1990, The British Journal of Radiology, 63, 512-516
Imaging for cochlear implants By Peter D. Phelps, M D , FRCS, FRCR, Jonathan A. D. Annis, FRCR and "Philip J . Robinson FRCS Departments of Radiology and Otolaryngology, Royal National Throat, Nose and Ear Hospital, London and *Royal Ear Hospital, Huntley Street, London WC1
{Received January 1990)
Abstract. Insertion of a sound amplification device into the round window niche (extracochlear implant) or into the coils of the cochlea (intracochlear implant) can give significant benefits to some carefully selected, severely deaf patients. Imaging has an essential role in selective and pre-operative assessment. Severe otosclerosis and post-meningitic labyrinthitis ossificans are common causes of deafness in these patients and can be demonstrated by computed tomography (CT). The most suitable side for operation can be assessed. We describe our experiences with 165 patients, 69 of whom were found suitable for implants. Thin (1 mm) section CT in axial and coronal planes is the best imaging investigation of the petrous temporal bones but the place of magnetic resonance scanning to confirm that the inner ear is fluid-filled and polytomography to show a multichannel implant in the cochlea is discussed. No implants were used for congenital deformities, but some observations are made of this type of structural deformity of the inner ear.
Conventional hearing aids amplify sound which is trans- for cochlear implants and, to a lesser extent, in the mitted to the cochlea via the normal conducting mech- follow-up. Implanting has proved most successful in anism of the eardrum and ossicular chain or, with bone post-lingual deafness, i.e. the small group of people who conduction aids, by transmitting directly through the have become severely deaf after speech acquisition. Unskull. Advances in modern electronics have made it fortunately, some of these disease processes not only possible to project sound further into the auditory damage the membraneous labyrinth but also result in reception pathway with significant benefit in carefully sclerosis and new bone formation in the sites of prospecselected cases of severe deafness. Such devices are tive implantation inhibiting or precluding insertion of referred to as cochlear implants and those in current the electrode. Previous publications have stressed the usage fall, broadly speaking, into two categories: the high incidence of abnormalities of the inner ear that can extracochlear single channel electrode, which is inserted be recognized by imaging (Harnsberger et al, 1987; into the round window niche, and the intracochlear Mueller et al, 1989). single channel or multichannel implant in which an array Thin section imaging of the highest quality is of electrodes is threaded into the cochlea via the round obviously necessary and, as we found a similar high window ideally reaching as far as the second coil incidence of middle and inner ear abnormalities, an (Fig. 1). account of our experiences and recommendations Radiology has a most important part to play parti- seemed appropriate as more implant programmes are cularly in the pre-operative selection of suitable subjects planned in Great Britain.
Figure 1. Post-implant coronal polytomography showing (a) single channel extracochlear electrode in the round window niche, (b) multichannel electrode in the coils of the cochlea. Conventional tomography appears more satisfactory than CT for showing the device in position, (v = vestibule; i = internal auditory meatus.)
512
The British Journal of Radiology, July 1990
Imaging for cochlear implants
Materials and methods
One hundred and sixty-five patients from the University College Hospital, the Royal National Institute for the Deaf and more recently from the Cambridge Cochlear Implant Programmes have been assessed both clinically and radiologically, and 69 were judged suitable for implants. Fifty-one patients have been given implants, 18 are waiting. Extracochlear single channel implants were inserted in 59 and an intracochlear multichannel implant in two patients. Radiological assessment was initially by polytomography and, more recently, by thin section 1 mm high-resolution computed tomography (CT) with the pictures processed on a bone algorithm. Post-operative imaging has been confined to polytomography for one multichannel implant (Fig. 1) and two single channel electrodes. We have not used magnetic resonance (MR) in the pre-operative assessment. The CT examination was done in axial and coronal planes to demonstrate cochlear coils, oval and round windows and the internal auditory meatus (Fig. 2). The facial recess and sinus tympani in the posterior aspect of the middle ear cavity were assessed on the same sections to ensure that there was adequate room for the surgeon to approach the round window niche. Mastoid pneumatization was also shown clearly on these sections. Abnormal soft-tissue opacification in the hypotympanum as from previous middle ear infection can also be shown and may inhibit the procedure. Results
The pathogenesis of the deafness may be known or suspected from the history and clinical findings, but in four cases we were able to confirm that long standing hearing loss was due to undiagnosed otosclerosis/ otospongiosis. There were no subjects with congenital deafness in our programmes, and those with significant pathology shown by imaging fell into the two groups of postmeningitic labyrinthitis ossificans (also called obliterans), where new bone formation narrows or obliterates
Figure 3. A patient with a sclerosing bone dysplasia of the skull, who had lost her hearing as a result of meningitis. The right ear showed evidence of labyrinthitis ossificans although the left cochlea was radiologically normal (arrow). However, the bone dysplasia had encroached on the middle ear which is fluid filled. Note the normal ossicles in this axial section.
