Diagnostic imaging in clinical neuro-otology

Retrosigmoid Craniotomy for Auditory Brainstem Implantation in Adult Patients with Neurofibromatosis Type 2

Objective To report our technique and experience using a retrosigmoid craniotomy approach for auditory brainstem implantation (ABI) placement in adult neurofibromatosis type 2 (NF2) patients. Design Retrospective case series. Setting Single-center study, Boston, Massachusetts, United States. Participants All NF2 patients who underwent evaluation at Massachusetts Eye and Ear Infirmary and surgery at Massachusetts General Hospital from 2009 to 2013 were reviewed. Six cases of retrosigmoid craniotomy for ABI surgery in five adult NF2 patients were identified. The clinical history, operative course, and outcomes in these patients were reviewed. Main Outcome Measures Postoperative complications and audiological outcomes. Results Indications for ABI surgery were profound hearing loss associated with growth or treatment of bilateral vestibular schwannomas. In all cases, a retrosigmoid craniotomy was performed for tumor resection and ABI placement without complication. Electrode placement was confirmed intraoperatively using electrical-evoked auditory brainstem responses. The ABI was activated in the awake patient 4 to 6 weeks postoperatively. Audiological testing was used to evaluate sound detection and speech perception with the ABI. There were no cases of cerebrospinal fluid leak. Conclusion Retrosigmoid craniotomy is a safe and effective means to provide access to the cochlear nucleus for ABI placement following tumor resection in the adult NF2 patient. Preliminary data indicate that this approach has few complications while offering benefits for hearing. The retrosigmoid craniotomy should be considered a reasonable alternative to the traditional translabyrinthine approach for placement of the ABI in deaf patients who are not candidates for the cochlear implant.

Visualization of human inner ear anatomy with high-resolution MR imaging at 7T: initial clinical assessment

BACKGROUND AND PURPOSE

In many centers, MR imaging of the inner ear and auditory pathway performed on 1.5T or 3T systems is part of the preoperative work-up of cochlear implants. We investigated the applicability of clinical inner ear MR imaging at 7T and compared the visibility of inner ear structures and nerves within the internal auditory canal with images acquired at 3T.

MATERIALS AND METHODS

Thirteen patients with sensorineural hearing loss eligible for cochlear implantation underwent examinations on 3T and 7T scanners. Two experienced head and neck radiologists evaluated the 52 inner ear datasets. Twenty-four anatomic structures of the inner ear and 1 overall score for image quality were assessed by using a 4-point grading scale for the degree of visibility.

RESULTS

The visibility of 11 of the 24 anatomic structures was rated higher on the 7T images. There was no significant difference in the visibility of 13 anatomic structures and the overall quality rating. A higher incidence of artifacts was observed in the 7T images.

CONCLUSIONS

The gain in SNR at 7T yielded a more detailed visualization of many anatomic structures, especially delicate ones, despite the challenges accompanying MR imaging at a high magnetic field.

In vivo imaging of middle-ear and inner-ear microstructures of a mouse guided by SD-OCT combined with a surgical microscope

We developed an augmented-reality system that combines optical coherence tomography (OCT) with a surgical microscope. By sharing the common optical path in the microscope and OCT, we could simultaneously acquire OCT and microscope views. The system was tested to identify the middle-ear and inner-ear microstructures of a mouse. Considering the probability of clinical application including otorhinolaryngology, diseases such as middle-ear effusion were visualized using in vivo mouse and OCT images simultaneously acquired through the eyepiece of the surgical microscope during surgical manipulation using the proposed system. This system is expected to realize a new practical area of OCT application.

Low-dose temporal bone CT in infants and young children: effective dose and image quality

BACKGROUND AND PURPOSE

The temporal bone is ideal for low-dose CT because of its intrinsic high contrast. The aim of this study was to retrospectively evaluate image quality and radiation doses of a new low-dose versus a standard high-dose pediatric temporal bone CT protocol and to review dosimetric data from the literature.

