Bell\'s palsy Pathogenesis, clinical features, and diagnosis in adults - Up To Date PDF

Preview

Summary

Download Bell's palsy Pathogenesis, clinical features, and diagnosis in adults - Up To Date PDF

Description

17/9/2020

You can now bookmark content for easy access later.

Bell's palsy: Pathogenesis, clinical features, and diagnosis in adults - UpToDate

rom UpToDate ® com ©2020 UpToDate, Inc. and/or its affiliates. All Rights Reserved.

Bell's palsy: Pathogenesis, clinical features, and diagnosis in adults Authors: Michael Ronthal, MD, Patricia Greenstein, MB, BCh Section Editor: Jeremy M Shefner, MD, PhD Deputy Editor: Richard P Goddeau, Jr, DO, FAHA All topics are updated as new evidence becomes available and our peer review process is complete. Literature review current through: Aug 2020. | This topic last updated: May 11, 2020.

INTRODUCTION Bell's palsy, also referred to as idiopathic facial nerve palsy or facial nerve palsy of suspected viral etiology, is the most common cause of acute spontaneous peripheral facial paralysis. Herpes simplex virus activation is the likely cause of Bell's palsy in most cases, although there is no established or widely available method of confirming a viral mechanism in clinical practice. The evaluation of suspected Bell's palsy therefore requires that other causes of peripheral facial palsy be considered and in some cases ruled out, depending on clinical suspicion. This review will discuss the pathogenesis, clinical features, and diagnosis of Bell's palsy. The prognosis and treatment of Bell's palsy are discussed elsewhere. (See "Bell's palsy: Treatment and prognosis in adults".)

ANATOMY OF THE FACIAL NERVE The facial nerve is a mixed nerve, containing the following (figure 1): ●

Motor fibers that innervate the facial muscles

●

Parasympathetic fibers innervating lacrimal, submandibular, and sublingual salivary glands

●

Afferent fibers from taste receptors from the anterior two-thirds of the tongue

●

Somatic afferents from the external auditory canal and pinna

https://www.uptodate.com/contents/bells-palsy-pathogenesis-clinical-features-and-diagnosis-in-adults/print?search=FACIAL&source=search_result&s… 1/23

17/9/2020

Bell's palsy: Pathogenesis, clinical features, and diagnosis in adults - UpToDate

The facial nerve arises from two roots (one motor root and one visceral mixed root known as the nervus intermedius) at the pontomedullary junction. It then courses laterally through the cerebellopontine angle together with the vestibulocochlear nerve to the internal auditory meatus, which is approximately 1 cm in length, and becomes encased in periosteum and perineurium. The two roots then enter the facial (fallopian) canal. The facial canal is approximately 33 mm in length, and consists of three consecutive segments: labyrinthine, tympanic, and mastoid. Because the canal is narrowest in the labyrinthine segment (average 0.68 mm in diameter), any swelling of the nerve is more likely to result in compression here. The nerve runs laterally toward the medial wall of the epitympanic recess where it bends sharply backward; at the bend or genu is a swelling, the geniculate ganglion, lying in the labyrinthine segment of the canal. The nerve then passes backward and downward to reach the stylomastoid foramen. The precise anatomy and relationship to the vestibular and auditory apparatus is of particular importance to surgeons operating in the area. The greater petrosal nerve branches off the facial nerve at the geniculate ganglion, enters the middle cranial fossa extradurally, and exits through the foramen lacerum toward the pterygopalatine ganglion (figure 1). It supplies the lacrimal and palatine glands. The next branch as the nerve passes downward is the nerve to the stapedius. More distally, the chorda tympani arises from the main trunk of the facial nerve approximately 6 mm above the stylomastoid foramen (figure 1). The chorda, the largest branch of the facial nerve, passes across the tympanic membrane, separated from the middle ear cavity only by a mucous membrane. It continues anteriorly to join the lingual nerve and is distributed to the anterior two-thirds of the tongue. The chorda tympani contains secretomotor fibers to sublingual and submandibular glands, and visceral afferent fibers for taste. The cell bodies of the unipolar gustatory neurons lie in the geniculate ganglion and travel via the nervus intermedius to the tractus solitarius. The remaining fibers of the facial nerve emerge at the stylomastoid foramen, turn anterolaterally, and pass through the parotid gland. These fibers divide into five groups of nerves between the deep and superficial lobes of the gland at the pes anserinus (Latin "goose foot") and are distributed to the facial muscles in a variable pattern (figure 1) [1].

EPIDEMIOLOGY Bell's palsy, defined as an acute peripheral facial nerve palsy of unknown cause, represents approximately half of all cases of facial nerve palsy [2]. The annual incidence rate is between 13 and 34 cases per 100,000 population [3]. There is no race, geographic, or gender predilection, but the risk is three times greater during pregnancy, especially in the third trimester or in the first postpartum week [4]. Diabetes is present in approximately 5 to 10 percent of patients [5,6]. https://www.uptodate.com/contents/bells-palsy-pathogenesis-clinical-features-and-diagnosis-in-adults/print?search=FACIAL&source=search_result&s… 2/23

17/9/2020

Bell's palsy: Pathogenesis, clinical features, and diagnosis in adults - UpToDate

The recurrence rate of Bell's palsy is discussed separately. (See "Bell's palsy: Treatment and prognosis in adults", section on 'Risk of recurrent Bell's palsy'.)

PATHOGENESIS Bell's palsy is the appellation commonly used to describe an acute peripheral facial palsy of unknown cause. However, the terms "Bell's palsy" and "idiopathic facial paralysis" may no longer be considered synonymous [7-9]. A peripheral facial palsy is a clinical syndrome of many causes, and herpes simplex virus activation is the likely cause of Bell's palsy in most cases. Nevertheless, most patients with peripheral facial palsy are labeled as having Bell's palsy because there is no established or widely available method of confirming herpes simplex virus as the mechanism in clinical practice. A herpes simplex-mediated viral inflammatory/immune mechanism was the subject of controversy for years but was suspected based upon serologic evidence [10]. Polymerase chain reaction DNA testing supports the notion of axonal spread and multiplication of a reactivated neurotropic virus leading to inflammation, demyelination, and palsy. Herpes simplex virus activation has become widely accepted as the likely cause of Bell's palsy in most cases [11-13], though the evidence is not entirely conclusive [14,15]. In one study, herpes simplex virus type 1 genomes were identified in facial nerve endoneurial fluid and auricular muscle in 11 of 14 patients undergoing decompression surgery for Bell's palsy but in no controls [16]. Herpes zoster is probably the second most common viral infection associated with facial palsy. In a large series of 1701 cases of Bell's palsy, 116 had herpes zoster [13]. Other infectious causes of acute peripheral facial palsy include cytomegalovirus, Epstein-Barr virus, adenovirus, rubella virus, mumps, influenza B, and coxsackievirus [17]. Two cases due to Rickettsial infection have also been reported [18], and ehrlichiosis can present as facial diplegia [19]. An inactivated intranasal influenza vaccine that was introduced and since withdrawn from the market in Switzerland was significantly associated with Bell's palsy in a case-control study [20]. The peak occurrence of Bell's palsy was between 31 and 60 days after intranasal vaccination, suggesting that the palsy was not due to a direct toxic response but rather an induced immune response, such as reactivation of latent herpes simplex or varicella-zoster virus [21]. Histopathology of the facial nerve in patients with Bell's palsy is consistent with an inflammatory and possibly infectious cause, and the appearance is similar to that found with herpes zoster infection, further supporting an infectious hypothesis. Specifically, the facial nerve has a thickened, edematous perineurium with a diffuse infiltrate of small, round, inflammatory cells between nerve bundles and around intraneural blood vessels. Myelin sheaths undergo degeneration [22]. These changes are seen throughout the bony course of the facial nerve, although nerve damage is maximal in the https://www.uptodate.com/contents/bells-palsy-pathogenesis-clinical-features-and-diagnosis-in-adults/print?search=FACIAL&source=search_result&s… 3/23

17/9/2020

Bell's palsy: Pathogenesis, clinical features, and diagnosis in adults - UpToDate

labyrinthine part of the facial canal where edema causes compression and the tenuous blood supply adds to the damage. Alternate postulated mechanisms of Bell's palsy include a genetic predisposition in some cases [2325] and ischemia of the facial nerve [26,27]. Diabetes is a risk factor for microangiopathy, which may lead to Bell's palsy via microcirculatory failure of the vasa nervosum [28]. A retrospective study found that 190 (74 percent) of 257 patients with Bell's palsy first noticed facial weakness in the morning, suggesting that actual development of facial palsy occurred during sleep [27]; the authors speculated that nocturnal onset suggested an ischemic mechanism. The increased risk of Bell's palsy associated with pregnancy, which is most marked in the third trimester and the first postpartum week, may be caused by pregnancy-related fluid retention leading to compression of the nerve or perineural edema. Other potential etiologic factors include hypercoagulability causing thrombosis of the vasa nervosum and relative immunosuppression in pregnancy [29]. Several studies have found an association of Bell's palsy with preeclampsia, again suggesting extracellular edema as the mechanism [30-32].

CLINICAL FEATURES Patients with Bell's palsy typically present with the sudden onset (usually over hours) of unilateral facial paralysis. Common findings include the eyebrow sagging, inability to close the eye, disappearance of the nasolabial fold, and drooping at the affected corner of the mouth, which is drawn to the unaffected side (picture 1). Decreased tearing, hyperacusis, and/or loss of taste sensation on the anterior two-thirds of the tongue may help to site the lesion in the fallopian canal, but these findings are of little practical use other than as indicators of severity. An acute facial palsy is often devastating for patients. Of 22,594 patients surveyed at the Edinburgh facial palsy clinic, one-half exhibited a considerable degree of psychological distress and restriction in social activities as a consequence of their facial palsy [33]. Additional cranial neuropathies — Patients with Bell's palsy typically have only isolated peripheral seventh cranial nerve neuropathy. However, clinical findings suggestive of additional cranial neuropathies may infrequently occur. A prospective case series of 51 patients diagnosed with Bell's palsy found four with additional cranial neuropathies; these included contralateral trigeminal (one), glossopharyngeal (two), and hypoglossal (one) [34]. In the same study, an additional 13 patients had ipsilateral facial sensory impairment suggestive of ipsilateral trigeminal neuropathy [34]. Such ipsilateral sensory loss in the setting of Bell's palsy has usually been attributed not to neuropathy, but to abnormal perception based on droopy facial https://www.uptodate.com/contents/bells-palsy-pathogenesis-clinical-features-and-diagnosis-in-adults/print?search=FACIAL&source=search_result&s… 4/23

17/9/2020

Bell's palsy: Pathogenesis, clinical features, and diagnosis in adults - UpToDate

muscles, skin, and associated tissue. Three additional patients had bilateral facial palsy, possibly from the same pathogenesis as typical unilateral Bell's palsy.

DIAGNOSIS The diagnosis of Bell's palsy is based upon the following criteria: ●

There is a diffuse facial nerve involvement manifested by paralysis of the facial muscles, with or without loss of taste on the anterior two-thirds of the tongue or altered secretion of the lacrimal and salivary glands. (See 'Peripheral versus central lesions' below.)

●

Onset is acute, over a day or two; the course is progressive, reaching maximal clinical weakness/paralysis within three weeks or less from the first day of visible weakness; and recovery or some degree of function is present within six months.

An associated prodrome, ear pain, or dysacusis is variable. A diagnosis of Bell's palsy is doubtful if some facial function, however small, has not returned within four months [3,35,36]. Additional evaluation to determine the etiology is warranted in this instance. Examination — Facial movement is assessed by observing the response to command for closing the eyes, elevating the brow, frowning, showing the teeth, puckering the lips, and tensing the soft tissues of the neck to observe for platysma activation. The evaluation also includes a general physical examination and neurologic examination. Particular attention is directed at the external ear to look for vesicles or scabbing (which indicates zoster) and for mass lesions within the parotid gland. (See 'Differential diagnosis' below.) Peripheral versus central lesions — Sparing of the forehead muscles on the affected side of the face is suggestive of a central (upper motor neuron) lesion because of bilateral innervation to this area. However, this finding does not exclude a peripheral site of pathology in all cases. As an example, a partial lesion of the facial nerve at the pes anserinus (between the deep and superficial lobes of the parotid gland) that spares the temporal branch to the frontalis muscle results in facial paralysis, but the patient is still able to wrinkle the forehead. Nonetheless, forehead sparing should stimulate further evaluation of a possible central etiology. A "peripheral" (lower motor neuron) pattern of facial weakness that involves the forehead is usually due to a lesion of the ipsilateral facial nerve, but also can be caused by a central (brainstem) lesion that involves the ipsilateral facial nerve nucleus or facial nerve tract in the pons.

https://www.uptodate.com/contents/bells-palsy-pathogenesis-clinical-features-and-diagnosis-in-adults/print?search=FACIAL&source=search_result&s… 5/23

17/9/2020

Bell's palsy: Pathogenesis, clinical features, and diagnosis in adults - UpToDate

Central activation of the facial nerve is both volitional and automatic or emotional in origin, and the facial nerve is the final common pathway. Thus, dissociation of movement of the face to command from spontaneous movement, as in smiling, indicates an upper motor neuron lesion. Lack of dissociation (absence of both voluntary and spontaneous movement) indicates a lower motor neuron (peripheral) lesion. Diagnostic tests — Electrodiagnostic studies help determine the prognosis, and imaging studies can define potential surgical causes of facial palsy. However, these tests are not necessary in all patients. Patients with a typical lesion that is incomplete and recovers do not need further study. By contrast, electrodiagnostic studies may be warranted in patients with clinically complete lesions for prognostic purposes. Imaging is warranted if the physical signs are atypical, if there is slow progression beyond three weeks, or if there is no improvement at four months. Screening blood studies for underlying systemic disease or infection should also be considered in these cases. However, no test provides prognostic information sufficiently early as to be used for determining who should or should not be treated. Attempting to localize the site of the lesion with tests such as the Schirmer test for lacrimation, stapedial reflex, and evaluation of taste and salivation have only moderate accuracy and are of little practical benefit [37]. Furthermore, in patients studied at surgery, only 6 percent had lesions distal to the geniculate ganglion [38], the site that these tests target. Electrodiagnostic studies — The simplest electrodiagnostic test is electromyography (EMG). In patients who have a clinically complete lesion, EMG may show some action potentials on active volition, which allows one to conclude that the nerve is still in continuity with a potential for regrowth. In the first days after symptom onset, the blink reflex (stimulation of the supraorbital nerve) can confirm the peripheral origin of weakness and assess the degree of axonal conduction block [39,40]. Motor nerve conduction studies (NCS) of the facial nerve are a technique whereby the facial nerve is supramaximally stimulated near the parotid, and the evoked potential is measured by surface recording electrodes over the orbicularis oculi, nasalis, or lower facial muscles, yielding a compound muscle action potential (CMAP) reflecting activity in the muscles beneath the electrodes. At approximately 10 days after the onset of symptoms, the amplitude of the CMAP on the paralyzed side compared with that on the normal side yields an estimate of the degree of axonal loss [40]. The CMAP value obtained by facial nerve stimulation correlates histologically with the number of degenerating motor neurons; a CMAP value of 10 percent of normal corresponds with a degeneration or loss of 90 percent of the motor axons on that side [41]. In one study, 90 percent degeneration was considered to be a critical value above which recovery was poor; degeneration of greater than 98 https://www.uptodate.com/contents/bells-palsy-pathogenesis-clinical-features-and-diagnosis-in-adults/print?search=FACIAL&source=search_result&s… 6/23

17/9/2020

Bell's palsy: Pathogenesis, clinical features, and diagnosis in adults - UpToDate

percent predicted a very poor result [42]. Recovery was variable with degeneration between 90 and 98 percent, and often weakness and synkinesis resulted. In another study, 75 percent was regarded as the critical cutoff [43]. At approximately 20 to 30 days after onset, needle EMG may provide confirmation of muscle denervation and the degree of axonal damage [40]. In patients with axonal loss, needle EMG at approximately three months after onset may be used to assess for evidence of subclinical reinnervation from the facial nerve. The patient is interested mainly in prognosis; the EMG in this instance serves to define an incomplete lesion with potential for recovery. Additional electrodiagnostic studies include the following: ●

The maximal stimulation test (MST) uses supramaximal stimulation to achieve maximal stimulation, and the response is subjectively observed by the tester. It is of doubtful value.

●

Nerve excitability threshold testing is similar, but the tester attempts to quantify the current required to produce a threshold response. A critical value of 3.5 milliamperes is said to correlate with 90 percent degeneration on motor NCS [44], and implies a poor prognosis [45].

Imaging studies — As mentioned above, imaging is warranted if the physical signs are atypical, if there is slow progression beyond three weeks, or if there is no improvement at four months. History of a facial twitch or spasm that precedes facial weakness suggests nerve irritation from tumor and should also prompt imaging [46]. For patients with acute onset of facial paralysis and negative imaging studies at four months who have continued complete flaccid paralysis at seven months, repeat imaging is warranted, and biopsy is suggested if repeat imaging is negative [47]. Imaging in these cases should be performed with high-resolution contrast-enhanced computed tomography (CT) or gadolinium-enhanced magnetic resonance imaging (MRI) and should include the brain, temporal bone, and parotid gland [47]. High-resolution CT scanning is excellent for bony detail and will demonstrate erosion. MRI delineates the soft tissue structures, and is the be...