Biomedical Engineering Reference
In-Depth Information
FIgUre 10.12
Teller acuity card.
the distance at which a given symbol must be recognized. For instance, 5/10 indicates that
the subject can read at a distance of 5 m what he/she should read at a 10-m distance. In
research, cycles/degree are usually used as unit of measurement. They indicate the num-
ber of black/white cycles in a grating. Knowing the subject's visual acuity is of utmost
importance because it provides for information in the dimension of the letters, icons, or
objects that have to be used.
10.2.2.1.1 Methods
The assessment of visual acuity in patients is generally performed by requesting the
patient to identify letters on a table appropriately set at the examined distance. The usual
procedure cannot be practiced with our patient, although they are able to recognize the
presented letters, because of simple problems in verbalization. The problem can be slightly
reduced using illustrations. Again it may happen that the name pronounced can be wrong
because of difficulty accessing vocabulary. The image of a bicycle was named “car” by
some patients because this is the prototypical word for a means of transport. In such situ-
ations, the exam must therefore be repeated a few times, and it is better to continue, even
if mistakes are made.
An expedient we frequently use with our patients with fixation disorders, both for
strictly visual problems and for poor head postural control, is to always present single
stimuli and avoid tables with more than one symbol because, after a loss of fixation, it is
much easier for the subject to get back to that single object instead of having to reorganize
his/her sight to find the symbol indicated by the operator among the others. This is why
the presentation on cardboard is preferred, instead of techniques involving projection,
because it is possible to place them in the exact point where the patient finds it easier to set
his/her look. It can thus be shifted up, down, or laterally as we wish. Particular attention is
given to the requested modality of response. As already indicated above, verbal response
can sometimes be inappropriate or unreliable. Usual praxis is to make the patient name
the images, presenting the bigger ones, generally 1/10, at a 30-cm distance from the patient,
and noting the name that the patient spontaneously uses. We will not say “This is a house”
and “This is a flower”: The patient must use those words that are easily evoked to him/her
by the images. Further tests are performed through coupling techniques. A few charts will
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