Eye-movement-based assessment of visual function in patients with infantile nystagmus syndrome.

Abstract

Infantile Nystagmus Syndrome (INS) is an ocular motor system dysfunction characterized by the rhythmic to-and-fro oscillations of the eyes. Traditionally, the assessment of INS visual function solely focused on null- or primary-position visual acuity. Our purpose is to use the past four decades of INS research to introduce a more complete assessment of visual function in patients with INS.
All eye-movement data were obtained using high-speed digital video, infrared reflection, or scleral search coil systems.
We have introduced four important aspects of a more complete INS visual function assessment: the eXpanded Nystagmus Acuity Function and visual acuity measurements in primary position; broadness of the eXpanded Nystagmus Acuity Function peak and high-acuity field; target acquisition time; and gaze-maintenance capability.
Visual function in patients with INS is multifactorial and the simple assessment of primary position visual acuity is both inadequate and may not be the most important characteristic in overall visual function. A more complete visual function assessment should also include primary and lateral gaze eye-movement and visual acuity examinations, target acquisition time and gaze holding.

Full text

Page 1

1040-5488/09/8608-0988/0 VOL. 86, NO.8, PP. 988--995
OPTOMETRY AND VISION SCIENCE
Copyright © 2009 American Academy of Optometry
Eye-Movement-Based Assessment of
Visual Function in Patients with Infantile
Nystagmus Syndrome
Zhong I. Wang* and Louis F. Dell'Osso*
ABSTRACT
Purpose. Infantile Nystagmus Syndrome (INS) is an ocular motor system dysfunction characterized by the rhythmic
to-and-fro oscillations of the eyes. Traditionally, the assessment of INS visual function solely focused on null- or
primary-position visual acuity. Our purpose is to use the past four decades of INS research to introduce a more complete
assessment of visual function in patients with INS.
Methods. All eye-movement data were obtained using high-speed digital video, infrared reflection, or scleral search coil
systems.
Results. We have introduced four important aspects of a more complete INS visual function assessment: the eXpanded
Nystagmus Acuity Function and visual acuity measurements in primary position; broadness of the eXpanded Nystagmus
Acuity Function peak and high-acuity field; target acquisition time; and gaze-maintenance capability.
Conclusions. Visual function in patients with INS is multifactorial and the simple assessment of primary position visual
acuity is both inadequate and may not be the most important characteristic in overall visual function. A more complete
visual function assessment should also include primary and lateral gaze eye-movement and visual acuity examinations,
target acquisition time and gaze holding.
(Optom Vis Sci 2009;86:988-995)
Key Words: nystagmus, visual function, visual acuity, high-acuity field, target acquisition, gaze maintenance
I
nfantile Nystagmus Syndrome (INS) I is an ocular motor sys­
tem dysfunction characterized by the rhythmic to-and-fro os­
cillations of the eyes, affecting about one in 3000 newborns.2•3
It may be inherited or spontaneous and either associated with a
sensory deficit or not. INS is usually noted within the first few
months of birth or at birth (especially in inherited cases) but can
rarely develop later in life. Those INS patients with poor vision·
usually have associated sensory deficits responsible for the greater
part of their vision loss. However, the nystagmus. eye movements
do affect visual function to a varying extent, especially when view­
ing lateral-gaze targets and fast-moving targets.
Approximately fifty percent ofINS patients also have impaired
stereovision, an indication of strabismus, which is sometimes hid­
den by the nystagmus eye movements4; strabismus can easily be
'PhD
The Daroff-Dell'Osso Ocular Motility Laboratoty, Louis Stokes Cleveland De­
partment of Veterans Affairs Medical Center and CASE Medical School, Cleve­
land, Ohio (ZIW, LFD), and Departments of Neurology (LFD), and Biomedical
Engineering (ZIW, LFD), Case Western Reserve University and University Hos­
pitals Case Medical Center, Cleveland, Ohio.
detected by accurate eye-movement recordings. Patients with INS
generally donot perceive oscillopsia, however, under certain cir­
cumstances, oscillopsia can be elicited. The physical attributes of
the stimulus, the experimental condition, and the role of individ­
ual attention may influence the perception of oscillopsia.5•6
Because INS usually develops from infancy, the sensory visual
and ocular motor systems both suffer from a low quality, unstable
input because of the constantly moving retina. Successful treat­
ment of nystagmus in infants and children will significantly
improve that input, thereby benefiting both systems in their devel­
opmental stages. The main goal of any nystagmus treatment
should be to increase the foveation quality,?·8 e.g., lengthening
foveation time, increasing foveation-period accuracy, and reducing
foveation-period velocity. Reducing nystagmus intensity is a sec­
ondary, cosmetic goal. Successful treatment is not limited to in­
creased nulI- or primary-position visual acuity; nor should that be
the sole focus of the treatment evaluation. In this review of the past
40 years ofINS research conducted in our Laboratory, we conclude
that accurate eye-movement recordings should be used for diagno­
sis and both planning and evaluating nystagmus treatments. Be-
Optometry and VISion Science, Vol. 86, No.8, August 2009

