This study examined the clinical course of 73 patients with intermittent exotropia over an average follow-up period of 10 years. The mean exodeviation decreased from 17.2 degrees at distance and 17.6 degrees at near initially, to 13.7 degrees at distance and 13.5 degrees at near at the final visit. However, these changes were not associated with any specific treatment and occurred regardless of follow-up time, suggesting the improvements may be partly due to statistical regression toward the mean. Overall, intermittent exotropia either improved or remained stable over time for many patients in this study.

1. ORIGINAL ARTICLE
The Clinical Course of Intermittent Exotropia
ROBERT P. RUTSTEIN, OD, MS, FAAO and DAVID A. CORLISS, OD, PhD
School of Optometry, University of Alabama at Birmingham, Birmingham, Alabama
ABSTRACT: Purpose. To report the clinical course for patients with intermittent exotropia. Methods. The clinical
records of patients diagnosed with intermittent exotropia from 1983 to 1991 who had at least 4 years of follow-up were
reviewed. All patients with neurological or medical abnormalities, developmental delays, ocular disease, or having
strabismus surgery during the follow-up period were excluded. Results. Of the 468 records reviewed, 73 met the
inclusion criteria. Forty-four patients were female, and 29 were male. Fifty-two patients had basic intermittent
exotropia, 11 patients had divergence excess intermittent exotropia, and 10 patients had convergence insufficiency
intermittent exotropia. The mean age at initial visit was 20 years (range, 1 to 63 years). The mean follow-up was 10
years (range, 4 to 23 years). Four patients had amblyopia of 20/30 or worse, nine patients had a vertical deviation in
the primary position, and 10 patients had undergone extraocular muscle surgery before coming to our clinic. Sixty
patients received some form of treatment during follow-up. The mean stereoacuity at the initial and final visits were 59
and 70 s arc, respectively. The initial mean spherical equivalent refraction was ؊0.48 D and increased to ؊1.15 D at
the end of the study. The mean exodeviation changed from 17.2 ⌬ at distance and 17.6 ⌬ at near at the initial visit to
13.7 ⌬ at distance and 13.5 ⌬ at near at the final visit. At the initial visit, 63 patients were exotropic and 10 patients
were either heterophoric or orthophoric at distance, whereas, 60 patients were exotropic and 13 patients were either
heterophoric or orthophoric at near. At the final visit, 37 patients were exotropic and 36 patients were either
heterophoric or orthophoric at distance whereas 33 patients were exotropic and 39 were either heterophoric or
orthophoric at near. One patient was esotropic at near at the last visit. Changes in the size and quality of the
exodeviation, although statistically significant (p < 0.001), were not associated with any specific treatment regimen or
with longer periods of follow-up. Measurements exhibited a regression toward the mean. Conclusion. Intermittent
exotropia improved for many patients quantitatively and qualitatively over time. That the improvement was unrelated
to any treatment and length of follow-up suggests that the changes at least quantitatively are not associated with any
physiologic process and may be due, in part, to regression toward the mean. (Optom Vis Sci 2003;80:644–649)
Key Words: intermittent exotropia, exophoria, follow-up, progression, regression toward the mean
I
ntermittent exotropia is an ocular deviation that at times is
completely controlled by positive fusional vergence and pre-
sents as an exophoria and at other times is not controlled by
positive fusional vergence and presents as an exotropia. Exophoria
supposedly precedes the development of the intermittent exotro-
pia, but this is probably not always the case. It is a relatively com-
mon form of strabismus occurring in about 25% of all strabismic
cases and in 1% of the general population.1–3
Its age of onset varies
but is often between 6 months and 4 years.4
The clinical course of intermittent exotropia is unclear. It has
been reported to be a progressive disorder.5
Progression or deteri-
oration over time may take different forms. The intermittent exo-
tropia may increase in size, may spread to another fixation distance,
may become more frequent, or may become constant with loss of
binocular vision and stereopsis. Some of the factors leading to
progression include decreased tonic convergence, decreased ac-
commodation, suppression, and increased separation of the orbits
with age.
