- Email: [email protected]
Establishment of normative cochlear and vestibular measurements to aid in the diagnosis of inner ear malformations
Establishment of normative cochlear and vestibular measurements to aid in the diagnosis of inner ear malformations DERK PURCELL,
BS,
JACOB JOHNSON,
MD,
NANCY FISCHBEIN,
OBJECTIVE: We sought to establish normative measurements of the inner ear using computed tomography (CT) of the temporal bone to aid in the diagnosis of inner ear malformations. STUDY DESIGN AND SETTING: Prospective measurements of the inner ear structures were made on axial and coronal temporal bone CT scans on 15 patients with normal hearing and 15 patients with sensorineural hearing loss. RESULTS: The vertical height of the cochlea on coronal scan and the size of the central bony island within the lateral semicircular canal on axial scan along with visual inspection identified 7 inner ear abnormalities in 6 patients: 5 cases of lateral semicircular canal dysplasia and 2 cases of cochlear hypoplasia. In contrast, visual inspection alone identified only 4 of the 7 abnormalities. CONCLUSIONS AND SIGNIFICANCE: Routine measurement of the cochlear height and bony island of the lateral semicircular canal, in conjunction with visual inspection of CT images, will increase recognition of common inner ear malformations. (Otolaryngol Head Neck Surg 2003;128:78-87.)
I n the evaluation of childhood sensorineural hearing loss (SNHL), a thorough physical and radiologic evaluation is undertaken by the otolaryngologist. A temporal bone computed tomography (CT) scan provides structural analysis of the external and middle ear, as well as bony labyrinth of From the Departments of Otolaryngology–Head and Neck Surgery (Mr Purcell and Drs Johnson and Lalwani) and Radiology (Dr Fischbein), University of California San Francisco. Presented at the Annual Meeting of the American Academy of Otolaryngology–Head and Neck Surgery, San Diego, CA, September 22-25, 2002. Reprint requests: Anil K. Lalwani, MD, Department of Otolaryngology–Head and Neck Surgery, University of California San Francisco, 400 Parnassus Ave, Room A730, San Francisco, CA 94143-0342; e-mail, [email protected]. Copyright © 2003 by the American Academy of Otolaryngology–Head and Neck Surgery Foundation, Inc. 0194-5998/2003/$30.00 ⫹ 0 doi:10.1067/mhn.2003.51 78
MD,
and ANIL K. LALWANI,
MD,
San Francisco, California
the inner ear. Approximately 25% of patients with congenital hearing loss will have bony inner ear malformations on CT of the temporal bone.1 Currently, visual inspection of the radiologic images alone is used to identify malformations of the intricate structures of the inner ear.2,3 Severe malformations such as complete labyrinthine aplasia (Michel deformity), cochlear aplasia, and common cavity deformity due to early arrest of inner ear development (before the fifth gestational week) are easy to diagnose via visual inspection but represent only 1% of radiographic abnormalities found in patients with SNHL.4 While sufficient for detecting severe inner ear malformations, visual inspection is often inadequate in the diagnosis of subtle abnormalities such as lateral semicircular canal (LSCC) dysplasia and cochlear hypoplasia as this approach is greatly dependent on the experience of the examiner. Nearly one third of these less severe dysplasias are missed by simple visual inspection of the radiologic images.5 The limitations of visual inspection and the reliance on clinical experience have been partially overcome by the development of normative radiographic measurements that have proved to be invaluable in the evaluation of disease processes in many parts of the body, including those occurring in the head and neck. For example, measurement of the prevertebral soft tissue thickness on lateral radiographs allows for the objective evaluation of prevertebral pathology (eg, abscess, hematoma).6,7 Measurement of the laryngeal anterior commissure helps assess the presence and spread of laryngeal cancer across the midline.8 In the temporal bone, measurement of the vestibular aqueduct is crucial in establishing the diagnosis of large vestibular aqueduct syndrome, especially in less florid cases.9 Surprisingly, similar measurements are not available for other inner ear structures, although CT of the temporal bone has been available for over 20 years. Unlike most anatomic structures, the inner ear labyrinth does not change in size
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after birth. Therefore the establishment of normative data for the dimensions of the inner ear structures using CT imaging will allow for consistent and accurate assessment for patients of all ages. The recent development of PACS (picture archiving and communications systems) greatly facilitates the precise digital measurements of the auditory and vestibular labyrinth that can be standardized across institutions at multiple locations. As in other areas of medicine and otolaryngology, the development of standardized measurements, to complement visual inspection, should improve the diagnostic accuracy and interpretation of radiologic studies. The objective of this study was to determine the normal dimensions of the inner ear structures using CT scans of the temporal bone of normal hearing adults and to develop a systematic approach to the diagnosis of inner ear malformations by applying these standard measurements to patients with SNHL. MATERIALS AND METHODS Patients Fifteen patients with otologic disease and normal hearing and 15 patients with SNHL underwent temporal bone CT scans as part of their routine radiologic evaluation. The CT scans were performed between June 2000 and December 2001. This study was reviewed and approved by the University of California San Francisco Institutional Review Board. CT Scan of the Termporal Bone All studies were performed at our institution using a standard temporal bone protocol. Contiguous 1-mm scans of the temporal bone were acquired in the axial and coronal planes with a GE (GE Medical Systems, Waukesha, WI) CT/i single-slice helical system. Images were acquired with a field of view (FOV) of 22 cm and with a high-resolution bone algorithm. Images were then retargeted to a 10-cm FOV for display in the axial and coronal planes. Studies were reviewed on Agfa (Agfa Corp, Ridgefield Park, NJ) PACS terminals in the neuroradiology reading room. These workstations have magnification and electronic caliper capabilities, allowing precise and reproducible measurements to be obtained.
