Singular Canal Research Articles

Topographic Radioanatomical Analysis of the Singular Canal: Computed Tomography Study

Purpose: The singular canal (SC) is where the singular nerve, a branch of the inferior vestibular nerve, which carries afferent information from the posterior semicircular canal (PSCC), passes and is important in the surgical approach of the presigmoid retrolabyrinthine. This study was carried out to evaluate the visibility of the SC on standard computed tomography (CT) images, its distance to the surrounding structures, and to investigate the variations of its anatomy and its relationship with the meatus acusticus internus. Materials and Methods: The study was carried out retrospectively using images of 194 temporal bones on temporal bone CT scans of 44 men and 53 women aged 18–65. In the study, various measurements were made, especially the presence of the SC, its length, its angle with the internal acoustic canal (IAC), and the distance between the internal acoustic pore (IAP) and the singular foramen. In addition, the presence of the high jugular bulb and PSCC dehiscence images were investigated. Results: The SC was detected in 85.1% of the analyzed images. The mean canal length was 3.93 ± 1.22 mm, the angle between the SC and the IAC was 22.68° ±3.60°, and the distance between the SC and the IAP was 7.70 ± 0.83 mm. While no difference was found between the sides, it was determined that the length and diameter of the SC did not differ according to gender. Conclusion: Detailed morphometric analysis of the SC and a thorough understanding of its relationship with the IAC, vestibulum, and PSCC will help to accurately define the posterior and lateral borders of the dissection for this region.

Early Development of the Heart and Great Vessels

The cardiovascular system is one of the first organ systems to develop in the embryo. By day 21, three germ layers: endoderm, mesoderm, and ectoderm are formed by the gastrulation process. Cells in the mesoderm coalesce through a process called vasculogenesis to form two parallel endocardial tubes. Folding of the gut tube brings these two parallel endocardial tubes towards the midline enabling them to fuse into a single linear‐shaped heart tube. By day 18, the lateral plate mesoderm splits to form two layers – splanchnic (visceral) and somatic (parietal). The cells of the splanchnic mesoderm will develop into the myocardial cells of a Primary (PHF) and Secondary Heart field (SHF). By day 22, the heart begins to beat and lengthen by the addition of cells from the SHF to the PHF. This develops into the outflow tract ‐ aortic and pulmonary vessels, and the right ventricle. By day 28, the linear‐shaped heart tube contains regions called the sinus venosus, primitive atrium, primitive ventricle, and bulbus cordis (outflow). The proximal and distal ends become “fixed” by the pericardium, the bulbus cordis and primitive ventricle grow faster than the other regions of the heart tube, and the heart bends upon itself to form a u‐shape. The primitive atrium and sinus venosus are moved dorsally, and the heart begins septation into 4 chambers. As early as day 14, two actively growing ridges called Endocardial Cushion (of the PHF) nearly divide the single lumen into two separate cavities connected by a singular (atrioventricular) canal. Subsequently, a septum grows from the cushions toward the roof of the common atrium, and from the cushions toward the apex of the primitive ventricle. Continued addition of cushion cells to the atrioventricular canal divides a single canal into a left and right canal, as well as valves attached to the ventricular wall by muscular cords. Abnormalities of Endocardial cushion can affect structures derived from the PHF. For example, Tricuspid Atresia involves the malformation of the right Atrioventricular Canal and is associated with: (i) Atrial, and (ii) Ventricular Septal Defect; (iii) hypotrophy of the right ventricle; and (iv) hypertrophy of the left ventricle. Cells of the SHF are involved in the formation and lengthening of the outflow tract. Interestingly, this growth is regulated by neural crest cells. Insults to neural crest cell development or migration can affect structures derived from the PHF. For example, Tetralogy of Fallot involves an unequal division and laterality of the bulbus cordis associated with: (1) narrow right ventricular outflow; (2) interventricular septal defect; (3) over‐riding aorta directly above the septal defect; (4) hypertrophy of the right ventricular wall because of high pressure in the right side of the heart. Neural crest cells also contribute to craniofacial development leading to facial and cardiac abnormalities in the same individual.

Evaluation of the depiction ability of the microanatomy of the temporal bone in quarter-detector CT: Model-based iterative reconstruction vs hybrid iterative reconstruction.

