Biomedical Engineering Reference
In-Depth Information
movements in the fixating eye or in the bandaged eye. These results confirm the absence of
strabismus.
Once the first level of visual skills was completed, after noticing a sufficient fixation
ability, the evaluation of the eye's slow pursuit movements was carried out. Horizontal
movements were made fluently with a postural accommodation of the head in the most
extreme positions. This answer is absolutely physiological in smaller children, but as they
grow up, a progressive segregation of the eye movement from the movement of the head
can be observed. The vertical pursuit movement is generally more complex because of the
anatomical structure of the extraocular muscles and because of the lower frequency of this
kind of movement.
The patient shows she can move vertically, but she cannot properly control her head and
this leads to a loss of fixation. This element should suggest the use of visual stimuli with
less vertical expansion and paying special attention to posture.
The next examination concerned the refractive state and was carried out through reti-
noscopy. Various precautions were used to lead the child keep her fixation at a distance.
If the parents were present, they were asked to reach a 3-m examination distance, call to
the child, and show her some toys, maybe even colorful and loud ones. An alternative tool
is represented by cartoons on the computer. Without a strong distant stimulus, the flash-
light of the instrument attracts the attention of the child, causing her to turn her gaze to
the operator. To neutralize the movement of the corneal reflex, lens racks (long guides on
which there are found many progressive power lenses) were often used, although they are
sometimes very cumbersome and can scare the children. For this reason, we instead use
single lenses from the test box, although this option requires more time for the test to be
completed.
The child showed a slight hypermetropia that was in the physiological range of age. The
greatest accuracy is not necessary, but it is critical that the anisometropias are recognized,
i.e., the differences in the refractive state between the LE and RE.
The same test was repeated by placing the fixation target at the matching distance, and
a change in focus was observed, thus indicating that the accommodating system had been
activated.
Moreover, placing the target closer produced unstable convergence movements. This
kind of unstable convergence movement can often occur and is generally linked to fac-
tors such as visual attention, which may be shifted to the peripheral area and is often not
linked to a convergence deficiency.
It is also necessary to note that very often hypermetropia is associated with esopho-
ria, which favors convergence movements. The saccadic movements showed an adequate
metric until a width of 20°, but with too much latency of response. This parameter can be
observed through single stimuli. A slight increase of the latency of response was not very
important in this case because the metrics were adequate. The visual acuity was tested
with two different methods with the aim of integrating and confirming the obtained data.
The first method consisted of the elicitation of the optokinetic nystagmus. This technique
was executed using a wide band with a square-wave grating printed on it that was slowly
moved from left to right and vice versa, rather than using the usual drum. Indeed, it was
observed that if a wider part of the field of vision was covered by the grating, the nystag-
mus was favored.
The second examination method was the preferential look. The results showed us that
the child responded up to 6-c/g gratings. This dimension corresponds approximately to
2/10, which is the font size of children books.