PURCELL et al 79
Inner Ear Measurements Measurements were taken using the electronic calipers and recorded in units of millimeters. After review of embryology of the inner ear and discussion with neuroradiologists and neurotologists, 25 different inner ear dimensions were designated as being reproducibly obtainable and potentially important: 16 measurements in the axial plane (Fig 1) and 9 measurements in the coronal plane (Fig 2) were obtained of each inner ear (cochlea, vestibule, internal auditory canal [IAC], and SCCs). All measurements were entered into an Excel datasheet, and averages and standard deviations were calculated (Table 1). A measurement was considered normal if it was within 2 SDs of the average measurement. RESULTS Patient Population Thirty patients (60 ears) were included in this study. The study population included an equal number of patients presenting with and without hearing loss. Fifteen patients without hearing loss underwent CT scanning for various reasons: cholesteatoma (6 patients), chronic otitis media (3), paraganglionic tumor (2), acquired external auditory canal stenosis (2), external otitis (1), and nasopharyngeal carcinoma (1). Inner Ear Measurements: Normal Hearing Data from the 15 patients without hearing loss were used to generate normative data with average values and standard deviations for each of the measurements (Table 1). The calculated standard deviations were ⱕ10% for most measurements, with the exception of the IAC measurements. The IAC proved to be too variable in its morphology and course to allow consistent measurements (Fig 3), and this was reflected in the standard deviation of the IAC measurement. Inner Ear Measurements: SNHL Measurements from the 15 patients with SNHL were compared to the normative data generated from the patients without SNHL. In 2 studies from the SNHL group, some measurements were not able to be collected due to severe malformation (axial vestibule measurements in one case of se-
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Fig 1. Axial inner ear measurements. (A-F) Inner ear measurements made on axial temporal bone CT scan. (A) SSCC bony width. (B) Diameter of SSCC ampulla. (C) LSCC bony island width (black arrow), vestibule width, vestibule height (large white arrows), IAC length, and IAC width (small white arrows). (D) PSCC inferior limb length. (E) Apical turns of cochlea width and height. (F) Basal turn of cochlea length and lumen diameter.
vere LSCC dysplasia and axial LSCC measurements in a second case of severe LSCC dysplasia). Of the 15 patients with SNHL, inner ear malformations were identified in 6 patients based on visual inspection combined with abnormal mea-
surements ([mt]2 SDs). These identified 6 cases of LSCC dysplasia, 4 cases of cochlear hypoplasia, and 2 cases of posterior SCC (PSCC) and superior SCC (SSCC) dysplasia. Of the 6 studies with abnormal measurements suggesting inner ear mal-
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LSCC dysplasia and 1 case of SSCC dysplasia). However, as seen later, not all of the inner ear abnormalities were identified by visual inspection in any given patient. Further, in 2 studies (patients 5 and 6); the inner ears were described as normal in the transcribed reports.
Fig 2. Coronal inner ear measurements. (A-C) Inner ear measurements made on coronal temporal bone CT scan. (A) SSCC bony height (black arrow), vestibule width, vestibule height (small white arrows), IAC length, and IAC opening width (large white arrows). (B) LSCC bony width. (C) Cochlear height.
formation, 4 patients had been identified as having inner ear malformations in the official transcription of the neuroradiologist’s report (4 cases of
Case Reports Patient 1. An 11-year-old boy with congenital hearing loss was diagnosed on visual inspection by a neuroradiologist with congenital abnormal inner ears bilaterally with dysplastic LSCCs, vestibules, and IACs (Fig 4A). The measurements corroborated this impression (Table 2). The “bony island” measurement alone was able to identify LSCC dysplasia, measuring only 1.6 mm on the right side and 2.5 mm on the left (normal, 3.67 ⫾ 0.35 mm). Measurements also demonstrated dilation of the LSCC lumens bilaterally (Table 2). The vestibules were not amenable to measurement given their severe malformation. Patient 2. A 5-year-old girl with congenital hearing loss was diagnosed on visual inspection with dysplastic inner ear structures bilaterally (Fig 4B). On measurement, the patient had severely hypoplastic LSCCs and SSCCs. LSSC bony island measurements on axial section were 2.1 mm and 1.6 mm on the right and left, respectively (normal, 3.67 ⫾ 0.35 mm). SSSC bony island measurements on axial section were 3.1 and 2.9 mm on the right and left, respectively (normal, 4.89 ⫾ 0.45 mm). The basal turns of the cochleae were also shorter than control, measuring 7.6 mm and 7.3 mm on the right and left, respectively (normal, 8.59 ⫾ 0.41 mm). Patient 3. A 1-year-old boy with congenital SNHL was diagnosed on visual inspection as having grossly dysplastic inner ear structures bilaterally, sparing the cochleae (Fig 4C). Axial LSCC measurements were not possible as the canal lumen appeared to taper off in the lateral segment. The basal turn length of the right cochlea was also shorter than control, measuring only 7.5 mm (normal, 8.59 ⫾ 0.41 mm). Patient 4. A 40-year-old man with SNHL underwent scanning to rule out a left-sided cholesteatoma. On initial visual inspection, bilateral patulous vestibules and bilateral LSCC dysplasia (left [mt] right) were noted (Fig 4d). The vestibule
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Table 1. Description of measuring technique, average values, and standard deviations for patients without sensorineural hearing loss Structure
Axial SSCC
PSCC
LSCC
Cochlea
IAC Vestibule Coronal SSCC LSCC Cochlea IAC Vestibule
Average (mm)
SD (mm)
Canal lumen
1.25
0.11
Bony island width
4.89
0.45
Ampulla lumen
2.26
0.26
Canal lumen
1.27
0.10
Inferior limb length
7.52
0.53
Lateral canal lumen Posterior canal lumen Bony island width Basal turn lumen
1.29 1.29 3.67 2.15
0.10 0.11 0.35 0.18
Basal turn length
8.59
0.41
Upper turn height
3.96
0.38
Upper turn width
6.24
0.40
6.93 11.10 3.40 5.83
1.42 1.72 0.28 0.58
4.94 1.20 1.22 3.45 5.31
0.56 0.10 0.11 0.42 0.52
4.52 11.86 3.13
1.00 1.39 0.25
4.91
0.37
Measurement
Opening width Length Width Length Bony island width Canal lumen Canal lumen Bony island width Height Opening width Length Width Length
Description
Canal lumen and bony island measured at maximum diameter of the turn, most often two cuts down from the first view of the SSCC. Ampulla identified at cuts between canal lumen and vestibule, most often one cut down from above measurement. Canal lumen measured one/two cuts up from inferior limb. Inferior limb identified by tracing the canal inferiorly and measuring longest distance of confluence. LSCC easily identified and all measurements made on same cut. Bony island measured medial to lateral on same line as lateral limb. Basal turn easily identified; lumen and length most often measured in same cut. Upper turn height measured from apex down to imaginary line separating basal and upper turns (base of modiolus to apex). Upper turns measured after identifying cut with largest length measurement. Measurements taken on longest cut. Vestibule identified and both measurements taken on same cut, most often on same cut as LSCC. Both measurements taken at greatest diameter of turn. Both measurements taken at greatest diameter of turn Easily identified cochlea, measurement taken on cut with maximum height that included basal and upper turn. Measurements taken on longest cut. Cut displaying largest area of vestibule identified and measurements taken (inconsistent relationship to other measurement levels).
