edge. An incision 15 mm posterior to this point along the tentorial edge would avoid injury to the fourth nerve.
Keywords trochlear nerve · tentorium cerebelli · tentorial segment · Skull base surgery
Introduction
trochlear nerve arises from the posterior surface of the midbrain and has the longest intracranial course. It is a slender nerve and therefore liable to injury during surgical maneuvers along the free edge of the tentorium. Surgical approaches to tumors in the upper petro-clival region and approaches to posterior circulation aneurysms often involve manipulation and incision of the free edge of the tentorium for adequate surgical exposure. Incision of the tentorium, therefore, has to be planned in such a way that the chances of trochlear nerve injury are minimized. Anatomy of the tentorial segment of the trochlear nerve therefore assumes importance. While there are many descriptions of the cis-ternal, cavernous and orbital segment of the trochlear nerve [1, 2, 5, 8], detailed anatomic description of the tentorial segment of the trochlear nerve is not available in the cur-rent literature. In the present work, we have studied the anatomical course of the trochlear nerve from the point it comes in contact with the free edge of the tentorium to the point it forms the lateral wall of the cavernous sinus.
Methods
the course of the trochlear nerve can be divided into the cisternal, tentorial, cavernous and orbital segments. the present study was focused on the tentorial segment, which
Abstract Background In traditional descriptions of the intracra-nial course of the trochlear nerve, the tentorial segment of this nerve has not been described. this segment assumes importance as it is at risk of injury during tentorial section-ing in skull base surgery. In the present study, the tentorial segment of the trochlear nerve was studied.Methods 30 cadaver sides were studied and the following parameters were measured: the parts and lengths of the ten-torial segments of the fourth nerve; the distances between the point where the third nerve came into contact with the tentorial edge and (a) the point where fourth nerve first touched the tentorium and (b) the point where fourth nerve pierced the tentorial dura; and the transverse separation between the fourth nerve and the gasserian ganglion.Results the tentorial part of the fourth nerve was found to have two segments: segment 1—from the point where the fourth nerve first came into contact with the tentorial edge to the point where it pierced the tentorium; segment 2—from the point of tentorial piercing to the point where the fourth nerve entered the cavernous sinus. the mean distance between the third nerve and the point of piercing along the tentorial edge was 9.9 ± 2.7 mm (5.29–15.32).Conclusions the most consistent and reliable anatomical landmark for avoiding injury to the fourth nerve was the point of contact between the third nerve and the tentorial
t. gupta (*) · D. Sahni Department of Anatomy, Postgraduate Institute of Medical education and Research, chandigarh 160012, Indiae-mail: [email protected]
S. K. gupta Department of neurosurgery, Postgraduate Institute of Medical education and Research, chandigarh 160012, India
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is blinded to the surgeon and comes into view after section-ing the tentorium.
cadaver heads of 15 bodies were studied. Skull cap was removed with the help of electric saw. Meninges were removed and the superior cerebral surface was exposed. the cerebral hemispheres were removed by transecting brain-stem above the level of the superior colliculi. the trochlear nerve was identified on each side in its cisternal part and the point where the trochlear nerve pierced the tentorial dura was noted. this study was focused on the part of the troch-lear nerve after its exit from the cistern. the course of the trochlear nerve from the point it comes under cover of tento-rial dura till it reaches the cavernous sinus was studied. this course could be divided into two segments.
Segment 1: the part of the trochlear nerve from the point it passed under the free edge of tentorium cerebelli (t) to the point (P) where it pierced the tentorial dura.
Segment 2: From the point where the trochlear nerve pierced the tentorial dura (P) till it reached the cavernous sinus.
Following parameters were measured with the help of dig-ital vernier caliper (Mitutoya, Japan, accurate up to 0.02 mm).
• length of the segment 1 (Fig. 1)• length of the segment 2 (Fig. 2)• Distance between the point where the third nerve first
came into contact with the tentorial edge and
(a) the point where the fourth nerve passed under the ten-torium (Fig. 1).
(b) the point where the fourth nerve pierced the tentorium (Fig. 3).
