By Raghu S. Athre, MD

Facial nerve disorders represent a wide variety of etiologic conditions manifesting in one end result, facial nerve weakness and/or paralysis.  No discussion of facial nerve disorders is complete without a short introduction on facial nerve anatomy; and hence, a short primer on facial nerve anatomy will serve as an introduction to the differential etiologies of facial nerve disorders.

The facial nerve is a complex nerve that primarily provides motor innervation to the muscles of the face.  However, the facial nerve also has parasympathetic outputs, for example to the parotid gland, special sensation functions such as taste to the anterior 2/3rds of the tongue, and general sensation as well.  Furthermore, the facial nerve has a circuitous course that makes it vulnerable to many different types of insults.  Since the otolaryngologist is well versed with the anatomic nuances of the facial nerve, he/she is the most qualified to deal with the majority of its disorders.

Facial nerve anatomy can be a very confusing topic.  The following offers a brief primer on the course of the facial nerve.  The facial nerve starts of at the facial motor nucleus in the anterior pons.  The facial nerve (FN) exits the brainstem where it is joined by fibers from the nervus intermedius (NI).  The NI carries efferent preganglionic parasympathetic fibers to the sublingual and submaxillary glands and afferent special sensory fibers from the chorda tympani.  The FN complex joins the 8th cranial nerve and crosses the cerebellopontine angle into the IAC.  In the IAC, the facial nerve sits in an anterior and cephalad position.  Inferior to the facial nerve sits the cochlear nerve, and posterior to the FN sits the superior and inferior vestibular nerves.  The IAC portion is approximately 8-10 mm.

The FN then enters the fallopian canal, which is a bony canal through which the FN and its adnexal structures travel.  This segment is called the labyrinthine segment.  It is at this point that the fallopian canal is at its narrowest point, thereby resulting in a physiologic bottleneck.  The labyrinthine segment includes the geniculate ganglion, where the greater superficial petrosal nerve carrying preganglionic parasympathetic fibers to the lacrimal gland is given off.  The FN then makes a 75 degree turn posteriorly (first genu).  This marks the beginning of the tympanic segment of the FN.  The FN travels above the oval window niche towards the antrum.  Here the FN makes its second turn (second genu) as it passes between the incus and the lateral semicircular canal to travel inferiorly along the anterior portion of the mastoid to the stylomastoid foramen.  In its course, the FN gives off the nerve to the stapedius muscle and the chorda tympani.  The chorda typani is responsible for taste to the anterior 2/3rds of the tongue, while the nerve to the stapedius muscle is involved in the stapedial reflex.

Once the facial nerve exits the stylomastoid foramen, it travels between the superficial and deep lobes of the parotid gland.  It then arborizes into five branches: the frontal, zygomatic, buccal, marginal, and cervical branches.

Facial nerve disorders can be broken down by various etiologies.  Despite the root etiology, the end symptom of facial paralysis must be treated with aggressive eye hygiene regimens to prevent exposure keratitis.  The first category to be discussed includes infectious causes of FN disorders.  Bell's palsy is the most common cause of facial paralysis.  The etiologic factor is thought to be Herpes infection of the FN.  As the FN is infected, it experiences a certain amount of edema secondary to inflammation.  As explained above, the FN travels in a bony canal that cannot expand.  Therefore, increased edema places increased restriction on the nerve leading to nerve weakness/paralysis.  Possible treatments for Bell's palsy include steroids, antiviral medications, and possible surgical decompression of the nerve in cases where >90% of nerve fiber degeneration has occurred.  Overall, the prognosis for Bell's palsy patients remains good with 80-100% of patients with less than 90% degeneration achieving a complete recovery.  A similar condition termed Ramsey-hunt syndrome can present with facial paralysis also.  Ramsey-Hunt represents Herpes zoster reactivation in the pathways of the facial nerve.  Characteristic symptoms include pain, vesicles in the EAC, SNHL, and facial paralysis.  The overall prognosis for Ramsey-Hunt is poorer than that for Bell's palsy in that only 50-70% of patients achieve a full recovery.  Acute and chronic ear disease can also cause facial paralysis.  Temporal bone studies have demonstrated that up to 50% of cadaveric temporal bone specimens may have a dehiscent facial nerve in the tympanic segment.  Acute or chronic effusion may contain inflammatory mediators, which cause inflammation and subsequent dysfunction of the FN.  Furthermore, chronic ear disease in itself may cause erosion of the bone overlying the FN and allow the FN to be exposed to inflammatory mediators.  Treatment of this type of facial paralysis is geared toward removing the etiologic inflammatory agent with a typanostomy, PET insertion, etc.

