The incidence of abscess formation as a result of pit prematurity, age at diagnosis and surgery, refractive

viper envenomation is 9%, and prophylactic antibiotics error, and initial angle of deviation.
are recommended.6 The infection rate of venomous snake- CONCLUSIONS: Intermittent exotropia was nearly twice as
bite in northern Taiwan is 6.2%.7 Although there is no common in girls compared with boys in this defined
specific data for D acutus snakebite in the literature, population. There were, however, no significant histori-
prophylactic systemic antibiotics were prescribed for our cal or clinical differences between the genders. (Am J
patient, and his condition was not complicated by local Ophthalmol 2005;140:546 –547. © 2005 by Elsevier
infection. Inc. All rights reserved.)
In summary, maintaining stable life signs and preven-
tion of infection remain the chief strategies of treatment
after venomous snakebite to the eye. In addition to
antivenom, early evisceration was our treatment of choice
for this patient’s blind eye.
I NTERMITTENT EXOTROPIA IS THE MOST COMMON FORM
of exodeviation and occurs more frequently than esotro-
pia in some populations.1 Although common, population-
based data on the epidemiology of this disorder are nearly
nonexistent. The primary objective of this study was to
REFERENCES
determine whether there are differences between genders
1. Gupta M, Sharma P, Jain A, Solanky J, Sharma KK, Basu S. in a population-based cohort of pediatric patients with
Unusual site of snake bite. Trop Doct 1995;25:134 –135. intermittent exotropia.
2. Kleinman DM, Dunne EF, Taravella MJ. Boa constrictor bite The medical records of all patients less than 19 years
to the eye. Arch Ophthalmol 1998;116:949 –950. of age who were residing in Olmsted County, Minnesota
3. Sheard RM, Smith GT. Penetrating eye injury following a and diagnosed by an ophthalmologist with intermittent
snake attack. Eye 2003;17:279 –280. exotropia from January 1, 1975, through December 31,
4. Brandao EO, de Bastos HC, Nishioka Sde A, Silveira PV. 1994, were retrospectively reviewed. Institutional Re-
Lance-headed viper (Bothrops moojeni) bite wounding the eye.
view Board approval was obtained for this study and
Rev Inst Med Trop Sao Paulo 1993;35:381–383.
5. Warrel DA. Injuries, envenoming, poisoning, and allergic
only patients who provided research authorization were
reactions caused by animals. In: Weatherall DA, Cox TM, included. Intermittent exotropia was defined in this
Firth JD, Benz EJ Jr, editors. Oxford textbook of medicine, 4th study as an acquired, intermittent exodeviation of 10 or
ed. Oxford: Oxford University Press, 2003:923–946. more prism diopters unassociated with other ocular,
6. Kerrigan KR. Bacteriology of snakebite abscess. Trop Doct paralytic, or neurologic defects. Potential cases of inter-
1992;22:158 –160. mittent exotropia were identified by means of the
7. Chen JC, Liaw SJ, Bullard MJ, Chiu TF. Treatment of resources of the Rochester Epidemiology Project, a
poisonous snakebites in northern Taiwan. J Formos Med medical records linkage system designed to capture data
Assoc 2000;99:135–139. on any patient-physician encounter in Olmsted County,
Minnesota.2,3 Exotropic patients not residing in Olm-
sted County at the time of their diagnosis were ex-
Female Predominance in Intermittent cluded.
Exotropia One hundred eighty-four new cases of childhood
intermittent exotropia were diagnosed in Olmsted
Kevin J. Nusz, MD, Brian G. Mohney, MD, County, Minnesota during the 20-year study period.
and Nancy N. Diehl, BS Sixty-four percent of the 184 patients were female
during a period in which the population of Olmsted
County patients less than 19 years were nearly equally
PURPOSE: To evaluate gender differences among children divided between the genders (Table). The age-adjusted
diagnosed with intermittent exotropia. incidence rate for intermittent exotropia in Olmsted
DESIGN: Retrospective, population-based cohort study. County, Minnesota was 38.3 per 100,000 for girls, which
METHODS: The medical records of all Olmsted County, is significantly greater than the 20.8 per 100,000 for
Minnesota residents younger than 19 years diagnosed boys (P ⬍ .0001). There were, however, no significant
with intermittent exotropia from January 1, 1975,
through December 31, 1994, were reviewed.
RESULTS: One hundred eighteen (64.1%) of the 184 Accepted for publication Mar 2, 2005.
study patients were girls with an age-adjusted inci- From the Department of Ophthalmology (K.J.N., B.G.M.) and Divi-
sion of Biostatistics (N.N.D.), Mayo Clinic and Mayo Foundation,
dence rate of 38.3 (95% CI: 31.4 – 45.2) per 100,000 Rochester, Minnesota.
compared to 20.8 (95% CI: 15.7–25.8) per 100,000 Supported in part by an unrestricted grant from Research to Prevent
for boys (P < .0001). There were no significant Blindness, Inc, New York, New York.
Inquiries to Brian G. Mohney, MD, Mayo Clinic, Department of
differences between girls and boys in their family Ophthalmology, 200 First Street Southwest, Rochester, MN, 55905; fax:
history of strabismus, birth weight, prevalence of (507) 284-4612; e-mail: mohney@mayo.edu

