Foramen magnum

AVMs of the perimedullary fistula type are direct AV shunts that are located on the ventral or dorsal surface of the spinal cord or the conus medullaris, usually in the thoracolumbar area, occasionally thoracic, and rarely cervical. Their location thus is intradural, intra- or extramedullary. They are always supplied by spinal cord vessels, either by the anterior spinal artery (ventrally) or by a posterolateral artery (dorsally), depending on their location. They drain into spinal cord veins (Fig. 17.12). Drainage may even ascend up to the foramen magnum or into the posterior fossa. 

The meninges are supplied by branches of the external carotid artery, internal carotid artery and vertebral arteries. The most prominent branches from the external carotid artery are the middle meningeal artery and tributaries of the ascending pharyngeal and occipital arteries. Most of the branches from the internal carotid artery arise near the cavernous sinus and from the ophthalmic artery in the orbit. Branches from the vertebral artery arise at the foramen magnum. There are numerous meningeal anastomoses between these small arteries. 

Expose the skull and cut along the sagittal suture from the foramen magnum to the forehead with a pair of surgical scissors (see Section 2.1.2.) the sharp pointed blade of the scissors should be placed on the inner side of the skull. 

Using a bone cutter, make one cut at the foramen magnum on each temporal side of the skull and expose the brain by carefully prying the skull open. 

Make an incision along the midline of the head and peel the skin away to expose the skull. Using the small scissors, remove the neck muscles and cut along the midline of the skull from the foramen magnum to the interhemispheric sulcus follow this with two lateral cuts... 

Fig. 20.8. The measurements used in assessment for atlanto-occipital dissociation. B, tip of basion C, spinolaminar junction of atlas A, posterior aspect of the anterior arch of atlas 0, posterior lip of foramen magnum D, tip of dens. Powers ratio BC/OA should normally be less than 1. The distance BD should normally be less than 12.5 mm...

DAVMs may occur anywhere within the cranium or in the spinal column. Intracranial DAVMs are located either in the anterior cranial fossa on or around the ethmoid groove (Fig. 4.3,1), in the middle cranial fossa at the cavernous sinuses (Fig. 4.3,2), in the posterior fossa at the transverse (Fig. 4.3,3) or the sigmoid (Fig. 4.3,4) sinuses, at the confluens sin-uum (Fig. 4.3,5), or around the foramen magnum... 

Fig. 4.3. Typical locations of intracranial dural arterial malformations. 1, anterior fossa 2, cavernous sinus 3, transverse sinus 4, sigmoid sinus 5, confluens sinuum 6, foramen magnum 7, tentorial incisura 8, base of the tentorium 9, straight sinus and vein of Galen...

Foramen magnum Clival venous plexus, spinal perimedullary veins... 

Quinones D, Duckwiler G, Gobin PY et al (1997) Embolization of dural cavernous fistulas via superior ophthalmic vein approach. AJNR Am J Neuroradiol 18 921-928 Ratliff J, Voorhies RM (1999) Arteriovenous fistula with associated aneurysms coexisting with dural arteriovenous malformation of the anterior inferior falx. Case report and review of the literature. J Neurosurg 91 303-307 Reinges MH, Thron A, Mull M et al (2001) Dural arteriovenous fistulae at the foramen magnum. J Neurol 248 197-203... 

Scan range Sagittal suture to foramen magnum ... 

Membranous structures in the body are all composed of coimective tissue derived from embryonic mesenchyme. AU membranes are continuous with all other mesenchymally derived connective tissues of the body. The intracranial membranes are intimately related to the fascias of the rest of the body through foramina in the cranial base and throughout the entire spine, the foramen magnum, and all fascial attachments to the undersurface of the cranial base. 

The spinal portion of the dura is continuous with the inner layer of the cranial dura. Only one layer extends into the spinal canal. The other layer Is continuous with the periosteum of the outside of the cranium. The spinal dura surrounds the spinal cord within the spinal canal. There are many attachments within the spinal canal. Firm attachments of spinal dura Include the foramen magnum, the posterior aspect of the dens, the posterior aspect of the body of C3, the posterior aspect of the body of 52, the posterior aspect of the coccyx via the filum terminate, and minor fibrous slips to the posterior longitudinal ligament along the entire spine, most predominant in the lunar area. 

Basilar occiput (basiocciput) anterior to and forming the anterior border of the foramen magnum. 

Squama body of the occiput, forming the posterior border of the foramen magnum. Ossification of all parts is not complete until approximately 5 years. This has important clinical significance in the timing of the treatment of certain problems in children. 

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