parts of the inner ear, and severe otosclerosis, which similarly encroaches upon the lumen of the labyrinth. One patient with post-meningitic labyrinthitis obliterans appeared to have the relevant parts of the inner ear unaffected but was rejected from the programme because of a congenital bone dysplasia which deformed the middle ear cavity (Fig. 3). Four patients were rejected from the programme because of the radiological findings. Discussion
Congenital abnormalities of the inner ear do not usually have progressive deafness and are therefore rarely suitable for implants as they do not have total, or near total, deafness allied to remembered speech ability. Exceptions would be the two types of deformity first
(a)
Figure 2. High resolution CT in (a) axial and (b) coronal planes to show the round window niche (white arrows) and the hook and full length of the basal cochlear coil in the axial plane (black arrows). Vol. 63, No. 751
513
P. D. Phelps, J. A. D. Annis and P. J. Robinson
(b)
Figure 4. (a) Axial and (b) coronal CT sections showing severe labyrinthitis ossificans. Only a faint outline of the cochlea coils can be seen on the axial section. The arrow indicates a patent round window niche on the right side only. Note also the proximity of the high anterior jugular fossa (J) on the right side and obliteration of the posterior semicircular canal (P) on the left.
described by Mondini (1791), namely deficient cochlea with normal basal turn and distal sac and a dilated vestibular aqueduct, both of which separately or in combination may be associated with fluctuant or progressive hearing loss. At least one case with such Mondini cochleas has had a successful multichannel implant (Silverstein et al, 1988). The more severe type of dysplasias with an amorphous cochlear sac, tapered internal auditory meatus (IAM) and risk of a cerebrospinal fistula (Phelps, 1987) would not be suitable for implantation as it is doubtful if a cochlear nerve is
present. Cochleas which are smaller than normal are difficult to implant (Mueller et al, 1989) and this would seem to preclude multichannel electrodes for cases of ear pits deafness syndrome in which there is a reduced cochlea (Slack & Phelps, 1985). A narrow IAM also suggests absence of the stato-acoustic nerve and is a contra-indication to implantation (Shelton et al, 1989). Labyrinthine ossification is the end result of suppurative labyrinthitis which may follow middle ear infection, meningitis or, rarely, septicaemia. The lumen of the labyrinth is narrowed or obliterated although very
(a) (b) Figure 5. Two adjacent 1 mm axial CT sections 1 mm apart through the round window niche hook and full length of the basal turn of the cochlea. The narrowed segment (arrows) was a result of ossification of the scala tympani which necessitated drilling for 3 mm. Two sections are shown to confirm that the reduced lumen represented genuine narrowing and not just an eccentric slice. Note the chain of aircells behind the cochlea. (Patient of Mr Roger Gray.) 514
The British Journal of Radiology, July 1990
Imaging for cochlear implants
Figure 8. The individual coils of the cochlea are shown on this normal 72-weighted axial MR scan (SE 2000/70). The bright signal confirms only that the coils are fluid filled.
Figure 6. Otospongiosis adjacent to the basal coil particularly obliterates the round window. The white dot represents a densitometric reading of 580 HU as opposed to normal readings for the otic capsule of 2000-2400 Hounsfield Units.