We have noted that some patients with intermittent exotropia
do not progress over time and may even improve. There are pa-
tients who manifest intermittent exotropia in early childhood and
are exophoric as adults. Hiles and associates6
followed 48 patients
with intermittent exotropia for an average period of 11.7 years.
During the follow-up period, no patient received strabismus sur-
gery. As many as 80% received some vision therapy. The average
deviation at distance decreased from 23 to 18 ⌬, and the average
deviation at near decreased from 11 to 8 ⌬. Forty patients (83%)
remained within 10 ⌬ of their original distance measurements at
the end of the study. In addition, 31 of the 48 patients (64%)
ceased to manifest exotropia and were heterophoric at the last visit.
von Noorden7
followed 51 patients with intermittent exotropia
who were not treated for an average of 3.5 years. In 25% of the
1040-5488/03/8009-0644/0 VOL. 80, NO. 9, PP. 644–649
OPTOMETRY AND VISION SCIENCE
Copyright © 2003 American Academy of Optometry
Optometry and Vision Science, Vol. 80, No. 9, September 2003

2. cases, the intermittent exotropia either remained unchanged or
improved. The purpose of this study was to report the clinical
course of patients with intermittent exotropia examined in our
clinic.
METHODS
After approval from our institutional review board, the clinical
records of all patients diagnosed with intermittent exotropia for
distance and/or near at the University of Alabama at Birmingham
School of Optometry Binocular Vision Clinic from 1983 to 1991
were reviewed. All patients had to have a follow-up for Ն4 years.
Records of patients with neurological or medical abnormalities,
developmental delays, ocular disease, or having had strabismus
surgery during the follow-up period were excluded. Information
retrieved from the records included patient gender, age at initial
visit, number of visits, length of follow-up, presence of amblyopia,
presence of a vertical deviation in primary position, history of
strabismus surgery before the initial visit, whether or not other
treatment was given during follow-up, and the type of treatment,
the magnitude, and type of ocular deviation at distance and near,
the stereoacuity, and the refractive error at the initial and final
visits.
For this study, amblyopia was defined as best-corrected visual
acuity of 20/30 or poorer in the deviating eye. The magnitude of
the deviation was determined with the alternate prism cover test at
distance and near utilizing accommodative targets and with the
patient wearing their refractive correction. Stereopsis was deter-
mined at near using either the Titmus or Randot stereotest. The
mean spherical equivalent for each patient was calculated and used
as the refractive error. All clinical measurements were taken either
by one of the authors (RPR), optometry clinical faculty experi-
enced in binocular vision anomalies, or senior optometry interns.
RESULTS
Of 468 records reviewed, the records of 73 patients, 44 females
and 29 males, were used in the study. Table 1 characterizes subject
age distribution and follow-up. The 73 patients, ranging in age
from 1 to 63 years at their first visits, were seen a total of 572 times.
The time between the first and last recorded visits ranged from 4 to
23 years, with an average of 10 years. The elapsed time between
successive visits averaged about 1 year.
The type of intermittent exotropia was classified according to its
magnitude at distance and near. Fifty-two patients (71%) had a
basic exodeviation with the deviation at both distances being
within 10 ⌬; 11 patients (15%) had a divergence excess exodevia-
tion with the deviation at distance exceeding the deviation at near
by 10 ⌬ or more; and 10 patients (14%) had a convergence insuf-
ficiency exodeviation with the deviation at near exceeding the de-
viation at distance by 10 ⌬ or more.
Four patients (5%) had amblyopia of 20/30 or poorer (range,
20/30 to 20/70). Three of these patients had anisometropia of 1 D
or more with the largest refractive error occurring in the deviating
eye. The other amblyopic patient had low myopic refractive error
in each eye. The low number of patients with amblyopia is consis-
tent with other studies.8, 9
Nine patients (12%) manifested a vertical deviation in primary
position in addition to the intermittent exotropia. One patient had
a dissociated vertical deviation with LN. She had strabismus sur-
TABLE 1.