Page 2

Visual Function in Infantile Nystagmus Syndrome-Wang and Dell'Osso
989
cause we have always recorded eye movements, we were in a unique
position to observe that clinical evaluations alone are often inaccu­
rate, inconsistent, and too insensitive to document therapeutic
changes; this conclusion is shared by other researchers.9 This article
will review multiple aspects of visual function that may be affected
by nystagmus treatments and form a more complete evaluation
paradigm for children and adults with INS.
Primary-Position Measured and Potential
Visual Acuity
Many INS patients have a "null" point, which indicates higher
visual acuity at a certain gaze angle? The null point is established
during the developmental period of the visual system and usually
does not change its nature over the years, i.e., a static gaze-angle
null stays in the same position; a time varying null retains its
(a) periodicity. The position of the null point mayor may not be in
primary gaze. Surgery or prisms can correct a lateral null to primary
position, alleviating the need for a head turn. Evaluation of treat­
ment efficacy usually compares the null- or primary-position acu­
ity pre- and posttreatment. In the case of a corrected lateral null,
the primary-position acuity would be vastly improved. Clinically,
visual acuity is usually measured with a distance line-letter chart.
The direct results of peripheral nystagmus therapy [e.g., prisms,
contact lenses, or extraocular muscle (EOM) surgery] are im­
proved INS waveforms. Measured visual acuity is only an indirect
and idiosyncratic measure of INS waveform characteristics.10-12
The improvement of measured acuity is dependent on a number of
factors in addition to improvement of nystagmus; these factors
include afferent visual deficits, patient age, associated strabismus,
amblyopia, uncorrected refractive errors, and associated central
nervous system disease. The quality ofINS foveation periods is the
single most important characteristic that directly affects visual
function (not nystagmus amplitude nor frequency). It is also the
characteristic that is directly affected by therapies that alter EOM
fimction. Therefure, an objective measure of the direct, eye-movement
effects of nystagmus therapies related to target foveation quality
was needed to assess INS and evaluate the results of therapy.
The Nystagmus Acuity Function (NAF) was developed to fill
that void. It measures nystagmus foveation quality at various gaze
angles. It is an objective measure of waveform foveation, and a
predictor of potential visual acuity for INS patients with no affer­
ent deficits (see the next paragraph for INS with afferent deficits).
The NAF measures directly the treatment-induced waveform
changes, specifically, the patient's ability to maintain fixation
within a "foveation window" of ±0.5° (position) and ±4.00/s
(velocity). 13 It combines the following factors of nystagmus-waveform
characteristics during target foveation: foveation time per cycle, the
standard deviations of eye position, and the standard deviations of
eye veloCity?,14,15 The NAF is defined by the following equation:
NAF
= (1 - CTpJ[l - E-�/T]
where the pooled estimator CT
0.125(SOJ, tris the foveation-period duration, and T = 33.3 ens.15-1?
The NAF is a function that is predicated on the assumption of
an intact visual system and is linearly proportional to potential
Snellen (decimal) visual acuity; Fig. 1 shows this age-dependant
=
I(S02 + SO '2)/2 SO'
'J
p.
=
pv
v' v
