Central Field Screening gives an invaluable 'early warning' for a variety
of general conditions.
Photo ©2001 Terence Mendoza
Visual Field Screening
The width of vision, known as the field of view, used to be plotted
by moving small white disks on black rods against a large black screen.
The method was tiring and time-consuming for the patient. These days these
manual methods have largely been replaced by computerised instrumentation
like the Henson screener above. The lighting levels can be accurately
controlled within the 'bowl' of the instrument, and the linkage to the
PC allows a variety of test routines to be carried out, to inspect either
the central area, or the periphery as well.
The Blind Spot
There are no light sensors at the point where the optic nerve enters the
back of the eye, and this gives rise to the normal blind spot we all have.
However, abnormality along the 'wiring pathway' between the eye and that
part of the brain responsible for vision will give rise to characteristic
patterns of defect. Different eye conditions or general diseases all have
their own characteristic patterns. For example, pressure within the eyeball
will affect the softest part of the eye. This is where the optic nerve enters.
The pressure will gradually 'nip' the blood supply, giving rise to a typical
arch shaped drop in sensitivity. The plot below, of a central field of a
left eye, shows an intense blind area to the right of the field.
As this eye was the only one to show the defect, it suggests damage to the
side of the optic nerve nearer the left ear. Had the other field been similarly
affected it would suggest damage nearer the brain, at the point where the
nerve fibres from each eye have started to become integrated with each other.
Because of its value in detecting conditions as diverse as glaucoma
and brain tumours, the field screening forms part of our routine examination
procedure for every adult. | 
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personal and professional eyecare service for over 30 years 
