Facial Nerve Injury

The course of the facial nerve from the brainstem to the facial musculature can be divided into three segments: intracranial, intratemporal, and extratemporal or peripheral.1 The intracranial portion of the facial nerve extends from the brainstem to the porus acusticus of the internal auditory canal. The intratemporal portion begins at the porous acousticus at the medial internal auditory canal, travels a tortuous course within the temporal bone and exits via the stylomastoid foramen. This segment is further subdivided into meatal, labyrinthine, tympanic, and mastoid segments. The meatal portion is located between the porus acusticus and the fundus and the labyrinthine segment travels from the fundus to the geniculate ganglion before the nerve turns at an acute angle to form the tympanic segment in the middle ear. It then makes a second genu and becomes the mastoid segment and travels toward the stylomastoid foramen. The petrous temporal bone protects the meatal and labyrinthine segments of the intratemporal facial nerve. From the mastoid segment, the chorda tympani, a branch of the facial nerve, travels anterolaterally to join the lingual nerve to provide taste sensation. The extratemporal or peripheral portion of the facial nerve begins at the stylomastoid foramen, from which it enters the parotid and branches to innervate the facial musculature.

During routine otologic surgery, the tympanic and mastoid segments are at increased risk for injury. However, in cochlear implantation, the mastoid segment with the second genu and chorda tympani nerve are of interest because they serve as the boundaries of the facial recess for the posterior tympanotomy. To visualize the cochlea adequately, a wide enough opening must be created. The proximity of the two nerves to the recess opening may result in injury to one or both nerves.

Variations in facial nerve anatomy can further increase the likelihood of inadvertent injury especially when the facial nerve is not easily visualized while entering into the facial recess. The most common variation is an anteromedial facial nerve distal to the oval window that runs inferior or over the location of the round window.2 Traumatic injuries to the facial nerve typically present as neural edema, especially if the facial palsy presents within the early postoperative days.3 Potential sources of injury include thermal injury by the bur and direct trauma. For that reason, copious irrigation is used when drilling around the facial nerve. Preoperative radiologic evaluation with a noncontrast computed tomography scan of the temporal bone will alert the surgeon to potential course deviations of the facial nerve to minimize inadvertent injury. Although commonly used during otologic surgery, intraoperative facial nerve monitoring has not been shown to decrease the incidence of facial palsy.

Other causes of facial paresis after cochlear implantation surgery include herpes virus reactivation, acute nerve compromise, and postoperative wound infection. Reactivation of herpes simplex virus type I secondary to surgical trauma, although rare, is a well-documented cause resulting in facial palsy after otologic surgery, especially when there is direct manipulation of the nerve root.4 Reactivation then results in inflammation, demyelination and palsy. Vrabec previously showed that increased manipulation of the sensory branches of the facial nerve resulted in an increased incidence of delayed facial palsy, and other studies have demonstrated that the raising of the tympanomeatal flap disrupts cutaneous branches of the facial nerve that could precipitate viral reactivation.5,6 As a result, several studies have suggested that injury to the chorda tympani nerve could serve as a likely trigger for reactivation.3,7 Confirmation that viral reactivation is the cause of the facial paresis can be obtained by sampling nerve tissue or endoneural fluid; however, these methods are not without risks. Because delayed facial palsy carries a favorable prognosis, surgical exploration or decompression with biopsy is not indicated.3 Acute nerve compromise can result from vasospasm triggered by local blood breakdown products, ischemia of the nerve, or venous outflow obstruction, resulting in neural edema.3,4 Despite these theories, the mechanism of delayed facial lesion remains elusive, and no etiologic factors have been found.3,6

Facial palsy after cochlear implantation tends to present in a delayed fashion. In a retrospective study of 705 patients who were implanted between 1980 and 2002, the incidence was 0.71% with all nerve injuries presenting in a delayed fashion. When the patient presents with this type of paresis, the treatment is medical with the prompt initiation of a high-dose corticosteroid taper, with or without antiviral medication.3 Additionally, if the weakness is severe, resulting in incomplete eye closure, lubrication with artificial tears and ointment is extremely important. The addition of a moisture chamber at night can also help to protect against corneal drying and subsequent ulceration. If the eye becomes painful or erythematous, a consultation with an ophthalmologist early in the course may be required.