linear relationship. The individual lines reflect the different maxi­
mum acuities of normal subjects for each age range.18,19 A subse­
quent version of the NAF, the eXpanded Nystagmus Acuity Function
(NAFX),15,20 extended the position window to ±6.0° and the
velocity window to ± 10.0° /s for patients incapable of controlling
fixation well enough for good foveation (this is a common scenario
in INS patients with afferent visual deficits). The NAFX is defined
by the following equation:
where the pooled estimator CT
(p/v)(SOJ, tris the foveation-period duration, T is calculated from the
average T surface. 15
The NAF and the NAFX (both unitless numbers between 0
and 1.0, 1.0 being the best) h.tve been successfully used to
evaluate the foveation quality of both INS and Fusion Malde­
velopment Nystagmus Syndrome (FMNS, formerly called la­
tent/manifest latent nystagmus) patients.10,12,13,21,22 Fig. 2
shows an example ofthe NAFX calculation performed by the
interactive user interface created in our lab (software available at
http://www.omlab.orglOMLAB_page/software/software.html). The
calculation startswith choosing a steady, blink-free fixation section
at the gaze angle of interest. Then, proper position and velocity
windows for this section are set within the user interface. Detailed
instructions are available online.2o The upper panel of Fig. 2 is the
velocity trace of the chosen fixation section, the bold part of the
waveform corresponding to the foveation periods detected by
the velocity criteria of the NAFX algorithm. The middle panel of
Fig. 2 is the position trace of the chosen fixation section; the bold
part of the waveform corresponding to the foveation periods de­
tected by the position criteria of the NAFX algorithm. The lower
panel of Fig. 2 is the final output of the NAFX, with the bold part
of the waveform satisfYing both the position and velocity criteria;
i.e., they are foveation periods and are the only data used to calcu­
late the NAFX. Thus, the amplitude of the INS waveform is not
correlated with visual acuity. It is suggested that multiple measure­
ments be performed and averaged at each gaze angle.
A more recent use of the NAFX is to estimate the upper limits of
visual acuity improvement that could be achieved with the reduc-
.
f
12 F
non 0 nystagmus.
or example, some INS patients already have
a high primary-position NAFX with a corresponding potential
acuity close to 20/20; however, their measured acuity might only
be 20/100 because of a deficit in the afferent visual system that
limits visual acuity improvement. Having obtained the primary­
position NAFX information of such a patient before hand, it is
unlikely that treatment of the patient's nystagmus could raise the
already-high NAFX values any higher, or increase the measured
peak visual acuity. Therefore, such patients should not expect in­
creased acuity as a result of any treatment. There are, however,
other aspects of visual function that can be improved (and, there­
fore, should be evaluated); they will be covered in the next sections.
In addition, the reduction or elimination of torticollis provides its
own functional benefits.
Combined with pretherapy measured acuity, the NAFX can also be
used to predict the measured acuity improvement postfour-muscle
tenotomy in patients with visual deficits.12 The ability to inform pa­
tients how much their nystagmus may improve is unique in the history
= �(S02 + SO'2)/2 SO'
p
=
pv
v) v
Optometry and Vuion Science, Vol. 86, No. 8, August 2009

