**4. Discussion**

A review of the search results showed that acute and subacute Bell's palsy respond favorably to PBM [32, 33, 37–42]. Chronic Bell's palsy, however, responds less satisfactory [34, 36, 38]. Therefore, PBM seems to be a good complementary treatment to the standard treatment with medication. A RCT by Shoman et al. [28] compared the effectiveness of LLLT with electrical stimulation in conjunction with medicine, facial massage, and exercise. Results showed that combination treatment with LLLT was more efficient than electrical stimulation in facial nerve regeneration, as determined by the nerve conduction velocity measurement and SB System scoring [28]. Several studies also recruited patients non-responsive to steroids and antivirals to investigate their response to PBM treatment [29, 36, 38]. Aghamohamdi et al. [35] studied 30 poorly controlled diabetic patients who did not receive other medications for the palsy, especially corticosteroids, due to their underlying disease. The results showed complete recovery in 18 patients (60%) and partial recovery in 6 patients (20%) [35], showing that LLLT could be a safe alternative approach for BP patients with other medical condition that is contraindicated to traditional medication treatment. However, more studies are warranted to justify its effectiveness.

### **4.1 Mechanism of actions**

PBM involves the application of red and near-infrared light, with wavelengths ranging from 600 to 1000 nm, over the area of injury or lesion. It is a non-invasive and painless therapy for patients contraindicated to corticosteroids or antiviral medication treatments [32]. PMB has anti-inflammatory effects. Light absorbed by cellular photo-receptors modulates reactive oxygen species (ROS) and promotes adenosine triphosphate (ATP) formation. Moreover, it increases micro and macro-circulation, thus increasing tissues' oxygen saturation. These improve cellular metabolism and promote faster regeneration or proliferation of the damaged tissue [47]. Furthermore, PBM can photo-dissociate the nitric oxide, which inhibits mitochondrial respiration, thus reversing mitochondrial inhibition [48]. Through these mechanisms, PBM thus dilates blood vessels, improves oxygenation, and increases immune cell traffic to the targeted structure [49].

Also, laser therapy is shown to have direct beneficial effects on the regeneration of peripheral nerves in sensory and motor deficits, such as trigeminal neuralgia, herpes zoster, and sciatica. The therapy improves the recovery of injured nerves, slows motor neuron degeneration, and promotes axonal growth and myelination. The treatment can also lower the pro-inflammatory cytokine level and raise the anti-inflammatory cytokine level [32, 35, 40]. In addition, evidence shows that the HSV-1 can be effectively suppressed by PBM [50]. Laser therapy is also found to affect tissues differently depending on the prescribed wavelengths, pulse duration, energy density, delivery system, and duration of the whole treatment [32].

#### **4.2 Parameters of PBM treatment**

Of interest is that Bell's palsy responds to light therapy with vastly different parameters, which include wavelengths, power, power density, frequency, and energy density (**Appendix 2**). The wavelengths used in the different studies vary from 660 nm [37, 43] to 1064 nm [29, 32], with most of the studies using lasers with a wavelength of 830 nm [32, 33, 42]. Also, the power of the laser used differs significantly, with some using low-level laser with a power lower than 500 mW [33, 35, 42, 43] while others use high power laser (up to 3 kW) [32]. The energy density of laser applied in different studies also varies widely, from 10 J/cm<sup>2</sup> [32, 33, 37] to 120 J/ cm2 [39].

Existing evidence suggests that LLLT acts on the mitochondria and cell membrane chromophores to initiate the biological response [51]. Studies have suggested the possibility of using defined wavelengths for particular biological responses, hence achieving specific therapeutic effects [52, 53]. Barbosa et al. [52] compared the effects of LLLT at 660 nm and 830 nm on sciatic nerve regeneration following crushing injuries in rat models. They found that laser at 660 nm provided early functional nerve recovery compared to other groups. Another animal study by Lee et al. [53] also showed that rats with facial nerve injuries responded positively to 633 nm light, with better facial palsy scores, larger axon diameter, and higher expression of Schwann cells, but not to 804 nm light. One of our included studies used a laser at 780 nm during the early stage of treatment, aiming for a deeper penetration [37]. Starting from the fifth session, they changed to a wavelength of 660 nm to accelerate neural recovery. Alternating the two wavelengths was suggested to balance the stimulatory and inhibitory effects and avoid plateau situations due to extended use of a single wavelength [37]. However, the standardization of laser wavelength has yet to be defined, as our review showed that BP patients respond positively to different wavelengths.

