Generic selectors
Exact matches only
Search in title
Search in content
Post Type Selectors
Search in posts
Search in pages
Filter by Categories
Biography
Case Report
Editorial
Letter to the Editor
Original Article
Originali Article
Recent Advances
Review Article
Updates in Neurotology
Generic selectors
Exact matches only
Search in title
Search in content
Post Type Selectors
Search in posts
Search in pages
Filter by Categories
Biography
Case Report
Editorial
Letter to the Editor
Original Article
Originali Article
Recent Advances
Review Article
Updates in Neurotology
View/Download PDF

Translate this page into:

Review Article
2025
:6;
e018
doi:
10.25259/AONO_9_2025

Cochlear Aqueduct and Its Clinical Implications

, ,
1Department of Otorhinolaryngology, Head and Neck Surgery, All India Institute of Medical Sciences, Bhubaneswar, Odisha, India

*Corresponding author: Santosh Kumar Swain, Department of Otorhinolaryngology, Head and Neck Surgery, All India Institute of Medical Sciences, Bhubaneswar, Odisha, India. santoshvoltaire@yahoo.co.in

Received: , Accepted: ,
© 2025 Published by Scientific Scholar on behalf of Annals of Otology and Neurotology
Licence
This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

How to cite this article: Swain SK, Pradhan P, Saleem S. Cochlear Aqueduct and Its Clinical Implications. Ann Otol Neurotol. 2025;6:e018. doi: 10.25259/AONO_9_2025

Abstract

The cochlear aqueduct (CA) is a narrow channel linking the basal turn of the cochlea to the subarachnoid space. It exhibits significant variation in appearance on CT imaging. Structurally, the CA is an osseous passage that extends from the posterior cranial fossa to the basal portion of the scala tympani of the cochlea. The CA is lined by connective tissue that forms the periotic duct, which serves as a conduit between the cerebrospinal fluid (CSF) in the subarachnoid space and the perilymph within the scala tympani. CA normally patents in humans and makes a balance among perilymphatic, endolymphatic, and CSF pressures. Enlargement of the CA can play a key role in dysfunctions of the inner ear. Anatomical anomalies of the CA and its accessory canals in the temporal bones of human beings have been studied with micro-CT and three-dimensional reconstruction techniques. A detailed understanding of these variations, especially near the basal turn of the cochlea and the round window niche, is crucial for preserving residual hearing and neural structures during cochlear implant procedures. There is little literature regarding CA and its clinical implications. This review article will discuss the embryology, clinical anatomy, functions, and clinical implications of CA.

Keywords

Cochlear aqueduct
CSF gusher
Inner ear
Perilymph fistula
Sensorineural hearing loss

INTRODUCTION

The cochlear aqueduct (CA) is a passage within the temporal bone that runs through the otic capsule, connecting the basal turn of the cochlea to the posterior cranial fossa.1 CA connects the scala tympani of the cochlea to the subarachnoid space.1 CA opens into the scala tympani next to the round window, close to the hook-shaped region at the base of the basilar membrane. So, meningitis may cause deafness by invading infection from the subarachnoid space to the cochlea.1 Also, in case of massive subarachnoid hemorrhage, subarachnoid blood can ascend through the CA, leading to a haemo labyrinth.2 The anatomy of CA in humans has been studied using both radiologic and histologic methods, revealing considerable variation in its size, shape, pathway, and lumen patency.2 The relationship between the anatomical characteristics of the CA and different pathological conditions remains unclear.3 The CA is commonly described as funnel-shaped, though some researchers have documented it as having an hourglass configuration, and a few have noted a mid-portion dilation.4 Here, the objective of this review article is to discuss the anatomy, physiology, imaging, and clinical implications of CA.

METHODS OF LITERATURE SEARCH

A search for research papers was conducted on the CA and its clinical implications using various methods. This began with searching online databases such as Scopus, PubMed, Medline, and Google Scholar. A search strategy was created based on Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines. The search approach found published article abstracts, and citations were used to manually find more research publications. The suitability of observational studies, comparative studies, case series, case reports, and randomized controlled trials for inclusion in this review was evaluated. A total of 58 articles (20 case reports, 17 case series, and 21 original articles) were found across various databases, with 44 being included in this review [Figure 1]. This article discusses embryology, clinical anatomy, functions, clinical implications, and imaging of the CA.