• transverse separation between medial edge of the trigeminal nerve and the trochlear nerve on the supe-rior petrosal border (attached margin of the tento-rium) (Fig. 4).
Fig. 1 course of the troch-lear nerve from the point it first comes in contact with the free tentorial edge (T) to the point it pierces the tentorium (P)—Segment 1, is depicted. Distance (OT) from the point where oculomotor nerve touches the tentorial edge (O) to the point where trochlear nerve first comes in contact with the tentorium (T)
Fig. 2 the segment 2 of the trochlear nerve is shown: from the point (P) it pierces the tentorium till it reaches the cavernous sinus
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Results (Table 1)
the length of the trochlear nerve from the point it came in relation with the free edge of the tentorium and traversed on its undersurface to the point where it pierced the tento-rium (segment 1) varied from 6.2 to 25.5 mm with a mean length of 12.5 mm. the mean length of the part of the fourth nerve course within the free edge of the tentorium till it reaches the lateral wall of the cavernous sinus (seg-ment 2) was 10 mm, again with a wide range 3.4–15 mm. the mean distance between the point where the third nerve came into contact with the tentorial free edge and the points where the fourth nerve came into contact with the tentorial edge was 23.3 mm, while the mean distance between the point of contact of the third nerve with the tentorial edge and where the fourth nerve pierced the ten-torium was 9 mm. In all cases except four sides, the point where the fourth nerve pierced the tentorium was posterior to the crossing over of the free and attached margins of the tentorium (Fig. 5). In these four sides (out of 30), the P point was on an average just a few mm (1–4 mm) anterior to the crossover (Fig. 6). the mean distance between the fourth nerve and the medial edge of the fifth nerve at the level of superior border of the petrous bone was 2.4 mm. Interestingly, in seven sides the fourth nerve was at the same level as the medial edge of the fifth nerve, that is, the horizontal separation between the two was 0, while in five sides the fourth nerve was in the same plane as the lateral edge of the fifth nerve.
Discussion
the intracranial course of the fourth nerve has been vari-ously described. traditionally, it has been divided into cister-nal, cavernous and orbital segments. Others have described the anatomy of the fourth nerve within the cisternal, tento-rial, cavernous sinus and orbital regions [1, 2, 6, 7]. Iacon-netta et al. [4] divided the course of the fourth nerve into five segments—cisternal, tentorial, cavernous, fissural and orbital. Recently, Mortazavi et al. [6] described a new seg-ment of the fourth nerve and labeled it a trigonal segment. this includes part of the fourth nerve from its entrance into posterior corner of the oculomotor trigone up to the posterior wall of the cavernous sinus. In surgical approaches involving orbito-zygomatic approaches and transtentorial approaches for skull base tumors and aneurysms, the fourth nerve is at risk of injury [3]. In its cisternal portion, the fourth nerve is readily visible in its subarachnoid space while once it reaches the free edge of the tentorium, it is hidden from the surgeon’s direct view and is, therefore, liable to injury during surgical maneuvers involving incision or sectioning of the tentorium. Knowledge of the details of the tentorial portion of the fourth nerve, therefore, may aid the surgeon. In the description of the tentorial segment by Ioconnetta et al. [4], this segment was described from the point the fourth nerve enters the tentorium to the anterior petro-clinoid fold. the cisternal anatomy of the trochlear nerve is widely described in literature [5], however, a detailed description of the tento-rial segment is not available.
