Anatomical variations of the human vestibular aqueduct. Part I. A radioanatomical study.

Variations in the size and shape of the human vestibular aqueduct were evaluated in 118 plastic casts of unselected specimens of human temporal bones. They were examined by conventional radiography and by high resolution CT. The degree of the mastoid and perilabyrinthine pneumatization was defined and classified into 3 types. The dimensions of the peripheral portion of the aqueduct were found to be related to the extent of the perilabyrinthine pneumatization.

Anatomical variations of the human vestibular aqueduct. Part II. A radioanatomical study.

The human vestibular aqueducts are classified into 3 types and into the types hyper-, normo- and hypoplastic. The types correspond with each other up to over 85%. For a better understanding of the radioanatomy and for the proper interpretation of radiograms, we describe the presence of a flat recess-like widening of the peripheral portion of the aqueduct, as well as other findings.

Anatomic variations of the chorda tympani canal.

The anatomic variations of the facial recess are of interest in certain otosurgical procedures. The medial border of the recess is the mastoid portion of the facial nerve canal, and the lateral border is the bony canal of the chorda tympani. These two structures were investigated in 64 polyester casts of temporal bone specimens. The point of exit of the chorda tympani canal from the facial canal was assessed together with the angle, formed between these two nerve channels. As a further way of describing the spaciousness at the facial recess, the distance between the sulcus of the stapedius muscle and the chorda tympani canal was evaluated.

The cochlea and the carotid canal.

The carotid canal conveys a large artery and lies in close topographic relationship to the basal turn of the cochlea. The bony wall between them consists in some cases only of the petrous bone of the otic capsule. It can be as thin as 0.2 mm. In other cases the distance can be more than 6 mm. A systematic assessment of the relationship between these two structures was performed on plastic casts of 173 human temporal bone specimens in order to investigate the range of normal variation. The topography in this region can be visualized by multidirectional tomography or CT.

Radioanatomy of cochlear and stapedial otosclerosis.

Otospongiosis of the otic capsule is a primary focal disease, which can be reproduced by both conventional multidirectional tomography and computed tomography. The final stage of otospongiosis, otosclerosis has a complete different appearance in radiography. When the otospongiosis focus is located at the oval window a progressive conductive hearing loss will result, which in its final stage will have the character of otosclerosis with fixation of the stapes in the oval window. The radiographic diagnosis with either conventional multidirectional tomography or CT is more or less pathognomonic and will easily confirm the audiologic examinations results. The diagnosis of otospongiosis by computed tomography is advantageous to conventional tomography. With both methods high resolution technique is mandatory for safeguarding both diagnosis and evaluation of the extent of the lesion. Minor otospongiotic or otosclerotic lesions of the oval window with fixation of the footplate are still a challenge to high resolution radiographic imaging. In most cases stapedial otosclerosis is advantageously diagnosed by otoscopy and audiometry. High resolution radiographic imaging is a valuable confirmative method to corroborate clinical diagnoses and differentiating the disease from other conditions based on other morphologic lesions.

The vestibular aqueduct in clinical radiography.

The gross morphology of the vestibular aqueduct and its clinical significance are better understood against the background of the endolymphatic duct and sac, which is still a matter of hypotheses. They are concerned with functions of the endolymphatic sac like secretion and absorption of endolymph, desintegration of cells and other debris from the inner ear and phagocytosis. The endolymphatic sac in the vestibular aqueduct is also thought to have a pressure regulation function for the labyrinth and also to deal with the metabolism of the inner ear and the desintegration of otoconia. Pathology in this area is also a matter of vital discussion. The indications for surgical intervention are consequently a matter of controversary. Radiographic research has concentrated both on the normal radioanatomy of the aqueduct and its appearances in cases of Meniere's disease and of inner ear malformations. Digression from the normal appearance can be demonstrated with multidirectional tomography and CT.

The temporal bone in patients with Meniere's disease.

