Morphometric analysis of the abducens nerve in the petroclival region

Abstract

Background: The abducens nerve (AN), our sixth cranial nerve, is responsible for the innervation of the lateral rectus muscle of the eye. The abducens nerve is a vulnerable structure at the skull base with its long intracranial course and complex topographic relationships. The AN anatomy in the petroclival region, where the nerve passes from the posterior to the middle cranial fossa, is of great interest for neurosurgical procedures. Despite detailed studies of its anatomy from the past 150 years, there is a need for more recent data on macroscopical and microscopical aspects of the AN in well defined populations.

Methods: We investigated macroscopical variations and the number of nerve fibers of the AN in the petroclival region in German body donors.

Results: In our histological samples (n = 24) we counted 4688 (+/-1,041) nerve fibers per AN. There was no correlation between sex, age and body side regarding the number of nerve fibers. In our macroscopic examination (n = 76 skull base sides), we found six duplications (four left-sided, two right-sided; 7.9%) and one triplication (right-sided; 1.3%) of the AN in the petroclival region. The AN triplication was further examined: Three nerve bundles pierce the dura mater separately and united before passing under the petrosphenoidal ligament (of Gruber).

Conclusion: Variations of the AN in the petroclival region are not a rare phenomenon but occur very frequently. Consequently, we have developed a new classification system for AN variations. This knowledge might help neurosurgeons, as it prepares them to be aware of such variations and adapt their surgical approaches accordingly.

Keywords: anatomical variation; cranial nerve; dura mater; neurosurgery; skull base.

Figure 1

Number of AN fibers in our histological samples. In the boxplots, comparison between the left and right side, as well as the total amount, is shown.

Figure 2

Histology of the duplicated AN. (A) Main fiber bundle (inferior bundle) of the AN. The main trunk is organized in twelve bundles that are separated by thin layers of connective tissue. (B) Accessory AN branch organized in three small bundles. Bar = 500 μm.

Figure 3

Number of AN dural pori. In most cases (90.8%), only one dural porus was present. Dural porus duplication was observed in 7.9%, and dural porus triplication in 1.3%.

Figure 4

Different types of abducens dural entrances. (A) Single left dural porus (white arrow) on the clivus at its textbook position (lateral to the midline of the clivus). (B) Duplicated left dural porus. The main dural entrance is marked with a white arrow. The white arrowhead points to the accessory porus, which is located superiorly and laterally to the main porus. Red dotted line = location of Gruber’s ligament. Red dot = location of the posterior clinoid process.

Figure 5

Macroscopic anatomy of AN trifurcation. (A) Right side of the clivus with main dural porus (arrow) and two accessory dural pori (arrowhead, asterisk). (B) Subdural course of the AN fibers. All three fiber bundles merge before entering Dorello’s canal. Red dotted line = location of Gruber’s ligament. Red dot = location of the posterior clinoid process.

Figure 6

Topography of AN triplication. (A) Pseudobranching in the cavernous sinus (CS). (B) Pseudobranching under the clival dura mater (DM) as described by (48). The AN pierces the DM through a single porus (C) trifurcation as described by (46). Three branches pierce the DM through three dural pori. While the most inferior fiber bunde lies below Gruber’s (GL), the two other bundle lie above the GL. All three bundles merge in the CS. (D) Present case. Three branches pierce the DM through three dural pori. All three fiber bundles merge before entering Dorello’s canal. (E) Trifurcation as described by (54). Three branches pierce the DM through three dural pori. All fibers lie below GL and merge within the CS at the posterior aspect of the internal carotid artery (ACI). PP, petrous pyramid; DS, dorsum sellae; SOF, superior orbital fissure; RLM, rectus lateral muscle.

Figure 7

Categories of the AN branching: A = loop shaped AN. AI: loop starts in the subarachnoid segment and the branches merge in the cavernous sinus (12, 46, 50, 54, 55, 69, 91). AII: looping starts in the petroclival segment and merging in the cavernous sinus (92). AIII: looping exclusively in the cavernous sinus (, , –55). AIV: looping starts in the subarachnoid region and merging in the petroclival segment (55). AV: Tripliation exclusively in the petroclival region DiDio et al. (48). AVI: pseudobranching into three or more rootlets exclusively in the cavernous segment (17, 47, 52, 53, 55). AVII: looping exclusively in the petroclival region (49). Type B: multiple trunks merging. BI: two trunks merging in the petroclival segment (49). BII: two trunks merging in the cavernous sinus (, , , , , –55, 69), BIII: two trunks merging in the prepontine cistern (49, 54). BIV: three trunks merging in the cavernous sinus (46, 54). BV: three trunks merging in the petroclival segment (present study). C = one strand splitting into multiple trunks. CI: one strand splitting into two trunks in the cavernous sinus (7, 53). CII: one strand splitting into two trunks in the fissural segment (57). D = two separate trunks travelling from the brainstem to the orbit. DI: two nerve strands travelling from brainstem to orbit (45, 86). DII: Variation in which the two branches travel separately and have a short connection at the fissural segment (87).