MATERIALS AND METHODS

Image quality and radiation doses were compared for 38 low-dose (80 kV/90-110 mAs) and 16 high-dose (140 kV/170 mAs) temporal bone CT scans of infants to 5-year-old children. The CT visualization quality of 23 middle and inner ear structures was subjectively graded by 3 neuroradiologists and 3 otologists by using a 5-point scale with scores 1-2 indicating insufficient and scores 3-5 indicating sufficient image quality. Effective doses of local and literature-derived protocols were calculated from dosimetric data by using NRPB-SR250 software.

RESULTS

Insufficient image-quality scores were more frequent in low-dose scans versus high-dose scans, but the difference was only statistically significant for otologists (6.0% versus 3.4%, P = .004) and not for neuroradiologists (1.2% versus 0.7%, P = .84). Image quality was critical for small structures (such as the stapes or lamella at the internal auditory canal fundus). Effective doses were 0.25-0.3 mSv for low-dose scans, 1.4-1.8 mSv for high-dose scans, and 0.9-2.6 mSv for literature-derived protocols.

CONCLUSIONS

The image quality of the new low-dose protocol remains diagnostic for assessing middle and inner ear anatomy despite a 3- to 8-fold dose reduction over previous and literature-derived protocols. However, image quality of small structures is critical and may be perceived as insufficient.

Volumetric in vivo imaging of intracochlear microstructures in mice by high-speed spectral domain optical coherence tomography

There is considerable interest in developing new methods for in vivo imaging of the complex anatomy of the mammalian cochlea for clinical as well as fundamental studies. In this study, we explored, the feasibility of spectral domain optical coherence tomography (SD-OCT) for 3-D in vivo imaging of the cochlea in mice. The SD-OCT system employed in this study used a broadband light source centered at 1300 nm, and the imaging speed of the system was 47,000 A-scans per second using the InGaAs camera. The system was capable of providing fully processed, high-resolution B-scan images [512 (axial) x 128 (lateral) pixels] at 280 frames per sec. The 3-D imaging acquisition time for a whole cochlea was approximately 0.45 sec. The traditional SD-OCT structural imaging algorithm was used to reconstruct 3-D cochlear morphology. We demonstrated that SD-OCT can be successfully used for in vivo imaging of important morphological features within the mouse cochlea, such as the otic capsule and structures within, including Reissner's membrane, the basilar membrane, tectorial membrane, organ of Corti, and modiolus of the apical and middle turns.

Congenital inner ear malformations without sensorineural hearing loss in children

Inner ear malformations are frequently found in patients with congenital hearing loss. It has been reported that normal hearing is rare in patients with severe inner ear vestibular malformations. A 9-year-old boy had had complained of recurrent dizziness and disequilibrium for 2 months. Clinical and neuro-otological examinations showed peripheral involvement of the vestibular system, while audiological investigation was normal. High-resolution magnetic resonance imaging, with three-dimensional reconstruction, showed dysplasia of the bilateral lateral semicircular canals (LSCCs). Isolated vestibular malformation might not be as rare as previously thought, and should be examined by imaging of the temporal bone.

Progress in restoration of hearing with the auditory brainstem implant

Fifty years ago auditory scientists were very skeptical about the potential of new prosthetic approaches that electrically stimulated the auditory nerve, the cochlear nuclei (CN), and the inferior colliculus (IC). In those decades, the basilar membrane was considered to play a fundamental and irreplaceable role as a fine spectrum analyzer in hearing physiology, and therefore it was thought that electrical stimulation of the auditory system would have never produced functionally useful hearing. Over the last 30 years, cochlear implants (CIs) have improved steadily to the point where the average sentence recognition with modern multichannel devices is better than 90% correct. More recently, similar performance has been observed with electric stimulation of the brainstem with auditory brainstem implants (ABIs). However, it is clear that to fully understand hearing and to design the next generation of prosthetic devices we must better understand the ear-brain relationship. Indeed some aspects of hearing do not require the intricate complexities of cochlear physiological responses, while other auditory tasks rely critically on specialized details of cochlear processing. The progress in electrical stimulation of the central auditory system requires us to reconsider the patient selection criteria for different implant devices, in particular to evaluate the possibility of ABIs for etiologies with poor outcomes with CIs. In the present review, the latest outcomes in restoration of hearing with ABI are presented. New guidelines are proposed for device selection for different etiologies and future research is suggested to further refine the process of matching an individual patient to the most appropriate implant device.