IAC, Internal auditory canal; LSCC, lateral semicircular canal; PSCC, posterior semicircular canal; SSCC, superior semicircular canal.
measurements were within normal limits bilaterally. Measurement of the LSCC demonstrated definite dysplasia, with bony island measurements of 2.3 and 1.7 mm on the right and left, respectively (normal, 3.67 ⫾ 0.35 mm). Of note, the PSCC was also hypoplastic on measurement but was not identified as such by visual inspection alone. PSCC inferior limb length measurements were 5.9 and 5.5mm on the right and left, respectively (normal, 7.52 ⫾ 0.53 mm). Patient 5. A 77-year-old woman with familial late-onset SNHL, being evaluated for cochlear implant placement, was diagnosed on initial visual
inspection by the neuroradiologist as having normal inner ear structures bilaterally (Fig 5). However, review of the printed scans in the otolaryngology clinic before interview of the patient suggested mild cochlear hypoplasia and LSCC dysplasia. Measurements demonstrated definite cochlear hypoplasia bilaterally, with coronal height measurements of only 3.6 and 3.3 mm on the right and left, respectively (normal, 5.31 ⫾ 0.52 mm). In addition, significant LSCC dysplasia was identified, with bony island measurements of only 2.3 and 2.5 mm on the right and left, respectively (normal, 3.67 ⫾ 0.35 mm). Of note, the
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Fig 3. Variability of the IAC. Axial CT scan images demonstrating the variable morphology of the IAC from 4 different patients. The IAC proved to be too variable in its morphology and course to allow consistent measurements.
axial sections, which enable the measurement of the basal and apical turns separately, demonstrated normal basal turns and small mid apical turns. Upper turn height measurements were only 3.0 and 2.9 mm on the right and left, respectively (normal, 3.96 ⫾ 0.38 mm). These findings are consistent with Mondini-type deformity, representing arrest of cochlear development before two-and-three-quarters turns are achieved. In this case, the inner ear measurements supplemented visual inspection of the CT images and led to the confirmation of clinician’s suspicion of cochlear hypoplasia and LSCC dysplasia. Patient 6. A 42-year-old woman with SNHL, being evaluated for cochlear implant placement, was diagnosed on initial visual inspection as having normal inner ear structures bilaterally. However, measurement demonstrated right-sided cochlear hypoplasia, with a coronal height
measurement of only 4.2 mm (normal, 5.31 ⫾ 0.52 mm). In addition, measurement of the cochleae on axial section demonstrated narrow basal turn lumens bilaterally: 1.4 mm on the right and 1.5 mm on the left (normal, 2.15 ⫾ 0.18 mm). Measurement also demonstrated dilation of the LSCC, SSCC, and PSCC lumens bilaterally (Table 2). In this case, the inner ear measurements alone identified cochlear hypoplasia. DISCUSSION Subtle abnormalities of the inner ear such as cochlear hypoplasia and SCC dysplasia, which account for the majority of inner ear malformations associated with SNHL, are often missed on CT of the temporal bone due to inexperience of the clinician and absence of normative data to aid in diagnosis. In this study, normative data for 25 different standardized measurements of the inner
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Fig 4. LSCC dysplasia. Axial CT scans demonstrating 4 different cases of LSCC dysplasia. Each of these dsyplasias was identified by visual inspection alone, as well as by abnormal central bony island width measurement. Case histories of patients 1 through ⫺4 are presented in the text.
ear were collected to develop strategies for recognition and confirmation of inner ear malformations using CT scan of the temporal bone. The average dimensions of the inner ear, with the exception of the IAC, were similar across individuals, with the standard deviation less than 10% of the average value; this suggests that there is little variability in the size of the human inner ear. However, the anatomy of the IAC was found to be too variable and difficult to reproduce from scan to scan to be of clinical use. The normative data were subsequently used to evaluate the temporal bone CT in 15 patients with SNHL. Inner ear malformations were identified in 6 (40%) of 15 patients based on the presence of visually obvious malformation or an inner ear measurement value greater than2 SDs from the mean. Visual inspection identified 4 (66%) of 6 patients with malformations but missed two cases
with more subtle malformations of the cochlea and LSCC. In patient 5, the cochlear and LSCC measurements were clearly over 2 SDs from the normal mean value, and in retrospective analysis by the neuroradiologist, the structures were considered abnormal in this patient with adult-onset, familial SNHL. In patient 6, measurement alone identified a previously unrecognized case of cochlear hypoplasia. Therefore, in two of six patients, measurement of the inner ear structures aided in the identification of an abnormality initially missed by visual inspection. In another study, the patient (patient 4) was correctly identified as having LSCC dysplasia; however, the accompanying dysplasia of the PSCC and SSCC was only discernible after measurements. The measurements were also valuable in defining the specific portion of the cochlea that was hypoplastic: basal turn in patients 2 and 6 and the apical turns
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Table 2. Measurements for patients 1 through 6 SSCC
PSCC
LSCC
Cochlea
Vestibule