Little is known regarding differences between model-based iterative reconstruction (MBIR) and hybrid iterative reconstruction (HIR) in temporal bone computed tomography (CT). This study compared the ability to depict microstructures in temporal bone in quarter-detector CT (QDCT) between MBIR and HIR.Sixty-two temporal bones in 31 consecutive adult patients who underwent QDCT were included. Reconstruction was performed with Forward projected model-based Iterative Reconstruction SoluTion (FIRST) BONE mild mode and Adaptive Iterative Dose Reduction 3D (AIDR3D) enhanced mild mode. Imaging quality was graded for 3 microstructures (spiral osseous lamina, tympanic membrane, and singular canal).Spiral osseous lamina was significantly well-delineated in the AIDR3D enhanced group, compared with the FIRST group. In nearly all cases with FIRST, spiral osseous lamina was poorly defined. Although there was no significant difference, depiction of the tympanic membrane and singular canal tended to be better with AIDR3D enhanced mode.Routine reconstruction for preoperative temporal bone CT should be performed with HIR, rather than MBIR.

A study of quantitative measurement of singular nerve canal with cone-beam computed tomography

Objective: To explore the feasibility of cone beam computed tomography (CBCT) in quantitatively assessing singular nerve canal and to propose a method to locate singular nerve canal in CBCT imaging. Methods: Eight cadaveric heads were scanned using CBCT and images of 16 temporal bones were obtained.The diameters of three points of singular nerve canal were measured which included the internal auditory canal aperture, the central turning area, and the aperture of ampulla of the posterior semicircular canal.The length of singular canal was measured.The distances from the three points to the inner edge of round window were measured.The best oblique axial plane to display the whole course of singular canal was adjusted and the angle between this plane and the lateral semicircular canal was measured.Two independent sample t-test was used to compare the left and right measurements. Results: The mean diameters of the three points were(0.98±0.51), (0.36±0.05) and(0.38±0.06) mm respectively.The mean length of singular nerve canal was(4.16±0.87) mm.The mean angle was 19.1°±10.2°.2D and 3D location graphs of singular nerve canal were made with the distance between the three points on singular nerve canal and the medial end of the round window.There was no significant statistical difference between left and right side in all the measurement (all P>0.05). Conclusion: CBCT can clearly display the whole course of singular nerve canal.Measurement can be done on CBCT images, and the specific location of singular nerve canal can be determined, which are important for surgery.

Vascular anomalies, sutures and small canals of the temporal bone on axial CT

Purpose: Subtle bony structures, small canals and fine sutures cause sometimes problems in the analysis of CTs of the temporal bone. The aim of this study was: to analyze the visibility of subtle structures and to estimate the incidence of vascular anomalies. Patients and method: We retrospectively analyzed axial scans of 223 high-resolution CTs of the temporal bone obtained as single slice or spiral CT with 1 mm slice thickness. All CTs had clinical indications. Two experienced radiologists studied CTs regarding the visibility of the fine sutures, fissures and small canals and the occurrence of vascular anomalies. Results: The following structures were seen commonly: sphenosquamosal suture (76%), arcuate artery canal (93%), vestibular aqueduct (89%), mastoid emissary vein (82%), singular canal (56%). Not so commonly were observed: tympanosquamosal suture (31%), mastoid canaliculus (28%), lateral sigmoid sinus (28%), petrotympanic fissure (24%), tympanomastoid suture (10%). Seldom we identified: the inferior tympanic canaliculus (6%), high jugular bulb (6%), anterior sigmoid sinus (5%), dehiscent internal carotid artery canal (2%), persistent petrosquamosal sinus (1%), dehiscent jugular bulb (1%). Persistent stapedial artery, aberrant internal carotid artery, dehiscent jugular bulb, high jugular bulb with diverticulum, anterior and dehiscent sigmoid sinus were detected in below 1% of the analyzed temporal bones. The frequency of asymmetry of the jugular foramen, which varied between 3% and 42%, depended on different criterions of size. Conclusion: A profound knowledge of normal anatomy and anomalies of the temporal bone avoids misinterpretation as pathological lesions and iatrogenic bleedings.