Page 3

990
Visual Function in Infantile Nystagmus Syndrome-Wang and Dell'Osso
1.4,--
--, -- -,- --,-- -,---,---,- -- , -- -,-- -,- --
20/15
Age >12-
1.2
>40-60
>-
·S
u
<C
(ij
:::J
II) 0.6
:>
-
0.8
0.4
0.2
20/200
0.1
0.2
0.3 0.4 0.5 0.6 0.7
0.8
0.9
Nystagmus Acuity Function (NAFINAFX)
FIGURE 1.
The age-dependent linear relationship between potential visual acuity and NAF/NAFX. The five lines correspond to the following age groups: <6, 6
to 12, 12 to 40,40 to 60, >60, as noted on the Figure. The lines for the age groups of 6 to 12 and >60 overlap each other. Snellen acuities are noted
on the right side of the figure. For any calculated NAFX value, the corresponding potential visual acuity can be read from the patient-appropriate age
line.
of nystagmus surgety and could not be achieved without the advent of
the NAFX measure of waveform foveation quality.
In other cases, although the foveation quality is improved
(shown by an increased average NAFX) and patients and parents
do report behavioral changes, the measured acuity is not.12 To
understand this apparent contradiction, it is important to note that
clinical examination differs from daily life experience in that pa­
tients may feel stressed by the former. Because IN is known to be
affected by psychological factors,23-25 it could have been exacer­
bated from its baseline level during the clinical examination.
Therefore, it is possible for patients to show no improvement in
measured acuity despite increases in NAFX values and in real­
world (unstressed) acuity. The complex relationship between nys­
tagmus, afferent visual deficits, and measured visual acuity makes it
challenging to obtain an adequate evaluation ofINS therapies.
To obtain eye-movement recordings for consistent comparison
of NAFX values, we suggest that a stress-free visual target (e.g.,
LED or laser spot) should be used, because distinguishing letters
and shapes adds another level of stress, especially for patients of a
younger age. In addition, the recording condition should be kept
consistent pre- and posttreatment for valid evaluation.26
Breadth of the High-Acuity Field
The null breadth (better measured by the NAFX "peak") of INS
individuals determines the high-acuity field they can utilize. We de­
fine the high-acuity field as the range of gaze angles where the acuity is
>90% of the best acuity (at the NAFX peak) .12 Improved lateral-gaze
vision (a broader NAFX peak) could greatly reduce the need for head
movements to align the best-acuity point to the target of interest. A
broader peak would be especially helpful to children with INS whose
developing visual system could still benefit from a better input within
a wider range of visual angles. Therefore, visual acuity at a number of
gaze angles should be assessed when evaluating treatment effective­
ness.27 Examinations focused solely on the null- or primary-position
NAFX and visual acuity document only one aspect of the posttreat­
ment changes and are not only inadequate but also may not measure
the patient's more important improvements in visual function.
Observed increases in the breadth of the null gave birth to the
four-muscle tenotomy and reattachment (T&R) procedure.8,28,29
A gaze-angle-dependent nystagmus intensity analysis of the post­
surgical effects of Kestenbaum procedure30,31 documented that
the nystagmus nulls were not only moved but also broadened; the
former was due to repositioning of the extraocular muscles and the
latter was attributed to the T&R of these muscles. It was then
hypothesized that a pure four-muscle T&R procedure would
damp the nystagmus in the corresponding plane, resulting in a
broader NAFX peak.29,32 This procedure greatly benefits those
patients with a primaty-position sharp peak, for whom the Kesten­
baum procedure was contraindicated, 10,12,33
NAFX vs. gaze-angle curves document foveation improvements
over the broad range of gaze angles tested in the experimental
paradigm, 12,22 Comparison of the pre- and postsurgical NAFX vs.
Optometry and Vision Science, Vol. 86, No. 8, August 2009