Patient age and follow-up in years.
Minimum Maximum Mean Median
Age at first visit 1 63 20 11
Age at last visit 8 72 30 22
Age difference between successive visits Ͻ1 14 1 1
Age difference between first and last visits 4 23 10 9
Number of visits per patient 2 21 8 7
TABLE 2.
Mean magnitudes of the deviation, the differences between the means of the magnitudes at the first and last visits, and
the dependent t-test results.a
Distance at First Visit (17.15 ⌬) Distance at Last Visit (13.72 ⌬) Near at First Visit (17.63 ⌬)
Distance at Last Visit (13.72 ⌬) Difference ϭ Ϫ3.43 ⌬ — —
t ϭ 3.46
p Ͻ 0.001
Near at First Visit (17.63 ⌬) Difference ϭ 0.49 ⌬ Difference ϭ 3.92 ⌬ —
t ϭ Ϫ0.48 t ϭ 3.30
p ϭ 0.633 p ϭ 0.001
Near at Last Visit (13.46 ⌬) Difference ϭ Ϫ3.70 ⌬ Difference ϭ Ϫ0.26 Difference ϭ Ϫ4.18 ⌬
t ϭ 2.77 t ϭ 0.22 t ϭ 4.11
p ϭ 0.007 p ϭ 0.823 p Ͻ 0.001
a
Differences are expressed as row minus columns df ϭ 73 for all cells.
Clinical Course of Intermittent Exotropia—Rutstein & Corliss 645
Optometry and Vision Science, Vol. 80, No. 9, September 2003

3. gery at 1 year for infantile esotropia. At the first visit in our clinic
when she was 8 years old, she manifested intermittent exotropia at
distance and near and had 80 s arc stereopsis.
Ten patients (14%) had undergone strabismus surgery before
being examined in our clinic. Two patients had surgery for esotro-
pia and subsequently developed intermittent exotropia. Eight pa-
tients had recurring intermittent exotropia.
Sixty patients (82%) had nonsurgical treatment prescribed at
some time during the follow-up period. Thirty-six patients (49%)
had vision therapy, 25 (34%) were prescribed prism, and 20 (27%)
were prescribed over-minus lenses. Two patients (3%) received
occlusion therapy for amblyopia. Twenty-four patients (33%) re-
ceived more than one type of treatment. Thirteen patients (18%)
received no specific treatment for the intermittent exotropia dur-
ing the follow-up period.
The mean stereoacuities at the first and final visits were 59 and
70 s arc. At the initial visit, 50 of the 52 patients tested had mea-
surable stereopsis compared with 44 of the 46 patients tested at the
final visit. Only one patient had no measurable stereopsis at both
the first and last visits. Another patient who had stereopsis initially
did not have stereopsis at the last visit. For both patients, the
magnitude of the exodeviation decreased by more than 10 ⌬ at one
fixation distance.
The mean spherical equivalent refraction changed from Ϫ0.48
to Ϫ1.15 D. At the first visit, 35 patients had a spherical equivalent
refraction ranging from Ϫ0.50 to ϩ0.50 D compared with 20
patients at the last visit. The number of patients having Ͼ1 D
anisometropia either in the spherical or astigmatic components of
their refraction was similar at both visits.
Table 2 shows the mean magnitudes of the exodeviations, the
differences between the mean magnitudes at the first and the last
visits, and the dependent t-test results. Overall, the intermittent
FIGURE 1.
Magnitude of the distance deviation as a function of age. The lines
represent the regression line and the 95% confidence limits for the regres-
sion line.
FIGURE 2.
Change in distance deviation between first and last visit vs. the time
between the first and last visits. There is no significant relation between
change in magnitude and time.
FIGURE 3.
Change in distance deviation between first and last visit vs. patient age at
first visit. There is no significant relation between change in magnitude
and imputed age of onset.
FIGURE 4.