Methods of literature search.
Figure 1
Methods of literature search.

EMBRYOLOGY

The CA is formed in the mesoderm surrounding the perilymphatic duct and gradually ossifies during the process of embryogenesis.5 The CA develops from the mesoderm around the perilymphatic duct and slowly undergoes ossification during embryonic development. The inferior cochlear vein separates from the primitive CA at the 20th week of pregnancy due to increasing otic capsule ossification.5 This results in the inferior cochlear vein’s own parallel bony canal, often known as Cotugno’s canal or the first accessory canal.6 The medial boundary of the CA is formed by the bony covering of the Cotugno’s canal. The lateral wall of CA forms and Hyrtl’s fissure is simultaneously obliterated as a result of the increasing otic capsule ossification.6 The tympano-meningeal fissure (Hyrtl’s fissure) also contains perilymph, and its functions may mimic CA. Between the 16th and 26th weeks of pregnancy, the diameter of the primitive CA decreases six times, which is consistent with this partition.6 A developmental arrest prior to the partitioning of the CA can interfere with normal-size regression, resulting in an abnormally enlarged structure that retains its embryonic components. This notion is further reinforced by the presence of a common cavity malformation, suggesting that the otic capsule developed abnormally prior to the 23rd week of gestation.7 Due to distinct embryological origins of the CA and membranous labyrinth, the correlation between a larger CA and inner ear deformity has been questioned.8 The membranous labyrinth originates from the otic vesicle. The perilymphatic duct first appears as a protrusion from the subarachnoid space and is regarded as a continuation of that space.9 During development, the CA progressively ossifies from its formation in the mesoderm surrounding the perilymphatic duct. Since the membranous labyrinth and the perilymphatic duct do not share a common embryologic origin, some researchers propose that congenital inner ear dysplasia is unrelated to CA malformation.9

ANATOMY

The CA is an osseous canal that connects the basal turn of the cochlea to the subarachnoid space.2 CA [Figure 2] is an hourglass-shaped structure with the narrowest part midway between the cochlear and cranial ends.2 CA is found around 7 mm inferior to the internal auditory meatus and at the superior edge of the jugular foramen.2 The CA has an elliptical-shaped lumen, with its widest dimension lying in the horizontal plane.10 Within it runs the perilymphatic duct, which links the scala tympani to the subarachnoid space. This duct is filled with a loose network of connective tissue that, while allowing fluid to pass through, restricts the full openness of the CA.9 Most of the CA is occupied by loose fibrous connective tissue, resembling the tissue found around the endolymphatic sac.9 This connective tissue layer, which varies in length and extends into the canal, is continuous medially with the dura and arachnoid and laterally with the endosteal covering of the scala tympani.11 Therefore, the CA lacks a proper epithelium-lined duct, in contrast to the vestibular aqueduct.11 Between the subarachnoid space and the cochlea, CA travels in an angled downward path. The CA can be broadly classified into four sections, from lateral to medial, including the cochlear opening, lateral otic capsule segment, medial petrous apex component, and cranial opening, despite differences in shape and course.12 The CA-round window membrane complex is formed by the cochlear opening, which is seen in the scala tympani of the cochlea’s basal turn.13 The petrosal fossa, located just below the external aperture, contains the glossopharyngeal nerve, which is sometimes enclosed within a complete or partial bony canal.14 The opening of the CA should not be mistaken for the pars nervosa of the jugular foramen. Its funnel-like end is covered by a dural sheath that extends inward to varying lengths. The remaining portion of CA contains a loose connective tissue meshwork surrounding a central lumen.4 Study with an electron microscope shows mesenchymal cell axes that are randomly oriented inside the CA. On the outside of the CA, fibrocytic tissue is found.15 The central duct of the CA contains perilymph, enabling communication between the perilymphatic space and the subarachnoid space. Perilymph and cerebrospinal fluid (CSF) share similar compositions, differing slightly in electrolyte concentrations since perilymph is believed to originate from CSF. This supports the hypothesis that the CA facilitates fluid transport into the inner ear.