Nerve Canals at the Fundus of the Internal Auditory Canal on High-Resolution Temporal Bone CT

Purpose: To identify and evaluate the normal anatomy of nerve canals in the fundus of the internal auditory canal which can be visualized on high-resolution temporal bone CT. Materials and Methods: We retrospectively reviewed high-resolution (1 mm thickness and interval contiguous scan) temporal bone CT images of 253 ears in 150 patients who had not suffered trauma or undergone surgery. Those with a history of uncomplicated inflammatory disease were included, but those with symptoms of vertigo, sensorineural hearing loss, or facial nerve palsy were excluded. Three radiologists determined the detectability and location of canals for the labyrinthine segment of the facial, superior vestibular and cochlear nerve, and the saccular branch and posterior ampullary nerve of the inferior vestibular nerve. Results: Five bony canals in the fundus of the internal auditory canal were identified as nerve canals. Four canals were identified on axial CT images in 100% of cases; the so-called singular canal was identified in only 68%. On coronal CT images, canals for the labyrinthine segment of the facial and superior vestibular nerve were seen in 100% of cases, but those for the cochlear nerve, the saccular branch of the inferior vestibular nerve, and the singular canal were seen in 90.1%, 87.4% and 78% of cases, respectively. In all detectable cases, the canal for the labyrinthine segment of the facial nerve was revealed as one which traversed anterolaterally, from the anterosuperior portion of the fundus of the internal auditory canal. The canal for the cochlear nerve was located just below that for the labyrinthine segment of the facial nerve, while that canal for the superior vestibular nerve was seen at the posterior aspect of these two canals. The canal for the saccular branch of the inferior vestibular nerve was located just below the canal for the superior vestibular nerve, and that for the posterior ampullary nerve, the so-called singular canal, ran laterally or posteolaterally from the posteroinferior aspect of the canal for the saccular branch. Conclusion: Five bony nerve canals in the fundus of the internal auditory canal were detected by high-frequency on high-resolution temporal bone CT. Familiarity with these structures can prevent confusion with, or misinterpretation as, a fracture line, and further study such as MR imaging may be required when any enlargement or erosion of these nerve canals is present.

Distribution of psammoma bodies in the internal auditory canal and its extended areas in the human temporal bone.

The internal auditory canal (IAC) and its extended areas of 27 normal human temporal bone specimens were investigated histologically for the distribution of psammoma bodies. A total of 145 +/- 25 (mean +/- SE) psammoma bodies were counted in series of every tenth 30-microm-thick section. Psammoma bodies were observed in the IAC and around the labyrinthine portion of the facial nerve (FN), the geniculate ganglion of the FN, and the posterior ampullary nerve in the singular canal. The number of psammoma bodies increases with age. We believe that psammoma bodies are a normal finding of aging in the IAC. The compression of the FN by psammoma bodies in the labyrinthine portion of the facial canal and the distribution of numerous psammoma bodies surrounding the posterior ampullary nerve in the narrow singular canal raise the questions of the involvement of psammoma bodies in the FN and in vestibular dysfunction and the presence of psammoma bodies in the subarachnoid space.

Normal Canals at the Fundus of the Internal Auditory Canal: CT Evaluation

Knowledge of the normal anatomy of the four bony canals located at the fundus of the internal auditory canal (IAC) is necessary during evaluation of temporal bone trauma, congenital anomalies affecting the individual nerves, and some neuro-otologic surgeries. The purpose of this work was therefore to characterize the normal appearance of the four bony canals and to measure their dimensions. A retrospective study was performed using CT studies of the temporal bones in 50 patients to identify and characterize the bony canals for the labyrinthine segment of the facial nerve (BCFN), superior vestibular nerve (BCSVN), cochlear nerve (BCNC), and the inferior vestibular nerve (singular canal; SC) located at the fundus of the IAC. All the patients underwent high resolution temporal bone CT for evaluation of uncomplicated inflammatory (n = 49) and neoplastic (n = 1) diseases involving the temporal bone. CT studies were done using 1-mm-thick contiguous sections in axial and coronal planes. Measurements of the canals were performed by one radiologist. No patient had a prior history of trauma, vertigo, and sensorineural hearing loss or facial nerve paralysis. The BCFN, BCSVN, and BCNC were identified in all studies, whereas the SC was seen in 93% of studies. The BCFN, BCSVN, and BCNC arise from the fundus of the IAC, whereas the SC arises medial to the fundus. Mean +/- SD measurements (in mm) of the length and width were as follows: BCFN = 2.92+/-0.48 and 0.91+/-0.28; BCSVN = 2.36+/-0.53 and 0.89+/-0.28; BCNC = 0.93+/-0.21 and 2.13+/-0.44; and SC = 3.22+/-0.73 and 0.50+/-0.14. These small canals are routinely visualized on thin section (1 mm) CT of the temporal bone and should not be confused with fractures. This study provides baseline measurements that may be used to evaluate congenital anomalies of these canals. These data may also be helpful in the presurgical evaluation of patients undergoing singular neurectomies for benign positional vertigo.

