The Optometrist - Assistive Technology Assessment - page 218

meters, or D = 1/ f (Catalano 2006). This is a physical indication and it does not provide any
information about the visual acuity of the patient.
10.2.2.8.1 Methods
The methods that can be used to quantify an ametropia and to determine the ophthalmic
prescription are numerous and they are very well known by all optometrists.
It is useful to recall some simple precautions. However, the use of a phoroptor is not
recommended in patients affected by nystagmus because this instrument forces a posture
that often does not allow patients to reach their own blockage point.
The most important evaluation in particularly severe clinical pictures is that obtained
in binocular conditions, in which interferences and mutual accommodations may produce
unexpected results. In situations of low cooperation of the patient, it may be that we only
are able to determine an ophthalmic correction by means of objective methods, but it must
be verified with test glasses whether the other visual abilities benefit or whether a dete-
rioration of such abilities occurs. This is the case with medium-high myopia; test glasses
allow a perfect vision at a proximal distance, but once they are corrected, the use of proxi-
mal vision is made difficult although long-distance visual ability is improved.
10.2.2.9 The Field of Vision
The field of vision is the part of the space that provides the visual information to the
patient.
The binocular field of vision has a wider horizontal range than the vertical one, both for
anatomical reasons and for the sum of the single monocular fields of vision (Figure 10.8).
The field of vision is divided into a central area and a peripheral area in which the visual
ability gradually decreases depending from the distance from the fovea.
In many retinal diseases and in central or visual tract diseases, defects in the field of
vision may occur. At a monocular level, this only happens in a few meridians, but there
could also be defects in the whole periphery of the field of vision so that the tubular vision
is limited to the central area where an adequate visual ability is preserved. The opposite
can be said for some maculopathies, in which the central visual ability is diminished,
whereas the peripheral vision is preserved.
In severe myopias or in other ocular diseases there can be scotomas, blind areas of
various extensions but always limited, in various parts on the retina. When the damage
occurs in the area from the chiasm toward the LGN, losses in the field of vision may affect
both eyes.
There also could be both homonymous or contralateral hemianopsies and quadrantop-
sies (Figure 10.18).
Lesions of cortical area V1 cause blindness in the area topographically linked to the
retina. The presence of damages in the field of vision affects the choices that have to be
made about the possible assistive technologies to be used with the patients, their features,
and their positioning.
10.2.2.9.1 Methods
The assessment of the field of vision is performed through various instruments that are
usually computerized. These instruments require a high attention level, a good knowledge
of the task, and sufficient manual skills.
For many of our patients the test is performed through behavioral methods by using the
orientation reflex toward objects appearing in the peripheral area. These techniques do not