Fig. 3 Distance of the point where trochlear nerve pierces the tentorial edge (P), to the point where oculomotor nerve comes in contact with the tento-rium (O) is shown
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Our cadaveric study focused on the segment of the trochlear nerve between its cisternal and cavernous parts. this part of the trochlear has been labeled as the tento-rial segment. We demonstrated that this tentorial part of the fourth nerve could be described as two distinctive seg-ments. the first segment (where the trochlear nerve comes in contact with the tentorium cerebelli to the point where it pierces the tentorium) is relatively free and, therefore, more likely to be stretched or distorted by pathologies in this region. However, the segment 2, i.e., the part of the fourth nerve from the point where it pierces the free edge of the tentorium to the point it enters the cavernous sinus is relatively fixed and its displacement occurs along with
the corresponding tentorial edge. therefore, knowledge of the anatomical relationships of this point of piercing of the tentorial edge by the fourth nerve will be of great help to avoid its injury during tentorial incision. During skull base surgical approaches in this region, the contact of the third nerve with tentorium is a readily visible and identifiable
Fig. 4 the trigeminal nerve and trochlear nerve are shown on the superior petrosal border. this is the segment 2 of the trochlear nerve which has been exposed by removing the tentorial edge and trigemi-nal nerve is exposed below the tent by removing the dura of the trigeminal cave. these two nerves are not in the same plane. Dot-ted line is drawn along the medial edge of the V nerve at the level of the IV nerve. transverse separation between these two nerves on the superior petrosal border is measured
Table 1 Morphometric measurements of various parameters in rela-tion to the trochlear nerve
Segment 1: the part of the trochlear nerve from the point it comes under the free edge of tentorium cerebelli (t) to the point (P) where it pierces the tentorial dura
Segment 2: From the point where the trochlear nerve pierces the ten-torial dura (P) till it reaches the cavernous sinus
P, the point where the trochlear nerve pierces the tentorial dura; O, the point where the oculomotor nerve first touches the tentorial edge
Parameter (n = 30) Mean (mm)
Standard deviation
Maximum (mm)
Minimum (mm)
length of segment 1 12.5 6.2 25.5 6.2
length of segment 2 10 2.8 15.09 3.36
Distance of P from third n (O)
9.9 2.7 15.32 5.29
Distance between third n (O) and point where fourth n touches tentorium (t)
23.3 4.3 30.8 15.1
Distance of fourth nerve from medial edge of fifth nerve at superior petrosal border
2.4 3.6 7.3 0
Fig. 5 Distance of the point where trochlear nerve pierces the tento-rial edge (P) to the point where free and attached margins of the ten-torium cross (C) each other is marked. In huge majority the trochlear nerve pierce the tent posterior to this crossover
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landmark. the point where the fourth nerve pierced the free edge of the tentorium was on an average 1 cm posterior to the point where the third nerve came into contact with the tentorial edge. this parameter can be of value when decid-ing where to start sectioning the tentorium. Since the maxi-mum distance between these two points was 15 mm, tento-rial incision about 1.5 cm posterior to the point of contact with the third nerve would avoid injury to segment 2 of the tentorial part of the fourth nerve. to avoid injury to seg-ment 1, the incision should start away from the free edge laterally, as this segment was seen to run just inferior to and parallel to the free tentorial edge. the lateral incision could then be extended medially with constant vision of the undersurface of the tentorium.
the lengths of the two segments as well as the morpho-metric measurements in this study showed a wide range stressing the point that there is no fixed anatomic land-mark which can be used to estimate the fourth nerve posi-tion and all surgical maneuvers require direct visualization of this nerve to avoid its injury. However, as described in the previous paragraph, the visualization of the third nerve contact with the tentorium could be of help. Another ana-tomic landmark which may be of value was the observation that in most cases, the point where the fourth nerve pierced the free tentorial edge was posterior to the crossing over of the free and attached margin. the position of the gasserian ganglion (gg) in the Meckel’s cave is relatively fixed. the
fourth nerve was found to be either medial to the medial border or just at the level of the medial or lateral border of the gg but never lateral to it. However, the gg and the trochlear nerve are in different horizontal planes.
Conclusion
As per findings of the present study, the most consistent reliable intraoperative maneuver in avoiding injury to the trochlear nerve is the identification of the third nerve at the point where it passes under the free tentorial edge. An inci-sion 15 mm behind this point along the free margin of the tentorium will always be behind the point where the fourth nerve pierces the tentorium. All the other parameters stud-ied demonstrated a wide range and, therefore, cannot be used as consistent landmarks for the purpose of avoiding trochlear nerve injury.
Conflict of interest the authors declare that they have no conflict of interest.
Ethical standard the experiments comply with the current laws of India.
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Fig. 6 Rare case where the trochlear nerve pierces the tent (P) ante-rior to the point where free and attached margins of the tentorium cross (C) each other