Iatrogenic causes of facial nerve paralysis remains a large category because of numerous surgical procedures ranging from parotidectomy to acoustic neuroma resection which involve anatomical structures in close proximity to the facial nerve.  According to an article in Laryngoscope 1995 April; 105(4-Part1): 444-5, the most common procedure in otologic surgery resulting FN paralysis was mastoidectomy (55%), followed by tympanoplasty (14%) and removal of exostoses (14%).  The most common area of injury to the facial nerve was in the tympanic segment, and in 79% of patients, the FN injury was not detected at the time of surgery.  Further characterization of iatrogenic injuries in otologic surgery shows that the most common location of injury in ME surgery is surrounding the oval window niche where the FN can be dehiscent, and the most common location of FN injury in mastoid surgery is during antrostomy.  Furthermore, according to J Neurosurg. 1997 Jul; 87(1): 60-66, surgical approach had no correlation with facial nerve outcome in acoustic neuroma extirpative surgery, with the primary factor affecting facial nerve outcome being size of tumor.  Parotidectomy is the most common cause of iatrogenic facial nerve injury.

Traumatic etiologies of facial nerve injuries are primarily caused by blunt trauma causing a temporal bone fracture or penetrating trauma to the face.  Penetrating trauma to the face lateral to a vertical line from the lateral canthus, represents an injury that can be possibly repaired via neurorrhaphy.  Medial to this imaginary line, the arborized facial nerve branches are too small to identify and repair.  Blunt facial trauma causing facial nerve injury is rare but can be caused due to contusion of the nerve.  More commonly, blunt, high-velocity trauma results in temporal bone fractures.  Transverse temporal bone fractures are more likely to cause SNHL and facial nerve injury due to the fact that the vector of the fracture lies perpendicular to the axis of the nerve and can possibly transect the nerve.

Neoplasms can definitely cause FN paralysis.  The most common neoplasm causing FN paralysis is an acoustic neuroma.  Acoustic neuromas are benign tumors that grow locally and exert pressure on the FN and surrounding structures in the CPA.  Similarly, FN neuromas and jugular foramen tumors can cause FN paralysis.  More distally, neoplasms of the EAC (e.g. SCCA of EAC) and cholesteatomas can exert pressure on or invade the FN resulting in FN paralysis.  Parotid tumors can result in FN paralysis.  More commonly, malignant parotid tumors can cause FN paralysis by direct extension and invasion of the nerve.  However, there are case reports of benign neoplasms that have stretched the FN and have subsequently resulted in FN paralysis.  More commonly, FN paralysis results from iatrogenic etiologies during parotid extirpative surgery, rather than the tumor itself.

Other etiologies of FN disorders/paralysis include stroke, postoperative weakness, Melkersson-Rosenthal syndrome and hemifacial spasms.  Postoperative weakness is an important etiology of facial nerve disorders.  It is important to recognize that this term represents the spectrum of injury ranging from conduction blockade secondary to contusion/stretch injury all the way to nerve section.  The primary factor that determines prognosis is nerve integrity.  If facial paralysis results following parotid surgery, for example, identification and preservation of all branches of the nerve intraoperatively will ensure that full recovery will occur.  It is important to remember that lidocaine can cause a temporary conduction blockade of the FN for several hours.  Melkersson-Rosenthal syndrome is an idiopathic condition with some genetic predisposition that affects primarily middle-aged females and results in recurrent facial paralysis, swelling of the lips and face, and development of furrows in the dorsal tongue.  Treatment is supportive.  Finally, hemifacial spasms can result from idiopathic etiologies, but also from an aberrant blood vessel that might be in close contact with the FN.  Ligation or transposition of the blood vessel is usually reparative.