Before discussing the etiological causes of facial paralysis, some peculiar anatomical features of the facial nerve deserve at least a casual mention. Because of the unusual pattern of fiber crossing in the motor nucleus of the facial nerve, the upper half of the face which is supplied by the ventral portion of the motor nucleus receives innervation from both sides of the motor cortex, while the muscles of the lower half of the face are supplied by the dorsal portion of the nucleus, which is mainly controlled by fibers from the contralateral side of the brain. Therefore, an important clinical sign distinguishing a central from a peripheral etiology affecting the facial nerve is that a supranuclear or upper motor neuron lesion would present with weakness in the contralateral lower face associated with normal tone and movement of both sides of the forehead. However, a peripheral nuclear or infranuclear lower motor neuron lesion will result in an ipsilateral abnormal movement of the entire side of the face including the forehead.⁶,¹⁴ The facial nerve also has the longest intraosseous route of any CN and is therefore particularly vulnerable to trauma or infections along its unusually long and tortuous course.⁶,⁷,¹⁰,¹¹

A comprehensive assessment of all the etiological factors causing FNP is beyond the scope of this chapter as more than 100 different causes have been identified so far,² but briefly speaking, they can be categorized as central or peripheral causes.⁶ Central causes include stroke, brain tumor, or trauma, while peripheral causes include congenital and hereditary causes, infections, Bell palsy, Ramsay Hunt syndrome, or compression of the facial nerve by a tumor mass.⁶,¹⁰,¹³ By far, the commonest etiology is Bell palsy, which is responsible for 51% of cases of facial weakness, followed by facial nerve trauma (22%), and Ramsay Hunt syndrome (7%).¹⁰ Because the classic textbook example of facial paralysis is Bell palsy, it will be the main focus of this chapter. Bell palsy is an acute peripheral facial mononeuropathy without a confirmed cause and is the most common cause associated with facial nerve weakness.¹,²,³,⁴,⁵,⁶,⁷,⁸,⁹,¹⁰ Although the evidence is piling up for a viral etiology that has long been suspected as the underlying cause, the exact mechanism of Bell palsy is currently unknown.⁸,⁹ Many viruses including herpes simplex virus types 1 and 2 (HSV-1, 2), varicella zoster virus (VZV), adenovirus, influenza B virus, coxsackie virus, and Epstein-Barr virus have been linked to the development of Bell palsy, but it is believed that HSV-1 is the one that is responsible
for idiopathic facial palsy.¹⁰ The proposed pathophysiology is that HSV-1 remains latent in the geniculate ganglia, and when it is reactivated, this causes inflammation, edema, and compression of CN V-VII as it exits the skull through the stylomastoid foramen.⁸,¹¹ While the evidence for a viral etiology in Bell palsy is anecdotal, herpes zoster virus is a confirmed viral agent responsible for FNP in Ramsay Hunt syndrome (herpes zoster oticus), which occurs due to reactivation of a latent herpes zoster virus in the geniculate ganglion.¹⁰ Other infective agents include Borrelia burgdorferi (Lyme disease), and rarely Epstein-Barr virus, cytomegalovirus, leprosy, HIV, and polio.¹⁰

Although laceration injuries/blunt trauma to the craniofacial region close to the course of the facial nerve or direct trauma to the temporal bone may be implicated in several cases of facial weakness, the two most commonly encountered traumatic causes include trauma during delivery either due to forceps delivery or birth canal trauma.⁷ Surgical trauma during removal of an acoustic neuroma is also the second most common cause of facial palsy because the facial nerve may be manipulated or occasionally sacrificed.¹⁵ Even before surgery, 5% and 17% of acoustic neuroma patients show symptoms and signs of facial nerve weakness, respectively.¹⁵ Other mass lesions that may cause facial weakness include meningiomas of the cerebellopontine angle, parotid tumors, facial nerve schwannomas, nasopharyngeal carcinomas, external meatus tumors, and even sarcoidosis.¹⁰ Rarely, patients may present with congenital or hereditary facial paralysis. The list includes familial facial paralysis, Treacher Collins syndrome, Moebius syndrome, and Goldenhar syndrome.¹⁰,¹⁶

