Tomography of the internal acoustic meatus. A critical evaluation of the radiological appearance in normals and in patients with acoustic neuromas

Morphology of the internal auditory canal: Deriving parameters from computer tomography scans. An observational STROBE-MR study

OBJECTIVES

The objective of this study is to identify and define landmarks to adequately describe the internal auditory canal for inclusion in three-dimensional computational models of the cochlea and its surrounding structures.

PARTICIPANTS

Computer tomography scans of live human cochleae were collected in the retrospective period. Descriptive and comparative statistics were used to describe the data obtained from the scans.

RESULTS

The mean anterior-posterior (AP) diameter at the base of the basal turn, the diameter of the AP at the midpoint of the IAC, and the anterior and posterior length of the internal acoustic canal were measured. 57.14% of the internal acoustic canals observed presented with a cylindrical, 40.48% was funnel-shaped and 2.38 % were bud-shaped. A statistically significant differences were found between the diameters of male and female internal acoustic meatus.

CONCLUSIONS

This paper serves as a reference that provides a set of references for the description of the internal acoustic canal for inclusion in three-dimensional computational reconstruction of the cochlea and surrounding structures.

The Normal Adult Human Internal Auditory Canal: A Volumetric Multidetector Computed Tomography Study

OBJECTIVE

The purpose of this study was to demonstrate that volumetric analysis of multidetector computed tomography (CT) images can be used to calculate the volume of the adult human internal auditory canal (IAC) reproducibly, and to describe the range of normal IAC volumes in the adult population with subgroup analysis of sex, age, and laterality.

BACKGROUND

Previous studies of the IAC have typically used measurements in two dimensions or by using casts of cadavers to measure IAC volumes. This study is the first to report the normal ranges of IAC volumes measured by CT.

METHODS

Two hundred eighty-one CT scans were assessed. Of the CT scans that met the inclusion criteria, a software package was used to manually contour the IACs in each subject to calculate the volumes in cubic millimeters. Subgroup analysis of laterality, sex, and age was evaluated. Interobserver agreement was calculated for the first 59 patients (118 canals).

RESULTS

Two hundred fifty-nine scans (518 canals) met the inclusion criteria. The volumes ranged from 74 to 502 mm, with no statistically significant difference between left and right (p value = 0.69). In males, the range of volumes measured 74 to 502 mm while in females it ranged from 78 to 416 mm. Males had larger IAC volumes than females (Wilcoxon rank-sum test: S = 14,845.0, p value = 0.01 on the right, and S = 14,646, p value = 0.004 on the left). No correlation was found with age (Spearman: -0.10, p value = 0.09 on the right and -0.04, p value = 0.50 on the left). Excellent interobserver agreement was found.

CONCLUSION

IAC volumes of normal adult subjects, measured by CT, were larger in males and not significantly different with respect to age or laterality.

Morphometric analysis of the internal auditory canal by computed tomography imaging

Background

Many clinical and experimental studies have been done to analyze the anatomical and functional aspects of the internal auditory canal (IAC) in human beings since there are great inter-individual variability and structural variations that may occur regarding the other adjacent structures.

Objectives

The purpose of this study was to characterize the morphology of the internal auditory canal (IAC) during development using high resolution computed tomography (CT) and to analyze its dimensions, which will be determined by measuring the nearby areas and structures using a system of digital image processing.

Patients and Methods

CT images of the IAC of 110 normal subjects aged 1 to 92 years (mean age, 46.5 years) of both genders were reviewed to determine the shape, area, opening width (OW), longitudinal length (LL), vertical diameter (VD) and distance from the vestibular aqueduct.

Results

The shapes observed in children and adults were funnel-shaped (74% and 58.3%, respectively), cylindrical (22% and 30.9%, respectively) and bud-shaped (4% and 10.8%, respectively). The measurements by CT in children were: area= 50.30 mm², OW = 7.53 mm, length = 11.17 mm, VD = 4.82 mm and the distance between the IAC and the vestibular aqueduct (VA) = 12.63 mm. In adults, the measurements were: area = 44.64 mm², OW = 7.10 mm, length = 9.84 mm, VD = 4.47 mm and the distance between IAC and VA = 11.17 mm.

Conclusions

CT images showed that the IAC has different shapes and when the measurements obtained for children were compared with those of adults, the parameters that presented statistically significant differences in either gender were length and diameter.

[Dilation of the internal auditory canal and intracanalicular vestibular schwannoma: what are the mechanisms involved?]

OBJECTIVES

To present a unique case of unilateral widening of the internal auditory canal (IAC) with no significant contact with an ipsilateral intracanalicular vestibular schwannoma (VS), raising the issue of the cause(s) of this IAC widening.