Inferior Lateral Post Bony Basal Basal Upper Upper Canal Ampulla Bony Canal limb canal canal island turn turn turn turn lumen lumen island lumen length lumen lumen width lumen length height length Width Length
Axial Average 1.25 (mm) SD 0.11 Patient 1 1.5 1.5 2 1.9 1.3 3 1.3 1.2 4 1.2 1.2 5 1.3 1.1 6 1.5 1.5
2.26
4.89
1.27
7.52
1.29
1.29
3.67
2.15
8.59
3.96
6.24
3.40
5.83
0.26
0.45
0.11
0.53
0.10
0.11
0.35
0.18
0.41
0.38
0.40
0.28
0.58
2.1 1.9 2.7 2.4 2.2 2.1 2.2 2.0 2.0 2.0 2.2 2.1
5.8 5.8 3.1 2.9 4.8 4.9 4.0 4.2 4.8 4.5 4.3 4.3
1.2 1.3 1.4 1.4 1.4 1.6 1.3 1.3 1.3 1.3 1.5 1.4
8.0 7.9 5.5 6.0 7.2 7.5 5.9 5.5 7.0 7.1 6.7 7.0
2.3 1.7 1.3 1.3 NC NC 1.2 1.5 1.3 1.3 1.5 1.5
2.4 1.9 1.3 1.3 1.5 1.5 1.2 1.5 1.5 1.3 1.5 1.5
1.6 2.5 2.1 1.6 NC NC 2.3 1.7 2.3 2.5 3.3 3.1
1.8 1.9 1.9 2.0 2.1 2.0 2.0 2.0 2.0 2.0 1.4 1.5
8.6 8.9 7.6 7.3 7.5 8.2 8.2 8.3 8.0 8.0 7.8 7.7
3.8 4.0 4.1 3.9 3.4 3.4 3.6 3.7 3.0 2.9 3.5 3.4
6.1 6.1 6.1 6.2 6.1 5.9 6.4 6.1 5.1 5.1 5.6 5.8
NC NC 3.1 2.7 3.1 3.2 3.4 2.7 3.6 3.4 4.1 3.5
NC NC 6.0 6.4 6.6 6.6 5.3 5.5 5.5 5.6 6.2 6.2
SSCC
Coronal Average (mm) SD Patient 1 2 3 4 5 6
LSCC
Vestibule
Lumen
Bony island
Lumen
Bony island
Cochlea height
1.20 0.10
4.94 0.56
1.22 0.12
3.45 0.42
5.31 0.52
3.13 0.25
4.91 0.37
1.3 1.3 1.2 1.3 1.2 1.0 1.3 1.3 1.3 1.3 1.1 1.3
5.0 4.9 3.7 3.7 4.9 5.2 4.1 4.1 4.4 4.3 4.4 4.4
2.5 1.9 1.3 1.2 .99 .99 1.3 1.2 1.3 1.1 1.5 1.5
1.4 2.2 1.7 1.8 3.1 3.1 1.7 1.5 1.6 1.7 3.1 2.9
5.9 5.8 4.6 4.8 4.7 4.5 5.1 5.2 3.6 3.3 4.2 4.5
3.7 3.4 2.6 2.8 2.9 2.7 2.9 3.3 3.0 3.2 3.0 3.2
5.4 5.4 4.5 4.2 5.2 5.2 4.7 5.2 4.3 4.2 4.8 4.4
Width
Length
NC, Measurement not able to be collected because of severe malformation. LSCC, Lateral semicircular canal; PSCC, posterior semicircular canal; SSCC, superior semicircular canal. Abnormal values are given in bold.
in patient 5. Therefore, the inner ear measurements not only aid in the diagnosis of inner ear malformation but also can provide additional information about which specific part is abnormal. In addition, the acquisition of routine inner ear measurements can aid in training the physician in the identification of inner ear malformations that would otherwise be missed because of inexperience or subtle nature of the abnormality.
Standardized measurements of inner ear structures to aid in the diagnosis of inner ear malformations are appealing; however, the routine measurements of 25 different dimensions, as in this study, would be too cumbersome. To determine which of the 25 measurements were the most useful, the embryology of inner ear was reviewed, the relative preponderance of the various known malformations was considered, and the ease and
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Fig 5. Identification of cochlear hypoplasia and LSCC dysplasia. The cochlear height on coronal CT (A, normal) was most sensitive for the detection of cochlear hypoplasia (B, patient 5). The measurement of central bony island on axial scan (C, normal) correctly identified LSCC dysplasia in all cases (D, patient 5). These 2 subtle abnormalities were not identified by visual inspection alone but became evident with the application of the standardized measurements.
reproducibility of different measurement were assessed. Our data suggest that the vertical height of the cochlea on coronal scan and the size of the central bony island within the LSCC on axial scan are the most useful measurements and are sufficient to diagnose common inner ear malformations. The most common inner ear malformations involve the LVA, LSCC, and cochlea. The major criterion for LVA, midpoint dimension of the vestibular aqueduct greater than 1.5 mm, has been previously defined.9 LSCC dysplasia is probably the second most common inner ear malformation. LSCC malformation is associated with CHL, mixed HL, and SNHL5 and most commonly occurs in isolation without associated malformation of the SSCC and PSCC. On the other hand, isolated dysplasia of the SSCC
or PSCC without abnormality of the LSCC is rare.4 Therefore, measurement of the LSCC (Fig 1c) would serve to identify the second most common inner ear malformation and as a screen for the other two SCCs; if the LSCC is malformed, then the SSCC and PSCC should be carefully evaluated. The mechanism of LSCC dysplasia involves the lack of formation of the central bony island, and that structure is easily seen and measured on the axial temporal bone CT scan. In our series, all patients with LSCC malformation (n ⫽ 6) had abnormal measurements of the bony island on the axial scan. Cochlear malformation, usually due to early developmental arrest leading to less than two and three-quarters turns, are also common. Premature arrest of cochlear development would be expected to decrease the base to apex height of
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the cochlea best assessed on the coronal CT scan (Fig 2c). The coronal height is easy to identify and measure and is easily reproducible. Two patients who initially were thought to have normal cochleae were correctly identified as having cochlear hypoplasia by the coronal height measurement. In summary, while visual inspection alone of temporal bone CT can reliably identify severe malformations of the inner ear, it can be insensitive in the detection of more subtle abnormalities. The sensitivity of detecting inner ear malformations can be greatly increased by routinely measuring the LSCC bony island on axial scan ([lt]2.97 mm or [mt]4.37 mm) and the cochlear height on coronal scan ([lt]4.27 mm or [mt]6.35 mm) to complement visual analysis. If the LSCC is malformed, then the SSCC and PSCC should be carefully evaluated. The accurate identification of inner malformations will have a direct impact on the diagnosis, prognosis, and management of patients with SNHL.
