Does Low-Level Laser Therapy Have an Antianesthetic Effect? A Review

Introduction

I t is widely known that, when local anesthetic solution is injected into an area of greater vasodilator activity, faster absorption occurs, which leads to a shorter duration of action. Because local anesthetics are vasodilators, they tend to be absorbed into the bloodstream from the operative field as a result of the vasodilatation of peripheral arterioles. To counteract this vasodilation, vasoconstrictive agents are often included in local anesthetic solutions to provide a longer duration of anesthesia. ¹ Vasoconstrictors in local anesthetic solutions are beneficial with regard to the duration, depth, and quality of anesthesia; they decrease blood loss and systemic local anesthesia toxicity, and they provide effective postoperative analgesia. ²

Although low-level laser therapy (LLLT) has been extensively used for treatment, its effects have not been completely understood. Moshkovska and colleagues ³ suggest that LLLT has the following benefits:

The effects of LLLT have been demonstrated both in vitro and in vivo. The reduction of interstitial fluid at the site of inflammation with marked increase in vasodilatation and an improvement of local circulation has been described; this is the result of a direct effect on the motoricity of the lymph vessels. These effects lead to a reduction of edema, the better oxygenation of tissues, and circulation enhancement. ⁴ Kami and colleagues ⁵ reported that a significantly higher blood flow was found in the skin of rats who had been exposed to 20 J/cm² of laser energy, as compared with nonirradiated control animals. In another double-blind, placebo-controlled study, the author's ⁶ group demonstrated a significant increase in skin circulation in patients with diabetic microangiopathy that started after half an hour of exposure to 20 J/cm² of laser energy. Improvements in local circulation and increased vasodilatation may increase the absorption of a local anesthetic agent. This may reduce the duration of the anesthesia, thereby allowing postoperative pain management to begin sooner.

The aim of this article is to review whether LLLT applied postoperatively to operated-on areas has an antianesthetic effect, that is, whether pain in the first hours after surgery was greater for patients who received LLLT than for control patients.

Materials and Methods

We aimed to find studies related to the use of LLLT after any type of surgical procedure. The keyword combinations used to search for relevant studies in the PubMed interface of MEDLINE (National Library of Medicine) were “low-level laser therapy” and “postoperative pain,” “low-power laser” and “postoperative pain,” “low-intensity laser” and “postoperative pain,” “LLLT” and “postoperative pain,” and “LILT” and “postoperative pain.” Related articles found in the database were also collected; however, articles not published in English were not reviewed. Further, studies that did not evaluate postoperative pain on the day of surgery were excluded from study.

Results

The results found through PubMed are presented in Table 1. The articles included in this review are the studies performed by Carrillo et al., ⁷ Roynesdal et al., ⁸ Clokie et al., ⁹ and Markovic and Todorovic. ¹⁰

Discussion

Several studies have noted the analgesic effect of LLLT. ^{11 –13} The ways in which laser irradiation attenuates pain are not yet fully understood, but they are becoming more clear. ¹⁴ Possible mechanisms of action include the alteration of endorphin secretion ¹² or of the metabolism of serotonin. ^11,13 If the pain sensation relieved with ischemia, then pain reduction is probably result of improvements in the microcirculation. In vivo studies of the analgesic effect of LLLT on the nerves that supply the oral cavity have demonstrated that LLLT decreases the firing frequency of nociceptors. LLLT selectively inhibits the range of the nociceptive signals that arise from peripheral nerves, including the neuronal discharges elicited by pinching, cold, heat, and chemical irritation. ¹⁵ In a double-blind study, it was reported that repeated irradiation with LLLT resulted in 19 of 26 study subjects experiencing the relief of chronic pain without the use of drugs. ¹³