rarely totally (Fig. 4). This can be assessed reliably by thin section CT, although considerable experience of petrous bone CT is required because of problems with partial volume averaging. Accurate appraisal of the cochlear coils is vital as a prelude to insertion of an intracochlear device (Jackler et al, 1987). Partial or complete obliteration of the cochlear coils by labyrinthitis ossificans would seem to us to be a contra-indication to a multichannel implant but at least one successful
Figure 7. Severe mixed otosclerosis/otospongiosis gives a ring of rarefaction around the cochlea which is greatly reduced by otosclerotic bone. Note the involvement of the stapes crura (arrow). Vol. 63, No. 751
implantation appears to have been done by drilling out an artificial channel for the electrode to wrap around the modiolus (Gantz et al, 1988). One case in the Cambridge series had a successful multichannel implant although ossification of the scala tympani necessitated drilling for 3 mm until the fully patent cochlear duct was reached (Fig. 5). Otosclerosis is not necessarily a contra-indication to cochlear implantation. In patients with fenestral otosclerosis, a bone plug in the round window can be successfully drilled and implantation can be performed if the surgeon knows, on the basis of CT findings, that the basal turn of the cochlea is patent beyond the obliterated round window (Harnsberger et al, 1987) (Fig. 6). Computed tomography of severe cases of otosclerosis often shows mixed otosclerosis and otospongiosis encroaching on the cochlear lumen. The border between the lumen and the rarefied bone may be difficult to assess if the zonal otospongiosis is severe (Fig. 7). There are two other important features assessed easily on the pre-operative CT: (i) the position of the jugular bulb which if high and anteriorly placed may reach up almost to the round window niche (Fig. 4); (ii) the presence of a retro- and infra-cochlear chain of aircells (Fig. 5). These may be mistaken for the round window niche and attempts made to introduce the electrode into aircells rather than the round window niche. Magnetic resonance may be a useful adjunct to CT but ideally requires appropriate surface coils to increase spatial resolution for the small area of the petrous pyramid under investigation. It is theoretically possible to tell whether the cochlear coils contain fluid or fibrous tissue from the strength of the signal on r2-weighted sequences (Fig. 8) as well as confirming the presence of a stato-acoustic nerve. It is said that degeneration in the nerve will be revealed by a brighter than usual signal on rrweighted images (Valvassori, 1987). It is likely that future development programmes, both in this country and worldwide, will tend towards more intracochlear implants and the importance of MRI in the pre-operative assessment will increase. Acknowledgments We are most grateful to all members of the University College Hospital and the Royal National Institute for the Deaf
515
P. D. Phelps, J. A. D. Annis and P. J. Robinson Cochlear Implant Programme and the Sir Jules Thorne Trust, and to Mr Roger Gray, FRCS, of the Cambridge Cochlear Implant Programme for their help and permission to publish details of their patients.
References GANTZ, B. J., MCCABE, B. F. & TYLER, R. S., 1988. Use of
multichannel cochlear implants in obstructed and obliterated cochleas. Otolaryngology Head and Neck Surgery, 98, 72-81. HARNSBERGER, H. R., DART, D. J., PARKIN, J. L., SMOKER, W.
R. K. & OSBORN, A. G., 1987. Cochlear implant candidates: Assessment with CT and MR imaging. Radiology, 164, 53-57.
MUELLER, D. P., DOLAN, K. D. & GANTZ, B. J.,
1989.
Temporal bone computed tomography in the preoperative evaluation for cochlear implantation. Annals of Otology, Rhinology & Laryngology, 98, 346-349. PHELPS, P. D., 1986. Congenital cerebrospinal fluid fistulae of the petrous temporal bone. Clinical Otolaryngology, 11, 79-92. SHELTON, C , LUXFORD, W. M., TONOKAWA, L. L. & Lo, W. W.
M., 1989. The narrow internal auditory canal in children: A contraindication to cochlear implants. Otolaryngology Head and Neck Surgery, 100, 227-231. SlLVERSTEIN,
H.,
SMOUHA,
E.
&
MORGAN,
N . , 1988.
Multichannel cochlear implantation in a patient with bilateral Mondini deformities. The American Journal of Otology, 9, 451-454.
JACKLER, R. K., LUXFORD, W. M., SCHINDLER, R. A. &
SLACK, R. W. T. & PHELPS, P. D., 1985. Familial mixed
MCKERROW, W. S., 1987. Cochlear patency problems in cochlear implantation. Laryngoscope, 97, 801-805. MONDINI, C , 1791. Anatomica surdi nati sectio, Bononiensi Scientarium et artium instituto atque academia commentarii. Bonaniae, VII, 419-428.
deafness with branchial arch defects (earpits deafness syndrome). Clinical Otolaryngology, 10, 271-277. VALVASSORI, G. E., 1987. Workshop: Surgical anatomy and radiographic imaging of cochlear implant surgery. American Journal of Otology, 8, 195-200.
