University of Sydney Rudi Schamschula Undergraduate Research Prize 1993-94

A scanning electron microscopic investigation of the human cavernous sinus

A. Wong;" R. Arnold, G. A. Doran (Supervisors)

Aim of the investigation The cavernous sinus and its dural wall is involved in

clinical problems such as meningioma, carotid-cavernous sinus fistula and thrombophlebitis. Therefore, this investigation was undertaken to:

a) Confirm or deny (using scanning electron microscopy) the venous plexiform nature of the human cavernous sinus.

b) Examine the precise relationships of cranial nerves 111, IV, V, and VI to one another and to the lateral wall of the sinus.

c) T o consider the possible function of the sinus in light of its unusual structure compared with that of the other dural sinuses.

Materials and methods Blocks containing the cavernous sinus were removed

from four cadaver heads. Two sinuses were sectioned coronally at 10 mm intervals, one sagittally and one horizontally. Some sections were critical point dried and one was dried using Peldri I1 (a fluorocarbon). All sections were platinum/gold coated and examined with a scanning electron microscope, either a JEOL 35C or 55C. t Results

Numerous venous plexuses were found surrounding the internal carotid artery. The oculomotor nerve was not embedded in the lateral dural wall as commonly described. It was surrounded by a thin meningeal envelope which divided the nerve into many fascicles, with blood vessels traversing the septa dividing the nerve.

The trochlear nerve was partially surrounded by a thin dural layer which in one instance was deficient, with endothelium of the cavernous sinus traversing the deficiency and surrounding the nerve with a meshwork similar in appearance to arachnoid tissue. The trochlear was not divided into fascicles like the oculomotor but gave off many small branches in close association with many small blood vessels.

The abducent nerve was, similarly, not embedded in the lateral dural wall but was covered laterally by a thin dural layer which divided the nerve into small fascicular bundles which were smaller and more regularly arranged than those of the oculomotor. Blood vessels and small autonomic fibres infiltrated the nerve between the fascicular bundles.

Closely related to the cavernous sinus, the cavum trigeminale is made up of the three meningeal layers; dura, arachnoid and pia with a rich vascular supply to the dura.

- *Recipient of a University of Sydney Dental Alumni Research Scholarship

tJEOL Ltd, Tokyo, Japan.

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1993-94.

The precise arrangement of the dural walls of the cavernous sinus was not as simply clear cut as described in the light microscopic literature. The lateral wall is generally described as having two distinct dural layers and the medial wall as merely periosteum. In this study there appeared to be many dural layers with the collagen fibres in various planes of orientation, comprising both lateral and medial walls of the cavernous space.

Discussion This study confirms the light microscopic, plexiform

theory of cavernous sinus structure and may help in determining its function in humans. The sinusoidal plexus nature has important surgical implications, as it would facilitate delicate surgical operations within the sinus in a relatively bloodless field, unlike the situation which would arise if the sinus was a blood lake as described in some texts.

It has been suggested that in oxen, counter-current heat exchange in the cavernous sinus between cool venous blood returning from the mucosa of the ventral nasal concha and internal carotid blood flowing to the brain may help to regulate brain temperature. Similarly, in dogs it has been shown that there are permeations in the cavernous part of the internal carotid artery.

Further support for this hypothesis is seen in one- humped camels in which some branches of the carotid rete share a common tunica adventitia with the veins of the cavernous sinus. Evidence also exists in support of an exchange system for some neuropeptides and steroid hormones in sheep and pigs.

It would seem that the human cavernous sinus might well function in much the same way, acting as a temperature regulating mechanism for neural and arterial structures passing through the sinus and maybe even hormonal exchange. After all, the pituitary is surrounded by inter- cavernous sinuses. If the function of the sinus was merely a collecting reservoir for blood from the orbits and anterior cranial fossae, why is it plexiform when other cranial venous sinuses are not, and why does the sinusoidal tissue invade the dura and surround some of the cranial nerves somewhat like an arachnoid layer?

This study is being continued in an attempt to find further structural evidence to support the hypotheses.

Address f o r correspondence: C/o G . A. Doran,

Department of Anatomy and Histology, Faculty of Medicine,

University of Sydney, Sydney, New South Wales 2006.

Australian Dental Journal 1994;39:4.