The anatomical and tomography evaluation of the optic nerve and optic canal decompression

The aim of this study was to find out the margins of safe optic canal and optic nerve decompression through cadaver anatomical and computed tomographical evaluations. 20 optic canals of 10 formaldehyde fixated cadavers and 50 optic canal measurements on previously done paranasal CT scans were evaluated anatomically. The histological examination was held on the optic nerves and the surrounding tissues dissected during the anatomical study. Findings; Optic canal has multiple projections. Canal axis and the angle between the sagittal plane can be used during surgical positioning. With 30 degrees of head external rotation and 15 degrees of extension, optic canal will be perpendicular to the operation table. Transcranial decompression should start from the posterior foramina of the optic canal. This area is in close contact with the internal carotid artery. Generally, the artery lies inthe inferolateral side of the canal. A tortiocity in the carotid syphon will place the artery to the area of decompression. Following the opening of the superior dural leaf of the canal, decompression should be done from the middle of the superior wall. This is the safes zone for decompression. The ophtalmic artery was not located on the superior part of the optic nerve in the cadavers studied and this issue is also stated in other papers, so there is no risk of arterial injury during optic nerve decompression. The annular tendon insertion point determines the margin of orbital decompression. In this region, the dura is strongly attached to bony structures. The superior wall of the optic canal in this region is thicker than the cranial part. There is possibility of optic nerve injury during decompression surgery of this region. While opening the superior wall of the optic canal, progression from the middle of the canal to the internal wall should be done with care. The sphenoid sinus wall forming the inner wall is very thin. With uncontrolled manipulation, entrance into sphenoid sinus is possible. Decompression should be done until the globe has been reached. The subdural area maesured in this study was decreasing in size from cranial to orbital part.The most efficient decompression of the nerve in the canal can be reached in the middle and orbital zones. The opening of the optic nerve dura should be done in the middle of the canal. The transection of the annular tendon outside the midline can injure the ocular muscles and cause ophtalmoparesis. The presence of an ophtalmic foramen may be important in ophtalmic segment aneurysm surgery. A 90 degrees angulation between the internal carotid artery and ophtalmic artery in angiography may suspect us on the presence of an accessory ophtalmic foramen, and this should be taken into consideration during surgery. Key Words: anatomy, optic canal, transcranial decompression