Cochlear aqueduct in computed tomography (CT) temporal bone.
Figure 2
Cochlear aqueduct in computed tomography (CT) temporal bone.

Types of CA

CA can be categorized into four types: Type 1: CA is fully visible from the cochlea to the external (medial) opening; Type 2: CA is visible in the inner two-thirds of the duct; Type 3: CA appears only at the external opening and/or inner third; and Type 4: CA is not seen in any part of the CT images.16 Type 1 CA is typically seen as a continuous structure extending from the basal turn of the cochlea to its opening into the subarachnoid space. Type 2 CA is generally observed within the medial two-thirds of the duct. In Type 3 CA, only the external opening and/or medial third are visible. Type 4 CA is not seen on CT imaging.

Patency of CA

Unlike the vestibular aqueduct, which contains the endolymphatic duct, the CA is often described by anatomists as lacking a true lumen and instead being filled with loose connective tissue meshwork.17 Based on histological analysis, the patency of the CA can be categorized into four types: (1) patent lumen, where a clear lumen is present along the entire length of the CA; (2) lumen containing loose connective tissue, where connective tissue occupies the lumen in at least one section; (3) lumen obstructed by bone, characterized by a bony plug completely filling the lumen and distinct from the CA itself in at least one area; and (4) CA obliteration, where the aqueduct is partially or entirely absent throughout its course.4 It is doubtful whether the role of age plays in modifying the patency or width of the lumen of CA.18

FUNCTIONS OF CA

The exact physiological role of the CA remains unclear, and the clinical implications of its enlargement are still debated. However, studies in laboratory animals have experimentally confirmed that a patent CA serves as a communication pathway between subarachnoid space and perilymph, allowing fluid exchange.19 CA serves several non-acoustic roles, including facilitating the exchange of biochemical and cellular components between the inner ear and CSF. It also helps regulate abnormal pressure fluctuations within the cochlea. Additionally, CA is thought to buffer respiratory- and cardiac-induced CSF pulsations, thereby safeguarding cochlear function. A reduction in CSF pressure following a lumbar puncture can result in sensorineural hearing loss. This procedure may cause a decrease in CSF volume, resulting in lower pressure within the CSF compartment. Since the CSF space is connected to the perilymphatic space via CA, a pressure difference can form, prompting perilymph to shift into CSF space. This fluid shift reduces the volume of both the perilymph and endolymph, potentially triggering a compensatory expansion or hydrops of endolymphatic space.20 The flow rate of fluid through the CA is directly proportional to pressure, viscosity, and the length of the duct, while the radius of the lumen influences flow to the fourth power.21

CLINICAL IMPLICATIONS

Although its precise function is unclear, the CA is crucial for preserving the fluid and pressure balance between the inner ear and CSF.22 It is believed that the CA’s small diameter protects the inner ear from the significant pressure changes found in the posterior fossa subarachnoid regions.23 It may play a role in the pathogenesis of disorders of the inner ear, such as bacterial meningitis, where it acts as a conduit for spreading infection from CSF to the inner ear.24 Enlarged CA may play a role in the pathogenesis of congenital hearing loss and in the risk of perilymphatic gusher in stapes surgery and cochleostomy during cochlear implantation. The anatomical relationship of CA has a significant surgical implication, as the transpromonitorial approach to the internal auditory canal from below the level of the CA may expose the lower cranial nerves and the jugular vein, thereby increasing the risk of damaging these structures. Because the internal auditory canal is located close to the jugular foramen, a high-riding jugular bulb can be a relative contraindication for transpromontorial approaches, as it limits how far downward the surgical dissection can go due to the vein’s position. A patent CA has been suggested as a potential path for allowing bacterial infection transmission between perilymph within the labyrinth and subarachnoid space.25 Following a subarachnoid hemorrhage, red blood cells may also go to the CA and reach the basal turn of the cochlea.26 Subarachnoid blood may ascend through CA in a severe subarachnoid hemorrhage, causing a hemolabyrinth.27 It has been proposed that larger and excessively patent CA causes increased inner ear pressure, which in turn causes CSF otorrhea, a complication of stapedectomy (stapes gusher).28 It is believed that a defect in the bony partition of the fundus of the internal auditory canal causes a direct link between the inner ear and the subarachnoid space, which is the pathophysiology of perilymph fistulas.29 Congenital enlargement of the internal auditory canal is also an expected site for CA associated with severe inner ear malformation. In the case of otosclerosis, where bone resorption and deposition of new bone occur, there are certain changes in CA. In otosclerosis, the length of the CA and funnel width are statistically longer and narrower compared to the control group.30 Patients with Meniere’s disease had thicker round window membranes, according to a study.31 Nonetheless, there is still uncertainty regarding the histological state of CA and its connection to Meniere’s illness. The bony conduit of CA in Meniere’s illness is readily seen in high-resolution computed tomography (HRCT) of the temporal bone.25 There are no differences in the bony width or length of the CA between individuals with Meniere’s disease and those without the condition.32 High-resolution magnetic resonance shows fluid in the CA among patients with Meniere’s disease.33 HRCT scan of the temporal bone and high-resolution magnetic resonance imaging (MRI) of the temporal bone shows fibrous tissue in the non-fluid part of CA.12