One study showed that HILT is significantly more effective than LLLT in improving the symptoms of Bell's palsy [32]. In addition, HB Scale and FDI improved more with the HILT than with LLLT [32]. Kuzmičić et al. [29] reported a case of Bell's palsy, which was treated medically with corticosteroid and antivirals 18 months prior with minimal improvement. They employed HILT, acupuncture, and mirror book therapy to treat the patient for 7 weeks. Mirror book therapy is an adapted version of the mirror box method used to treat phantom pain and paralysis. It consists of a bi-fold mirror to reflect twice the unaffected patient's face so that the patient sees a full, unaffected face. The patient then performs a series of facial exercises. The intervention resulted in significant improvement of the facial palsy. The HB Scale reduced from IV to II, the SB System improved from 24/100 to 71/100, and the FDI improved from 43/200 to 173/200 [29]. The latter study did not compare the effects of HILT with LLLT. Also, the combination therapy makes it difficult to draw conclusions relating to the effectiveness of HILT in chronic Bell's palsy. Thus, whether HILT is superior to LLLT in managing Bell's palsy requires further investigation.

#### **4.3 Irradiation area**

Apart from the different parameters of the laser applied, the treatment method differed in various studies. Six studies used the laser on seven to nine points (**Figure 1**) along the facial nerve branches of the affected side [28, 32, 33, 35, 36, 41]. A laser is applied near the mastoid process in the stylomastoid foramen, where the facial nerve exits. Also, light is used on points along the facial nerve branches, including the temporal branch across to forehead, the zygomatic branch along the zygomatic arc, the buccal nerve in the middle portion of the cheek, and the mandibular branch along the chin [35, 41]. Ton et al. [34] applied lasers on the acupuncture points; seven were on the affected face, covering all the facial nerve branches, while two were on both sides of the extremities. Other than facial nerve branches, some studies irradiate

*Management of Bell's Palsy with Phototherapy DOI: http://dx.doi.org/10.5772/intechopen.106617*

**Figure 1.** *Common laser application points along the facial nerves.*

the facial muscles. Tanganeli et al. [39] applied laser on ten points, including the frontal, temporal, zygomatic, buccinator, lip elevator, orbicularis of the lips, lip depressor, and masseter muscles. Poloni et al. [42] only applied laser at the origin and insertion of the masseter muscle, and Fornaini et al. [40] also irradiated only the parotid gland area about the former. Bernal Rodriguez et al. [43] and Fontana and Bagnato [37] applied laser on 59 points and up to 80 points, respectively, covering all the area on the affected face. The treatment was applied to many application points because the laser beam spot is small [37, 43]. In Ng and Chu's [38] report, the four pads for MIRE therapy were placed in the post-auricular area, pre-auricular area, the temple, and the mandibular area of the affected side.

While laser therapy at the site of compression in the facial canal is understandable, the same applied over the peripheral nerve branches and muscles over the face requires further justification. Moriyama et al. [54] observed a change in gene expression in BP with the degree of facial nerve palsy. Most genes in energy and muscle categories of severe BP (HB Scale V) were downregulated, while they were upregulated in moderate BP (HB Scale III). The facial nerve conduction is largely abolished in severe BP, hence there is very little regeneration and energy production in the affected muscles and nerves [54]. This may partly explain the total or partial unresponsiveness of patients to medication [36]. On the basis of these molecular biological findings, treatment to promote muscle regeneration and energy production is hence considered. Given the ability of PBM to improve cellular metabolism and promote ATP formation [47], irradiation of the affected facial muscles and nerve branches could potentially improve the muscle regeneration and enhance the paralytic condition. Further investigations are required to compare the effects of PBM on different irradiation areas.