Chorda Tympani Injury

The chorda tympani nerve carries preganglionic secretomotor fibers that innervate the submandibular and sublingual glands and taste fibers from the anterior two-thirds of the tongue. They are carried with the lingual nerve through the infratemporal fossa and enter the tym-panic cavity through the petrotympanic fissure and iter chordae anterius, also known as the Canal of Hugier. It crosses the tympanic cavity along the posterior superior middle ear space, medial to the neck of the malleus, and then exits the tympanic cavity via the iter chordae posterius as it proceeds inferiorly to join the vertical portion of the facial nerve within the mastoid bone.

By virtue of its location, during the cochlear implant surgery, more specifically, during the drilling of the facial recess, this nerve is at increased risk of injury, resulting in dysgeusia, a change in sensation of taste. These alterations in taste are often described as a metallic, bitter or salty taste in the mouth. Patients occasionally complain of tongue numbness. Lloyd et al8 compared the outcomes of patients who underwent cochlear implantation with preservation of the chorda tympani nerve, with the outcomes of patients whose nerves were sectioned. They showed that about half of the patients with an intact nerve presented with alterations in taste and 42% of these patients had subsequent resolution of their symptoms. However, in patients whose nerve was sectioned, although 86% had taste disturbances, 67% had resolution of their symptoms. These results were found to be similar to other studies that described postoperative dysgeusia in otologic surgery.

Meningitis

Meningitis is a serious infection that can present after cochlear implantation with the majority presenting within the first year of implantation.12 Many papers have been published about the risk of meningitis after cochlear implants, but the precise cause is still under debate. It has been suggested that the incidence of meningitis in profoundly deaf patients is higher than in the general population.13 That same study showed that the incidence of postimplant meningitis due to Streptococcus pneumoniae was 138.2 cases per 100,000 person-years, at least 30 times greater than that of the general population in 2000.13 Risk factors include an age less than 5 years, impaired immune status, presence of intracranial foreign bodies such as a ventricular shunt, and a history of meningitis. Additionally, a history of otitis media as well as inner ear malformations may place patients at increased risk.14 In many cases, meningitis, especially when it occurs 30 days postimplantation, is often preceded by an episode of acute otitis media.15 Previously, an additional consideration that predisposed cochlear implant patients to meningitis was the use of positions that created further trauma to the osseous spiral lamina and modiolus. This allowed pneumococcal meningitis to spread via an otogenic route.16 As a result such implants have been withdrawn from the market.

Various theories have been proposed regarding the pathophysiology of meningitis after cochlear implantation. The two most common theories are via otogenic or hematogenous spread. Otogenic spread can be from direct invasion of the bacteria from the middle ear to the meninges through a tegmen defect or via the inner ear in patients with cochlear implants. The latter is the more commonly accepted view.12 By creating a cochleostomy and inserting an implant, the barriers separating the middle ear from the inner ear are compromised, potentially allowing bacteria easier access into the inner ear, especially when the cochleostomy seal is inadequate. Alternatively, bacteria can enter the inner ear through the round window membrane. Bacterial toxins, antibiotics and other substances have been shown to pass through the round window membrane.17 Once in the inner ear, bacteria access the meninges and the cerebrospinal fluid (CSF) through the labyrinth, infiltration of the cochlear turns, along the electrode into the bony channels of Schuknecht, or through perineural or perivascular routes into the internal auditory canal.15 Additionally, the presence of cochlear malformation is often associated with an enlarged cochlear aqueduct, providing a more open channel between the cochlea and central nervous system.18–20 Evidence also exists for the hematogenous spread of bacteria as the cause of meningitis with a subsequent retrograde spread of infection toward the inner and middle ears.21–23 Infection in patients with bacteremia may seed in compromised tissues within the inner ear, such as areas of tissue necrosis related to the electrodes or positioner, with subsequent spread to the meninges and CSF.24 The likelihood of meningitis may be related to the duration and severity of the bacteremia, as these variables determine the concentration of bacteria reaching the subarachnoid space.25

