Acta Neurochirurgica

, Volume 159, Issue 11, pp 2147–2148 | Cite as

Petrobasilar, petroclival, or petrosphenoidal canal of the abducens nerve

  • Gianfranco Pizzolorusso
  • Andrea Cirotti
  • Felice Pizzolorusso
Letter to the Editor (by Invitation) - Neurosurgical Anatomy

In the recently published case report entitled Anatomic variation of the abducens nerve in a single cadaver dissection: thepetrobasilar canal” we described the exceptional course of the abducens nerve, cranial nerve VI (CNVI) [5]. Generally, CNVI exits the pontomedullary sulcus as a single trunk, ascending in a rostral and lateral direction to reach the petrous apex. At the petrous apex the nerve pierces the visceral dura, passing over the temporal bone and below the dural sheath of the petrosphenoidal ligament in most cases, with the exception that a branch of a duplicated CNVI or the nerve itself can course above the ligament [1, 2, 3, 4].

In our report the dissection of the skull base showed an unusual course of CNVI at the level of the petroclival region. The cadaver under study was from an elderly female of 74 years with no reported clinical data relevant to cranial nerves. A diagram of the cranial base (Fig. 1) has been drawn along the same lines of the dissected skull to shed light on the course of CNVI. As shown in the diagram (Fig. 1), the right CNVI presented with one single branch, did not enter Dorello’s canal and ran within the petrobasilar suture through a narrow canal that we called the ”petrobasilar canal” [5].
Fig. 1

Schematic diagram of the cranial base. The course of both abducens nerves (yellow) is shown. After leaving the brainstem (orange), at the level of the petroclival region, the right abducens nerve passes below the petrous bone (red) through the petrobasilar canal (black hatch). Sphenoid bone in brown. Petrobasilar and occipitomastoid sutures in gray

To the best of our knowledge, our finding is difficult to place in the current literature since similar anatomic variations of CNVI have never been described. Only the reports by Wegner [6] and Özveren [4] are worth being considered carefully. More specifically, in 1920 Wegner showed that the posterior clinoid process of a gibbon extended as a broad plate posteriorly and, underneath it, the left CNVI traveled through a particular closed canal that he named “canalis nervi abducentis” [6]. In such manner, the “petrobasilar canal” resembles the “canalis nervi abducentis” described by Wegner and comprised in the posterior clinoid process of the sphenoid bone.

Moreover, in a study on the duplication of CNVI at the petroclival region, Özveren [4] described one CNVI of 50 under study with an unusual course. It presented with two branches having different courses: the smaller branch traveled over the apex of the petrous bone, the larger through a bony canal named the “petrosphenoidal canal”. The “petrosphenoidal canal”, as outlined by Özveren, was formed by the junction of the petroux apex with the superolateral border of the clivus [4].

As a result, we reported a similar variation noted by Özveren with the difference that the nerve passing through the canal between the petrous bone and the clivus had a single branch. However, our study brings into question whether the course of CNVI under the petrous bone could have been described using the terms “petrosphenoidal” or “petroclival”.

Given the findings of our study, we did not consider the term “petrosphenoidal” since no anatomic relationship was found with the sphenoid bone, and the course of CNVI below the petrous bone was significantly below the dorsum sellae. We acknowledge that the location of the petrobasilar canal, as we have described, is at the level of the petroclival region, but the passage of the nerve through the petrobasilar suture is visible and not questionable. Thus, we think that the so-termed “petrobasilar canal”, comprised in the suture, reflects what we noticed and documented when we studied the course of CNVI.


Compliance with ethical standards

Conflict of interest

All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript.

Ethical approval

For this type of study formal consent is not required.


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Copyright information

© Springer-Verlag GmbH Austria 2017

Authors and Affiliations

  1. 1.Research DepartmentAccademia di Osteopatia e Anatomia (A.O.A.)AndriaItaly
  2. 2.Research DepartmentAccademia Italiana Osteopatia Tradizionale – Marche (A.I.O.T. Marche)Civitanova MarcheItaly
  3. 3.ChietiItaly

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