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Cochleostomy management in patients with enlarged vestibular aqueduct receiving cochlear implants
Operative Techniques in Otolaryngology (2005) 16, 82-85
Cochleostomy management in patients with enlarged vestibular aqueduct receiving cochlear implants Joni K. Doherty, MD, PhD, William M. Luxford, MD From the House Clinic and House Ear Institute, Los Angeles, California. KEYWORDS Cochlear implant; Cochleostomy; Enlarged vestibular aqueduct; Inner ear malformation; Cerebrospinal fluid gusher; Cerebrospinal fluid oozer; Mondini deformity; Common cavity; Incomplete partition
Enlarged vestibular aqueduct (EVA) is the most common, radiographically evident abnormality of the inner ear in association with congenital or early onset sensorineural hearing loss. It can occur in isolation or in association with other inner ear malformations, such as the Mondini malformation. Cochlear implantation is an effective hearing rehabilitation option for patients with profound hearing loss, even in the presence of EVA and many other inner ear anomalies. However, congenital anomalies of the inner ear are associated with an increased incidence of both cerebrospinal fluid (CSF) leak at cochleostomy and aberrant facial nerve course, which can lead to postoperative CSF leak, meningitis, and facial nerve complications in association with cochlear implantation. In isolated EVA, a CSF leak is presumably caused by the dilated endolymphatic sac, which defectively allows communication with the subarachnoid space CSF in the posterior fossa. Additionally, occult osseous modiolar and interscalar septum deficiencies may exist and, thus, allow communication with the subarachnoid space CSF at the distal internal auditory canal. Therefore, modifications of the cochleostomy technique may be necessary in patients with EVA and other inner ear anomalies. We routinely use temporalis muscle to pack the cochleostomy site after electrode insertion. Additionally, oxidized nitrocellulose and temporalis muscle are used to pack the eustachian tube before cochleostomy to prevent CSF rhinorrhea in such patients. © 2005 Elsevier Inc. All rights reserved.
Enlarged vestibular aqueduct (EVA) is the most common, radiographically evident malformation of the inner ear associated with hearing loss. EVA is bilateral twice as often as unilateral, occurs in a 3:2 ratio of females-to-males, and leads to progressive sensorineural hearing loss (SNHL) during early childhood.1 EVA frequently occurs in combination with other inner ear anomalies, such as incomplete partition type II (the classic Mondini dysplasia) and the common cavity deformity.2 Furthermore, it has been suggested that even in isolated EVA, there may be concurrent bony defects in the osseous interscalar septum, modiolus, or cribriform area that are not detectable radiographically, yet predispose the cochlea to internal microfractures and/or cerebrospinal fluid (CSF) communication.3-6 Such osseous deficiencies may be lead to SNHL with minor head trauma.3 The EVA inner ear malformation, among others, has been Address reprint requests and correspondence: William M. Luxford, MD, House Clinic and House Ear Institute, 2100 West Third Street, Los Angeles, CA 90057. E-mail address: [email protected]. 1043-1810/$ -see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.otot.2005.03.002
associated with a high incidence of both CSF gusher at cochleostomy and an aberrant facial nerve course, leading to an increased risk of postoperative CSF leak, meningitis, and facial nerve complications in patients with a cochlear implant.7,8 In addition, complete insertion of the electrode array may not be feasible. Thus, modifications of the cochleostomy technique may be necessary in patients with EVA. CSF gusher at cochleostomy occurs in approximately 40% of cases with inner ear malformation, with the reported incidence ranging from 15% to 100%.9-11 CSF gushers are usually a result of osseous deficiency at the cribriform plate, causing communication between the subarachnoid space and the cochlea. Often, preoperative high-resolution computerized tomography reveals a patulous cribriform area, which may alert the surgeon to an impending CSF gusher, but this radiographic finding, or the absence thereof, does not always correlate with surgical findings.6 Furthermore, deficiency of the modiolus and/or interscalar septum may be undetectable using routine radiographic techniques but may be detected on magnetic resonance imaging using a fast spin echo technique.4,5,12
Doherty and Luxford
Cochleostomy for Enlarged Vestibular Aqueduct