Although not necessarily causative, the presence of an electrode within the inner ear has been shown to be associated with an increased risk of meningitis, especially within the first 2 months after implantation.24 The presence of the implant has been shown to reduce the threshold level of bacteria required to induce meningitis.1 It has been postulated that the presence of the electrode may reduce local inner ear immunity, making it more susceptible to inoculation with subsequent spread to the central nervous system. Alternatively, the presence of the implant could reduce the global central nervous system immunity, breaking down the blood–brain barrier and allowing for hematogenous spread from the systemic circulation.1 Traumatic insertions of the cochlear implant have been shown to be a risk factor for pneumococcal meningitis, especially in those patients whose implants had a positioner before 2002. Reefhuis et al11 demonstrated in a study of 4,264 children that although only 19% of these children had implants with a positioner, a wedge that rests in the cochlea next to the electrodes, 71% of these children developed meningitis.24 Despite achieving a better electrode position for stimulation of the cochlear nerve, the positioner may result in increased trauma to the inner ear with subsequent necrosis and absorption of the modiolus and osseous spiral lamina resulting in higher susceptibility to infection.2 Even though the risk of meningitis is elevated in these patients beyond 24 months of implantation, removal of these implants or their positioners is not recommended.24

Preventive Measures

The most common pathogens responsible for meningitis in cochlear implant recipients are Streptococcus pneumoniae (most frequent) and, Haemophilus influenzae, including type b and nontypeable species. Because this is a preventable infection that could result in undesirable consequences, the U.S. Food and Drug Administration (FDA) and many other government organizations around the world have recommended universal immunization for all implant recipients in an effort to reduce the incidence of meningitis. Because all children in the United States are required to have the Hib (H. influenza type b conjugate) vaccine in infancy and early childhood, the only additional vaccination recommended by the Center for Disease Control and Prevention is against S. pneumoniae. The most commonly used vaccines are the heptavalent pneumococcal conjugate vaccine (PCV7; Prevnar, Wyeth-Lederle Vaccines, Madison, NJ, USA) and the 23-valent pneumococcal polysaccharide vaccine (PPV23; pneumovax 23, Merck & Co. Inc., Whitehouse Station, NJ, USA; and Pnu-Immune 23; Lederle Laboratories, Madison, NJ, USA).12,26 It is expected that all children considered for cochlear implantation will be up-to-date for all their vaccinations. The immunization guidelines for cochlear implant candidates and recipients are summarized as follows:

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  Children under the age of 24 months are expected to have completed the pneumococcal conjugate vaccine (Prevnar) series. If their last dose was given before 12 months of age, an additional Prevnar dose should be given between 12 and 15 months of age. When they turn 24 months old, they should be given the PPV23 vaccine.

  
  
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  Children older than 24 months old should have completed the Prevnar series already. If they have not, they should be immunized according to a high-risk schedule. All patients should receive the PPV23 vaccine, with at least 2 months after the last Prevnar dose.27

  
  
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  Children between 24 and 59 months who have not yet been immunized should receive two doses of the Prevnar vaccine spaced at least 2 months apart, followed by one dose of the PPV23 vaccine 2 months later.27

  
  
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  Persons older than 5 years old, should receive a single dose of PPV23.23,28

  
  
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  Patients planning to receive a cochlear implant should have their pneumococcal vaccination up-to-date.27

It is recommended that the patient complete all vaccinations at least 2 weeks before implantation. Some would even argue that the patients should be immunized a month before surgery, because a study in 2005 of 120 cochlear implant patients between the ages of 5 and 27 years, who received the PPV23 vaccine, demonstrated that serum antibody levels to vaccine-specific serotypes increased significantly within 4 weeks after vaccination.29