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REFERENCES
1. Mafong DD, Shin EJ, Lalwani AK. Use of laboratory evaluation and radiologic imaging in the diagnostic evaluation of children with sensorineural hearing loss. Laryngoscope 2002;112:1-7. 2. Davidson HC, Harnsberger HR, Lemmerling MM, et al. MR evaluation of vestibulocochlear anomalies associated with large enodlymphatic duct and sac. AJNR Am J Neuroradiol 1999;20:145-1. 3. Lemmerling M, Vanzieleghem B, Dhooge I, et al. CT and MRI of the semicircular canals in the normal and diseased temporal bone. Eur Radiol 2001;11:1210-9. 4. Jackler RK, Luxford WM, House WF. Congenital malformations of the inner ear: a classification based on embryogenesis. Laryngoscope 1987;97:2-14. 5. Johnson J, Lalwani AK. Sensorineural and conductive hearing loss associated with lateral semicircular canal malformation. Laryngoscope 2000;110:1673-9. 6. Haug RH, Wible RT, Lieberman J. Measurement standards for the prevertebral region in the lateral soft-tissue radiograph of the neck. J Oral Maxillofac Surg 1991;49: 1149-51. 7. Dai LY, Jia LS. Radiographic measurement of the prevertebral soft tissue of cervical vertebrae. Chin Med J (Engl) 1994;107:471-3. 8. Kallmes DF, Phillips CD. The normal anterior commissure of the glottis. AJR Am J Roentgenol 1997;168:1317-9. 9. Valvassori GE, Clemis JD. The large vestibular aqueduct syndrome. Laryngoscope 1978;88:723-8.
BS,
JACOB JOHNSON,
MD,
NANCY FISCHBEIN,
OBJECTIVE: We sought to establish normative measurements of the inner ear using computed tomography (CT) of the temporal bone to aid in the diagnosis of inner ear malformations. STUDY DESIGN AND SETTING: Prospective measurements of the inner ear structures were made on axial and coronal temporal bone CT scans on 15 patients with normal hearing and 15 patients with sensorineural hearing loss. RESULTS: The vertical height of the cochlea on coronal scan and the size of the central bony island within the lateral semicircular canal on axial scan along with visual inspection identified 7 inner ear abnormalities in 6 patients: 5 cases of lateral semicircular canal dysplasia and 2 cases of cochlear hypoplasia. In contrast, visual inspection alone identified only 4 of the 7 abnormalities. CONCLUSIONS AND SIGNIFICANCE: Routine measurement of the cochlear height and bony island of the lateral semicircular canal, in conjunction with visual inspection of CT images, will increase recognition of common inner ear malformations. (Otolaryngol Head Neck Surg 2003;128:78-87.)
I n the evaluation of childhood sensorineural hearing loss (SNHL), a thorough physical and radiologic evaluation is undertaken by the otolaryngologist. A temporal bone computed tomography (CT) scan provides structural analysis of the external and middle ear, as well as bony labyrinth of From the Departments of Otolaryngology–Head and Neck Surgery (Mr Purcell and Drs Johnson and Lalwani) and Radiology (Dr Fischbein), University of California San Francisco. Presented at the Annual Meeting of the American Academy of Otolaryngology–Head and Neck Surgery, San Diego, CA, September 22-25, 2002. Reprint requests: Anil K. Lalwani, MD, Department of Otolaryngology–Head and Neck Surgery, University of California San Francisco, 400 Parnassus Ave, Room A730, San Francisco, CA 94143-0342; e-mail, [email protected]. Copyright © 2003 by the American Academy of Otolaryngology–Head and Neck Surgery Foundation, Inc. 0194-5998/2003/$30.00 ⫹ 0 doi:10.1067/mhn.2003.51 78
MD,
and ANIL K. LALWANI,
MD,
San Francisco, California
the inner ear. Approximately 25% of patients with congenital hearing loss will have bony inner ear malformations on CT of the temporal bone.1 Currently, visual inspection of the radiologic images alone is used to identify malformations of the intricate structures of the inner ear.2,3 Severe malformations such as complete labyrinthine aplasia (Michel deformity), cochlear aplasia, and common cavity deformity due to early arrest of inner ear development (before the fifth gestational week) are easy to diagnose via visual inspection but represent only 1% of radiographic abnormalities found in patients with SNHL.4 While sufficient for detecting severe inner ear malformations, visual inspection is often inadequate in the diagnosis of subtle abnormalities such as lateral semicircular canal (LSCC) dysplasia and cochlear hypoplasia as this approach is greatly dependent on the experience of the examiner. Nearly one third of these less severe dysplasias are missed by simple visual inspection of the radiologic images.5 The limitations of visual inspection and the reliance on clinical experience have been partially overcome by the development of normative radiographic measurements that have proved to be invaluable in the evaluation of disease processes in many parts of the body, including those occurring in the head and neck. For example, measurement of the prevertebral soft tissue thickness on lateral radiographs allows for the objective evaluation of prevertebral pathology (eg, abscess, hematoma).6,7 Measurement of the laryngeal anterior commissure helps assess the presence and spread of laryngeal cancer across the midline.8 In the temporal bone, measurement of the vestibular aqueduct is crucial in establishing the diagnosis of large vestibular aqueduct syndrome, especially in less florid cases.9 Surprisingly, similar measurements are not available for other inner ear structures, although CT of the temporal bone has been available for over 20 years. Unlike most anatomic structures, the inner ear labyrinth does not change in size
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after birth. Therefore the establishment of normative data for the dimensions of the inner ear structures using CT imaging will allow for consistent and accurate assessment for patients of all ages. The recent development of PACS (picture archiving and communications systems) greatly facilitates the precise digital measurements of the auditory and vestibular labyrinth that can be standardized across institutions at multiple locations. As in other areas of medicine and otolaryngology, the development of standardized measurements, to complement visual inspection, should improve the diagnostic accuracy and interpretation of radiologic studies. The objective of this study was to determine the normal dimensions of the inner ear structures using CT scans of the temporal bone of normal hearing adults and to develop a systematic approach to the diagnosis of inner ear malformations by applying these standard measurements to patients with SNHL. MATERIALS AND METHODS Patients Fifteen patients with otologic disease and normal hearing and 15 patients with SNHL underwent temporal bone CT scans as part of their routine radiologic evaluation. The CT scans were performed between June 2000 and December 2001. This study was reviewed and approved by the University of California San Francisco Institutional Review Board. CT Scan of the Termporal Bone All studies were performed at our institution using a standard temporal bone protocol. Contiguous 1-mm scans of the temporal bone were acquired in the axial and coronal planes with a GE (GE Medical Systems, Waukesha, WI) CT/i single-slice helical system. Images were acquired with a field of view (FOV) of 22 cm and with a high-resolution bone algorithm. Images were then retargeted to a 10-cm FOV for display in the axial and coronal planes. Studies were reviewed on Agfa (Agfa Corp, Ridgefield Park, NJ) PACS terminals in the neuroradiology reading room. These workstations have magnification and electronic caliper capabilities, allowing precise and reproducible measurements to be obtained.