Bony dehiscence between singular canal and round window niche

The incidence of bony dehiscence of the human singular canal was studied by examining 409 human temporal bones obtained from 300 individuals. Such a dehiscence was noted in three (0.7%) of the bones from three (1%) of the individuals, all male, aged 2, 6, and 76 years. All of these bony dehiscences were located between the round window niche and the cribrose area of the singular canal. The finding of a microfissure in the vicinity of the bony dehiscence in two of the three cases indicates that such dehiscences occur independently of microfissures. Although the incidence of bony dehiscences of the singular canal is small, these dehiscences appear to have considerable clinical significance in that they form a communication between the middle and inner ears or possibly between the middle ear and the cerebrospinal fluid space.

Three-dimensional computer-aided reconstruction and measurement of the temporal bone for singular neurectomy.

A computer reconstruction method was used to study five normal temporal bones for the three-dimensional anatomy necessary for singular neurectomy. The length of the singular canal was 4.9 +/- 0.6 mm. The distal portion of the singular canal courses at a sharp angle (48.2 degrees) to the direction of the transmeatal surgical approach for this operation. The reconstructions indicated that the singular canal can be exposed by introducing a drill through the external auditory meatus at a point 0.71 mm posteroinferior to the posteromedial margin of the round window on the saucerized medial wall of the round window niche. The distance from this site to the distal end-portion of the singular canal averaged 1.47 mm. The point on the drilling course (from the drilling site to the singular canal) closest to the vestibular end of the hook portion of the cochlea was 0.48-1.00 mm from the drilling site. The reconstructions also indicated that the dissection should proceed superomedially, if necessary, after a 1-mm-deep straight dissection along the transmeatal approach. The ampulla of the posterior semicircular canal, cochlear aqueduct and inferior cochlear vein were all found to lie within 2 mm of the drilling point.

Singular neurectomy update. II. Review of 102 cases

During the past 18 years, 102 singular neurectomies (SN) were performed on 95 patients with chronic cupulolithiasis of the posterior canal sense organ. Bilateral sequential singular neurectomies were performed in 7 patients and unilateral singular neurectomies in 88 patients. Ninety-nine (97%) of the 102 singular neurectomies completely eliminated the positional nystagmus and vertigo produced by the involved (undermost) ear in the Hallpike maneuver. Following three singular neurectomies (3%), there was improvement but incomplete relief from the positional nystagmus. A sensorineural hearing loss caused by the singular neurectomies occurred in four patients (4%). A thorough knowledge of the surgical and histologic anatomy of the round window niche and singular canal, together with the use of local anesthesia to allow monitoring of a vestibular response to singular neurectomy are essential for effectiveness and safety.

Radioanatomy of the singular nerve canal

The singular canal conveys vestibular nerve fibers from the ampulla of the posterior semicircular canal to the posteroinferior border of the internal auditory meatus. Radiographic identification of this anatomic structure helps to distinguish it from a fracture. It is also a landmark in certain surgical procedures. Computed tomography (CT) examinations of deep-frozen temporal bone specimens were compared with subsequently prepared plastic casts of these bones, showing good correlation between the anatomy and the images. The singular canal and its variable anatomy were studied in CT examinations of 107 patients. The singular canal could be identified, in both the axial and in the coronal planes. Its point of entry into the internal auditory meatus varied considerably.

The singular canal: a valuable landmark in surgery of the internal auditory canal.

The singular canal transmits the posterior ampullary nerve between the inferior part of the internal auditory canal (IAC) and ampulla of the posterior semicircular canal. The anatomy of the singular canal was studied in temporal bone dissections, in surgical dissections, and in high-resolution computerized tomography scans. Measurements were taken for distances between the origin of the singular canal in the IAC, the porus acousticus, the vestibule, and posterior canal ampulla. The location and importance of the singular canal are demonstrated for retrosigmoid-IAC vestibular neurectomy, retrosigmoid acoustic neuroma surgery, and transcochlear cochleovestibular neurectomy. The main purpose for the use of the retrosigmoid approach to the internal auditory canal during vestibular neurectomy and excision of acoustic neuromas is preservation of hearing. A major concern when the contents of the internal auditory canal are exposed through this approach is fenestration of the labyrinth, which results in sensorineural hearing loss. In the retrosigmoid approach, the singular canal has been found to be a vital landmark in prevention of fenestration during surgery of the internal auditory canal.