Change in distance deviation between first and each successive visit vs.
the time between the first and each successive visit. There is no significant
relation between change in magnitude and time within patients.
646 Clinical Course of Intermittent Exotropia—Rutstein & Corliss
Optometry and Vision Science, Vol. 80, No. 9, September 2003

4. exotropia became smaller, the changes being significantly different.
The difference in magnitudes of the distance and near deviations
represent changes of 20% and 24%, respectively. The exodeviation
decreased by 5 ⌬ or more for 28 patients at distance and for 33
patients at near. It stayed the same or decreased by Ͻ5 ⌬ for 21 and
26 patients at distance and near, respectively. For the remaining
patients, the deviation increased. However, the increase was larger
than 5 ⌬ at either distance or near for only six patients.
The effect of treatment regimen and length of follow-up on the
changes in the exodeviation were analyzed. As shown by repeated-
measures analysis of variance, there was no significant difference (F
ϭ 0.0002, p ϭ 0.99) in the magnitude of the changes in the
exodeviation at the first and final visits for patients who were
treated at some point during the follow-up period (N ϭ 60) vs.
patients who were not treated (N ϭ 13). This could be the result of
poor patient compliance because the analysis is based on whether
treatment was prescribed, not on the type of treatment or the
amount of treatment actually done.
For the 10 patients who had strabismus surgery before being
evaluated in our clinic, the effect of surgical history on the changes
in the exodeviation over time was also analyzed. As shown by
repeated-measures analysis of variance, surgical history had no ef-
fect on the magnitude of change in the distance deviation (F ϭ
0.523, p ϭ 0.472) and the near deviation (F ϭ 0.825, p ϭ 0.367).
Fig. 1 shows that the exodeviation at distance had a tendency to
decrease with increasing patient age. Despite the high degree of
scatter, the relation is significant (r ϭ Ϫ0.168, p Ͻ 0.001), al-
though age explains little of the variance.
Fig. 2 illustrates the change in magnitude between the first and
last visits as a function of time between those visits. There is no
significant relation between these two variables. Fig. 3 shows that
the change in the magnitude of the deviation between the first and
last visit is also independent of the age at which the patient was first
evaluated in our clinic. In addition, Fig. 4 illustrates the change in
magnitude of the distance deviation at each visit from the magni-
tude at the first visit vs. the time between the first and each succes-
sive visit. Again, there is no significant relation.
The observed differences in the magnitude of the exodeviation
are possibly due, at least in part, to a measurement effect. In par-
ticular, where there are functions in any kind of physiologic mea-
surement due either to physiologic variability or measurement er-
ror, there may exist the phenomenon of regression toward the
mean.10
This means that if at one visit the magnitude is higher than
the mean, then at the next visit there is good likelihood that the
magnitude will be smaller than the first measure. Conversely, if the
initial measurement is low at one visit, it may be higher at the next.
Fig. 5 demonstrates the existence of this measurement phenom-
enon in our data. Fig. 5 shows the probability of an increase, a
decrease, and no change in the magnitude of the exodeviation from
one visit to the next as a function of the magnitude of the deviation
at a particular visit. It is clear that as the magnitude of the exode-
viation increases, the probability of measuring a smaller value at the
next visit increases, whereas the probabilities of measuring a
smaller value or no change decreases. The opposite is true at low
values of the deviation.
Concomitant with but independent of the magnitude of the
deviation, the type of deviation is classified as either an intermit-
tent exotropia or phoria. Initially, 63 patients (86%) were exo-
tropic and 10 patients (14%) were either heterophoric or or-
thophoric at distance, whereas 60 patients (82%) were exotropic
and 13 (18%) were either heterophoric or orthophoric at near.
Fifty patients (68%) manifested intermittent exotropia at both
distance and near. At the last visit, 37 patients (51%) were exo-
tropic and 36 (49%) were either heterophoric or orthophoric at
distance, whereas 33 (45%) were exotropic and 39 (53%) were
either heterophoric or orthophoric at near. Twenty-eight patients
(38%) manifested intermittent exotropia at both distance and
TABLE 3.