Fallgruben in der Magnetresonanztomographie

The constantly extending indication spectrum of magnetic resonance imaging (MRI) is a challenge for the anaesthesiologist, who is being increasingly more consulted for assistance during the examination. Due to the special technology of MRI the anaesthetic technique differs substantially from that in the operating theatre. In addition to the permanent strong magnetic field the intermittently used high frequency impulses are also a potential danger for the patient. Patients with metal implants (e.g. cardiac pacemaker) are particularly at risk. For the safe treatment of patients during MRI a special MRI compatible anaesthesia equipment is necessary. Unsuitable devices can lead to malfunctioning or to projectile effects (attracting ferromagnetic objects into the magnet) causing injury to the patients. This paper describes the MRI technology and the associated dangers for the patient as well as the characteristics of the anaesthetic techniques.

Cochlear nerve size evaluation in children with sensorineural hearing loss by high-resolution magnetic resonance imaging

PURPOSE

To determine differences in size of cochlear nerves among subjects with deafness due to connexin 26 (Cx26) mutations, subjects with deafness of unknown origin, and normal hearing subjects by sagittal high-resolution magnetic resonance (HRMR) imaging of the temporal bone.

MATERIALS AND METHODS

Cross-sectional and surface areas and volumetric measurements of the cochlear nerve and modiolus were made on HRMR images of the internal auditory canal (IAC) and inner ear in the 3 groups of children (groups 1, 2, and 3). Three-way comparisons of in vivo cochlear nerve measurements on HRMR imaging were made among 17 children with sensorineural hearing loss (SNHL) and no obvious etiology for the hearing loss (group 1), 7 children with profound SNHL due to a Cx26 mutation (group 2), and 10 normal hearing children (group 3).

RESULTS

Children with profound SNHL of unknown cause and children with profound SNHL due to a connexin mutation displayed hypoplastic cochlear nerves as compared with normal controls. HRMR imaging of the temporal bone was accurately delineated potential problems with cochlear nerves in 2 of 17 instances where high-resolution computed tomography did not do so.

CONCLUSIONS

Accurate and specific measurements of the cochlear nerve and related structures is possible on HRMR imaging of the temporal bone. The size of the cochlear nerve is mildly hypoplastic in children with profound SNHL of unknown causes or children with a deafness-causing Cx26 mutation. HRMR imaging is superior to high-resolution computed tomography in the investigation of profound SNHL in children.

T2-weighted turbo spin-echo images, maximum-intensity projections, and three-dimensional volume-rendering for delineation of pathologies and anatomic details of the inner ear

PURPOSE

The aim of the current study was to compare 3-dimensional T2-weighted turbo spin-echo (TSE) axial slices, maximum-intensity projections (MIPs), and volume-rendered images with respect to the delineation of pathologic findings and anatomic structures of the inner ear. Each of the compared image types was available at 2 different resolutions.

MATERIALS AND METHODS

In 64 patients, 3-dimensional T2-TSE images were acquired with a matrix of 128 x 100 and 230 x 230 (field-of-view 90 x 90 mm). Direct axial images, MIPs, and volume-rendered images from the 2 datasets were evaluated in terms of the delineation of pathologic findings and anatomic structures. Analysis was performed by 2 radiologists in consensus.

