During
ophthalmoscopic examination of the optic nerve disk by our sight we are
affected by optic illusion in both plane and stereoscopic perception of the
disk.
According to the literature the area of disk varies from 0,8 to 6,0 mm,
thus it varies 7 times. But according to our experience we would say it varies
only 3x. This is caused by our natural perception of the disk diameter. When we think one disk is 3x larger than
the other, actually it is 7x.
In C/D ratio we often suppose that increasing C/D ratio is in straight
relation with degree of glaucoma neuropathy. But it is not true, because
glaucoma functional changes are in straight relation with decrease of
neuroretinal rim area, which corresponded with increase of area C/D ratio.
Relationship between linear and area C/D ratio will be demonstrated. In linear
C/D 0,5 the area of the rim is 75% of the disk. In linear C/D 0,7 is area of
the rim 50% of the disk.
At stereoscopic examination of
the disk we suppose, that we see the plane of the disk in 90 degree angle to
the axis of our view. But naturally the plane of the disk is tilted its
temporal part toward back. The result of this is that we perceive the disk more
vertically oval that really is, the temporal slope of the cup we perceive less
steep and the nasal slope of the cup we perceive more steep that really is.