Changes in the classification of the exodeviation between the first and the last visits for both the distance and
near deviations.a
Distance Near
Exotropia Exophoria Orthophoria or Esophoria Exotropia Exophoria Orthophoria or Esophoriab
First Visit 63 (86%) 4 (6%) 6 (8%) 60 (82%) 10 (14%) 3 (4%)
Last Visit 37 (51%) 26 (35%) 10 (14%) 33 (45%) 32 (44%) 7 (10%)
a
All changes in deviation types are statistically significant.
b
One patient was esotropic at near for the last visit.
FIGURE 5.
Probability of an increase, decrease, or no change in the magnitude of the
distance deviation at a subsequent visit as a function of the magnitude of
the deviation at the current visit. Note that the probability of observing a
decrease in magnitude at the next visit increases as the magnitude of the
deviation of the current visit increases.
Clinical Course of Intermittent Exotropia—Rutstein & Corliss 647
Optometry and Vision Science, Vol. 80, No. 9, September 2003

5. near. One patient manifested intermittent esotropia at near during
the last visit and was prescribed bifocals.
Table 3 shows cross tabulations of the exodeviation types at the
first and last visits. All the changes are statistically significant (Mc-
Nemar ␹2
ϭ 8.4, p ϭ 0.001 for distance, and McNemar ␹2
ϭ
14.6, p Ͻ 0.001 for near). Fig. 6 shows the mean magnitudes of the
distance deviations associated with these quality changes. There are
no changes from exophoria to intermittent exotropia. Exotropias
Ͻ20 ⌬ had a higher incidence of converting to exophoria than did
exotropias exceeding 20 ⌬.
DISCUSSION
The results of this retrospective study indicate that intermittent
exotropia does not progress with advancing age for many patients
and may even improve. For our patients, the first and last stereopsis
measurements were similar; the magnitude of the deviation be-
came smaller by an average of 3.5 ⌬ at distance and 4.1 ⌬ at near,
and for 36% of the patients, the intermittent exotropia converted
either to exophoria or orthophoria at the last visit.
These findings are in agreement with those of Hiles and associ-
ates6
and Fournier and associates11
(Table 4). Unlike the earlier
retrospective studies, the present study was not limited to children
and also included patients who had undergone strabismus surgery
before being examined in our facility. In addition, most of our
patients (71%) had a basic intermittent exotropia, whereas most
patients in the earlier studies had a divergence excess intermittent
exotropia.
If there is a physiologic process that caused the overall improve-
ment, one might expect that it would be affected by treatment
regimen and/or length of follow-up. The intermittent exotropia
would be more likely to have become smaller for patients who were
treated during follow-up vs. those who were not treated. Similarly,
greater decreases in the exodeviation would be more likely for
patients having longer periods of follow-up vs. shorter periods of
follow-up. We could, however, find no time dependency that one
would expect of a physiologic process to explain the lack of pro-
gression and overall improvement of intermittent exotropia. Fur-
thermore, the reduction in the magnitude was not related to
whether patients were prescribed treatment such as vision therapy,
prisms, or over-minus lenses during the follow-up period.
Fournier and associates11
treated 35 of their 65 patients with
either vision therapy, prisms, or over-minus lenses and reported
that the average distance exotropia decreased from 21 ⌬ to 15.6 ⌬,
whereas for the 30 patients not receiving treatment, the magnitude
remained relatively stable. Hiles and associates6
also prescribed
vision therapy for approximately 80% of their patients. Sixty pa-
tients in the present study were prescribed nonsurgical treatment
such as vision therapy, over-minus lenses, and/or prism at some
FIGURE 6.
Mean distance deviations at first and last visits associated with deviation
type at these visits. XT, intermittent exotropia; XP, exophoria; OR,
orthophoria.
FIGURE 7.
Probability of an increase, decrease, or no change in the magnitude of the
distance deviation at a subsequent visit as a function of the magnitude of
the deviation at the current visit (from Hiles and associates6
).