516
The British Journal of Radiology, July 1990
Imaging for cochlear implants By Peter D. Phelps, M D , FRCS, FRCR, Jonathan A. D. Annis, FRCR and "Philip J . Robinson FRCS Departments of Radiology and Otolaryngology, Royal National Throat, Nose and Ear Hospital, London and *Royal Ear Hospital, Huntley Street, London WC1
{Received January 1990)
Abstract. Insertion of a sound amplification device into the round window niche (extracochlear implant) or into the coils of the cochlea (intracochlear implant) can give significant benefits to some carefully selected, severely deaf patients. Imaging has an essential role in selective and pre-operative assessment. Severe otosclerosis and post-meningitic labyrinthitis ossificans are common causes of deafness in these patients and can be demonstrated by computed tomography (CT). The most suitable side for operation can be assessed. We describe our experiences with 165 patients, 69 of whom were found suitable for implants. Thin (1 mm) section CT in axial and coronal planes is the best imaging investigation of the petrous temporal bones but the place of magnetic resonance scanning to confirm that the inner ear is fluid-filled and polytomography to show a multichannel implant in the cochlea is discussed. No implants were used for congenital deformities, but some observations are made of this type of structural deformity of the inner ear.
Conventional hearing aids amplify sound which is trans- for cochlear implants and, to a lesser extent, in the mitted to the cochlea via the normal conducting mech- follow-up. Implanting has proved most successful in anism of the eardrum and ossicular chain or, with bone post-lingual deafness, i.e. the small group of people who conduction aids, by transmitting directly through the have become severely deaf after speech acquisition. Unskull. Advances in modern electronics have made it fortunately, some of these disease processes not only possible to project sound further into the auditory damage the membraneous labyrinth but also result in reception pathway with significant benefit in carefully sclerosis and new bone formation in the sites of prospecselected cases of severe deafness. Such devices are tive implantation inhibiting or precluding insertion of referred to as cochlear implants and those in current the electrode. Previous publications have stressed the usage fall, broadly speaking, into two categories: the high incidence of abnormalities of the inner ear that can extracochlear single channel electrode, which is inserted be recognized by imaging (Harnsberger et al, 1987; into the round window niche, and the intracochlear Mueller et al, 1989). single channel or multichannel implant in which an array Thin section imaging of the highest quality is of electrodes is threaded into the cochlea via the round obviously necessary and, as we found a similar high window ideally reaching as far as the second coil incidence of middle and inner ear abnormalities, an (Fig. 1). account of our experiences and recommendations Radiology has a most important part to play parti- seemed appropriate as more implant programmes are cularly in the pre-operative selection of suitable subjects planned in Great Britain.
Figure 1. Post-implant coronal polytomography showing (a) single channel extracochlear electrode in the round window niche, (b) multichannel electrode in the coils of the cochlea. Conventional tomography appears more satisfactory than CT for showing the device in position, (v = vestibule; i = internal auditory meatus.)
512
The British Journal of Radiology, July 1990
Imaging for cochlear implants
Materials and methods
One hundred and sixty-five patients from the University College Hospital, the Royal National Institute for the Deaf and more recently from the Cambridge Cochlear Implant Programmes have been assessed both clinically and radiologically, and 69 were judged suitable for implants. Fifty-one patients have been given implants, 18 are waiting. Extracochlear single channel implants were inserted in 59 and an intracochlear multichannel implant in two patients. Radiological assessment was initially by polytomography and, more recently, by thin section 1 mm high-resolution computed tomography (CT) with the pictures processed on a bone algorithm. Post-operative imaging has been confined to polytomography for one multichannel implant (Fig. 1) and two single channel electrodes. We have not used magnetic resonance (MR) in the pre-operative assessment. The CT examination was done in axial and coronal planes to demonstrate cochlear coils, oval and round windows and the internal auditory meatus (Fig. 2). The facial recess and sinus tympani in the posterior aspect of the middle ear cavity were assessed on the same sections to ensure that there was adequate room for the surgeon to approach the round window niche. Mastoid pneumatization was also shown clearly on these sections. Abnormal soft-tissue opacification in the hypotympanum as from previous middle ear infection can also be shown and may inhibit the procedure. Results
The pathogenesis of the deafness may be known or suspected from the history and clinical findings, but in four cases we were able to confirm that long standing hearing loss was due to undiagnosed otosclerosis/ otospongiosis. There were no subjects with congenital deafness in our programmes, and those with significant pathology shown by imaging fell into the two groups of postmeningitic labyrinthitis ossificans (also called obliterans), where new bone formation narrows or obliterates
Figure 3. A patient with a sclerosing bone dysplasia of the skull, who had lost her hearing as a result of meningitis. The right ear showed evidence of labyrinthitis ossificans although the left cochlea was radiologically normal (arrow). However, the bone dysplasia had encroached on the middle ear which is fluid filled. Note the normal ossicles in this axial section.