IMPACT OF ENLARGED CA

The enlargement of the CA is a topic of controversy. Many experienced investigators are suggesting that enlarged CA is perhaps a non-existent malformation.9 The evaluation of CA on HRCT scans of temporal bones using 1.5 mm slice thickness focuses on its normal appearance.9 The diameter of the CA varies and depends on the particular segment being examined. This finding aligns with those reported in previous research.34 An enlarged CA has been proposed as a potential factor contributing to sensorineural hearing loss and perilymphatic fistula.35 Enlarged CA is also associated with stapes gusher and trans-otic CSF leak.

IMAGING OF CA

The CA is regarded as a challenging anatomical entity to visualize in imaging.36 The assessment of normal CT pictures of CA in HRCT temporal bone (1 mm thick sections) reveals that the diameter of CA is based on the specific segment being studied.36 Micro-CT with three-dimensional reconstruction provides new possibilities for the evaluation of minor details of CA.36 In CT scans of 100 temporal bones, the diameter of the CA media aperture was found to be highly variable, ranging from 0 to 11 mm, with a mean of 4.5 mm.9 In contrast, the otic capsule segment is very narrow in all cases and can be visualized in only 56% of cases, none of which exceed 2 mm in diameter.9 The difficulty in identifying parts of the CA is most likely attributed to its very small diameter, which anatomical studies have shown to measure between 0.1 and 0.2 mm at the midsection.2 CT imaging has made it possible to divide the course of the CA into four segments. The lateral opening is a narrow entrance where the bony aqueduct connects to the basal turn of the cochlea. It is located along the anteroinferior margin of the scala tympani, just in front of the crest where the round window membrane attaches.37 The lateral orifice opens into the otic capsule segment, which runs medially through the dense bone of the labyrinth. This segment is directly connected to the petrous apex segment on the medial side. The petrous apex part travels through the bone and may contain air cells (pneumatized) or bone marrow. It eventually opens into the subarachnoid space near the pars nervosa of the jugular foramen via a funnel-shaped opening on the medial side.9 The importance of high-resolution MRI techniques in the assessment of CA is not yet done in current literature. The bony dimensions of CA are normal in Meniere’s disease. The fluid length of CA might be reduced in Meniere’s disease.38

CONCLUSION

The CA plays a role in equalizing the pressures between the perilymph, endolymph, and CSF. It serves as a connection between the subarachnoid space of the posterior fossa and the basal turn of the cochlea. Enlargement of CA is a distinct entity that may be diagnosed by an HRCT scan of the temporal bone. CA can play an important role in transmitting bacterial infection to the inner ear in an individual with meningitis. The proper understanding of the surgical anatomy of CA and its anatomical relationship is crucial for performing safe, minimally invasive lateral skull base surgery while avoiding damage to the lower cranial nerves and other vital structures.

Acknowledgments

The acknowledgments for this review are extended to those who have previously contributed to and conducted research on the same topic. This recognition acknowledges the existing body of work and the collective efforts of researchers in the field. The author expresses gratitude to those who contributed to the development and execution of this research, acknowledging their invaluable support and collaboration. Use of Artificial Intelligence is not applicable to this manuscript.