TABLE 4.
Comparison of studies on course of intermittent exotropia.
Study
No. of
Patients
Average Age
(yr)
Average Follow-Up
(yr)
D1
a
(⌬) D2 (⌬) N1 (⌬) N2 (⌬)
Present Study (2003) 73 20 10 17.2 13.7 17.6 13.5
Hiles et al. (1968)6
48 4.8 11.7 23 18 11 8
Fournier et al. (2000)11
65 4.1 5.3 21.3 (21.0)b
20.2 (15.6)b
16.1 18.1
a
D1, magnitude at distance for first visit; D2, magnitude at distance for last visit; N1, magnitude at near for first visit; N2, magnitude
at near for last visit.
b
Magnitudes for 35 patients who received treatment during follow-up.
648 Clinical Course of Intermittent Exotropia—Rutstein & Corliss
Optometry and Vision Science, Vol. 80, No. 9, September 2003

6. time during the follow-up period. Our analysis was based on
whether treatment was prescribed as determined by record review
rather than the actual amount of therapy that was actually done. A
prospective study that carefully monitors the amount of therapy
undertaken is needed to determine the efficacy of nonsurgical
treatment in reducing the magnitude of intermittent exotropia.
Although the magnitude of the intermittent exotropia varied
somewhat with the age of the patient, its overall reduction was
unrelated to the duration of the follow-up. In part, these changes
can be attributed to a statistical phenomenon known as regression
toward the mean.10
If at one visit the magnitude is higher than the
mean, than at the next visit it is likely that the magnitude will be
smaller than the mean. Conversely, if the first measurement is low,
it is likely that it will be higher at the next visit. This is illustrated
for our patients in Fig. 5. Regression toward the mean is not
limited to measuring ocular deviations and has also been reported
in studies monitoring intraocular pressure.12
The findings of Hiles and associates6
were digitized and replot-
ted to determine whether regression toward the mean was also a
factor in their study. As illustrated in Fig. 7, it is evident that as the
magnitude of the deviation at the first visit increased for their
patients, the probability of a decrease at the last visit also increased.
It should be noted that the data of Hiles and associates6
involved
analysis comparing only the first and last patient visits, whereas our
data analyzed the magnitude of the exodeviation for each succes-
sive patient visit.
Over one-third of our patients no longer manifested intermit-
tent exotropia and were either heterophoric or orthophoric at the
last visit. For the 48 patients reported by Hiles and associates,6
64% were heterophoric at the last visit. However, Chia and asso-
ciates13
recently reported improved control of intermittent exotro-
pia in only 18% of their patients at distance and in only 6% of their
patients at near. The magnitude of the exotropia for their patients
was generally larger and averaged 36 ⌬ at distance and 26 ⌬ at near.
Combining the findings of the present study with those of Hiles
and associates,6
it appears that as many as half of patients with
intermittent exotropia may achieve better control over time for at
least one fixation distance. This appears more likely when the
exodeviation is Ͻ20 ⌬ (Fig. 6).
In summary, intermittent exotropia improved both quantita-
tively and qualitatively for many of our patients. The quantitative
changes were possibly due, in part, to the effect of regression to-
ward the mean. Intermittent exotropia may not be a progressive
disorder for many patients. A long-term prospective study that
carefully monitors the magnitude and quality of intermittent exo-
tropia is needed to add to our knowledge of this type of strabismus.
ACKNOWLEDGMENTS
Presented, in part, at the annual meeting of the American Academy of Op-
tometry, San Diego, California, December 2002.
Received October 17, 2002; revision received May 26, 2003.
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Robert P. Rutstein
College of Optometry
University of Alabama at Birmingham
1716 University Blvd.
Birmingham, AL 35294-0010
e-mail: rrutstein@icare.opt.uab.edu
Clinical Course of Intermittent Exotropia—Rutstein & Corliss 649
Optometry and Vision Science, Vol. 80, No. 9, September 2003