Aetiology
Strong evidence points to reactivation of herpes simplex virus type 1 (HSV-1) within the geniculate ganglion as the underlying cause of Bell's palsy. A prospective controlled trial demonstrated HSV-1 DNA in endoneurial fluid and mimetic muscle specimens from 11 of 14 patients with Bell's palsy undergoing facial nerve decompression, but in none of the specimens from nine patients with Ramsay Hunt syndrome nor from three patients with temporal bone fractures undergoing the same procedure, nor from specimens from other controls.[18] In a murine model, the presence of HSV-1 DNA in the facial nerve and HSV-1 capsids in the geniculate ganglion of mice inoculated with HSV-1 who developed facial palsy after streptozotocin-induced diabetes mellitus has been demonstrated.[19] Evidence from animal models and clinical research suggests that cellular immune suppression may be a precipitating factor in HSV-1 reactivation and the subsequent onset of Bell's palsy.[20][21][22] Data from the US show a recent rise in incidence, possibly linked to increasing rates of herpes infections.[23]
Correlation between facial palsy and coronavirus disease 2019 (COVID-19) has been demonstrated.[24][25][26] One review reported facial palsy as the first clinical manifestation in patients with COVID-19.[25] Two systematic reviews reported Bell’s palsy as the only major neurological manifestation in patients with COVID-19.[24][26] However, robust evidence is needed to confirm the association between the two. Further studies have suggested a possible association between Bell’s palsy and COVID-19 vaccination.[27][28][29] One systematic review found that facial nerve palsy was the most common neurological adverse effect experienced by individuals receiving COVID-19 vaccines.[27] Another study demonstrated no association between recent COVID-19 vaccination and Bell’s palsy.[30] The benefits of vaccination outweigh the risks of adverse effects, and more studies are required to prove or disprove the association.[28][29]
Pathophysiology
Re-activation of HSV-1 results in the destruction of ganglion cells and infection of Schwann cells, leading to demyelination and neural inflammation.[31] In addition to special visceral efferent axons to the muscles of facial expression, the stapedius muscle, the posterior belly of the digastric muscle, and the stylohyoid muscle, the facial nerve consists of three other components bound in a distinct region known as the nervus intermedius: general visceral efferent axons that provide pre-ganglionic parasympathetic innervation to the lacrimal, submandibular, sublingual, and minor salivary glands; special visceral afferent axons that relay gustation from the oral tongue; and general somatic afferent axons that relay cutaneous sensation from the posterior wall of the external auditory canal, concha, and postauricular area. Evidence has demonstrated that the initial site of conduction blockade in Bell's palsy occurs at the narrowest portion of the bony fallopian canal (i.e., the meatal foramen),[32] which is immediately proximal to the labyrinthine segment and geniculate ganglion. Consequently, Bell's palsy may affect all four fibre types carried by the facial nerve. Insult to somatomotor axons to the muscles of facial expression leads to the most obvious presentation of facial palsy, while insult to axons supplying the stapedial muscle results in hyperacusis, insult to gustatory fibres results in hypo- or dysgeusia, insult to sensory fibres results in otalgia and post-auricular pain, and insult to parasympathetic axons results in lacrimal and salivary gland dysfunction.
Classification
House-Brackmann scale (HBS)
The HBS is a grading scale of facial nerve function, developed as a means to measure long-term facial mimetic outcomes following vestibular schwannoma resection.[5] The simplicity and commonality of the scale has led to its widespread - and somewhat inappropriate - use in quantifying the degree of severity of all causes of acute and long-term facial palsy. When used to quantify the severity of an acute facial palsy such as Bell's palsy, the HBS is interpreted by clinicians in the following manner:
Grade I = normal
Grade II = slight weakness/asymmetry
Grade III = obvious weakness with movement but absence of disfigurement at rest; intact ability to close the eye
Grade IV = obvious weakness with movement and disfigurement at rest; inability to fully close the eye
Grade V = barely perceptible movement
Grade VI = no movement.[5]
Sunnybrook Facial Grading System (FGS)
The FGS is a higher-resolution measure of facial function in comparison to the HBS, consisting of ordinal-scale scoring of multiple parameters within three domains:
Resting symmetry (eye, nasolabial fold, and mouth)
Symmetry of voluntary movement (brow elevation, gentle eye closure, open mouth smile, snarl, and lip pucker)
Synkinesis (same parameters as voluntary movement).[6]
FGS domain scores are combined to form a weighted composite score from 0 (complete flaccid paralysis) to 100 (normal function). While the complexity of the FGS typically necessitates the use of a paper or digital form for scoring and documentation, its use permits tracking of clinical recovery with higher precision over time, making it better suited for research use compared with the HBS.[6]
Electronic facial paralysis assessment (eFACE)[7]
The eFACE tool provides an even higher-resolution means of tracking facial outcomes than the FGS, through clinician-graded continuous visual analogue scales within three domains:
Static symmetry (resting brow, resting palpebral fissure width, resting nasolabial fold depth, oral commissure resting position)
Dynamic symmetry (brow elevation, gentle and full eye closure, nasolabial fold depth and orientation with smile, oral commissure excursion with smile, lower lip movement with 'eeeee' [the photographer will ask the person to say 'eeeee' when capturing movement of the lower lip]).[7]
Synkinesis (ocular, midfacial, mentalis, platysmal).
The tool is available as a downloadable application whose graphical user interface allows for touch-screen scoring and, when linked to a database, immediate graphical comparison to previous scores. This grading tool has also been validated for use with standardised patient photos and videos.[8][9] One systematic review concluded that eFACE was the most comprehensive tool available for the assessment of blink function.[10]
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