Sixty-three patients with Meniere's disease were compared with a group of healthy individuals with respect to findings at tomography and plain radiography. The temporal bone in patients differed in many respects from that in the healthy controls. Characteristic features in patients were a lack of periaqueductal pneumatization, a lack of pneumatization medial to the arcuate eminence, a short vestibular aqueduct, a narrow external aperture of the vestibular aqueduct, and a reduction in size of the mastoid air cell system. The mean mastoid area in the diseased ear in patients was 7.93 cm2 and in controls 11.59 cm2. The vestibular aqueduct was visible in its full length on tomograms in 81% of the healthy controls and in 65% of the diseased ears in patients. The mean volume of the mastoid air cell system in Meniere patients, determined at operation, was 5.3 ml.

Temporal bone characteristics in Meniere's disease.

The petrous portion of the temporal bone in patients with Meniere's disease differs from that of healthy individuals mainly in its lack of periaqueductal pneumatization and its consequently short and narrow vestibular aqueduct. Diminished pneumatization may have an impact upon the tomographic reproducibility of the aqueduct. A total lack of periaqueductal pneumatization is prevalent in long-standing Meniere's disease. Tomography may serve as a tool by providing a basis for the choice of surgical procedure. Roentgenologic and histologic studies have indicated that the pars rugosa of the endolymphatic sac in normals mainly is housed inside the distal part of the vestibular aqueduct. In patients with Meniere's disease, the sac might be located outside the aqueduct and therefore deprived of the functions of the loose and highly vascular tissue normally surrounding it within the aqueduct. This might influence the total vascular supply of the sac, thereby interfering with its resorptive and immunodefensive functions.

Menière's disease - roentgenologic diagnosis.

Radiologic diagnosis in Menière's disease should not be based on the vestibular aqueducts tomographic reproducibility. Instead, valuable preoperative information should be given about the size and location of the aqueducts external aperture in relation to the posterior and lateral semicircular canals. In saccotomy the semicircular canals are landmarks for identification of the endolymphatic sac. The periaqueductal pneumatization is demonstrated to be partly decisive for the vestibular aqueducts gross morphology. In Menière's disease either a sparse or a total lack of periaqueductal pneumatization is observed in most of all cases. The development of peri- and infralabyrinthine pneumatization is also decisive for the position of the foveate impression, which houses the extraosseous part of the endolymphatic sac. Its location and size can be evaluated by tomographic characterization of the vestibular aqueducts external aperture.

Multidirectional tomography of the facial canal.

The course of the facial canal in the complicated structure of the temporal bone is best demonstrated by multidirectional (hypocycloid or spiral) tomography. A description of its normal anatomy is delivered, based on experiences from dissected temporal bone specimens, plastic moulds and tomograms. Detailed knowledge of the anatomy is required for distinction of defects and anomalies, particularly pre-operatively (fractures, reconstructive middle ear surgery). In malformation the pre-operative localization of the course of the facial canal is important, since its relation to the oval window and the posterior wall of the tympanic cavity is decisive for surgical procedures.

Multidirectional tomography in reconstructive middle ear surgery.

The tomographic demonstration of essential structural details before reconstructive middle ear surgery with osseous auto- and homografts is described, and the preoperative information requirements are briefly discussed. The tomographic appearance of the different types of reconstruction is presented. The halfaxial and true lateral projections are to be preferred, if necessary complemented by the axial-pyramidal projection. Tomography may disclose obvious morphologic causes of absence of postoperative hearing improvement or secondary hearing impairment.

Multidirectional tomagraphy of defects in the facial canal. An experimental investigation.

Defects on the tympanic wall of the facial canal may endanger the nerve in middle ear surgery. Their preoperative tomographic demonstration is therefore valuable. In an experimental investigation an attempt was made to assess the discernibility of artificial defects of different size by hypocycloid and spiral tomography of temporal bone specimens (both isolated and mounted in a tissue-equivalent skull phantom). The thickness of the wall was measured and its influence on the discernibility of the defects discussed.

A tomographic test object.

Testing of the tomographic and photographic system is a prerequisite for successful reproduction of minor details particularly in tomography of the temporal bone. A test phantom has been developed, consisting of a single layer of small metal balls which can be adjusted to varying inclinations to the tomographic plane. It involves a simple procedure for examining the homogeneity and stability of the tomographic layer at wide movement angles and may also be used for calculating the layer thickness.