In association with isolated EVA, a CSF gusher is presumably caused by the dilated endolymphatic sac, which defectively allows communication with CSF in the subarachnoid space of the posterior fossa.10 Additionally, occult osseous modiolar and interscalar septum deficiencies may exist and, thus, allow CSF communication with the subarachnoid space at the distal internal auditory canal (IAC).4,5 Nonetheless, a CSF leak encountered with isolated EVA tends to be less profuse than with other inner ear malformations,5,11 and a slower leak or “oozer” is encountered more often than a gusher. In general, a lesser degree of CSF leak is encountered with less severe inner ear malformations. With either a gusher or an oozer, the surgeon may proceed with gentle insertion of the cochlear implant, but it is advisable to wait for slowing of the CSF flow after a gusher is encountered. The typical oozer is easily controlled with soft tissue plugging of the cochleostomy in the majority of cases. However, the risks of postoperative CSF leak and meningitis are increased in patients with inner ear malformation, and these complications may present even several months after implantation.10 Facial nerve anomalies occur in 16% to 32% of patients with inner ear anomalies.6,8,9 The tympanic portion of the facial nerve has been reported to lie in a more anterior, lateral, and inferior position, and usually involves displacement of the second genu.6 Aberrant facial nerve location has led to postoperative facial paralysis, and facial nerve monitoring during cochlear implantation is essential.7,8 The incidence of postoperative facial nerve stimulation by the cochlear implant is significantly higher in patients with inner ear malformations, estimated at 11%.6 This has been postulated to result from the anomalous facial nerve course in the tympanic and mastoid segments. It has also been suggested that stimulation of the facial nerve in the IAC could occur because of disruption of the soft tissue barrier or migration of the electrode into the IAC in patients with a deficient cochlear modiolus. Thus, the technique of electrode insertion could affect this complication, although this has not been investigated. Finally, electrode insertion may be incomplete in an estimated 21% of patients with inner ear malformations.6 Although isolated EVA does not generally lead to incomplete electrode insertion, associated inner ear anomalies, such as the common cavity deformity, may preclude complete insertion of the electrode array. Incomplete electrode insertion has resulted in lower average performance scores on auditory measures compared with complete insertion; however, even in the presence of incomplete insertion, auditory performance testing reveals marked improvement after implantation in the vast majority of patients.7,8,13 Additionally, in common cavity cases, the choice of electrode is essential for optimal performance. For a right ear, one should use a left cochlear implant device and vice versa.14 This is performed because orientation of the electrode array is designed to hug the modiolus, with the electrodes in close proximity to the spiral ganglion cells. In the common cavity deformity, spiral ganglion cells will presumably be in the periphery and, therefore, best stimulated by eversion of the typical modiolar hugging design. Patients with both isolated EVA and EVA associated with incomplete partition type II perform better with co-
83
chlear implants than patients with other types of inner ear malformations, such as common cavity and the nonpartitioned cochlea.14-20 In fact, the auditory performance of EVA implant recipients resembles that of age-matched patients without inner ear anomalies.15 In general, the more severe the malformation, the worse the hearing outcome will be, but exceptions to this rule occur. Thus, cochlear implantation can be an effective hearing rehabilitation option for patients with EVA as well as other inner ear malformations. The only anatomic contraindications to cochlear implantation are the absence of the cochlear nerve (i.e., cochlear nerve agenesis) and absence of the cochlea (i.e., Michel aplasia).
Preoperative evaluation A discussion of the audiometric criteria for cochlear implant recipient candidacy is beyond the scope of this work. However, once candidacy is established, high resolution computerized tomography of the temporal bones in both the coronal and axial planes is obtained. Before selection of the ear for implantation, these films are reviewed with emphasis on identifying: the extent of the inner ear anomaly; fenestration of the cribriform area of the cochlear modiolus; ossification of the cochlea, especially when a history of meningitis exists; facial nerve course; and location of the jugular bulb, with respect to the round window.