Page 4

Visual Function in Infantile Nystagmus Syndrome-Wang and Dell'Osso
991
Velocity points within ±1 ° by 4°/sec window
U 100
Q)
�
'"-
�
·13
0
Qj
>
Q)
>-
w
50
0
0
0.5
1.5
2
2.5
Position points within ±1 ° by 4°/sec window
2
�
l?..-
e
g
·iii
0
a...
Q)
>-
w
0
-2
-4
0
0.5
1.5
2
2.5
9 Foveations identified by algorithm
2
l?..-
e
g
·iii
0
a...
Q)
>-
w
0
-2
NAFX=O.696
-4
0
0.5
1
1.5
Time (sec)
2
2.5
FIGURE 2.
NAFX-program outputs for a fixation section. Velocity traces (upper row) and position traces (middle and lower row) are shown. In each subplot, the
NAFX algorithm-determined foveation periods satisfying the foveation-window criteria are shown thickened. The NAFX value for this section is 0.696.
gaze-angle curves determines if the nystagmus treatment increased
potential acuity across a wider range of gaze angles. In a number of
previous studies, we have successfully utilized the NAFX vs. gaze­
angle curves to provide a way of measuring the "broadening" effect
of T&R.12,22 Fig. 3 is a demonstration of one such patient, with a
postsurgical elevated and broadened high visual function field.
Clinically, we agree that visual acuity should be measured at dif­
ferent lateral gaze angles before and after nystagmus treatments to
better characterize their effectiveness. 27
Indeed, with a broader NAFX peak, INS individuals have a
wider high-visual-function field in which they can perform tasks
with sufficient accuracy. This results in better performance in ac­
curacy/speed critical events, which leads to the dynamic measure­
ment that we will introduce in the next section.
Target Acquisition Time
To mimic the real-world scenario of acquiring and tracking moving
targets, another realistic assessment of postsurgical changes is to mea­
sure the eye-movement responses to dynamic, jumping/moving visual
targets. This aspect is critically impottant for visual function during
locomotion and sports. Researchers have studied multiple propetties
ofINS target foveation and acquisition,34-38 and recently, a dynamic
measurement of INS target acquisition time was established.39 That
time is measured from the target initiation to the beginning of the first
foveation period on the target (i.e., the first foveation period in the
patient's foveation window that is followed by subsequent foveation
periods within that window). It has been demonstrated that INS tar­
get acquisition time has an intricate relationship with the timing of
target onset vis-a.-vis its occurrence within the INS cycle, i.e., when
target onset is near to the intrinsic foveating/braking saccades, a lon­
ger target acquisition time results,39-41
The target-acquisition-time measurement is also an important as­
pect of evaluating treatment effects. An earlier study by Sprunger et
al.,42 measured the recognition time of a fixed optotype target at an
INS patient's threshold visual acuity postfour-muscle recession sur­
gery. Recently, we published a study demonstrating the postT&R,
target-acquisition-time improvement ofINS patients.40 The five pa­
tients in that study, aged ftom 9 to 45, all had a decrease in target
acquisition time ranging from 200 to 500 ms. Fig. 4 contains plots of
Optometry and Vision Science, Vol. 86, No.8, August 2009

Page 5

992
Visual Function in Infantile Nystagmus Syndrome-Wang and Dell/Ossa
•
-30 -20
-10
FIGURE 3.
Post: Broad & High Peak
0.2
0.15
Pre: Sharp & Low
o
10
Gaze Angle (0)
20
30
Two NAFX vs. gaze-angle curves showing a marked improvement of static visual function postfour-muscle T&R. The patient had an elevated and
broadened NAFX peak region. Triangles denote presurgical NAFX values; squares denote postsurgical NAFX values; rightward gaze angles are
positive.
FIGURE 4.
1.3,----- -- ---------------------------------- -------- ---- ---- ---,
..
1.1
0.9
iii
- 07
:i'
0,5
Post: Fast
•
0.3
0.1 +---
o
10 20
30
40
50
60
-r- -�- -�---r_� -��-_r-�_--�- --r_--�
100
Tc%
70
80
90
Two target-acquisition-time curves showing a marked improvement of dynamic visual function postfour-muscle T&R. The patient had decreased
target-acquisition-time values vs. target timing. Lt: target acquisition time; Tc%: normalized target onset time within the current nystagmus cycle.
Triangles denote presurgical Lt values; squares denote postsurgical Lt values; both are in seconds .
. Optometry and Vision Science, Vol. 86, No.8, August 2009