Several hypotheses have been put forward to try to explain the synkinetic movements that may accompany an incomplete recovery of the facial nerve, which will be discussed later in detail. The most accepted theory is that misdirected peripheral regeneration of the damaged facial nerve causes such aberrant movements.¹⁷ After nerve injury, proximal axons regenerate, and then subsequently reroute or sprout to overinnervate an anatomically correct muscle or aberrantly innervate an incorrect one.²,¹⁷ Other less plausible mechanisms include hyperexcitability of the facial nucleus or the possibility that neighboring axons may be stimulating each other.²

Because the list of etiologies responsible for facial nerve palsy is long, an accurate estimate of its overall incidence is difficult.¹⁰ It is estimated that between 7 and 50 patients per 100,000 person-years are affected annually by facial paralysis in different populations⁴,⁵,¹⁰; however, the incidence of idiopathic causes ranges between 13.1/100,000 and 20.2/100,000.¹⁰ Children are less frequently affected with a gradual rise in incidence with increasing age, but with a higher incidence in females across all age strata.⁴ In children younger than 10 years, the incidence is significantly lower (2.7 per 100,000 annually), compared to children older than 10 years, where the annual incidence is estimated to be 10.1 per 100,000.⁴ The most common age at presentation is between 15 and 40 years,¹⁰,¹² but there is a disagreement regarding the sexual preponderance in FNP. While some authors failed to demonstrate a sex preference, other conflicting reports claimed more prevalence in either males or females.⁵,¹⁰,¹²,¹³

Detailed history taking and a thorough clinical examination are of paramount importance in patients with facial paralysis and may spare the patient unnecessary radiological or laboratory investigations.⁸ Ideally, history taking should establish whether the onset was acute or chronic, unilateral or bilateral, and whether the lesion is proximal or distal.¹⁰ Alarmed Bell palsy patients may complain that the onset of facial paralysis was sudden, and in such a scenario, the symptoms peak after 2 to 3 days.¹⁰ Severe facial or postauricular pain, which typically precedes the onset of facial paralysis by 2 to 3 days or occurs at the day of onset, usually suggests Ramsay Hunt syndrome. This may or may not be associated with the typical vesicular rash.¹⁰ Ramsay Hunt syndrome may also be associated with vestibuloauditory symptoms because of the proximity of the vestibulocochlear nerve (CN VIII) to the geniculate ganglion.⁷ Patients may also present with a painful red eye, watering, or complain of cosmetic disfigurement.

The functional and debilitating effects of facial palsy are variable from mild to severe (Figure 23.1). This makes facial paralysis challenging to quantify, and up to the present day, the main obstacle to achieving evidence-based and reproducible management decisions is the subjective nature of assessment.¹⁸ Therefore, numerous grading systems for FNP have been devised to establish standardization of facial functional assessment, improve the reporting of outcomes, and facilitate communication between professionals.¹⁸ The most popular grading systems among clinicians are the House-Brackmann scale, the Sunnybrook grading system, and the Facial Nerve Grading Scale 2.0 (FNGS 2.0).¹⁸ The House-Brackmann grading scale, which was adopted by the American Academy of Otolaryngology in 1985, is widely used because of its relative ease and simplicity and subjectively classifies facial nerve function into six grades: grade I, normal function; grade II, mild dysfunction; grade III, reduced forehead movement, noticeable synkinesis, and contracture; grade IV, no forehead movement, incomplete eye closure, asymmetric mouth, and disfiguring asymmetry; grade V, minimal movement; and grade VI, no movement.¹⁰,¹⁸,¹⁹,²⁰ Other systems like the Sunnybrook grading system and the FNGS 2.0 incorporate synkinetic movements as one of their grading criteria or characteristics. Any of these grading systems would have been ideal if the sole purpose of such grading instruments was diagnostic, although preferably any grading system should also help in the management plan as well.¹⁸

Unfortunately, at least from an ophthalmological perspective, those grading systems would not aid in the management
plan because they mostly grade the whole face, as they were not designed by ophthalmologists; therefore, corneal assessment is usually neglected.²⁰ To adequately encompass the ocular sequelae of facial nerve dysfunction, the CADS grading system was recently proposed. It attempts to quantify the ophthalmic sequelae based on several parameters including Corneal involvement, static Asymmetry in the periorbital region, Dynamic functioning of periorbital muscles, and Synkinesis involving the orbital region.²¹ In a separate study by the same group of authors, the CADS grading system reliability was rated from good to very good.²² There are several perceived limitations with all these systems. None of the proposed generalized or arguably regional grading systems are truly quantitative, and instead, at best, they merely rely on or attempt to adopt some sort of subjective methodology.¹⁸ The perfect facial nerve grading system should make use of current advances in digital photography to quantitate ocular and periocular defects, and to the best of our knowledge, this has not been designed yet.