Surgical technique The patient undergoing a cochlear implant is placed in the supine position and induced with general anesthesia. Once peripheral intravenous access is established, a short acting paralytic is used for intubation to facilitate facial nerve monitoring during the procedure. The table is reversed, and the facial nerve monitor is connected with needle electrodes placed into the ipsilateral orbicularis oris muscle. Minimal shaving is performed, and the area is prepared and draped using sterile technique. A small inverted lazy “S” incision extending from the temporal area (approximately 2-3 cm above the root of zygoma) into the postauricular sulcus (to the level of the inferior border of the external auditory canal) is drawn (Figure 1), and local anesthetic is injected. The skin is incised, and skin flaps are elevated anteriorly and posteriorly superficial to the temporalis fascia. The temporalis muscle and mastoid periosteum are then incised and elevated anteriorly until the spine of Henle is in view, as a landmark for the mastoid antrum. A posterior superior elevation exposes the temporal squama to allow for seating of the implant receiver. The temporalis muscle incision should be approximately 1.5 cm anterior to the skin incision to allow for complete coverage of the device on closure with a minimal risk of contamination from the skin. A piece of temporalis muscle is harvested and set aside for plugging the eustachian tube and cochleostomy. A self-retaining Weitlaner retractor is placed. The standard transmastoid facial recess approach is performed as previously described.20 When opening the facial recess, the chorda tym-
84
Operative Techniques in Otolaryngology, Vol 16, No 2, June 2005
Figure 1 Placement of incision. An inverted “lazy S” incision is made beginning just posterior to the postauricular crease and is extended superiorly over the temporal squama area, curving posteriorly at the superior-most aspect. (Reprinted with permission.20)
pani nerve should be preserved to avoid violating the annulus of the tympanic membrane, which lies just lateral to it (Figure 2). After the facial recess is opened, the squamous part of the temporal bone is prepared for seating of the cochlear implant internal receiver. The implant receiver is placed and secured to bone using two 4-0 neurilon sutures. In patients with inner ear malformation, when an oozer or gusher of CSF is expected, the incudostapedial joint is
Figure 2 Transmastoid facial recess approach. A thin covering of bone should be left over the facial nerve and chorda tympani to protect both the nerve and adjacent tympanic membrane, respectively. The incus buttress is thinned, allowing creation of a larger facial recess opening. Note that to facilitate secure placement of the electrode wire, the mastoid cavity margins are not saucerized. (Reprinted with permission.20)
Figure 3 Removal of the anterosuperior aspect of the round window overhang facilitates cochleostomy at the anterosuperior aspect of the true round window membrane to enter the scala tympani. (Reprinted with permission.20)
separated, and the incus is removed to allow for better visualization of the eustachian tube. The eustachian tube is packed with oxidized cellulose (Surgicel, Johnson & Johnson, Piscataway, NJ) and temporalis muscle before cochleostomy. Next, the round window niche is identified, the overhang of the hook portion is removed using either a 1.5 or 1-mm diamond bur (Figure 3), and cochleostomy is made with a 1-mm diamond bur at the anterior inferior aspect, which facilitates entry into the scala tympani. At this point, CSF may leak profusely from the cochleostomy site (i.e., gusher), and the surgeon should wait for the leak to subside before passing the electrode array. The electrode is then inserted according to manufacturer’s instructions. Once full insertion is achieved, the cochleostomy is tightly packed with small pieces of temporalis muscle, which completely surround the electrode in the cochleostomy (Figure 4). The final piece of temporalis muscle should be placed in the lateral-most position to prevent the electrode from applying pressure against the tympanic membrane, possibly leading to extrusion. The surgeon and patient should be prepared to over-sew the external auditory canal, and obliterate the mastoid and middle ear space if profuse and unrelenting CSF leakage is encountered. In the authors’ experience, this is rarely necessary. The ground electrode, if present, is tucked into the temporalis muscle, and the electrode array is securely arranged in the mastoid cavity. The temporalis muscle fascia and mastoid periosteum are then closed with interrupted absorbable suture over the implant, and the skin incision is closed as a separate layer over this using absorbable subcuticular sutures. Steristrips (3M, St. Paul, MN) are applied to the surface of the incision. The wound is dressed with steristrips, and a compressive mastoid-type dressing is applied.