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Inner Ear Measurements Measurements were taken using the electronic calipers and recorded in units of millimeters. After review of embryology of the inner ear and discussion with neuroradiologists and neurotologists, 25 different inner ear dimensions were designated as being reproducibly obtainable and potentially important: 16 measurements in the axial plane (Fig 1) and 9 measurements in the coronal plane (Fig 2) were obtained of each inner ear (cochlea, vestibule, internal auditory canal [IAC], and SCCs). All measurements were entered into an Excel datasheet, and averages and standard deviations were calculated (Table 1). A measurement was considered normal if it was within 2 SDs of the average measurement. RESULTS Patient Population Thirty patients (60 ears) were included in this study. The study population included an equal number of patients presenting with and without hearing loss. Fifteen patients without hearing loss underwent CT scanning for various reasons: cholesteatoma (6 patients), chronic otitis media (3), paraganglionic tumor (2), acquired external auditory canal stenosis (2), external otitis (1), and nasopharyngeal carcinoma (1). Inner Ear Measurements: Normal Hearing Data from the 15 patients without hearing loss were used to generate normative data with average values and standard deviations for each of the measurements (Table 1). The calculated standard deviations were ⱕ10% for most measurements, with the exception of the IAC measurements. The IAC proved to be too variable in its morphology and course to allow consistent measurements (Fig 3), and this was reflected in the standard deviation of the IAC measurement. Inner Ear Measurements: SNHL Measurements from the 15 patients with SNHL were compared to the normative data generated from the patients without SNHL. In 2 studies from the SNHL group, some measurements were not able to be collected due to severe malformation (axial vestibule measurements in one case of se-
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Fig 1. Axial inner ear measurements. (A-F) Inner ear measurements made on axial temporal bone CT scan. (A) SSCC bony width. (B) Diameter of SSCC ampulla. (C) LSCC bony island width (black arrow), vestibule width, vestibule height (large white arrows), IAC length, and IAC width (small white arrows). (D) PSCC inferior limb length. (E) Apical turns of cochlea width and height. (F) Basal turn of cochlea length and lumen diameter.
vere LSCC dysplasia and axial LSCC measurements in a second case of severe LSCC dysplasia). Of the 15 patients with SNHL, inner ear malformations were identified in 6 patients based on visual inspection combined with abnormal mea-
surements ([mt]2 SDs). These identified 6 cases of LSCC dysplasia, 4 cases of cochlear hypoplasia, and 2 cases of posterior SCC (PSCC) and superior SCC (SSCC) dysplasia. Of the 6 studies with abnormal measurements suggesting inner ear mal-
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LSCC dysplasia and 1 case of SSCC dysplasia). However, as seen later, not all of the inner ear abnormalities were identified by visual inspection in any given patient. Further, in 2 studies (patients 5 and 6); the inner ears were described as normal in the transcribed reports.
Fig 2. Coronal inner ear measurements. (A-C) Inner ear measurements made on coronal temporal bone CT scan. (A) SSCC bony height (black arrow), vestibule width, vestibule height (small white arrows), IAC length, and IAC opening width (large white arrows). (B) LSCC bony width. (C) Cochlear height.
formation, 4 patients had been identified as having inner ear malformations in the official transcription of the neuroradiologist’s report (4 cases of
Case Reports Patient 1. An 11-year-old boy with congenital hearing loss was diagnosed on visual inspection by a neuroradiologist with congenital abnormal inner ears bilaterally with dysplastic LSCCs, vestibules, and IACs (Fig 4A). The measurements corroborated this impression (Table 2). The “bony island” measurement alone was able to identify LSCC dysplasia, measuring only 1.6 mm on the right side and 2.5 mm on the left (normal, 3.67 ⫾ 0.35 mm). Measurements also demonstrated dilation of the LSCC lumens bilaterally (Table 2). The vestibules were not amenable to measurement given their severe malformation. Patient 2. A 5-year-old girl with congenital hearing loss was diagnosed on visual inspection with dysplastic inner ear structures bilaterally (Fig 4B). On measurement, the patient had severely hypoplastic LSCCs and SSCCs. LSSC bony island measurements on axial section were 2.1 mm and 1.6 mm on the right and left, respectively (normal, 3.67 ⫾ 0.35 mm). SSSC bony island measurements on axial section were 3.1 and 2.9 mm on the right and left, respectively (normal, 4.89 ⫾ 0.45 mm). The basal turns of the cochleae were also shorter than control, measuring 7.6 mm and 7.3 mm on the right and left, respectively (normal, 8.59 ⫾ 0.41 mm). Patient 3. A 1-year-old boy with congenital SNHL was diagnosed on visual inspection as having grossly dysplastic inner ear structures bilaterally, sparing the cochleae (Fig 4C). Axial LSCC measurements were not possible as the canal lumen appeared to taper off in the lateral segment. The basal turn length of the right cochlea was also shorter than control, measuring only 7.5 mm (normal, 8.59 ⫾ 0.41 mm). Patient 4. A 40-year-old man with SNHL underwent scanning to rule out a left-sided cholesteatoma. On initial visual inspection, bilateral patulous vestibules and bilateral LSCC dysplasia (left [mt] right) were noted (Fig 4d). The vestibule
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Table 1. Description of measuring technique, average values, and standard deviations for patients without sensorineural hearing loss Structure
Axial SSCC
PSCC
LSCC
Cochlea
IAC Vestibule Coronal SSCC LSCC Cochlea IAC Vestibule
Average (mm)
SD (mm)
Canal lumen
1.25
0.11
Bony island width
4.89
0.45
Ampulla lumen
2.26
0.26
Canal lumen
1.27
0.10
Inferior limb length
7.52
0.53
Lateral canal lumen Posterior canal lumen Bony island width Basal turn lumen
1.29 1.29 3.67 2.15
0.10 0.11 0.35 0.18
Basal turn length
8.59
0.41
Upper turn height
3.96
0.38
Upper turn width
6.24
0.40
6.93 11.10 3.40 5.83
1.42 1.72 0.28 0.58
4.94 1.20 1.22 3.45 5.31
0.56 0.10 0.11 0.42 0.52
4.52 11.86 3.13
1.00 1.39 0.25
4.91
0.37
Measurement
Opening width Length Width Length Bony island width Canal lumen Canal lumen Bony island width Height Opening width Length Width Length
Description
Canal lumen and bony island measured at maximum diameter of the turn, most often two cuts down from the first view of the SSCC. Ampulla identified at cuts between canal lumen and vestibule, most often one cut down from above measurement. Canal lumen measured one/two cuts up from inferior limb. Inferior limb identified by tracing the canal inferiorly and measuring longest distance of confluence. LSCC easily identified and all measurements made on same cut. Bony island measured medial to lateral on same line as lateral limb. Basal turn easily identified; lumen and length most often measured in same cut. Upper turn height measured from apex down to imaginary line separating basal and upper turns (base of modiolus to apex). Upper turns measured after identifying cut with largest length measurement. Measurements taken on longest cut. Vestibule identified and both measurements taken on same cut, most often on same cut as LSCC. Both measurements taken at greatest diameter of turn. Both measurements taken at greatest diameter of turn Easily identified cochlea, measurement taken on cut with maximum height that included basal and upper turn. Measurements taken on longest cut. Cut displaying largest area of vestibule identified and measurements taken (inconsistent relationship to other measurement levels).
