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What do Infantile
Nystagmus Syndrome (INS) waveforms look like?
The
illustration below contains some commonly seen Infantile Nystagmus
[nĭ-STAG-muss] Syndrome (INS) waveforms. Usually a person with INS would
have a combination of several waveforms, depending on the direction in which
they are looking. For example, when they look right, they might have one
waveform; when they look straight ahead, they might have another. Some people
with a time-varying INS would have changing nystagmus waveforms even when they
are looking at the same spot.
Eye-movement
recordings can easily determine which waveforms an INS individual has, and how
they change over different gaze angles and over time. This provides extremely
important information for making a treatment plan.
Some
of the names for nystagmus waveforms include: pendular, pendular with foveating
saccades, pseudopendular with foveating saccades, jerk, jerk with extended
foveation, dual jerk, and so on. DonÕt be offended when you hear that you have
ÒjerkÓ waveforms, it is only an engineering term to describe the waveform
shape!
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How do INS waveforms
determine visual acuity?
Basically,
two factors are at play when determining the visual acuity in INS: the
waveforms and the deficits in the visual system. If the visual system is
perfectly OK and the only problem is the nystagmus, looking at the waveforms
will predict the personÕs visual acuity.
ÒGoodÓ waveforms have long, aligned
Òfoveation periodsÓ. When you are looking at a target, the image of the target
is held steady on the fovea, the part of the retina (the back of your eye) with
the best possible vision. Your eye movements need to be steady for looking at
details of the target. In the Figure below, the arrows are pointing to the
relatively steady moments in the nystagmus cycles. It is only during these
moments that people with nystagmus can see a stable world. The brain
conveniently disregards the rest of the nystagmus cycle. The brain knows the
visual information is the best during those steady moments, and makes use of
that information! Scientifically, we call these moments Òfoveation periodsÓ.
The longer the Òfoveation periodsÓ are, the better people see. In people
without nystagmus, their eye position recording is always a straight line when
they are staring at one spot. They can use all that visual information at any
time.
So,
take a guess on line 1 in the Figure: Does the waveform on the left or the
right produce better vision? The answer is the left, because it has much longer
Òfoveation periodsÓ than the right.
In
order to see well, you not only need long Òfoveation periodsÓ, but also need
these periods to be lined up well. Take a look at line 2 in the Figure; if the
left and the right have the same amount of steady time, which one gives better
vision? The left one is better because it is much more aligned (less jagged)
than the right one! What about the line 3 in the Figure? Left is better than
right because it is again much more aligned.
Could
a large nystagmus have the same Òfoveation periodsÓ as a small nystagmus? It is
entirely possible! Look at line 4 in the Figure; they are equal in terms of
ÒfoveationÓ, although different in their actual sizes. It is also possible that
a person with a large nystagmus has better ÒfoveationÓ than a person with a
small nystagmus (for example, line 1 in the Figure); you canÕt judge by the
size of the nystagmus, you have to look at the Òfoveation periodsÓ– they
are the only functional part of the waveform!
Only
through eye-movement recordings can we accurately measure the Òfoveation
periodsÓ in INS. Everyone has a different set of waveforms. When determining if
a nystagmus surgery has achieved a positive functional effect, we have to refer to the Òfoveation periodsÓ, not just the
cosmetic nystagmus size (that is, how big the wiggle is).