Doherty and Luxford
Cochleostomy for Enlarged Vestibular Aqueduct
85
toring should be used routinely in cochlear implantation, and it is especially important in patients with inner ear anomalies because of the higher incidence of concurrent facial nerve anomalies. The surgeon should be prepared to pack tightly the cochleostomy site with soft tissue after electrode placement, and temporalis muscle provides a readily available and easily accessible graft for this purpose. In addition, Surgicel and muscle are used to occlude the eustachian tube. Although rarely necessary, the surgeon should also be prepared to over-sew the external auditory canal and obliterate the mastoid if necessary.
References
Figure 4 Packing of cochleostomy with soft tissue (temporalis muscle) after electrode insertion. The final piece of tissue is placed at the lateral aspect of the electrode to prevent extrusion through the tympanic membrane (Illustration courtesy of Robert Fenn.)
Postoperative care Postoperative care is similar to that of patients who have undergone chronic ear surgery. The mastoid dressing is removed on the first postoperative day, and the patient may shower on the fifth day. Steristrips are removed in the office one week after the operation. The patient is instructed to avoid any heavy exertion or lifting for 3 to 4 weeks after surgery to prevent unnecessary increases in intracranial pressure. Nausea and vomiting, as well as constipation are controlled medically as needed. Nose blowing is to be avoided, and the patient should sneeze with the mouth open to prevent positive pressure through the eustachian tube. In our experience of implanting more than 50 children with inner ear malformations, we have had only one postoperative persistent CSF leak that required reoperation for closure in a 4-year-old patient. This patient had an oozer at the initial implantation procedure, and had significant immediate postoperative violent behavior and also nausea and vomiting, which may have increased her intracranial pressure to the extent that the cochleostomy muscle plug was extruded. CSF rhinorrhea immediately developed, which did not respond to conservative measures (a lumbar drain was not attempted). At reoperation, a CSF oozer was again noted. Therefore, the eustachian tube was packed, the cochleostomy repacked without removing the functional electrode, and the middle ear space was obliterated with soft tissue. The patient is now asymptomatic and doing well with her implant device.
Conclusion Cochleostomy in patients with EVA receiving cochlear implantation is performed similarly to that in patients with normal inner and middle ear anatomy. Facial nerve moni-
1. Valvassori GE, Clemis JD: The large vestibular aqueduct syndrome. Laryngoscope 88:723-728, 1978 2. Sennaroglu L, Saatci I: A new classification for cochleovestibular malformations. Laryngoscope 112:2230-2241, 2002 3. Jackler RK, Luxford WM, House WF: Congenital malformations of the inner ear: A classification based on embryogenesis. Laryngoscope 97:2-14, 1987 (suppl 40) 4. Davidson HC, Harnsberger HR, Lemmerling MM, et al: MR evaluation of vestibulocochlear anomalies associated with large endolymphatic duct and sac. AJNR Am J Neuroradiol 20:1435-1441, 1999 5. Sennaroglu L, Saatci I: Unpartitioned versus incompletely partitioned cochleae: Radiologic differentiation. Otol Neurotol 25:520-529, 2004 6. Buchman CA, Copeland BJ, Yu KK, et al: Cochlear implantation in children with congenital inner ear malformations. Laryngoscope 114: 309-316, 2004 7. Slattery WH III, Luxford WM: Cochlear implantation in the congenital malformed cochlea. Laryngoscope 105:1184-1187, 1995 8. Hoffman RA, Downey LL, Waltzman SB, et al: Cochlear implantation in children with cochlear malformations. Am J Otol 18:184-187, 1997 9. Mylanus EA, Rotteveel LJ, Leeuw RL: Congenital malformation of the inner ear and pediatric