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Prevent
Blindness America
Policy Statement on the Use of Photorefraction for Children's Vision Screening
The
prevalence of amblyopia and strabismus among children has been estimated
to be 5%. Only 21% of preschool children and even fewer children below
preschool age are screened for these conditions. Because early detection
and treatment of amblyogenic and other ocular conditions are believed
to lead to improved functional outcomes, vision screening of all preschool
children is advocated by ophthalmologic, optometry, and pediatric professional
organizations. The reality is, however, that the majority of children
are not being screened. Limited professional and financial resources hinder
efforts at reaching the goal of universal preschool vision screening.
A lack of adequate technology and agreed-upon methodology for screening
some groups is also a major obstacle, particularly for children under
the age of 3 years. Vision testing techniques for preverbal children,
such as certain preferential-looking techniques or the recording of visually
evoked cortical potentials, tend to be expensive or labor-intensive and
thus not presently suitable for mass screening.
Photorefraction is a technique in which the simultaneous recording of
corneal and fundus reflexes by flash or video photography may allow detection
of conditions that can give rise to amblyopia such as ametropia, anisometropia,
strabismus, and cataracts. Photorefraction does not test directly for
the presence of amblyopia, but rather for eye problems that can cause
amblyopia if untreated. The primary advantage of photorefraction is that
is can be used with children who are otherwise unable to be screened ¾
infants, preverbal children, and developmentally disabled children.
Several different photorefraction instruments are, or soon will be, commercially
available. Photorefraction screening instruments can be divided into two
broad categories, on-axis (co-axial) and off-axis (eccentric), which differ
in the spatial arrangement of the camera's optical axis and the flash
mechanism.
An on-axis photorefractor of the orthogonal or isotropic type requires
two pictures or images to define the sign of a refractive error, and as
a result is relatively sensitive to astigmatism. The isotropic type is
not useful for the detection of strabismus and ocular media opacities
unless a third photograph is taken, because an in-focus red reflex is
not produced with the two photographs taken for analysis of refractive
error.
Off-axis photorefractive screening can be done with instruments that take
one photograph with one flash, one photograph with two simultaneous flashes,
or two separate photographs. One photograph-one flash photorefraction
is not as sensitive to astigmatism as two-flash or two-photograph screening.
Off-axis photorefraction is very sensitive for the detection of strabismus
and ocular media opacities. Although all photorefractors are said to be
able to be used without cycloplegia, controlling accommodation is nonetheless
a problem; the child's state of accommodation remains an unknown variable.
For instance, in two-photograph photorefraction, either on or off-axis,
a change in the state of accommodation between photographs can confound
the assessment of astigmatism. Pharmacologic cycloplegia can be used to
control accommodation but would add a tremendous logistical and medicolegal
burden to vision screening programs.
Photorefraction screening can be done with videotape, 35-mm film, or instant
film. The principle advantage of a videotape system is low marginal cost
per screening, although the equipment tends to be expensive and not easily
portable. Thirty-five millimeter film requires off-site processing, and
thus, a delay occurs in obtaining results and making referrals. Furthermore,
an inadequate picture cannot be discovered in time to take another. Results
can be obtained immediately with instant film, and inadequate pictures
can be repeated. More training and expertise is required of the screeners
if the photographs are read at the time of screening, and the film cost
is greater.
Reported sensitivity rates for detecting abnormal ocular status range
from 77% to 91% for off-axis systems.3,4,6,7 Specificity rates range from
72% to 89%.3,4,6,7 Sensitivity and specificity rates of 85% and 54%, respectively,
have been reported for an on-axis system. Sensitivity and specificity
rates and the ability to obtain useable photographs clearly decline with
younger ages. Direct comparison of various photorefraction screening studies
and types of systems is difficult because of the use of different patient
populations and threshold criteria. As a result, a consensus regarding
the optimal threshold for screening failure with respect to hyperopia,
myopia, astigmatism, and anisometropia has not yet emerged.
The issue of what patient populations to screen with photorefraction has
also not been settled. Children from 0-3 years old and the developmentally
disabled are logical groups to consider because they are not now being
adequately screened.
The use of photorefraction for preschool children from 3-5 years of age
is more controversial than for preverbal children because, if photorefraction
is to be used, it must be shown to be superior to the screening being
done now with visual acuity and stereopsis tests. Furthermore, this superiority
must be shown in studies where the photorefractive screening is accomplished
by trained volunteers in a regular school environment, not by highly trained
professionals in a controlled, laboratory-like setting.
For schoolchildren older than 6 years of age, who can be screened very
reliably with current methods, photorefraction would appear to be a much
less compelling and probably more expensive technology.
To prove the superiority of photorefraction over currently used techniques
in any age group, several questions must be considered: (1) Are the sensitivity
and specificity rates with photorefraction better than those with traditional
methods, such as visual acuity and stereopsis testing? (2) Can a greater
number of children be screened than with other methods? (3) Are cost and
personnel requirements compatible with available resources?
Photorefraction is a promising technology that has potential for allowing
expansion of screening programs to include children who are not being
screened effectively (children below the age of 3 years and the developmentally
disabled) and possibly to improve upon methods now used for preschool
children from 3 through 5 years of age. To assure that quality screening
programs result from this emerging technology, further research needs
to be done in several areas: (1) the benefit from photorefractive screening
of either preverbal or preschool children, beyond what is obtained from
current preschool vision screening practices, must be firmly established;
(2) competing photorefraction systems must be compared with each other
and with currently available screening methods for each age group; (3)
appropriate referral criteria must be established; (4) training and certification
standards for screening personnel must be developed; and (5) cost must
be evaluated. Commercial considerations must not be allowed to drive the
careful and painstaking process of accomplishing these tasks. Prevent
Blindness America wholeheartedly supports and encourages further work
in this area.
Statement
approved by the board of directors, June, 1994.
Addendum
Prevent Blindness America's photorefraction task force recognizes photoscreening
to be an acceptable method for screening preverbal and developmentally-delayed
children for vision problems when traditional methods are ineffective.
The task force also recognizes the importance of comparing photoscreening
technology to current children's vision screening methods and has begun
to work with the national office to develop and implement a scientifically
based pilot study.
Until the findings of this and possibly other studies are published, the
task force suggests that affiliates/divisions interested in utilizing
photoscreening technology in their local screening programs coordinate
with the national office to determine appropriate target audiences, screening
techniques, training requirements, referral criteria, follow-up procedures,
evaluation and other critical components to ensure effective screening.
Addendum
approved by the board of directors, November, 1994.
Policy
Note
Prevent Blindness America does not endorse specific products or manufacturers.
Any products named in this document are mentioned for informational purposes
only.
Prevent Blindness America policy U.5.
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