parts of the inner ear, and severe otosclerosis, which similarly encroaches upon the lumen of the labyrinth. One patient with post-meningitic labyrinthitis obliterans appeared to have the relevant parts of the inner ear unaffected but was rejected from the programme because of a congenital bone dysplasia which deformed the middle ear cavity (Fig. 3). Four patients were rejected from the programme because of the radiological findings. Discussion
Congenital abnormalities of the inner ear do not usually have progressive deafness and are therefore rarely suitable for implants as they do not have total, or near total, deafness allied to remembered speech ability. Exceptions would be the two types of deformity first
(a)
Figure 2. High resolution CT in (a) axial and (b) coronal planes to show the round window niche (white arrows) and the hook and full length of the basal cochlear coil in the axial plane (black arrows). Vol. 63, No. 751
513
P. D. Phelps, J. A. D. Annis and P. J. Robinson
(b)
Figure 4. (a) Axial and (b) coronal CT sections showing severe labyrinthitis ossificans. Only a faint outline of the cochlea coils can be seen on the axial section. The arrow indicates a patent round window niche on the right side only. Note also the proximity of the high anterior jugular fossa (J) on the right side and obliteration of the posterior semicircular canal (P) on the left.
described by Mondini (1791), namely deficient cochlea with normal basal turn and distal sac and a dilated vestibular aqueduct, both of which separately or in combination may be associated with fluctuant or progressive hearing loss. At least one case with such Mondini cochleas has had a successful multichannel implant (Silverstein et al, 1988). The more severe type of dysplasias with an amorphous cochlear sac, tapered internal auditory meatus (IAM) and risk of a cerebrospinal fistula (Phelps, 1987) would not be suitable for implantation as it is doubtful if a cochlear nerve is
present. Cochleas which are smaller than normal are difficult to implant (Mueller et al, 1989) and this would seem to preclude multichannel electrodes for cases of ear pits deafness syndrome in which there is a reduced cochlea (Slack & Phelps, 1985). A narrow IAM also suggests absence of the stato-acoustic nerve and is a contra-indication to implantation (Shelton et al, 1989). Labyrinthine ossification is the end result of suppurative labyrinthitis which may follow middle ear infection, meningitis or, rarely, septicaemia. The lumen of the labyrinth is narrowed or obliterated although very
(a) (b) Figure 5. Two adjacent 1 mm axial CT sections 1 mm apart through the round window niche hook and full length of the basal turn of the cochlea. The narrowed segment (arrows) was a result of ossification of the scala tympani which necessitated drilling for 3 mm. Two sections are shown to confirm that the reduced lumen represented genuine narrowing and not just an eccentric slice. Note the chain of aircells behind the cochlea. (Patient of Mr Roger Gray.) 514
The British Journal of Radiology, July 1990
Imaging for cochlear implants
Figure 8. The individual coils of the cochlea are shown on this normal 72-weighted axial MR scan (SE 2000/70). The bright signal confirms only that the coils are fluid filled.
Figure 6. Otospongiosis adjacent to the basal coil particularly obliterates the round window. The white dot represents a densitometric reading of 580 HU as opposed to normal readings for the otic capsule of 2000-2400 Hounsfield Units.
