**1.2 Relationship between COVID-19 and Bell's Palsy**

Besides the unknown etiology of BP, a statistically significant higher incidence of BP is observed in 2020 compared to 2019. The increased incidence is associated with the COVID-19 pandemic [21]. A systematic review showed significant evidence that BP is the only major neurological manifestation in COVID-19 patients [12]. Another systematic review conducted by the same authors reviewed 46 COVID-19 cases. Around 70% of the cases showed complete recovery from BP symptoms, while over 20% of them had residual symptoms after combination treatment, mainly of corticosteroids and antiviral drugs [11]. Although the evidence of BP being the major neurological symptom in COVID-19 is significant, further studies are required to identify how the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) exactly causes the neurological symptoms [11, 12].

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

Along with a higher risk of developing BP with COVID-19 infection, BP has also been recognized as a common adverse event after COVID-19 vaccination. A systematic review recognized BP as one of the most common adverse events after mRNA and viral vector-based immunization [13]. In addition, a study in Hong Kong reported a significantly increased risk of Bell's palsy following the vaccination of inactivated SARS-CoV-2 vaccine but not the mRNA vaccine [22]. However, another systematic review, which studied 61 vaccinated patients immunized with different vaccines, reported that more than half of the cases developed BP after vaccination. Interestingly, BP only occurs in patients who received mRNA and viral vector-based vaccines but not in inactivated SARS-CoV-2 vaccines [23].

Both COVID-19 and COVID-19 vaccination are suspected to be a potential cause of BP. However, the relationship between COVID-19 and COVID-19 vaccine on BP has yet to be determined and requires further investigation.

#### **1.3 Prognosis**

The current standard treatment for BP includes corticosteroids alone or combined with antiviral agents, although the condition is considered idiopathic. However, different studies reported conflicting results [3]. Some studies reported inadequate recovery from the treatments [24]. Around 70% of BP patients will recover spontaneously without treatment; 10–29% of patients will have persistent dysfunction in the facial nerve [24] with an 8% recurrence rate for those who had previous episodes of BP [3].

Comparing the standard treatment of corticosteroids alone or in combination with antiviral agents, the effects of corticosteroids alone are found to be no different from placebo [25], while a higher rate of full recovery is observed with the use of antiviral drugs in combination with corticosteroids [24]. The recovery rate of using corticosteroids alone was reported to be 67% while that of the combination treatment was 78% [24]. A randomized controlled trial (RCT) indicated that when the medical treatment was delayed for more than 4 days after the onset of symptoms, the treatment has no benefit to the patients [26].

However, the treatment of corticosteroids and antiviral agents is contraindicated in some patients (e.g., hypertensive or diabetic patients) [27]. Alternative and complementary therapies may thus provide other options.

Along with the standard treatment, physicians have been using other therapeutic methods, for instance, electrical stimulation, facial exercise, massage [28], acupuncture [29], and photobiomodulation (PBM) as supplementary treatments. While the effects of electrical stimulation, facial exercise and massage were insignificant [28], acupuncture treatment potentially showed some benefits in patients with BP [30]. However, exercises, electrical stimulation and acupuncture are not recommended to be used in patients with acute BP in several clinical guidelines due to the lack of good quality evidence [29].

#### **1.4 Role of Photobiomodulation in Bell's Palsy**

Among these complementary therapies, PBM is a non-invasive treatment, which has been investigated in recent years for its effect in treating BP. Of the different types of PBM, low-level laser therapy (LLLT) has been the most studied; it is effective in regenerating peripheral nerves in the preclinical study [31]. Our study investigates whether sufficient evidence supports PBM treatment in BP patients.