IAC, Internal auditory canal; LSCC, lateral semicircular canal; PSCC, posterior semicircular canal; SSCC, superior semicircular canal.
measurements were within normal limits bilaterally. Measurement of the LSCC demonstrated definite dysplasia, with bony island measurements of 2.3 and 1.7 mm on the right and left, respectively (normal, 3.67 ⫾ 0.35 mm). Of note, the PSCC was also hypoplastic on measurement but was not identified as such by visual inspection alone. PSCC inferior limb length measurements were 5.9 and 5.5mm on the right and left, respectively (normal, 7.52 ⫾ 0.53 mm). Patient 5. A 77-year-old woman with familial late-onset SNHL, being evaluated for cochlear implant placement, was diagnosed on initial visual
inspection by the neuroradiologist as having normal inner ear structures bilaterally (Fig 5). However, review of the printed scans in the otolaryngology clinic before interview of the patient suggested mild cochlear hypoplasia and LSCC dysplasia. Measurements demonstrated definite cochlear hypoplasia bilaterally, with coronal height measurements of only 3.6 and 3.3 mm on the right and left, respectively (normal, 5.31 ⫾ 0.52 mm). In addition, significant LSCC dysplasia was identified, with bony island measurements of only 2.3 and 2.5 mm on the right and left, respectively (normal, 3.67 ⫾ 0.35 mm). Of note, the
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Fig 3. Variability of the IAC. Axial CT scan images demonstrating the variable morphology of the IAC from 4 different patients. The IAC proved to be too variable in its morphology and course to allow consistent measurements.
axial sections, which enable the measurement of the basal and apical turns separately, demonstrated normal basal turns and small mid apical turns. Upper turn height measurements were only 3.0 and 2.9 mm on the right and left, respectively (normal, 3.96 ⫾ 0.38 mm). These findings are consistent with Mondini-type deformity, representing arrest of cochlear development before two-and-three-quarters turns are achieved. In this case, the inner ear measurements supplemented visual inspection of the CT images and led to the confirmation of clinician’s suspicion of cochlear hypoplasia and LSCC dysplasia. Patient 6. A 42-year-old woman with SNHL, being evaluated for cochlear implant placement, was diagnosed on initial visual inspection as having normal inner ear structures bilaterally. However, measurement demonstrated right-sided cochlear hypoplasia, with a coronal height
measurement of only 4.2 mm (normal, 5.31 ⫾ 0.52 mm). In addition, measurement of the cochleae on axial section demonstrated narrow basal turn lumens bilaterally: 1.4 mm on the right and 1.5 mm on the left (normal, 2.15 ⫾ 0.18 mm). Measurement also demonstrated dilation of the LSCC, SSCC, and PSCC lumens bilaterally (Table 2). In this case, the inner ear measurements alone identified cochlear hypoplasia. DISCUSSION Subtle abnormalities of the inner ear such as cochlear hypoplasia and SCC dysplasia, which account for the majority of inner ear malformations associated with SNHL, are often missed on CT of the temporal bone due to inexperience of the clinician and absence of normative data to aid in diagnosis. In this study, normative data for 25 different standardized measurements of the inner
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Fig 4. LSCC dysplasia. Axial CT scans demonstrating 4 different cases of LSCC dysplasia. Each of these dsyplasias was identified by visual inspection alone, as well as by abnormal central bony island width measurement. Case histories of patients 1 through ⫺4 are presented in the text.
ear were collected to develop strategies for recognition and confirmation of inner ear malformations using CT scan of the temporal bone. The average dimensions of the inner ear, with the exception of the IAC, were similar across individuals, with the standard deviation less than 10% of the average value; this suggests that there is little variability in the size of the human inner ear. However, the anatomy of the IAC was found to be too variable and difficult to reproduce from scan to scan to be of clinical use. The normative data were subsequently used to evaluate the temporal bone CT in 15 patients with SNHL. Inner ear malformations were identified in 6 (40%) of 15 patients based on the presence of visually obvious malformation or an inner ear measurement value greater than2 SDs from the mean. Visual inspection identified 4 (66%) of 6 patients with malformations but missed two cases
with more subtle malformations of the cochlea and LSCC. In patient 5, the cochlear and LSCC measurements were clearly over 2 SDs from the normal mean value, and in retrospective analysis by the neuroradiologist, the structures were considered abnormal in this patient with adult-onset, familial SNHL. In patient 6, measurement alone identified a previously unrecognized case of cochlear hypoplasia. Therefore, in two of six patients, measurement of the inner ear structures aided in the identification of an abnormality initially missed by visual inspection. In another study, the patient (patient 4) was correctly identified as having LSCC dysplasia; however, the accompanying dysplasia of the PSCC and SSCC was only discernible after measurements. The measurements were also valuable in defining the specific portion of the cochlea that was hypoplastic: basal turn in patients 2 and 6 and the apical turns
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Table 2. Measurements for patients 1 through 6 SSCC
PSCC
LSCC
Cochlea
Vestibule
Inferior Lateral Post Bony Basal Basal Upper Upper Canal Ampulla Bony Canal limb canal canal island turn turn turn turn lumen lumen island lumen length lumen lumen width lumen length height length Width Length
Axial Average 1.25 (mm) SD 0.11 Patient 1 1.5 1.5 2 1.9 1.3 3 1.3 1.2 4 1.2 1.2 5 1.3 1.1 6 1.5 1.5
2.26
4.89
1.27
7.52
1.29
1.29
3.67
2.15
8.59
3.96
6.24
3.40
5.83
0.26
0.45
0.11
0.53
0.10
0.11
0.35
0.18
0.41
0.38
0.40
0.28
0.58
2.1 1.9 2.7 2.4 2.2 2.1 2.2 2.0 2.0 2.0 2.2 2.1
5.8 5.8 3.1 2.9 4.8 4.9 4.0 4.2 4.8 4.5 4.3 4.3
1.2 1.3 1.4 1.4 1.4 1.6 1.3 1.3 1.3 1.3 1.5 1.4
8.0 7.9 5.5 6.0 7.2 7.5 5.9 5.5 7.0 7.1 6.7 7.0
2.3 1.7 1.3 1.3 NC NC 1.2 1.5 1.3 1.3 1.5 1.5
2.4 1.9 1.3 1.3 1.5 1.5 1.2 1.5 1.5 1.3 1.5 1.5
1.6 2.5 2.1 1.6 NC NC 2.3 1.7 2.3 2.5 3.3 3.1
1.8 1.9 1.9 2.0 2.1 2.0 2.0 2.0 2.0 2.0 1.4 1.5
8.6 8.9 7.6 7.3 7.5 8.2 8.2 8.3 8.0 8.0 7.8 7.7
3.8 4.0 4.1 3.9 3.4 3.4 3.6 3.7 3.0 2.9 3.5 3.4
6.1 6.1 6.1 6.2 6.1 5.9 6.4 6.1 5.1 5.1 5.6 5.8
NC NC 3.1 2.7 3.1 3.2 3.4 2.7 3.6 3.4 4.1 3.5
NC NC 6.0 6.4 6.6 6.6 5.3 5.5 5.5 5.6 6.2 6.2
SSCC
Coronal Average (mm) SD Patient 1 2 3 4 5 6
LSCC
Vestibule
Lumen
Bony island
Lumen
Bony island
Cochlea height
1.20 0.10
4.94 0.56
1.22 0.12
3.45 0.42
5.31 0.52
3.13 0.25
4.91 0.37
1.3 1.3 1.2 1.3 1.2 1.0 1.3 1.3 1.3 1.3 1.1 1.3
5.0 4.9 3.7 3.7 4.9 5.2 4.1 4.1 4.4 4.3 4.4 4.4
2.5 1.9 1.3 1.2 .99 .99 1.3 1.2 1.3 1.1 1.5 1.5
1.4 2.2 1.7 1.8 3.1 3.1 1.7 1.5 1.6 1.7 3.1 2.9
5.9 5.8 4.6 4.8 4.7 4.5 5.1 5.2 3.6 3.3 4.2 4.5
3.7 3.4 2.6 2.8 2.9 2.7 2.9 3.3 3.0 3.2 3.0 3.2
5.4 5.4 4.5 4.2 5.2 5.2 4.7 5.2 4.3 4.2 4.8 4.4
Width
Length
NC, Measurement not able to be collected because of severe malformation. LSCC, Lateral semicircular canal; PSCC, posterior semicircular canal; SSCC, superior semicircular canal. Abnormal values are given in bold.
in patient 5. Therefore, the inner ear measurements not only aid in the diagnosis of inner ear malformation but also can provide additional information about which specific part is abnormal. In addition, the acquisition of routine inner ear measurements can aid in training the physician in the identification of inner ear malformations that would otherwise be missed because of inexperience or subtle nature of the abnormality.
Standardized measurements of inner ear structures to aid in the diagnosis of inner ear malformations are appealing; however, the routine measurements of 25 different dimensions, as in this study, would be too cumbersome. To determine which of the 25 measurements were the most useful, the embryology of inner ear was reviewed, the relative preponderance of the various known malformations was considered, and the ease and
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Fig 5. Identification of cochlear hypoplasia and LSCC dysplasia. The cochlear height on coronal CT (A, normal) was most sensitive for the detection of cochlear hypoplasia (B, patient 5). The measurement of central bony island on axial scan (C, normal) correctly identified LSCC dysplasia in all cases (D, patient 5). These 2 subtle abnormalities were not identified by visual inspection alone but became evident with the application of the standardized measurements.
reproducibility of different measurement were assessed. Our data suggest that the vertical height of the cochlea on coronal scan and the size of the central bony island within the LSCC on axial scan are the most useful measurements and are sufficient to diagnose common inner ear malformations. The most common inner ear malformations involve the LVA, LSCC, and cochlea. The major criterion for LVA, midpoint dimension of the vestibular aqueduct greater than 1.5 mm, has been previously defined.9 LSCC dysplasia is probably the second most common inner ear malformation. LSCC malformation is associated with CHL, mixed HL, and SNHL5 and most commonly occurs in isolation without associated malformation of the SSCC and PSCC. On the other hand, isolated dysplasia of the SSCC
or PSCC without abnormality of the LSCC is rare.4 Therefore, measurement of the LSCC (Fig 1c) would serve to identify the second most common inner ear malformation and as a screen for the other two SCCs; if the LSCC is malformed, then the SSCC and PSCC should be carefully evaluated. The mechanism of LSCC dysplasia involves the lack of formation of the central bony island, and that structure is easily seen and measured on the axial temporal bone CT scan. In our series, all patients with LSCC malformation (n ⫽ 6) had abnormal measurements of the bony island on the axial scan. Cochlear malformation, usually due to early developmental arrest leading to less than two and three-quarters turns, are also common. Premature arrest of cochlear development would be expected to decrease the base to apex height of
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the cochlea best assessed on the coronal CT scan (Fig 2c). The coronal height is easy to identify and measure and is easily reproducible. Two patients who initially were thought to have normal cochleae were correctly identified as having cochlear hypoplasia by the coronal height measurement. In summary, while visual inspection alone of temporal bone CT can reliably identify severe malformations of the inner ear, it can be insensitive in the detection of more subtle abnormalities. The sensitivity of detecting inner ear malformations can be greatly increased by routinely measuring the LSCC bony island on axial scan ([lt]2.97 mm or [mt]4.37 mm) and the cochlear height on coronal scan ([lt]4.27 mm or [mt]6.35 mm) to complement visual analysis. If the LSCC is malformed, then the SSCC and PSCC should be carefully evaluated. The accurate identification of inner malformations will have a direct impact on the diagnosis, prognosis, and management of patients with SNHL.
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