Chapter 190

Bell Palsy

Wanda J. Handel

Definition and Epidemiology

Defined as an acute, unilateral weakness or paralysis of the facial nerve with an onset of less than 72 hours and unknown cause, Bell palsy is the most commonly diagnosed peripheral facial nerve condition.¹ Although the condition is typically self-limiting, some patients have persistent facial paralysis and are at risk for eye injury.¹ People of all ages are affected, but incidence is most common in young and middle-aged adults (ages 15 to 45), with an even distribution between men and women and an annual incidence of 11 to 40 cases per 100,000.²^–⁴ Either side of the face may be affected.⁴ Incidence is higher during pregnancy, particularly in the last trimester, in the first week postpartum, and in women with preeclampsia. Other risk factors that increase incidence include diabetes, hypothyroidism, recent upper respiratory infections, obesity, family history, and hypertension.¹^,⁵

Pathophysiology

The facial nerve (cranial nerve [CN] VII) is mixed: afferent fibers from the anterior two thirds of the tongue and the external auditory canal; efferent fibers to the facial muscles; and parasympathetic fibers to the lacrimal, sublingual, and submandibular glands. Knowledge of the topographic anatomy of the facial nerve can provide clinical clues to sites of injury. Sparing of the forehead muscles typically suggests an upper motor neuron or central lesion because the forehead is bilaterally innervated.³ The typical unilateral facial paralysis of Bell palsy is assumed to be initiated by a triggering event that places physiologic stress on the body (e.g., an upper respiratory tract infection or ischemia to the nerve). This stressor promotes the body’s protective inflammatory response with its release of acute-phase reactants. The intraneural inflammatory response results in edema of the facial nerve. If the edema is not alleviated, there is ischemia of the nerve, with resulting axonal demyelination and inevitable nerve degeneration. Varying degrees of motor control loss become obvious about 3 days after nerve demyelination.⁵

Although the cause of Bell palsy remains idiopathic, and causative factors such as genetic, vascular, nerve compression, infectious, and metabolic changes have been discussed, the two most accepted hypotheses are a viral and an autoimmune pathomechanism. Viruses such as human herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) and varicella-zoster virus (VZV) all have the ability to cause latent infections in a single peripheral nerve distribution for the life of the host.³ Reactivation of any one of these viruses could cause Bell palsy in an individual. However, current polymerase chain reaction testing can only confirm that the virus exists within the nerve; it cannot delineate whether the virus is in a latent or active state.⁶ Lyme disease has also been implicated. Immunologic theory is the peripheral demyelination of Bell palsy is a cell-mediated response such as the demyelination in Guillain-Barré syndrome,³ Bell palsy remains a diagnosis of exclusion, meaning a complete history and thorough physical examination are needed.

Clinical Presentation

The typical onset is acute and progressive; maximum paralysis is attained in about half of the cases within 48 to 72 hours and in nearly all cases by day 5. Individuals may report pain behind the ipsilateral ear preceding the facial paralysis by 1 to 2 days. Typically, a smooth forehead, widened palpebral fissure, inability to close the eye, flattened nasolabial fold, and asymmetric smile are characteristic. Tearing, drooling, postauricular pain, tinnitus, and a mild hearing deficit may occur. Complaints of altered taste (dysgeusia) and an increased sensitivity to sound (hyperacusis) as well as hypoesthesia in one or more branches of the trigeminal nerve may also be present.¹^,³^,⁵ Timing of onset is key in the diagnosis; slowly progressive or relapsing courses suggest other entities.¹

Other associated symptoms include a history of recent infections, especially viral illnesses such as chickenpox, mumps, mononucleosis, coxsackievirus, cytomegalovirus, human immunodeficiency virus (HIV), and influenza. The presence of chronic illnesses, such as diabetes mellitus, hypertension, or hypothyroidism, should be ascertained, and the patient should be queried about pregnancy, rashes or skin lesions, and insect bites. Any history of facial trauma should be carefully noted.¹

Physical Examination

A complete physical and neurologic examination is warranted to rule out more serious central nervous system conditions such as stroke, tumor, and multiple sclerosis. The CN examination is key in identifying CN VII as the peripheral source of the facial weakness. The patient is asked to smile, show the teeth, puff out cheeks, raise eyebrows, and close eyes tightly. Any facial asymmetry is noted, paying close attention to whether the facial weakness is upper and lower or lower alone. Bell palsy causes a unilateral, full-face paresis or paralysis, with an ipsilateral source indicating a peripheral nerve problem. The patient is observed for lack of eyelid closure and absence of wrinkling of the forehead. Drooling and continuous tearing of the eye may also be present. A central nervous system lesion may manifest as a lower facial weakness with sparing of the forehead, and other deficits may be noted on full neurologic examination.⁵^,⁷^,⁸ Attention to otologic and head and neck examination is warranted to assess for decreased hearing and vesicles on the face or in and around the external ear canal, which may indicate herpes zoster oticus (Ramsay Hunt syndrome), although absence of vesicles does not rule out zoster sine herpete.⁵

Special attention to the sensory and motor functions of the branches of the facial nerve is also necessary. Minor asymmetry of the lower face may be a normal deviation. The degree of facial weakness should be documented. A number of grading systems have been developed to objectively define the severity of the palsy. Clinicians may find the House-Brackmann tool helpful in gauging the severity of neural degeneration and in establishing objective measures of recovery.¹^,⁹ A photographic record is also helpful in establishing the extent of facial muscle weakness and documenting progressive neural regeneration.

Diagnostics

Routine diagnostic tests and imaging are not recommended for new-onset Bell palsy. Diagnostic studies may be useful to exclude identifiable conditions such as Lyme disease in the differential diagnosis and to determine prognosis. Imaging is warranted in atypical presentation, such as bilateral facial nerve palsies.¹

Diagnostics

Bell Palsy

Laboratory