Because cochlear trauma is thought to increase the risk of developing meningitis, some have advocated the use of soft surgical techniques whenever possible.2 The advance off-stylet technique minimizes force on the lateral wall of the cochlea to reduce structural damage to the inner ear.30 Cochleostomy placements that are more inferior and anterior to the round window have also been shown to decrease displacement of electrodes from the scala tympani.31,32

Wound Infection

Wound infections after cochlear implantation can potentially have severe consequences, especially if it requires the removal of the device. Often, after removal of the implant, it cannot be immediately replaced. During this time, there is the possibility that the scala tympani can become scarred making future reimplantation more difficult or even, if possible, resulting in poorer performance. For that reason, the electrode array is often severed at the cochleostomy and left in place until the time of reimplantation.

Infectious complications range from 1.7% to 8.2%, although typically lower for severe complications.14,25,33 Clinically they present as tenderness over the receiver site, progressing with edema, erythema, and eventually sometimes to an abscess. Because all efforts should be made to minimize the chance of implant removal, conservative medical treatment with broad-spectrum intravenous antibiotics is often initiated. If cultures can be obtained without compromising implant integrity, the antibiotics can be tailored accordingly. Appropriate intraoperative wound debridement with culture-guided antibiotics can often salvage the implant.

In treating infectious complications of cochlear implants, one must consider the presence of bacterial biofilms. These are composed of bacterial communities that produce a polymeric matrix of exopolysaccharides with the ability to attach and persist on the surface of biomaterials. The presence of a biofilm requires a minimum of 6 weeks of antibiotic treatment to eradicate the offending organisms. However, the persistence of the biofilm may require removal of the device, leaving only the electrode within the cochlea.25 Antonelli et al34 demonstrated that Staphylococcus aureus has been the most common pathogen identified in such infections, suggesting a non-otologic source. Sterile technique with intraoperative antibiotic irrigation may minimize the seeding of bacteria at the time of implantation.

Middle Ear and Mastoid Disease

Middle ear and mastoid disease are common not only in children but also in adults. As cochlear implantation becomes increasingly popular in young children and infants with profound sensorineural hearing loss, prompt treatment of middle ear or mastoid infections over the course of the lifetime of patients with cochlear implants becomes increasingly important. As the incidence of acute otitis media is 60 to 80% within the first 6 years of life, even though there are no studies that demonstrate a higher incidence of acute otitis media in children with cochlear implants, its management remains a challenge in these patients.35 Previously, cochlear implantation was contraindicated in patients with otitis media because of the increased risk of tympanic membrane perforation, recurrent cholesteatoma, meningitis and electrode extrusion.28 However, recent studies have shown that cochlear implantation is safe in these patients. One prospective study even demonstrated that the incidence of acute otitis media decreased postimplantation after adequate control of the disease preimplantation.28 The literature emphasizes the importance of controlling acute otitis media before implantation to minimize bacterial contamination of the device and meningitis.

Patients with a dry perforation can undergo cochlear implantation after a successful tympanoplasty. Some argue that the tympanoplasty should precede the cochlear implantation by at least 3 months to ensure adequate healing as a persistent perforation at the time of implantation may increase postoperative complications from an exposed electrode. In the setting of a dry perforation, some have advocated a one-stage closure and implantation. Others have advocated procedures such as meatal closure or radical mastoidectomy to perform the cochlear implant in a single-stage surgery. However, after a meatal closure, otoscopic surveillance is impossible, and therefore, early diagnosis and prompt treatment of middle ear diseases is limited, thereby increasing the risk of meningitis.28

Patients with cholesteatoma have an elevated risk of intracranial complications, such as meningitis, and other complications such as device extrusion.36 Therefore, complete eradication of the disease should be obtained before implantation. Intact canal wall mastoidectomy and radical mastoidectomy are both options to remove the disease depending on the extent and likelihood of recurrence. It is recommended that the implant be placed at least 3 to 6 months after eradication of the disease.28 As revision surgery is performed 6 to 12 months after the first surgery, the same waiting period should apply toward cochlear implantation surgery.28