cochlear implantation. Otol Neurotol 25: 308-317, 2004 10. Woolley AL, Jenison V, Stroer BS, et al: Cochlear implantation in children with inner ear malformations. Ann Otol Rhinol Laryngol 107:492-500, 1998 11. Aschendorff A, Marangos N, Laszig R: Large vestibular aqueduct syndrome and its implications for cochlear implant surgery. Am J Otol 18:S57, 1997 (suppl) 12. Weber BP, Dillo W, Dietrich B, et al: Pediatric cochlear implantation in cochlear malformations. Am J Otol 19:747-753, 1998 13. Tucci DL, Telian SA, Zimmerman-Philips S, et al: Cochlear implantation in patients with cochlear malformations. Arch Otolaryngol Head Neck Surg 121:833-838, 1995 14. McElveen JT Jr, Carrasco VN, Miyamoto RT, et al: Cochlear implantation in common cavity malformations using a transmastoid labyrinthotomy approach. Laryngoscope 107:1032-1036, 1997 15. Miyamoto RT, Bichey BG, Wynne MK et al: Cochlear implantation with large vestibular aqueduct syndrome. Laryngoscope 112:11781182, 2002 16. Bichey BG, Hoversland JM, Wynne MK, et al: Changes in quality of life and the cost-utility associated with cochlear implantation in patients with large vestibular aqueduct syndrome. Otol Neurotol 23:323327, 2002 17. Jackler RK, Luxford WM, House WF: Sound detection with the cochlear implant in five ears of four children with congenital malformations of the cochlea. Laryngoscope 97:15-17, 1987 (suppl 40) 18. Bent JP III, Chute P, Parisier SC: Cochlear implantation in children with enlarged vestibular aqueducts. Laryngoscope 109:1019-1022, 1999 19. Fahy CP, Carney AS, Nikolopoulos TP, et al: Cochlear implantation in children with large vestibular aqueduct syndrome and a review of the syndrome. Int J Pediatr Otorhinolaryngol 59:207-215, 2001 20. Luxford WM: Surgical technique of cochlear implantation, in Brackmann DE, Shelton C, Arriaga MA (eds): Otologic Surgery (ed 2). Philadelphia, PA, Saunders, 2001, pp 341-348
Cochleostomy management in patients with enlarged vestibular aqueduct receiving cochlear implants Joni K. Doherty, MD, PhD, William M. Luxford, MD From the House Clinic and House Ear Institute, Los Angeles, California. KEYWORDS Cochlear implant; Cochleostomy; Enlarged vestibular aqueduct; Inner ear malformation; Cerebrospinal fluid gusher; Cerebrospinal fluid oozer; Mondini deformity; Common cavity; Incomplete partition
Enlarged vestibular aqueduct (EVA) is the most common, radiographically evident abnormality of the inner ear in association with congenital or early onset sensorineural hearing loss. It can occur in isolation or in association with other inner ear malformations, such as the Mondini malformation. Cochlear implantation is an effective hearing rehabilitation option for patients with profound hearing loss, even in the presence of EVA and many other inner ear anomalies. However, congenital anomalies of the inner ear are associated with an increased incidence of both cerebrospinal fluid (CSF) leak at cochleostomy and aberrant facial nerve course, which can lead to postoperative CSF leak, meningitis, and facial nerve complications in association with cochlear implantation. In isolated EVA, a CSF leak is presumably caused by the dilated endolymphatic sac, which defectively allows communication with the subarachnoid space CSF in the posterior fossa. Additionally, occult osseous modiolar and interscalar septum deficiencies may exist and, thus, allow communication with the subarachnoid space CSF at the distal internal auditory canal. Therefore, modifications of the cochleostomy technique may be necessary in patients with EVA and other inner ear anomalies. We routinely use temporalis muscle to pack the cochleostomy site after electrode insertion. Additionally, oxidized nitrocellulose and temporalis muscle are used to pack the eustachian tube before cochleostomy to prevent CSF rhinorrhea in such patients. © 2005 Elsevier Inc. All rights reserved.