RESULTS

Though axial images acquired with a matrix of 230 x 230 showed more anatomic details than images acquired with a matrix of 128 x 100, pathologic findings (33 in total) were assessable with the same confidence on images of either resolution. Pathologies completely surrounded by fluid were best assessed on axial slices. The delineation of pathologies not embedded in a fluid was almost equally good in axial slices, MIPs, or 3-dimensional volume-rendered images.

CONCLUSION

Reading the axial images cannot be replaced by viewing postprocessed reconstructions for initial diagnosis, because only some of the pathologies can be delineated on MIPs and volume reconstructions. However, reconstructions can be valuable for assessing the extent of pathologies and their spatial relation to the anatomic structures.

General vestibular testing

A dysfunction of the vestibular system is commonly characterized by a combination of phenomena involving perceptual, ocular motor, postural, and autonomic manifestations: vertigo/dizziness, nystagmus, ataxia, and nausea. These 4 manifestations correlate with different aspects of vestibular function and emanate from different sites within the central nervous system. The diagnosis of vestibular syndromes always requires interdisciplinary thinking. A detailed history allows early differentiation into 9 categories that serve as a practical guide for differential diagnosis: (1) dizziness and lightheadedness; (2) single or recurrent attacks of vertigo; (3) sustained vertigo; (4) positional/positioning vertigo; (5) oscillopsia; (6) vertigo associated with auditory dysfunction; (7) vertigo associated with brainstem or cerebellar symptoms; (8) vertigo associated with headache; and (9) dizziness or to-and-fro vertigo with postural imbalance. A careful and systematic neuro-ophthalmological and neuro-otological examination is also mandatory, especially to differentiate between central and peripheral vestibular disorders. Important signs are nystagmus, ocular tilt reaction, other central or peripheral ocular motor dysfunctions, or a unilateral or bilateral peripheral vestibular deficit. This deficit can be easily detected by the head-impulse test, the most relevant bedside test for the vestibulo-ocular reflex. Laboratory examinations are used to measure eye movements, to test semicircular canal, otolith, and spatial perceptional function and to determine postural control. It must, however, be kept in mind that all signs and ocular motor and vestibular findings have to be interpreted within the context of the patient's history and a complete neurological examination.

Cochlear Implant Candidacy and Surgical Considerations

Numerous changes continue to occur in regard to cochlear implant candidacy. In general, these have been accompanied by concomitant and satisfactory changes in surgical techniques. Together, this has advanced the utility and safety of cochlear implantation. Most devices are now approved for use in patients with severe to profound rather the prior requirement of a bilateral profound loss. In addition, studies have begun utilizing short electrode arrays for shallow insertion in patients with considerable low frequency residual hearing. This technique will allow the recipient to continue to use acoustically amplified hearing for the low frequencies simultaneously with a cochlear implant for the high frequencies. New hardware, such as the behind-the-ear speech processors, require modification of existing implant surgery. Similarly, the new perimodiolar electrodes require special insertion techniques. Bilateral implantation clearly requires modification of the surgical techniques used for unilateral implantation. The surgery remains mostly the same, but takes almost twice as long, and requires some modification since at a certain point, when the first device is in contact with the body, the monopolar cautery may no longer be used. Research has already begun on the development of the totally implantable cochlear implant (TICI). This will clearly require a modification of the surgical technique currently used for the present semi-implantable devices. In addition to surgically burying the components of the present cochlear implant, we will also have to develop techniques for implanting a rechargeable power supply and a microphone for the TICI. The latter will be a challenge, since it must be placed where it is capable of great sensitivity, yet not exposed to interference or the risk of extrusion. The advances in design of, and indications for, cochlear implants have been matched by improvements in surgical techniques and decrease in complications. The resulting improvements in safety and efficacy have further encouraged the use of these devices. We anticipate further changes in the foreseeable future, for which there will likely be surgical problems to solve.