546 AMERICAN JOURNAL OF OPHTHALMOLOGY SEPTEMBER 2005

 TABLE. Historical and Initial Clinical Characteristics of 184 Patients With Intermittent Exotropia Less Than 19 Years From
Olmsted County, Minnesota

Characteristic Females Males P Value

Incident cases (%) 118 (64.1%) 66 (35.9%) ⬍.0001

Age-adjusted incidence per 100,000 (95% confidence 38.3 (31.4–45.2) 20.8 (15.8–25.8) ⬍.0001
interval)
1990 Population of Olmsted County, MN, ages birth to 19 296, 201 (49%) 309, 316 (51%)
years (%)
Number with positive family history of strabismus (%) 41 (34.8) 20 (30.3%) .83
Number born at ⬍37 weeks gestational age (%) 6 (5.1%) 4 (6.1%) .72
Median birth weight in grams (range) 3374 (953–4620) 3382 (1640–4545) .76
Mean age at diagnosis in years (range) 6.2 (0.9–18.5) 6.5 (0.7–18.6) .92
Number with amblyopia at first examination (%) 2 (1.70%) 2 (3.0%) .62
Median angle of distance deviation in prism diopters (range) 20 (10–40) 20 (10–40) .86
Median angle of near deviation in prism diopters (range) 12 (0–45) 10 (0–45) .73
Median initial refractive error (range) ⫹0.50 (⫺3.25–⫹2.63) ⫹0.25 (⫺8.00–⫹3.13) .11
Number who underwent at least one surgery (%) 41 (34.8%) 18 (27.3%) .33
Mean age of first surgery in years (range) 7.2 (3.3–22.1) 8.5 (3.6–22.8) .42

historical or clinical differences between the genders This population-based study found that intermittent
(Table). exotropia is significantly more common among girls
No prior reports on strabismus have demonstrated a than boys. Two of every three children diagnosed with
predominance of girls among children with intermittent intermittent exotropia in this 20-year study were female.
exotropia.1,3– 6 Former studies of prevalence either failed In contrast, there were no historical or clinical differ-
to distinguish between the intermittent variety and ences between the genders. Further studies are needed
other forms or exotropia,4 or reported no appreciable to validate the gender differences of this common form
difference between the genders.1,3,5,6 A recent study of childhood strabismus.
from Olmsted County, surveying a period of 10 years, a
duration shorter than the current study, found that girls
tended to be more likely than boys to develop exotropia REFERENCES
during the first decade, whereas boys were more com-
monly exotropic during the teenage years (P ⫽ .10).3 1. Yu CB, Fan DS, Wong VW, et al. Changing patterns of
However, they did not indicate gender for each of the strabismus: a decade of experience in Hong Kong. Br J
individual forms of exotropia. Ophthalmol 2002;86:854 – 856.
2. Kurland LT, Molgaard CA. The patient record in epidemiol-
Why intermittent exotropia may be more common
ogy. Sci Am 1981;245:54 – 63.
among females is unclear. The gender distribution in
3. Govindan M, Mohney BG, Diehl NN, Burke JP. Incidence
this study suggests an X-linked dominant inheritance
and types of childhood exotropia: a population-based study.
that would affect females nearly twice as often as males.
Ophthalmology 2005;112:104 –108.
However, an X-linked affected male would be expected 4. Chew E, Remaley NA, Tamboli A, et al. Risk factors for
to transfer the trait to all his daughters and none of his esotropia and exotropia. Arch Ophthalmol 1994;112:1349 –
sons. This seems unlikely, although Lyonization, incom- 1355.
plete penetrance, and environmental factors could ex- 5. Graham PA. Epidemiology of strabismus. Br J Ophthalmol
plain some of the discrepancies. Additionally, we 1974;58:224 –231.
cannot discount the possibility that parents of exotropic 6. Friedman Z, Neumann E, Hyams SW, Peleg B. Ophthalmic
girls were more likely to seek medical attention for their screening of 38,000 children, age 1 to 2 and one half years, in
child than were the parents of boys with the same child welfare clinics. J Pediatr Ophthalmol Strabismus 1980;
condition. 17:261–267.

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