Ethical approval

Institutional Review Board approval is not required.

Declaration of patient consent

Patient’s consent not required as there are no patients in this study.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Use of artificial intelligence (AI)-assisted technology for manuscript preparation

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

REFERENCES

  1. . The Cochlear Aqueduct and Its Significance in Hearing Pathology. Literature Review and a Clinical Case. Neurosci Behav Physiol. 2025;55:1-5.
    [Google Scholar]
  2. , , , , , , , , . Computed Tomographic 3D Analysis of the Cochlear Aqueduct—Potential and Limitations of Clinical Imaging. Acta Otolaryngol. 2023;143:931-5.
    [CrossRef] [PubMed] [Google Scholar]
  3. , , , , . Diameter of the Medial Side of the Cochlear Aqueduct Is Narrower in Meniere’s Disease: A Radiologic Analysis. J Int Adv Otol. 2016;12:156-60.
    [Google Scholar]
  4. , , . Anatomy of the Normal Human Cochlear Aqueduct with Functional Implications. Hear Res. 1997;107:9-22.
    [CrossRef] [PubMed] [Google Scholar]
  5. , , , , . Cochlear Aqueduct Post-Natal Growth: A Computed Tomography Study. J Assoc Res Otolaryngol. 2024;25:611-7.
    [Google Scholar]
  6. , , , , , . Cochlear Aqueduct Revisited: A Histological Study Using Human Fetuses. Ann Anat. 2024;253:152236.
    [CrossRef] [PubMed] [Google Scholar]
  7. . Embryology and Developmental Anatomy of the Ear. In:. In: , , , eds. Pediatric Otolaryngology.. Vol Vol. 1. Philadelphia, Pa: Saunders; . p. :77-87.
    [Google Scholar]
  8. , . The Morphologic Basis for Perilymphatic Gushers and Oozers. Adv Otorhinolaryngol. 1988;39:1-12.
    [CrossRef] [PubMed] [Google Scholar]
  9. , . Enlargement of the Cochlear Aqueduct: Fact or Fiction? Otolaryngol Head Neck Surg. 1993;109:14-25.
    [Google Scholar]
  10. , , , , , . Cochlear Aqueduct Morphology in Superior Canal Dehiscence Syndrome. Audio Res. 2023;13:367-77.
    [Google Scholar]
  11. , , , , . Evaluating Common Cavity Cochlear Deformities Using CT Images and 3D Reconstruction. Laryngoscope. 2021;131:386-91.
    [CrossRef] [PubMed] [Google Scholar]
  12. , , , , . Radiological Findings of the Cochlear Aqueduct in Patients with Meniere’s Disease Using High-Resolution CT and High-Resolution MRI. Eur Arch Otorhinolaryngol. 2014;271:3325-31.
    [CrossRef] [PubMed] [Google Scholar]
  13. , , , . The Relationship of the Round Window Membrane to the Cochlear Aqueduct Shown in Three-Dimensional Imaging. Hear Res. 2005;209:19-23.
    [CrossRef] [PubMed] [Google Scholar]
  14. , , , , . A Radiologic-Anatomical Comparative Study of the Cochlear Aqueduct. Clin Radiol. 2000;55:288-91.
    [Google Scholar]
  15. , , , . Systematic Review and Meta-Analysis of Cochlear Implantation in Deaf Patients with Large Vestibular Aqueduct Deformity. Ann Palliat Med. 2021;10:12598-606.
    [CrossRef] [PubMed] [Google Scholar]
  16. , . Radiology of the Cochlear Aqueduct. Ann Otol Rhinol Laryngol. 2005;114:863-6.
  17. , , , , , . Review of Temporal Bone Microanatomy: Aqueducts, Canals, Clefts and Nerves. Clin Neuroradiol. 2020;30:209-19.
    [CrossRef] [PubMed] [Google Scholar]
  18. . Studies on Cochlear Aqueduct Patency. Ann Otol Rhinol Laryngol. 1978;87:22-8.
    [Google Scholar]