rarely totally (Fig. 4). This can be assessed reliably by thin section CT, although considerable experience of petrous bone CT is required because of problems with partial volume averaging. Accurate appraisal of the cochlear coils is vital as a prelude to insertion of an intracochlear device (Jackler et al, 1987). Partial or complete obliteration of the cochlear coils by labyrinthitis ossificans would seem to us to be a contra-indication to a multichannel implant but at least one successful
Figure 7. Severe mixed otosclerosis/otospongiosis gives a ring of rarefaction around the cochlea which is greatly reduced by otosclerotic bone. Note the involvement of the stapes crura (arrow). Vol. 63, No. 751
implantation appears to have been done by drilling out an artificial channel for the electrode to wrap around the modiolus (Gantz et al, 1988). One case in the Cambridge series had a successful multichannel implant although ossification of the scala tympani necessitated drilling for 3 mm until the fully patent cochlear duct was reached (Fig. 5). Otosclerosis is not necessarily a contra-indication to cochlear implantation. In patients with fenestral otosclerosis, a bone plug in the round window can be successfully drilled and implantation can be performed if the surgeon knows, on the basis of CT findings, that the basal turn of the cochlea is patent beyond the obliterated round window (Harnsberger et al, 1987) (Fig. 6). Computed tomography of severe cases of otosclerosis often shows mixed otosclerosis and otospongiosis encroaching on the cochlear lumen. The border between the lumen and the rarefied bone may be difficult to assess if the zonal otospongiosis is severe (Fig. 7). There are two other important features assessed easily on the pre-operative CT: (i) the position of the jugular bulb which if high and anteriorly placed may reach up almost to the round window niche (Fig. 4); (ii) the presence of a retro- and infra-cochlear chain of aircells (Fig. 5). These may be mistaken for the round window niche and attempts made to introduce the electrode into aircells rather than the round window niche. Magnetic resonance may be a useful adjunct to CT but ideally requires appropriate surface coils to increase spatial resolution for the small area of the petrous pyramid under investigation. It is theoretically possible to tell whether the cochlear coils contain fluid or fibrous tissue from the strength of the signal on r2-weighted sequences (Fig. 8) as well as confirming the presence of a stato-acoustic nerve. It is said that degeneration in the nerve will be revealed by a brighter than usual signal on rrweighted images (Valvassori, 1987). It is likely that future development programmes, both in this country and worldwide, will tend towards more intracochlear implants and the importance of MRI in the pre-operative assessment will increase. Acknowledgments We are most grateful to all members of the University College Hospital and the Royal National Institute for the Deaf
515
P. D. Phelps, J. A. D. Annis and P. J. Robinson Cochlear Implant Programme and the Sir Jules Thorne Trust, and to Mr Roger Gray, FRCS, of the Cambridge Cochlear Implant Programme for their help and permission to publish details of their patients.
References GANTZ, B. J., MCCABE, B. F. & TYLER, R. S., 1988. Use of
multichannel cochlear implants in obstructed and obliterated cochleas. Otolaryngology Head and Neck Surgery, 98, 72-81. HARNSBERGER, H. R., DART, D. J., PARKIN, J. L., SMOKER, W.
R. K. & OSBORN, A. G., 1987. Cochlear implant candidates: Assessment with CT and MR imaging. Radiology, 164, 53-57.
MUELLER, D. P., DOLAN, K. D. & GANTZ, B. J.,
1989.
Temporal bone computed tomography in the preoperative evaluation for cochlear implantation. Annals of Otology, Rhinology & Laryngology, 98, 346-349. PHELPS, P. D., 1986. Congenital cerebrospinal fluid fistulae of the petrous temporal bone. Clinical Otolaryngology, 11, 79-92. SHELTON, C , LUXFORD, W. M., TONOKAWA, L. L. & Lo, W. W.
M., 1989. The narrow internal auditory canal in children: A contraindication to cochlear implants. Otolaryngology Head and Neck Surgery, 100, 227-231. SlLVERSTEIN,
H.,
SMOUHA,
E.
&
MORGAN,
N . , 1988.
Multichannel cochlear implantation in a patient with bilateral Mondini deformities. The American Journal of Otology, 9, 451-454.
JACKLER, R. K., LUXFORD, W. M., SCHINDLER, R. A. &
SLACK, R. W. T. & PHELPS, P. D., 1985. Familial mixed
MCKERROW, W. S., 1987. Cochlear patency problems in cochlear implantation. Laryngoscope, 97, 801-805. MONDINI, C , 1791. Anatomica surdi nati sectio, Bononiensi Scientarium et artium instituto atque academia commentarii. Bonaniae, VII, 419-428.
deafness with branchial arch defects (earpits deafness syndrome). Clinical Otolaryngology, 10, 271-277. VALVASSORI, G. E., 1987. Workshop: Surgical anatomy and radiographic imaging of cochlear implant surgery. American Journal of Otology, 8, 195-200.
516
The British Journal of Radiology, July 1990