Enlarged vestibular aqueduct (EVA) is the most common, radiographically evident malformation of the inner ear associated with hearing loss. EVA is bilateral twice as often as unilateral, occurs in a 3:2 ratio of females-to-males, and leads to progressive sensorineural hearing loss (SNHL) during early childhood.1 EVA frequently occurs in combination with other inner ear anomalies, such as incomplete partition type II (the classic Mondini dysplasia) and the common cavity deformity.2 Furthermore, it has been suggested that even in isolated EVA, there may be concurrent bony defects in the osseous interscalar septum, modiolus, or cribriform area that are not detectable radiographically, yet predispose the cochlea to internal microfractures and/or cerebrospinal fluid (CSF) communication.3-6 Such osseous deficiencies may be lead to SNHL with minor head trauma.3 The EVA inner ear malformation, among others, has been Address reprint requests and correspondence: William M. Luxford, MD, House Clinic and House Ear Institute, 2100 West Third Street, Los Angeles, CA 90057. E-mail address: [email protected]. 1043-1810/$ -see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.otot.2005.03.002
associated with a high incidence of both CSF gusher at cochleostomy and an aberrant facial nerve course, leading to an increased risk of postoperative CSF leak, meningitis, and facial nerve complications in patients with a cochlear implant.7,8 In addition, complete insertion of the electrode array may not be feasible. Thus, modifications of the cochleostomy technique may be necessary in patients with EVA. CSF gusher at cochleostomy occurs in approximately 40% of cases with inner ear malformation, with the reported incidence ranging from 15% to 100%.9-11 CSF gushers are usually a result of osseous deficiency at the cribriform plate, causing communication between the subarachnoid space and the cochlea. Often, preoperative high-resolution computerized tomography reveals a patulous cribriform area, which may alert the surgeon to an impending CSF gusher, but this radiographic finding, or the absence thereof, does not always correlate with surgical findings.6 Furthermore, deficiency of the modiolus and/or interscalar septum may be undetectable using routine radiographic techniques but may be detected on magnetic resonance imaging using a fast spin echo technique.4,5,12
Doherty and Luxford
Cochleostomy for Enlarged Vestibular Aqueduct
In association with isolated EVA, a CSF gusher is presumably caused by the dilated endolymphatic sac, which defectively allows communication with CSF in the subarachnoid space of the posterior fossa.10 Additionally, occult osseous modiolar and interscalar septum deficiencies may exist and, thus, allow CSF communication with the subarachnoid space at the distal internal auditory canal (IAC).4,5 Nonetheless, a CSF leak encountered with isolated EVA tends to be less profuse than with other inner ear malformations,5,11 and a slower leak or “oozer” is encountered more often than a gusher. In general, a lesser degree of CSF leak is encountered with less severe inner ear malformations. With either a gusher or an oozer, the surgeon may proceed with gentle insertion of the cochlear implant, but it is advisable to wait for slowing of the CSF flow after a gusher is encountered. The typical oozer is easily controlled with soft tissue plugging of the cochleostomy in the majority of cases. However, the risks of postoperative CSF leak and meningitis are increased in patients with inner ear malformation, and these complications may present even several months after implantation.10 Facial nerve anomalies occur in 16% to 32% of patients with inner ear anomalies.6,8,9 The tympanic portion of the facial nerve has been reported to lie in a more anterior, lateral, and inferior position, and usually involves displacement of the second genu.6 Aberrant facial nerve location has led to postoperative facial paralysis, and facial nerve monitoring during cochlear implantation is essential.7,8 The incidence of postoperative facial nerve stimulation by the cochlear implant is significantly higher in patients with inner ear malformations, estimated at 11%.6 This has been postulated to result from the anomalous facial nerve course in the tympanic and mastoid segments. It has also been suggested that stimulation of the facial nerve in the IAC could occur because of disruption of the soft tissue barrier or migration of the electrode into the IAC in patients with a deficient cochlear modiolus. Thus, the technique of electrode insertion could affect this complication, although this has not been investigated. Finally, electrode insertion may be incomplete in an estimated 21% of patients with inner ear malformations.6 Although isolated EVA does not generally lead to incomplete electrode insertion, associated inner ear anomalies, such as the common cavity deformity, may preclude complete insertion of the electrode array. Incomplete electrode insertion has resulted in lower average performance scores on auditory measures compared with complete insertion; however, even in the presence of incomplete insertion, auditory performance testing reveals marked improvement after implantation in the vast majority of patients.7,8,13 Additionally, in common cavity cases, the choice of electrode is essential for optimal performance. For a right ear, one should use a left cochlear implant device and vice versa.14 This is performed because orientation of the electrode array is designed to hug the modiolus, with the electrodes in close proximity to the spiral ganglion cells. In the common cavity deformity, spiral ganglion cells will presumably be in the periphery and, therefore, best stimulated by eversion of the typical modiolar hugging design. Patients with both isolated EVA and EVA associated with incomplete partition type II perform better with co-
83
chlear implants than patients with other types of inner ear malformations, such as common cavity and the nonpartitioned cochlea.14-20 In fact, the auditory performance of EVA implant recipients resembles that of age-matched patients without inner ear anomalies.15 In general, the more severe the malformation, the worse the hearing outcome will be, but exceptions to this rule occur. Thus, cochlear implantation can be an effective hearing rehabilitation option for patients with EVA as well as other inner ear malformations. The only anatomic contraindications to cochlear implantation are the absence of the cochlear nerve (i.e., cochlear nerve agenesis) and absence of the cochlea (i.e., Michel aplasia).
Preoperative evaluation A discussion of the audiometric criteria for cochlear implant recipient candidacy is beyond the scope of this work. However, once candidacy is established, high resolution computerized tomography of the temporal bones in both the coronal and axial planes is obtained. Before selection of the ear for implantation, these films are reviewed with emphasis on identifying: the extent of the inner ear anomaly; fenestration of the cribriform area of the cochlear modiolus; ossification of the cochlea, especially when a history of meningitis exists; facial nerve course; and location of the jugular bulb, with respect to the round window.
Surgical technique The patient undergoing a cochlear implant is placed in the supine position and induced with general anesthesia. Once peripheral intravenous access is established, a short acting paralytic is used for intubation to facilitate facial nerve monitoring during the procedure. The table is reversed, and the facial nerve monitor is connected with needle electrodes placed into the ipsilateral orbicularis oris muscle. Minimal shaving is performed, and the area is prepared and draped using sterile technique. A small inverted lazy “S” incision extending from the temporal area (approximately 2-3 cm above the root of zygoma) into the postauricular sulcus (to the level of the inferior border of the external auditory canal) is drawn (Figure 1), and local anesthetic is injected. The skin is incised, and skin flaps are elevated anteriorly and posteriorly superficial to the temporalis fascia. The temporalis muscle and mastoid periosteum are then incised and elevated anteriorly until the spine of Henle is in view, as a landmark for the mastoid antrum. A posterior superior elevation exposes the temporal squama to allow for seating of the implant receiver. The temporalis muscle incision should be approximately 1.5 cm anterior to the skin incision to allow for complete coverage of the device on closure with a minimal risk of contamination from the skin. A piece of temporalis muscle is harvested and set aside for plugging the eustachian tube and cochleostomy. A self-retaining Weitlaner retractor is placed. The standard transmastoid facial recess approach is performed as previously described.20 When opening the facial recess, the chorda tym-
84
Operative Techniques in Otolaryngology, Vol 16, No 2, June 2005
Figure 1 Placement of incision. An inverted “lazy S” incision is made beginning just posterior to the postauricular crease and is extended superiorly over the temporal squama area, curving posteriorly at the superior-most aspect. (Reprinted with permission.20)
pani nerve should be preserved to avoid violating the annulus of the tympanic membrane, which lies just lateral to it (Figure 2). After the facial recess is opened, the squamous part of the temporal bone is prepared for seating of the cochlear implant internal receiver. The implant receiver is placed and secured to bone using two 4-0 neurilon sutures. In patients with inner ear malformation, when an oozer or gusher of CSF is expected, the incudostapedial joint is
Figure 2 Transmastoid facial recess approach. A thin covering of bone should be left over the facial nerve and chorda tympani to protect both the nerve and adjacent tympanic membrane, respectively. The incus buttress is thinned, allowing creation of a larger facial recess opening. Note that to facilitate secure placement of the electrode wire, the mastoid cavity margins are not saucerized. (Reprinted with permission.20)
Figure 3 Removal of the anterosuperior aspect of the round window overhang facilitates cochleostomy at the anterosuperior aspect of the true round window membrane to enter the scala tympani. (Reprinted with permission.20)
separated, and the incus is removed to allow for better visualization of the eustachian tube. The eustachian tube is packed with oxidized cellulose (Surgicel, Johnson & Johnson, Piscataway, NJ) and temporalis muscle before cochleostomy. Next, the round window niche is identified, the overhang of the hook portion is removed using either a 1.5 or 1-mm diamond bur (Figure 3), and cochleostomy is made with a 1-mm diamond bur at the anterior inferior aspect, which facilitates entry into the scala tympani. At this point, CSF may leak profusely from the cochleostomy site (i.e., gusher), and the surgeon should wait for the leak to subside before passing the electrode array. The electrode is then inserted according to manufacturer’s instructions. Once full insertion is achieved, the cochleostomy is tightly packed with small pieces of temporalis muscle, which completely surround the electrode in the cochleostomy (Figure 4). The final piece of temporalis muscle should be placed in the lateral-most position to prevent the electrode from applying pressure against the tympanic membrane, possibly leading to extrusion. The surgeon and patient should be prepared to over-sew the external auditory canal, and obliterate the mastoid and middle ear space if profuse and unrelenting CSF leakage is encountered. In the authors’ experience, this is rarely necessary. The ground electrode, if present, is tucked into the temporalis muscle, and the electrode array is securely arranged in the mastoid cavity. The temporalis muscle fascia and mastoid periosteum are then closed with interrupted absorbable suture over the implant, and the skin incision is closed as a separate layer over this using absorbable subcuticular sutures. Steristrips (3M, St. Paul, MN) are applied to the surface of the incision. The wound is dressed with steristrips, and a compressive mastoid-type dressing is applied.
Doherty and Luxford
Cochleostomy for Enlarged Vestibular Aqueduct
85
toring should be used routinely in cochlear implantation, and it is especially important in patients with inner ear anomalies because of the higher incidence of concurrent facial nerve anomalies. The surgeon should be prepared to pack tightly the cochleostomy site with soft tissue after electrode placement, and temporalis muscle provides a readily available and easily accessible graft for this purpose. In addition, Surgicel and muscle are used to occlude the eustachian tube. Although rarely necessary, the surgeon should also be prepared to over-sew the external auditory canal and obliterate the mastoid if necessary.
References
Figure 4 Packing of cochleostomy with soft tissue (temporalis muscle) after electrode insertion. The final piece of tissue is placed at the lateral aspect of the electrode to prevent extrusion through the tympanic membrane (Illustration courtesy of Robert Fenn.)
Postoperative care Postoperative care is similar to that of patients who have undergone chronic ear surgery. The mastoid dressing is removed on the first postoperative day, and the patient may shower on the fifth day. Steristrips are removed in the office one week after the operation. The patient is instructed to avoid any heavy exertion or lifting for 3 to 4 weeks after surgery to prevent unnecessary increases in intracranial pressure. Nausea and vomiting, as well as constipation are controlled medically as needed. Nose blowing is to be avoided, and the patient should sneeze with the mouth open to prevent positive pressure through the eustachian tube. In our experience of implanting more than 50 children with inner ear malformations, we have had only one postoperative persistent CSF leak that required reoperation for closure in a 4-year-old patient. This patient had an oozer at the initial implantation procedure, and had significant immediate postoperative violent behavior and also nausea and vomiting, which may have increased her intracranial pressure to the extent that the cochleostomy muscle plug was extruded. CSF rhinorrhea immediately developed, which did not respond to conservative measures (a lumbar drain was not attempted). At reoperation, a CSF oozer was again noted. Therefore, the eustachian tube was packed, the cochleostomy repacked without removing the functional electrode, and the middle ear space was obliterated with soft tissue. The patient is now asymptomatic and doing well with her implant device.
Conclusion Cochleostomy in patients with EVA receiving cochlear implantation is performed similarly to that in patients with normal inner and middle ear anatomy. Facial nerve moni-
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