METHODS

The medical record and radiologic data were reviewed of a patient presenting an enlarged unilateral IAC, which led to the diagnosis of an intracanalicular VS that could not account for the dilation.

RESULTS

The patient had a unilateral dilation of the IAC that did not match the ipsilateral VS he had. As a result, this case motivated discussion of whether such dilation of the IAC was congenitally asymmetrical or the result of the mechanisms involved in the widening of the IAC.

CONCLUSIONS

Although asymmetry of IAC is a current notion, this case demonstrates a contrario that increased pressure exerted on the walls of the IAC cannot be the only mechanism in such widening.

[Usefulness of the opposite direction for Stenvers' method]

In otorthinolaryngology, Stenvers' method is employed in radiography of the pyramid (internal acoustic meatus and semicircular canals). However, in cases of dizziness, where the prone position is difficult to achieve, we occasionally use the opposite Stenvers' method instead of the conventional one. This makes it possible to perform radiography with the patient in the supine position. Compared with the conventional Stenvers' method, the problems of this method were increases not only in the rate of magnification but also in lens dose. In this study, we evaluated these problems by employing computed tomography (CT) and a glass dosimeter with phantom as well as by clinical evaluation. The results showed no statistically significant difference between Stenvers' method and the opposite Stenvers' method in both the rate of magnification and the clinical evaluation. The increase in lens dose with the opposite Stenvers' method was not significant. We concluded that the opposite Stenvers' method was useful.

Prognostic significance of changes in the internal acoustic meatus caused by vestibular schwannoma

The prognostic significance of the variety of changes in the internal auditory meatus (IAM) caused by vestibular schwannoma was retrospectively analyzed in 69 consecutive patients with vestibular schwannoma. Preoperative bone-window computed tomography was used to classify IAM changes into extensive destruction (17%), widening (46%), and normal IAM (36%). Extensive destruction (47 +/- 19 years) and widening (48 +/- 13) occurred in significantly younger patients than normal IAM (59 +/- 9). Preoperative hearing was significantly more severely disturbed in patients with extensive destruction than in those with widening or normal IAM. IAM change was significantly related to the tumor consistency, as normal IAM was more common in patients with cystic tumor than in those with solid tumor. Postoperative hearing and facial function were worse in patients with severe IAM change, although the relationship between the IAM change and the surgical result was not significant. One patient with extensive destruction developed postoperative cerebrospinal fluid (CSF) leakage through the air cells around the IAM, and needed surgical repair. Severe IAM change occurs with solid tumor and causes severely disturbed preoperative hearing in younger patients, which reflects the tumor aggressiveness. Severe IAM change increases the technical difficulty of tumor removal and the risk of postoperative CSF leakage, and is associated with a poorer prognosis for patients with vestibular schwannoma.

Postnatal development of the internal auditory canal studied by computer-aided three-dimensional reconstruction and measurement

Postnatal development of the internal auditory canal (IAC) was investigated in 20 normal human temporal bones obtained from individuals 1 month to 72 years old. Computer-aided three-dimensional reconstruction and measurement of bones showed that the superior, inferior, anterior, and posterior walls of the IAC lengthen significantly from birth until about 10 years of age, with development mainly attributable to lengthening of the part of the IAC medial to the foramen singulare. The lengths of the part of the IAC lateral to the foramen singulare and of the transverse crest and Bill's bar did not appear to develop postnatally. The IAC diameter increased slightly at the porus for about the first year after birth, but not at the fundus or the middle portion of the canal. This finding was confirmed by studying the shape of the IAC. Postnatal increases in the volume of the IAC followed patterns similar to that of increases in length of studying the shape of the IAC. Postnatal increases in the volume of the IAC followed patterns similar to that of increases in length of the IAC walls. These results show that postnatally the IAC increases significantly in length until about 10 years of age and slightly in diameter until about 1 year of age, especially medial to the foramen singulare. This concentration of growth of the IAC medially implies that its postnatal development is mainly due to growth of the bone around the otic capsule, which has implications for IAC surgery.

Landmark structures to approach the internal auditory canal: a dimensional study related to the middle cranial fossa approach

For the purpose of assisting the accurate topological identification of the internal auditory canal (IAC) during surgery through the middle cranial fossa, we conducted a dimensional study around the IAC using histologically prepared temporal bones. The anterior margin of the porus acoustics, the posterior margin of the porus acousticus, and the center of the fundus were located 19.7 mm, 16.8 mm, and 8.2 mm from the center of the malleus head, respectively. The angles formed by lines connecting those three points of the IAC and the ossicles were also measured. These values could be of help to identify the accurate localization of the IAC using the ossicles as surgical landmarks, when available. The common crus, i.e. a critical inner ear structure for hearing preservation, was also studied. The distance from the medial side of the common crus to the posterior wall of the IAC was relatively constant. However, the distance from the medial side of the common crus to the surface of the posterior cranial fossa was variable. Measurements in the vertical sections showed that the thickness of the superior bony wall of the IAC was similar to that of the cochlear otic capsule which should be kept unexposed for hearing preservation.