  19. . Cochlear Deformities and Its Implication in Cochlear Implantation: A Review. Int J Res Med Sci. 2022;10:2339-45.
    [Google Scholar]
  20. , , , , , . Hearing Loss and Cerebrospinal Fluid Pressure: Case Report and Review of the Literature. Ear Nose Throat J. 2008;87:144-7.
    [Google Scholar]
  21. . Fluid Flow in the Cochlear Aqueduct and Cochlea-Hydro Dynamic Considerations in Perilymph Fistula, Stapes Gusher, and Secondary Endolymphatic Hydrops. Am J Otol. 1987;8:319-22.
    [PubMed] [Google Scholar]
  22. . Audiovestibular Manifestations During Pregnancy: A Review. Int J Res Med Sci. 2022;10:1809-14.
    [Google Scholar]
  23. . Enlargement of the Cochlear Aqueduct: Does It Exist? Eur Arch Otorhinolaryngol. 2011;268:1655-61.
    [Google Scholar]
  24. . Pathology of the Ear. 1993:7-11.:216-11.
    [Google Scholar]
  25. , , , , , . Hearing Loss and Pneumococcal Meningitis: An Animal Model. Laryngoscope. 1991;101:1285-92.
    [CrossRef] [PubMed] [Google Scholar]
  26. , , , , . Cochlear Implantation in Children with Enlarged Vestibular Aqueduct: A Systematic Review of Surgical Implications and Outcomes. Ear Hear. 2023;44:440-7.
    [CrossRef] [PubMed] [Google Scholar]
  27. , . The Otic Capsule and Otodystrophies. In:. Imaging of the Temporal Bone. 1998:240-317.
    [Google Scholar]
  28. , , , . Endoscopic Transcanal Stapedotomy: Our Experiences at a Tertiary Care Teaching Hospital of Eastern India. Indian J Otol. 2020;26:80-4.
    [Google Scholar]
  29. . Congenital Cerebrospinal Fluid Fistula of the Petrous Temporal Bone. Clin Otolaryngol. 1986;11:79-82.
    [CrossRef] [PubMed] [Google Scholar]
  30. , , , , , . High-Resolution Computed Tomography of the Inner Ear: Effect of Otosclerosis on Cochlear Aqueduct Dimensions. Ann Otol Rhinol Laryngol. 2019;128:749-54.
    [Google Scholar]
  31. , , , , , . Round Window Membrane in Meniere’s Disease: A Human Temporal Bone Study. Otol Neurotol. 2011;32:147-51.
    [CrossRef] [PubMed] [Google Scholar]
  32. , , , , , . Computed Tomography of the Inner Ear: Size of Anatomical Structures in the Normal Temporal Bone and in the Temporal Bone of Patients with Meniere’s Disease. Eur Radiol. 2005;15:1505-13.
    [Google Scholar]
  33. , , , , , . The Jugular Foramen: Imaging Strategy and Detailed Anatomy at 3T. Am J Neuroradiol. 2009;30:34-41.
    [Google Scholar]
  34. , , . High-Resolution Computed Tomographic Appearance of Normal Cochlear Aqueduct. AJNR Am J Neuroradiol. 1984;5:715-20.
    [Google Scholar]
  35. , , , , , . Cochlear Implantation in Pediatric Patients with Enlarged Vestibular Aqueduct: A Systematic Review. Laryngoscope. 2022;132:1459-72.
    [CrossRef] [PubMed] [Google Scholar]
  36. , , , , , . The Human Cochlear Aqueduct and Accessory Canals: A Micro-CT Analysis Using a 3D Reconstruction Paradigm. Otol Neurotol. 2018;39:429-35.
    [CrossRef] [PubMed] [Google Scholar]
  37. , , . High-Resolution Computed Tomographic Appearance of Normal Cochlear Aqueduct. AJNR Am J Neuroradiol. 1984;5:715-72.
    [Google Scholar]
  38. . Meniere’s Disease in the Pediatric Age Group - A Review. Int J Contemp Pediatr. 2022;9:693-7.
    [Google Scholar]

Fulltext Views
2,280

PDF downloads
1,434
View/Download PDF
Download Citations
BibTeX
RIS
Show Sections

Suggested read for related articles: