The facial nerve carries the motor fibers for the facial expression and mastication and the sensory fibers to the external ear and the special sensation of taste. Partial weakening or loss of function of the facial nerve due to paralysis, therefore, affects innervated structures and associated functions depending on where the paralysis of the facial nerve is located. This can be presented as a unilateral or bilateral loss of voluntary and involuntary facial muscle movements. An asymmetric smile, dry eyes, involuntary muscle movements, reduced tolerance to sounds, abnormal blinking, issues in speech articulation, involuntary drooling, alterations in taste perception, and facial pain are some of the symptoms of such paralysis. With a broad management variability, health practitioners initially investigate the etiological nature of the condition. An oral health practitioner can encounter a patient with a facial paralysis, be the first one to diagnose a patient with facial paralysis or induce facial paralysis iatrogenically during simple or complex dental procedures. It is therefore important to involve the medical practitioner and relevant medical specialties in determining and confirming the specific cause and etiology of the facial paralysis as it may be multifactorial and require treatment from other disciplines in order to provide the best possible outcome for the patient.
Background of Facial Paralysis
The facial nerve (cranial nerve VII) is composed of two roots, a motor and sensory root.1 The large motor root carries the motor fibers to the muscles of facial expression and mastication. The sensory root carries general sensory fibers to parts of the external ear and the special sensation of taste to the anterior two-thirds of the tongue and the palate. The sensory root contains some parasympathetic secretomotor fibers as well which supply the submandibular and sublingual salivary glands, as well as the lacrimal glands and the minor mucous glands of the nose and palate. Since facial paralysis causes the partial weakening or loss of function of the facial nerve due to the cessation or inhibition of nerve impulse propagation,1 any structures and their associated functions innervated by the facial nerve can also be affected depending on where the paralysis of the facial nerve is located.
Facial paralysis presents as a unilateral or bilateral loss of voluntary and nonvoluntary facial muscle movements. This can cause symptoms such as an asymmetric smile, dry eyes, involuntary muscle movements, reduced tolerance to sounds, abnormal blinking, issues in speech articulation, involuntary drooling, alterations in taste perception, and facial pain.2 Since symptoms cause deficiencies in taste, mastication, speaking, and expressing emotions, the patient’s physical and emotional health is affected.3
Management for facial paralysis can vary broadly. This is due to the complex etiological nature of the condition. There are a number of known causes for facial paralysis, the most common being Bell palsy, an idiopathic disease that can only be diagnosed by exclusion. Treatment plans differ, depending on cause and severity of injury.4 Bell palsy presents as unilateral drooping of the face. An asymmetrically sloping mouth is the landmark feature of Bell palsy. Bell palsy is a peripheral paralysis of the 7th cranial nerve. Central paralyses can occur as well, often as a result of a stroke. Central and peripheral palsies present differently. Central paralysis only involves the unilateral side of the lower face while peripheral palsy most commonly affects the entire, unilateral side of the face.5
Description of Facial Paralysis
Paralysis of the face has been an observed phenomenon for millennia. Ancient Greek, Roman, Egyptian, and Persian cultures all have documented conditions that resemble facial paralysis.6 Hippocrates was one of the earliest to record such conditions. In his writings from around the fifth century BCE, he vaguely described distortions of the face that would resolve without treatment.7
For a long time Charles Bell was credited with the first comprehensive description of idiopathic facial paralysis in 1821, hence its name Bell palsy.6 However, it was later discovered that a number of individuals before Bell recorded accounts of facial paralysis.8 The first comprehensive account of facial paralysis is that of the Persian Physician known as Razi. In his sixth book of al-Hawi, Razi was able to differentiate between facial nerve spasms, central paralysis, and peripheral paralysis. Razi is credited with being the first to describe the clinical features of Bell palsy. His findings were documented around the ninth century CE.9 Dutchman Cornelis Stalpart van der Wiel is now largely regarded as the first to comprehensively document the clinical features of Bell palsy, rather than Charles Bell himself. He described the palsy as being one-sided and healing in several weeks without treatment. He documented this description in 1683 in his second publication of “Hundred Rare Observations.”10 Following Stalpart van der Wiel, Nikolaus Anton Friedreich (1798) and James Douglas (1704) also appear to have observed and described facial paralysis before Bell in the 18th century.11
Epidemiology of Facial Paralysis
Facial paralysis varies depending on the level of facial nerve lesion. This phenomenon can be reversed spontaneously or via clinical or surgical treatment. About 20% of patients develop some form of sequelae, with unilateral and bilateral complete paralysis of facial muscle movements being the most severe outcome.3 The global annual incidence of facial paralysis is approximately 70 cases per 100,000 population. In a study published by Bleicher et al, the global annual number of cases of permanent facial paralysis was estimated to be 127,000.12 The prevalence, which refers to the number of existing cases or lesions within a defined population at a specific point in time,13 of facial paralysis, was greater among patients that are younger than 20.3 Also, according to a study conducted at Murtala Muhammed Specialist Hospital, Kano, Nigeria between January 2000 and December 2005,14 males had a higher incidence of facial paralysis than females at 56.2%, it was most common among businessmen at 31.6%, and that right-sided facial palsy was slightly more predominant compared to the left.
Facial paralysis remains a challenge diagnostically due to the high number of causes. Multiple causes of facial paralysis have been described in the literature. In two-thirds of the cases studied, the cause is unknown and are referred to as idiopathic paralysis or Bell palsy.3 In the United States, the annual incidence, which refers to the number of new cases or lesions of a specific disease arising over a given period of time within a specified population,13 of Bell palsy is approximately 23 cases per 100,000 population. Bell palsy is thought to account for approximately 60 to 75% cases of acute unilateral facial paralysis.15 Hence, Bell palsy is the most common cause of facial nerve paralysis. However, there are many other causes for facial paralysis that makes up for the remaining one-third of the cases. All the major causes will be explained in detail later in subsequent sections, but for now, the distribution of facial paralysis by cause shall be discussed. According to a retrospective study by Batista (2011), 122 (42.8%) of the patients admitted to the hospital with facial paralysis were due to Bell palsy characteristics, 54 (18.9%) due to stroke, 48 (16.8%) due to congenital paralysis, 9 (3.2%) due to facial trauma, 17 (6%) due to traumatic brain injuries, 7 (2.4%) due to vestibular schwannoma, 9 (3.2%) due to tumors, and 19 (6.7%) due to other etiologies (Fig. 9.1).3
Fig. 9.1 Processed quantitative data showing the percentage of patients diagnosed with facial paralysis due to Bell palsy, congenital paralysis, traumatic brain injuries, stroke, facial trauma, tumors, vestibular schwannoma, and other etiologies admitted to the study of Batista in 2011.3
In an attempt to explain the differences in these values between the different causes of facial paralysis (analytical epidemiology), Bell palsy (most common) and other causes of facial paralysis, which are less common, are compared. Bell palsy is significantly higher than other causes (42.80% compared to second most common 16.80% in congenital paralysis) because there are many factors that lead to Bell palsy in comparison to other causes: Bell palsy occurs when the nerves of facial muscles become compressed or inflamed, when viral infection such as viral meningitis or herpes simplex are present, and also when associated with diabetes, chronic middle ear infection, headaches, influenza, tumors, high blood pressure, Lyme disease, sarcoidosis, and facial trauma.16 The second most common congenital paralysis in comparison is only affected by traumatic injury or developmental deformities of the brain or cranial nerve VII of the facial nerve.17
Etiology and Pathogenesis of Facial Paralysis
The most commonly diagnosed cause of facial paralysis is Bell palsy (Fig. 9.2).18 This condition is poorly understood and often thought to be of idiopathic origin and thus diagnosed by exclusion.19 However, recent scientific advances have and are still exploring a possible link between numerous infectious agents and Bell palsy, but as this is not completely understood yet, they will be handled separately, given the assumption that a causal relationship does not exist between them.20
Some of these infectious agents leading to the development of facial paralysis include varicella zoster virus; herpes simplex virus; spirochete Borrelia burgdorferi giving rise to Lyme disease; and Streptococcus pneumonia, Haemophilus influenzae, and Moraxella catarrhalis as the most frequent causative agents of otitis media.19
The varicella zoster virus is a member of the Herpesviridae family and following the primary varicella infection establishes a ganglionic latency.21 When this latency is established in the sensory fibers of the geniculate ganglion, reactivation of the virus, often precipitated by factors including stress, trauma, and immune deficiency, results in Ramsay Hunt syndrome type 2.16 This reactivation as herpes zoster leads to inflammation in the geniculate ganglion, and it is this inflammation that results in disrupted motor function of the facial nerve as visceral efferent fibers from the motor nucleus of CN VII pass through the geniculate ganglion and are thus affected.22 In addition, visceral afferent taste fibers from the anterior two-thirds of the tongue and visceral efferent parasympathetic fibers that innervate the salivary and lacrimal glands are affected as they also pass through the geniculate ganglion leading to decreased taste sensation, salivation, and lacrimation.22
The herpes simplex virus (HSV) is another member of the Herpesviridae family and, similar to the varicella zoster virus, establishes latency following primary infection.23 HSV-1 is most often associated with cold sores and HSV-2 with genital herpes. The virus lies dormant in peripheral nerve cell axons and when activated leads to an inflammation of the endoneurium and demyelination of the nerve fiber.19 Facial nerve demyelination leads to slower conduction of action potentials along the axon of the nerve fiber and depending on the extent of the demyelination, paralysis. Moreover, when the endoneuritis spreads to the facial canal, facial nerve paralysis results from the compression of the nerve due to the local swelling.24
Lyme disease is spread through vector-borne transmission, primarily ticks, and is classified into acute and chronic stages of the disease, both of which represent periods for facial nerve paralysis to occur.25 When the spirochete bacteria directly invade the nervous system, neural abnormalities develop.19 One such abnormality, cranial neuralgia, which has been found to manifest within 2 days to 2 months of illness, may result in the paralysis of any cranial nerve.25 When this neuralgia affects the facial nerve, facial paralysis results.
Otitis media is an infection of the middle ear. While facial nerve paralysis is an important complication, little is understood on the mechanism through which this palsy results.26 In acute otitis media, facial paralysis is believed to be a consequence of bacterial toxins associated with the infection affecting the facial nerve in the facial canal and resulting in inflammatory edema.27 In chronic suppurative otitis media, it is thought that facial paralysis may be a result of either the spreading of the body’s chronic inflammatory response from the middle ear to the facial nerve or from direct pressure on the facial nerve by a pathology, most frequently a cholesteatoma.28
Another cause of facial paralysis is stroke. Ischemic stroke occurs because of an inadequate supply of blood and oxygen to tissue in the brain and it is in this ischemic environment that the brain cells cease to function and may die if deprived of oxygen for too long. Hence it is the location of the ischemia that will determine whether the facial nerve is affected and the duration that will determine whether facial paralysis results.29 In hemorrhagic stroke, it is the rupture of a blood vessel in the brain and the subsequent bleeding that may cause facial paralysis depending on whether the bleeding is placing pressure on the facial nerve.30
Facial paralysis may also be a result of a tumor either growing within the nerve or compressing the nerve anywhere along its course.20 Neoplasms of the parotid gland, most often adenoid cystic carcinoma, are one such source through which facial paralysis may manifest.31 Paralysis is a result of the expansion of the neoplasm within the parotid gland leading to either compression of the nerve with subsequent ischemia or perineural invasion.31 , 32 The part of the nerve that is paralyzed will depend on the location of the neoplasm within the gland.
Trauma is another origin from which facial paralysis can develop.20 Of particular dental relevance is the trauma to the facial nerve sustained during dental surgery.33 When performing an inferior alveolar nerve block as part of a dental surgery, it is the close association between the inferior alveolar nerve and the facial nerve that makes physical trauma to the facial nerve by the needle a possibility.34 Traumatizing the facial nerve, through perforating it with the needle, results in facial paralysis.35 It has also been suggested that the deposition of lignocaine into or around the nerve, by injecting into the parotid gland, may lead to local anesthetic toxicity of the facial nerve and thus paralysis.34
Genetic syndromes may also play a role in the development of facial paralysis. These include Melkersson-Rosenthal syndrome, Albers-Schönberg disease (osteopetrosis), Möbius syndrome, and Goldenhar syndrome (oculoauriculovertebral dysplasia).19 The summary of causes of facial paralysis is presented in Fig. 9.3.
Genetic Component of Facial Paralysis
Möbius syndrome (MBS) is a rare congenital condition that affects every 1 in 50,000 live births. The clinical findings of MBS can vary significantly in terms of both symptoms and their severity. However, one typical characteristic involves the paralysis of the facial (VII) nerve either unilaterally or bilaterally. The most widely accepted theory regarding facial paralysis from MBS is thought to occur due to an interruption to vascular supply leading to ischemia and malformation of the facial nerve. Recent studies have produced findings which indicate that MBS may result from rhombencephalon developmental defects rather than isolated facial nerve dysfunction as in hereditary congenital facial palsy (HCFP).36
FLTI/VEGFR1 is one gene thought to be involved in abnormal vascular growth, associated with MBS and resulting facial paralysis.36 Mutations in two homeobox genes, HOXA1 and HOXB1, also have been found to contain overlapping features with MBS. These said mutations produce phenotypes that are typical of MBS including facial weakness, indicating an association with the syndrome.37 Two genes, PLXND1 and REV3L, present within the HCFP1 and HCFP2 loci, respectively, have been confirmed to be associated with MBS. PLXND1 encodes for a multitude of amino acids, and mutations to this gene cause defects in motoneuron migration, affecting neural fiber structures. The gene REV3L is associated with hypoplasia of neural structures in mice and is also linked to DNA damage.38
Goldenhar Syndrome (Oculoauriculovertebral Dysplasia)
Goldenhar syndrome is congenital defect with incidence of 1 in 5000 to 25,000 live births with a prevalence in males. Facial nerve palsy is one characteristic associated with the typical craniofacial abnormalities found in Goldenhar syndrome, although this has not been confirmed and remains largely unstudied.39
Albers-Schonberg Disease (Osteopetrosis)
Albers-Schonberg disease is a disorder of autosomal dominance and is the most common type of osteopetrosis, a disease that affects the functioning ability of the body’s osteoclasts.40 This disease typically onsets during adulthood with a defect in the osteoclast’s ability for resorption, resulting in increased mass of skeletal bone. Facial nerve paralysis of bilateral range typically arises in Albers-Schonberg disease due to facial canal stenosis, leading to ischemia or rupture of the facial canal due to overgrowth of the epitympanum.41 Recent studies have confirmed that mutations in the genes encoding for ClCN7 chloride channels on chromosome 16p13.3 are responsible for Albers-Schonberg disease. This chloride channel allows for osteoclasts to resorb bone through acidification of the extracellular lacuna and ceases to function in patients with this disease.40
Melkersson-Rosenthal syndrome (MRS) is a rare, granulomatous disorder that may present with intermittent or relapsing facial paralysis. This disease typically onsets at 25 years old; however, it can affect all age groups.42 Although much of the genetics remains relatively unknown, studies have been conducted showing a hereditary link between those with existing MRS and a familial history of facial palsy. Further study in 2015 implicated that MRS-induced facial palsy is of autosomal-recessive inheritance.43
Hereditary Congenital Facial Paresis
Hereditary congenital facial paresis (HCFP) is an autosomal-dominant disorder that exists separately from the previously mentioned genetic syndromes.44 This disease is classified as a congenital cranial dysinnervation disorder (CCDD) alongside MBS and is characterized by weakness or paralysis of the facial muscles and dysfunction of the facial nerve (CN VII). However, unlike MBS, the paralysis results from either unilateral or bilateral malformation of the facial branchiomotor (FBM) nucleus.17
There are three genetic subdivisions of this disease: HFCP1, HFCP2, and HFCP3, the latter of which has only recently been discovered. The former two subtypes were initially thought to be subtypes of MBS. However, this has been proved to be incorrect, and the loci suggested to be associated with the syndrome on chromosomes 3q21.2-q22.1 and 10q21.3-q22.1 have now been renamed from MBS2 and MBS3 to HFCP1 and HFCP2. Studies have been conducted to identify genes from the mentioned loci that are involved in HCFP. From the HFCP1 and HFCP2 loci, three genes have been shown to display expressional associations to FBM nucleus development. Mgll is one the first loci that codes for lipase and is expressed in zones that largely coincide with the migratory route of facial motoneurons, indicating an association. The two suggested associated genes from HCFP2 are REEP3 and LRRTM3. REEP3 has shown expression in the FBM nucleus, and mutations to certain segments of REEP3 are known to be associated with other diseases such as autism. In contrast, LRRTM3 is involved with development of neurons, hence possibly relatable to the facial nerve. However, it is necessary to conduct further studies in order to prove its role in the specific development of the FBM.17 From the HCFP3 locus, the gene HOXB1, which also correlates to MBS, has been indicated to be associated with HCFP, where mutations in the Arg5 homeodomain are thought to be responsible.44
Diagnostic Evaluation of Facial Paralysis
Identification of facial paralysis is mostly done clinically. There are factors that act as a good indicator of facial paralysis such as facial drooping, eyelid weakness, puffing out the cheeks, and raising both eyebrows. In facial paralysis, there are two types of lesions, which are central and peripheral. The central lesion, which is a stroke, spares forehead and drooping lips whereas peripheral lesion, which is Bell palsy, leads to patients not being able to spare forehead but droops forehead and lips.45
The physical examination should include a complete head-and-neck examination with an emphasis on ear, mastoid, and parotid glands. When performing the physical examination, it is necessary to assess the patient’s motor function.46 Thus consider if the patient has peripheral facial palsy, if the patient closes the eyes tightly, if the smile is symmetric, if the patient is able to puff out the cheeks, and such, because most of the times with facial paralysis patients, they present difficulties doing those movements.45
Medical history of patients provides clues to narrow the differential diagnosis.46 When examining an acute event, there are a couple of factors to be considered; duration of paralysis, onset time, past history of facial paralysis, the presence of skin rashes, and any potential tick exposure. For example, if the patient has gone camping and could have been tick-exposed, the patient could be suspicious of Lyme disease causing the facial paralysis.47
In relation to the facial paralysis caused by Lyme disease, people who live in an area where Lyme disease is endemic are likely to be exposed to ticks, which can lead to facial paralysis. In this case, patients will need to have a laboratory testing to further investigate. How the testing works is that patients would have to have the enzyme-linked immunosorbent assay run for the screening of the disease of interest. However, other options are possible, which is the indirect fluorescent antibody test to do so. In case of a positive result from the assay, the diagnosis of Lyme disease will have to be double-checked by protein immunoblot known as Western blot.47
For other differential diagnoses, through an examination for herpes zoster, if vesicles are present, clinicians need to check serum antibodies. It is also possible that ACE, HIV, and inflammatory indicators could be tested in certain clinical settings. Cerebrospinal fluid (CSF) is typically not helpful in the diagnosis for facial paralysis, but it can differentiate it from diseases involving the CNS.