Experiments on cells have shown that He–Ne laser irradiation activates the synthesis of macroergic compounds ¹⁶ and induces reversible membrane depolarization that correlates with the intensity of the irradiation; the effects of LLLT were found to depend on the initial membrane potential of the study neurons. ¹⁷ Thus, it is evident that laser irradiation can affect ion channels that are dependent on membrane voltage. The actions of local anesthetics can be mediated mainly by the suppression of sodium ions and potassium channels in electrically excitable cells. Normally, anesthetic solutions sharply decrease or completely suppress ion currents, and these actions are accompanied by increases in nonspecific leakage currents (membrane destabilization) and the incomplete reversibility of the effects. ¹⁸ Some studies of the effects of the laser irradiation of cells have demonstrated that LLLT induces shifts in the oxidative reductive potential of cells. Ignatov and colleagues ¹⁸ suggest that irradiation with an He–Ne laser induces changes in the functional activity of the neuron by acting on ion channels in the cell membrane. Those authors reported that laser irradiation at a dose of 0.7 × 10^–4 J increased the amplitude of the ion current and accelerated the process of channel inactivation. However, a tenfold increase in the dose of laser irradiation (0.7 × 10^–3 J) led to a decrease in the amplitude of the ion current, a decrease in the time constant of the inactivation of the potassium channels, and, most likely, a decrease in the number of functioning ion channels. The slowing or accelerating of the process of potassium channel inactivation probably identifies changes in the functioning of inactivation gates in the channels. Thus, He–Ne laser irradiation has modifying effects on the actions of the local anesthetic by acting on voltage-dependent slow potassium channels. In short, it can be concluded that LLLT may both help and hinder the effects of anesthesia: it may both increase and decrease the duration of the anesthetic effects.

The maximal intensity of pain occurs during the first hours after surgery, when the local anesthetic has worn off. Theoretically, postoperative pain control can be increased with the use of a local anesthetic with a more-prolonged action. ¹⁹ The ideal local anesthetic should cause minimal or no side effects or allergic reactions, it should exert its effect in the shortest time possible, it should not produce irreversible damage to nerve fibers while providing a lasting effect to complete any planned procedures, and it should be effective for an adequate duration. ²⁰ If a treatment method has both analgesic and antianesthetic effects, then the method may block its own effects.

In previous studies, LLLT has been used to prevent postoperative pain. Although some studies reported positive effects of the laser treatment, others reported no effects. ^{7 -10, 21 –27} (Table 1).

Our search for related literature produced only four studies that were relevant to this review. ^{7 –10} Of these four studies, Carrillo et al. ⁷ and Roynesdal et al. ⁸ reported that LLLT after surgery had no beneficial effect on pain on the day of surgery, whereas Clokie et al. ⁹ and Markovic and Todorovic ¹⁰ reported a positive effect of LLLT on postoperative pain. In their randomized double-blind study, Carillo et al. ⁷ reported that the percentage of postoperative pain among individuals treated with LLLT was higher than that of the placebo group after the third and fourth hours, whereas, after the first hour, the percentage of pain was lower. Although the differences are not significant, the results of the study by Carillo et al. ⁷ regarding level of pain after the third and fourth hours after surgery may be evidence of the antianesthetic effects of the laser therapy. However, Carrillo et al. ⁷ suggested that further dose-finding controlled trials were required to establish possible laser analgesic effects. However, Roynesdal et al., ⁸ who applied 6 J of laser energy at 40 mW of power and reported no beneficial effect of the LLLT on postoperative pain, did not provide information about the amount of laser energy applied per square centimeter. Hence, the energy level of laser may not have been adequate. Still, neither of the laser parameters in these studies may have been appropriate to produce an effect, and the antianesthetic effect of the laser might have been inhibited by the analgesic effect of the laser.

Interestingly, Markovic and Todorovic ¹⁰ determined that the postoperative use of LLLT after surgical extraction significantly reduced postoperative pain when compared with pain levels in control patients and patients treated with nonsteroidal antiinflammatory drugs (NSAIDs). However, Markovic and Todorovic ¹⁰ used a control group rather than a placebo group. Therefore, this analgesic effect may be a placebo effect. In addition, this effect was found to be dose dependent, and laser output doses of <4 J/cm² did not significantly affect postoperative pain intensity. By contrast, Clokie et al. ⁹ applied 1.8 J (0.97 J/cm²) of laser energy to the surgical site for 3 min immediately after the third molar surgery and reported that postoperative pain was reduced on the day of surgery.

Although LLLT has been used to prevent pain postoperatively, the results are variable and controversial. These results may be cause by different things, such as varying study designs; differences in or difficulties with the measurement of variables related to postoperative sequelae; and different lasers, handpiece types, and irradiation parameters. ^{7 -10,21,25 -27} Alternatively, these differences may be the result of the opposing and dose-dependent effects of the LLLT.

Conclusion

As a result, not too much evidence supports the antianesthetic effects of LLLT. Additional experimental and clinical studies must be performed to investigate the effects of LLLT on the duration of anesthesia.