#### **4.4 Timing of treatment**

Also of interest is that the outcome of chronic cases is less favorable than acute and subacute cases. Ng and Chu [38] described a chronic BP case that only partially responded to MIRE. The 46 years old lady with systemic lupus erythematosus and BP for two and a half years was treated by MIRE. After 45 treatment sessions, the patient reported 50% improvement compared with baseline. Residual facial palsy was still evident [38]. Pasquale et al. [36] reported similar findings; the researchers applied HILT to a cohort of 14 patients who were non-responsive to standard treatments. The number of treatments varied from 6 to 20. Of the fourteen patients, eleven who had subacute BP improved uneventfully. However, three patients with chronic BP did not improve [36]. Similarly, Ton et al. [34] showed that LLLT did not improve chronic Bell's palsy. Seventeen chronic BP patients were randomized to treatment by laser acupuncture and sham laser. The patients were treated three times a week for 6 weeks. The outcome was measured by HB Scale, FDI, and SB System. Results showed that only HB Scale improved slightly after 3 weeks of laser acupuncture treatment compared to sham laser treatment. No significant differences were detected for HB Scale and FDI at 6 weeks between the intervention group and the sham laser treatment group [34]. One study, however, showed that LLLT improved the symptoms of chronic BP [43]. The female patient had 8 years of history of BP. LLLT treated fiftynine points on the face of the affected side; each point was treated for 20 seconds, with 4 J per point. The patient was treated three times a week for 8 weeks. After the treatment, the HB Scale was reduced from IV to II, suggesting a partial response to LLLT [43]. Kuzmičić et al. [29] reported a chronic BP responding to combination therapy of HILT, acupuncture, and mirror book therapy.

As the outcome of these studies on chronic BP differs significantly, with some reporting favorable outcomes and others minimal changes, the use of laser therapy in chronic BP remains undetermined.

Clinically, we suggest combining phototherapy with the standard treatment to manage acute and subacute cases. Medical practitioners can refer patients to other healthcare practitioners for PBM treatment while prescribing the medications. Though chronic BP does not respond as reliably as acute and subacute cases, they should be treated for three to 6 weeks by LLLT, as some patients may respond favorably to the treatment [43]. As we are unaware of any studies on the effects of PBM on BP caused by COVID-19, we cannot provide any recommendations. However, we consider the method worth trying, particularly when the medication treatment is contraindicated.

#### **4.5 Possibility of self-administered PBM**

PBM treatments are time-consuming [55]. Clinical practice usually requires patients to return for short treatment sessions about three times a week to achieve a satisfactory result. Although laser treatments administered by trained professionals were shown to be good options for accelerating the recovery of BP [56], several studies have proposed the use of "at home" PBM devices [40]. One study prescribed a patient with a self-administered class II laser device, emitting at 808 nm and 250 mW output power, to be used twice daily, each for 15 minutes [40]. The patient simply placed the device in contact with the skin on the designated areas. After 2 weeks of treatment, the patient reported a complete recovery (HB Scale improved from IV to I) [40]. Another study reported the effects of MIRE, an array of 60 gallium aluminum

*Management of Bell's Palsy with Phototherapy DOI: http://dx.doi.org/10.5772/intechopen.106617*

arsenide light-emitting diodes, on patients with BP; it showed positive results, especially in acute patients [38]. The pads of this MIRE device could irradiate a larger area at one time compared with an infrared laser, hence reducing the need for accurate localization of facial nerve [38]. In both studies, no adverse events were reported for the devices used, and protective glasses are unnecessary, making them more favorable for self-application [38, 40]. Nevertheless, patient evaluation by practitioners before its use remains mandatory.

#### **4.6 Contraindications and adverse effects of PBM**

Phototherapy is a generally safe and non-invasive treatment. Yet, the North American Association for Laser Therapy Conference 2010 issued cautionary statements and stipulated several contraindications [48]. The operator must not point the laser beam at the eyes, and all participants in the treatment should wear appropriate safety spectacles. Practitioners must pay extra attention while operating the device as a low frequency pulsed visible light of less than 30 Hz may trigger a seizure in photosensitive and epileptic patients.

Also, it is recommended not to apply light therapy on any known primary carcinoma or secondary metastasis site, as its effects on cancer have not been elucidated [48]. Animal studies have shown PBM can be detrimental to cancer. However, other studies have shown that PBM benefits cancer treatment. It can directly damage the tumor, enhance the effects of cancer therapies, stimulate the host immune system, and increase cancer patients'survival rate [57]. In addition, if the patient is receiving chemotherapy or is defined as terminally- ill, PBM can be used to relieve the side effects of treatment and for palliative relief [48].

The evidence on the use of LLLT on cancer is limited. However, practitioners should be aware of this in the management of BP. If the patient is suffering from BP and has a known medical history of cancer somewhere else in the body, other than directly on the face, we would still consider treating the patient with LLLT. The adverse effects of the PBM are reported to be no different from placebo treatment [48].
