabbing symptom that interferes with sleep, work, and other activities of daily life. It can lead to social withdrawal and depression.2 The duration of PHN can last from 30 days to long after the disappearance of the dermatological lesions. In some cases, the pain persists for many years.7
As shown in Figure 1, PHN results in inflammation of the sensory root ganglia, the dermis of the associated dermatome, and frequently, the posterior and anterior horns of the grey matter, meninges, and both dorsal and ventral nerve roots.
For more than 40 years, LLLT has been effectively used in the treatment of wounds and in acute and chronic musculoskeletal conditions, including degenerative disc disease, repetitive stress injuries, muscle strains, and arthritis.8 LLLT has been cleared by the FDA for the relief of minor muscle and joint pain, arthritis, muscle spasm, relieving stiffness, and promoting relaxation of muscle tissue. Certain devices have also been approved for treating carpal tunnel syndrome and rotator cuff injuries specifically. The therapy is traditionally not covered by insurance and patients may pay between $30 and $60 per half hour of treatment, depending on the healthcare practitioner.
LLLT is a noninvasive, pain-free, light-based therapy that uses a combination of red and infrared light in the form of laser and superluminous light-emitting diodes. The power output of these devices is below the level of surgical or other high-intensity lasers. Photon particles of light are absorbed by the mitochondria through cytochrome c oxidase and result in increased cellular adenosine triphosphate levels.8
In the case of neuropathic pain, LLLT has been proposed to mediate analgesia by releasing local neurotransmitters such as serotonin,9 promoting the release of endorphins,10 while simultaneously decreasing prostaglandin E2 and bradykinin levels. An increase in anti-inflammatory cytokines, interleukin (IL)-10, along with a decrease in pro-inflammatory cytokines, tumor necrosis factor-a, and IL-1b, allows for the rapid resolution of the inflammatory process.11 LLLT has been well documented to stimulate tissue regeneration including angiogenesis, collagen production, muscle and nerve regeneration, cartilage production, and even bone formation.8 The inflammation that causes the abnormal stimulation of the nerves in the case of PHN is reduced rapidly with LLLT.
In a double-blind crossover trial of LLLT for PHN, all patients who received treatment demonstrated a reduction in pain severity of between 40% and 95% (mean value, 74%) and a reduction in pain distribution (as measured by body surface perimetry using a universal goniometer) of 49% to 84% (mean value, 64%).12 Patients all began with a pain level of 10 out of 10, as measured on the visual analogue scale (VAS)—with 0 representing no pain, 2 - mild pain, 4 - moderate pain, 6 - severe pain, 8 - very severe pain, and 10 - intolerable pain. Therefore, a reduction of 40% to 95% on VAS represents a significant impact on the quality of life of these patients.
Follow-up of patients between 3 to 6 months after therapy revealed that 80% of the patients had maintained their improvement in pain levels and pain distribution. The large age range of the patients (55 to 82 years) and duration of PHN (6 months to 8 years) could account for the relatively wide range of pain reduction demonstrated in this study. In particular, 50% of the patients were over the age of 70 years, and 25% had suffered from the condition for more than 4 years. A detailed analysis of age and severity of condition compared to treatment response was not presented in the article. Despite this variability, all patients saw an improvement of at least 40%, which is comparable to the most successful pharmacological treatment, but without the harmful side effects (Table 1).