A modified screening protocol for the diagnosis of acoustic neuromas

In many centres the diagnosis of acoustic neuromas relies on a preliminary screening protocol. The current method is a combination of caloric tests, tomography of the internal auditory meati and brain stem evoked response audiometry (BSERA) with 2 positive results indicating the need for further investigation. This protocol, although sensitive lacks specificity and results in a high incidence of normal CT air meatograms. As this procedure is associated with a significant morbidity, we have devised a 'weighted' system of scoring to avoid subjecting large numbers of normal patients to CT air meatography. The proposed system has been derived from screening 61 patients with unilateral sensorineural hearing loss, 24 of whom had an acoustic neuroma. This method reduced the false positive rate of CT air meatography from 68%, using the 2 out of 3 criteria, to 18%, whilst the false negative incidence remained at 8%.

Diagnostic strategies in search for acoustic neuromas. Findings in 300 acoustic neuroma patients

The diagnostic findings in 300 patients with acoustic neuromas are reported. Because of a centralized treatment of acoustic neuromas, we have a uniform work-up of all the patients, which enables us to make a non-selected comparison of the diagnostic efficiency of the various tests. Diagnostic dilemmas in patients with normal hearing, anakusis, normal auditory brainstem recordings, normal vestibular function and normal tomography of the internal acoustic meatus are discussed. In patients with hearing better than 80 dB, a normal ABR, the presence of recruitment and a normal caloric reaction can exclude the presence of a tumour, making tomography superfluous. In patients with poor hearing, the need for tomography is imperative. We perform CT if two procedures among the following produce tumour-positive results: ABR, recruitment tests, caloric test, and tomography. Those who have only one tumour-positive finding at the screening are re-examined after one year. If, even with contrast enhancement, CT proves negative, we continue to perform air CT. Only then do we know for sure whether the patient has a tumour or not. An advantage with using many tests for the same physiological function is that they complement each other, though, on the other hand this often adds to the confusion. With fewer tests, the confusion is reduced, but the requirements regarding technical quality must be very stringent. ABR, the Metz recruitment test and tomography are purely objective, while the caloric test is subjective on the part of the investigator. The Hallpike procedure, however, has stood the test of time in separating pathologic from normal.

The internal acoustic meatus. Anatomic variations and relations to other temporal bone structures

The internal acoustic meatus was evaluated in 97 temporal bone specimens, half of which were radiographed in different projections. Part of the material was also examined by multidirectional and computed tomography. Casts of the specimens were prepared and the meatuses were measured in different planes. The anatomic variations, especially in the transverse plane, and their relations to other structures in the petrous bone, were assessed. The meatus varied in shape, being mainly either long and thin, short and broad, funnel-shaped or pear-shaped. The pear-shaped meatus was practically always broad. The angle formed by the long axis of the meatus and the posterior wall of the pyramid varied inversely with the meatal length, a long meatus forming a small angle and vice versa. Large anteroposterior diameters were correlated to large meatal angles, but there was no direct correlation between meatal length and width. In addition, the meatal length was correlated to the perilabyrinthine and mastoid pneumatization and also to the dimensions of the pyramid.

Pitfalls in the diagnosis of acoustic neuroma

The patient with an acoustic neuroma may present to the otologist with a variety of clinical features. Classically these include a retrocochlear pattern of sensorineural hearing loss, reduced vestibular response on caloric testing and radiological asymmetry of the internal auditory canals (IAC). The absence of any or all of these features, however, does not exclude the presence of tumour. Five cases are presented to illustrate the potential for diagnostic delay unless a routine battery of investigations is undertaken in patients with neuro-otological disorders. All patterns of subjective audiometry are encountered, auditory brain stem response testing may be unreliable where hearing loss is profound, vestibular testing is normal in half of small tumours and the intracanalicular tumour may be radiologically undetectable unless IAC meatography is employed.

Computerized x-ray tomography of fossil hominid skulls

The newest generation of Computerized Tomography (CT) scanning appears to provide x-ray pictures of the internal structures of a rather heavily mineralized Homo erectus skull, i.e., Pithecanthropus IV, notably of its temporal bones, that reveal more details than traditional x-ray procedures. An analysis of the right temporal bone shows that it is extremely pneumatized, that the oval shape of the external meatus does not result from fractures as previously suggested, and that the middle ear and the internal auditory meatus have probably been damaged during restoration. Labyrinthine structures can be seen, and the facial nerve canal appears to be of a relative small size.