Anatomic basis for labyrinthine preservation during posterior fossa acoustic tumor surgery

Earlier diagnosis of acoustic tumors promises to increase our opportunity to identify patients with serviceable hearing. Critical to a posterior fossa transmeatal approach for acoustic tumor resection is preservation of the underlying labyrinth. Although the labyrinth has been recognized as a limiting factor in exposure of tumor in the internal auditory canal, few reports have detailed the microscopic surgical anatomy posterior to the internal auditory canal. An anatomic study was undertaken to determine consistent relationships between critical structures within the temporal bone relevant to hearing preservation surgery. The results of this study indicate that, whereas topographic landmarks are helpful for orientation, the more consistent relationship of the labyrinth to the vestibular aqueduct and singular canal allows a more accurate localization of the underlying labyrinth. Although the vestibule frequently prevents direct visualization of the transverse crest, a dissection based upon the microsurgical anatomy will maximize visualization of the lateral fundus while preserving the integrity of the labyrinth.

Singular neurectomy update.

The results of a poll revealed that 96 singular neurectomies have been performed by ten surgeons in this country. Eighty-eight of these (91.7%) resulted in complete relief of benign paroxysmal positional vertigo (BPPV). In seven patients (7.3%) there was a sensorineural hearing loss as a result of this procedure. The average hospital stay for patients having this procedure ranged from 2-6 days, and the return to work time ranged from 1 to 3 weeks. These results indicate that singular neurectomy is an effective selective vestibular ablation procedure for BPPV and that the risks and disability are comparable to other routine middle ear procedures. The causes for failure of this procedure to relieve positional vertigo are a) misdiagnosis of BPPV and b) failure to recognize the singular nerve in the middle ear. Knowledge of the pathophysiology of the disorder and of the anatomical variation in the location of the singular canal will reduce these causes of failure.

Singular Neurectomy: Hypotympanotomy Approach

A modified surgical technique is described for singular neurectomy. The hypotympanotomy approach provides greater ease in exposing the singular canal. Delayed round window overhang removal provides decreased risk of round window trauma. Eight cases done by this technique indicated a lower complication rate than any reported transmeatal series. Selection criteria, results, and complications are discussed. The risk of cochlear damage limits singular neurectomy to patients who are severely incapacitated by benign paroxysmal positional vertigo. Compared with vestibular neurectomy, singular neurectomy avoid risks inherent in intracranial procedures, and the vertigo caused by surgical intervention is less pronounced.

Further observations on posterior ampullary nerve transection for positional vertigo.

An evaluation was made on ten patients with benign paroxysmal positional vertigo (BPPV) in whom transection of the posterior ampullary nerve was performed by the middle ear approach under local anesthesia. The undermost ear in the provocative test position was selected for surgery. All ten patients were relieved of positional vertigo by the procedure. Of the five patients who were relieved of BPPV by posterior ampullary nerve transection prior to 1974, the long-term follow-up on four revealed continued relief of vertigo. Five additional patients treated by this surgical procedure since 1975 have also experienced relief from BPPV, but a moderate sensorineural hearing loss occurred in one patient. Two additional patients with BPPV were explored surgically but the singular canal could not be located. Persistence of the vertigo in these two patients strengthens the conclusion that the posterior semicircular canal sense organ is largely responsible for BPPV.

Transection of the posterior ampullary nerve for the relief of benign paroxysmal positional vertigo.

Five patients with incapacitating benign paroxysmal positional vertigo (BPPV) for one year or longer were managed surgically by transection of the posterior ampullary nerve. An approach to this nerve was devised based upon a careful study of the posterior ampullary nerve in temporal bone dissections and specimens of the inner ear. Using the round window membrane as the principal landmark, the singular canal was exposed in the round window niche via a tympanotomy approach under local anesthesia. Transection of the posterior ampullary nerve promptly and completely relieved the signs and symptoms of BPPV in all five patients. A severe sensorineural hearing loss was produced in one case as a result of traumatic labyrinthitis. Transection of the posterior ampullary nerve in the round window niche is recommended to: 1) relieve severe BPPV of one or more years duration, and 2) add to the completeness of transcanal labyrinthectomy by denervating the inaccessible posterior canal crista.