Biomedical Engineering Reference
In-Depth Information
be submitted to the patient at a distance so that he/she is able to touch them. The charts
represent images presented at an exam distance. In this case, the number of images must
be adapted to the motor abilities of the patient: the lower the motor control is, the fewer
are the images he/she is called to pick. Generally, the number of images ranges from a
minimum of two to a maximum of six. The fewer the alternatives presented, the more tests
that must be formed for every single visual acuity level to avoid false positives. In some
patients affected by a serious motor situation impairment (e.g., severe tetraparesis), the
indication of the gaze has been successfully used as a response. Visual acuity is generally
evaluated at a distance because in the nonpresbyotic patients, it is exactly correlated with
the visual acuity at a proximal distance. However, this is true if an appropriate accom-
modation is present. If accommodation deficiency occurs during the test, the visual acuity
must also be reassessed at a proximal distance.
Visual acuity values can be then related to known dimension of fonts (Figure 10.13).
In the case of noncooperative patients, the preferential look and the optokinetic reflex
techniques are used. In the first case, a grating is placed in front of the subject on a plain
background and the test is based on whether the patients uses his/her gaze to find his/
her way on the grating.
On the other hand, the optokinetic nystagmus is elicited through cylinders or monitors
on which grids are reproduced with a definite dimension. They are then slowly moved
in front of the patient only when the resolution system of the eye perceives the bands as
separate. Of course, such a technique cannot be used with a subject that is affected by
pathologic nystagmus and with patients affected by epilepsy.
10.2.2.2 Fixation
The visual system acquires information during the so-called fixation period, a short period
in which the eyes remain still after a saccade. Some subjects cannot keep their eyes com-
pletely still, and some spurious movements occur that reduce visual ability (Phillips and
Cycles/degree
Ex. of fonts
Visus
Visus
Visus
Labels insurance
policies
20/20
6/6
10/10
30
20/22
6/6,7
9/10
27
Bible, dictionary
Economic
announcements
20/25
6/7,5
8/10
24
20/28
6/8,6
7/10
21
Phone book
Newspaper
20/32
6/10
6/10
18
6/12
5/10
15
20/40
Cheap topics
20/50
6/15
4/10
12
Books
20/66
6/20
3/10
9
Children's book
Children's book
Books for
low vision
20/10
6/30
2/10
6
20/20
6/60
1/10
3
FIgUre 10.13
Visus font. (Data partially from Rossetti, A. and Gheller, P.
Manuale di Optometria e Contattologia
, Zanichelli,
Bologna, Italy, 1997.)
