Year : 1978 | Volume
: 26 | Issue : 4 | Page : 18--22
The relationship between the third cranial nerve and posterior clinoid process
B Joseph, KK Bisaria, A Halim
Deparment of Anatomy, King George's Medical College, Lucknow, (U.P.), India
K.G.«SQ»s Medical College, Lucknow, U.P
|How to cite this article:|
Joseph B, Bisaria K K, Halim A. The relationship between the third cranial nerve and posterior clinoid process.Indian J Ophthalmol 1978;26:18-22
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Joseph B, Bisaria K K, Halim A. The relationship between the third cranial nerve and posterior clinoid process. Indian J Ophthalmol [serial online] 1978 [cited 2013 May 20 ];26:18-22
Available from: http://www.ijo.in/text.asp?1978/26/4/18/31496
It is generally accepted that increased intracranial pressure may cause cerebral herniation and brain stem diplacement and distortion, accompanied by characteristic changes in the patients' neurological picture and vital signs. The most convincing evidence of herniation is a dilated pupil or other signs of third nerve dysfunction. Hutchinson's pupil or most often results from cerebral oedema; subdural or intracerebral haematoma forces the uncus of the hippocampus downwards, thus pressing on the superior aspect of the adjacent third nerve. The third nerve palsy has been variously attributed to direct pressure by herniated tissue, midbrain ischaemia, compression by the petroclinoid ligament and compression or distortion of the nerve as it enters the cavernous sinus.
The most frequent explanation is that the nerve is kinked across a displaced posterior cerebral artery or trapped between the posterior cerebral and superior cerebellar arteries. This nerve is described to run an exposed course while crossing the hiatus of the tentorium and also on the dural roof of the cavernous sinus against the firm surface of which it may be compressed'. Another explanation of palsy is that there may be shifting backward of brain stem.
The oculomotor nerve may be pressed on by hardening or an aneurysm of any of the arteries with which it is in close relation, namely, the posterior cerebral, superior cerebellar, basilar, posterior communicating, and internal carotid.
The present study was undertaken to find out if the oculomotor nerve is likely to be stretched over the top or side of the posterior clinoid process or the superior border of the petrous temporal bone. In the case of abducent nerve palsy the sharp upper border of the petrous temporal over which the nerve beads has been held responsible when the intra cranial pressure is increased and the brain stem herniates through the foramen magnum and moves downwards.
Material and methods
We studied cranial cavities of 50 adult cadavers. Out of these 33 were male and 17 female. The skull cap was removed by a cut, anteriorly, fully three quarters of an inch above the margins of the orbits, and posteriorly about half an inch or less above the external occipital protuberance. The cerebal hemispheres were exposed by making four flaps of the duramater. The falx cerebri was detached from the crista galli and pulled backwards. The frontal lobes of the brain were gently raised with the fingers. The optic nerves, internal carotid arteries and infundibulum were then divided. The oculomotor nerves were seen passing forwards by the sides of the dorsum sellae. The temporal lobe was then raised from the floor of the middle cranial fossa and from the tentorium cerebelli. The mid brain was cut from its lateral surface inwards to the median plane immediately above the level of the oculomotor nerve. The operation was repeated on the opposite side. The cerebrum was removed from the skull dividing the veins at the sinuses.
The anterior and posterior clinoid processes, the oculomotor nerves, the free and attached borders of the tentorium and the superior border of petrous temporal bone were clearly visible. Observations were taken on both the sides and important variations were photographed. The petro-sphenoidal ligament which is a fibrous band connecting the lateral maigin of the dorsum sellae to the under border of the petrous part of temporal bone near its medial end was dissected and its relationship to the third nerve was studied. Afterwards the skull bases were severed from the rest of the body and were boiled and cleaned to see the types of posterior clinoid processes and medial portion of the superior border of petrous temporal bone.
The distance rom posterior clinoid process to the point of crossing of free and attached margins of tentorium cerebelli (a to b) was as under:
The variation was found to be due to the differences in the types of posterior clinoid processes.
The oculomotor nerve crossed any where between these two extreme points. By measuring the distance a to c the oculomotor nerve was found to be having the following relationship in this region.
No significant difference was noticed in the relationship in males and females.
The part of the oculomotor nerve lying exposed on the top of cavernous sinus for a distance from the tip of the posterior clinoid process C to the point of its entry d is shown in [Table 3].
Earlier authors have attributed the third nerve palsy due to compression by the petro clinoid ligament in the region on which we have taken observations. Petro-clinoid or petro sphenoidal ligament is situated quite low down and much below the attached border of tentorium cerebelli. We did not find the oculomotor nerve related to this ligament. It was found to be related to the side of the dorsum sellae in 53% of cases. The attached border of tentorium cerebelli was seen extending upto the posterior clinoid process and was seen being crossed by the oculomotor nerve at about its middle in 61% of cases. The nerve was seen lying on the attached border of tentorium near its crossing with free border in 25% cases. In this situation it was on the bony superior border of petrous temporal bone separated only by the thickness of the duramater. Only in 1% of the cases the third nerve crossed medial to the tip of the posterior clinoid processes.
In cases where the third nerve was skirting the dorsum sellae it was noticed that dorsum sellae had no posterior clinoid process worth mentioning (53%. In cases where the third nerve was crossing about the middle of the attached border of tentorium, the posterior clinoid processes were prominent, but presented blunt end (21 %). The posterior clinoid processes were sharp in 25% of cases where the nerve was lying away from them at the crossing of free and attached borders of tentorium. In 1% where the third nerve crossed medial to the tip, the posterior clinoid process presented a backward projecting spine and was itself seen going forwards towards the anterior clinoid process. In Kthis case the middle clinoid process was present and formed a carotid foramen.
Our study clearly demonstrates that the third nerve is liable to be stretched on the sharp attached border of tentorium cerebelli and the superior border of petrous temporal bone. Only in a very small percentage it can be kinked by the posterior clinoid process.
[Table 3] shows that the part of the oculomotor nerve lying exposed on the top of the cavernous sinus is on an average 6.6 mm. This is the part which can be pressed by cerebral oedema or displaced uncus.
On the basis of the evidence from the present study we conclude that.
The third nerve skirts the side of dorsum sellae in 53% of cases and in these cases the posterior clinoid process is absent.In 25% of cases the posterior clinoid process is very sharp and pointed and in these bodies the third nerve is nearer the attached and free margins of tentorium cerebelli.In 21% of cases the third nerve lies midway between the posterior clinoid process and the crossing of free and attached margins of tentorium cerebelli. The posterior clinoid processes are present but are blunt.In 1 % of cases the third nerve crosses medial to the tip of the posterior clinoid process and is precariously related to it. The posterior clinoid process in this case was very large and prominent.
It is therefore possible that in all these four positions the nerve can get stretched by displacement of brain stem downwards.
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