In the periocular region, FNP typically presents with several worrying signs, including upper eyelid retraction from loss of orbicularis muscle tone, lower eyelid laxity, paralytic ectropion (Figure 23.2), adynamic ptotic brow with loss of forehead wrinkling, a variable degree of
lagophthalmos on attempted eyelid closure with or without a good Bell phenomenon, excessive tearing or occasionally dry eyes, and chronic inferior exposure keratopathy. If neglected, this could result in corneal abscess/perforation or endophthalmitis.²³ The size of the palpebral fissure in acute FNP is variable. Typically, although far from being the rule, the palpebral fissure is wider than the contralateral normal side not just because the upper eyelid is higher than the normal side but because the lower eyelid also sits below its normal position.²⁴

Evaluation of a patient with acute facial paralysis should include a thorough assessment of all muscles of facial expression to determine if the patient has paresis or paralysis,²³ and the degree of paralysis could be graded with one of the abovementioned general or locoregional grading systems. The frontalis muscle should be assessed dynamically to differentiate an upper from a lower motor neuron lesion,¹⁴ and the eyebrow position should be noted in the resting state as well. The degree of passive lagophthalmos, as well as the extent of forced eyelid closure, is examined as well (Figure 23.3). The degree of upper eyelid retraction, dermatochalasis, lower eyelid laxity/ectropion, and Bell phenomenon are also noted.²³,²⁵,²⁶ The sensation along the distribution of the trigeminal nerve is also evaluated,²³ as well as the ocular motility because the abducens nerve lies in close proximity intracranially to the facial nerve nucleus.²⁶ A red eye is a red alert sign in patients with facial palsy⁸,²⁵ and requires a careful slit lamp examination, that should include assessment of corneal sensation, which may be reduced after acoustic neuroma surgery.²³ The tear film and the tear meniscus should be examined with Schirmer test or Lissamine green to quantify tear production, and exposure keratopathy should be ruled out with fluorescein staining. The slit lamp is also an ideal tool to closely observe the impact of the patient’s incomplete blink and Bell phenomenon on the cornea.²³,²⁵,²⁶

As was mentioned earlier, one of the more common ophthalmic presenting symptoms is epiphora, which has a multifactorial etiology in facial palsy patients. Lagophthalmos can cause an increase in tear production but decreased tear drainage due to atonicity of the orbicularis oculi and weakness or failure of the lacrimal pump, as well as eversion of the lower lacrimal punctum, can all contribute.²⁴,²⁶ Epiphora usually regresses with a resolution of the palsy. It is pertinent to evaluate the lacrimal system carefully in all cases to avoid missing a coexistent nasolacrimal duct pathology. Conversely, tear production may be reduced in facial palsy patients if the lesion is proximal to the geniculate ganglion. Severe dryness and not tearing may be observed in those patients.²⁶

Several imaging procedures and diagnostic tests have been used to evaluate patients with acute facial paralysis to help look for an identifiable cause, but a gold standard is yet to be defined, and the choice of radiology or laboratory investigations should be individualized.⁸,¹⁰ The take-home message is that “any presentation of facial paresis/paralysis inconsistent with Bell palsy should be further evaluated by imaging.”⁸ Computed tomography may help in localizing the whereabouts of a nerve injury in cases of trauma, while magnetic resonance imaging (MRI) of the entire course of the facial nerve may help if a tumor mass or an infectious process is suspected and should include imaging of both the internal auditory canal and face.⁷,⁸,¹⁰ However, if the diagnosis of Bell palsy is established, radiology does not usually provide any added benefit and may confuse the clinician
because MRI may show enhancement along the involved facial nerve, which may be confused with a small mass compressing the nerve.¹⁰