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| An abnormal Hirschberg test demonstrating esotropia via |
In Bruckner Testing we are using the reflex of light from the
retina to determine if the patient is using both eyes equally to focus and
process visual information. Hirschberg
Testing also uses a light reflex to screen for asymmetry in ocular performance, but that’s where the similarities end. Instead of a retinal light reflex, the Hirschberg
Test assesses a corneal light reflex (called a Purkinje image) to assess
alignment of the eyes. Where Bruckner
Testing screens for visual potential and aims to find eyes with reduced visual
ability (amblyopia), Hirschberg Testing isn’t assessing vision at all, but
instead ocular alignment or potential strabismus.
Strabismus can cause amblyopia, so there is definitely a relation
between the two. But it is possible to
have a normal Hirschberg test but an abnormal Bruckner test, and also to have a
normal Bruckner test but an abnormal Hirschberg test.
What it Tells You:
The aim of this screening test is to determine the presence of
strabismus (an eye turn or misalignment of the eyes), and can also be used to
roughly quantify the amount of deviation.
Equipment:
A light-- can be a transilluminator, pen light, or a direct or indirect
ophthalmoscope used as a hand held light source.
Method:
1. Position your light about 1 foot from the patient and aim the
light right at the center of the bridge of their nose.
2, Ask the patient to look at your light.
2, Ask the patient to look at your light.
3. Assess where the light reflex from the patient’s cornea falls
compared to the center of the patient’s pupil (both direction and approximate
distance of deviation) in each eye.
Interpretation:
Normally the eyes will align together to focus equally on a
visual target. The amount of deviation from the center
should be equal between the two eyes in a normal test. If one eye is
misaligned, then we would expect an unequal position of the corneal reflex
between the eyes.
If the light reflex positions more nasal to the pupil in one eye, then that eye
is turned outward (or Exotropia).
If the light reflex positions more temporal to the pupil in one eye, then that eye is turned inward (or Esotropia).
A general rule of thumb: For every 1 mm of deviation, there are
22 prism diopters of eye turn.
Modification:
The Krimsky Method is a modification of the Hirschberg Test that
allows for more accurate measurement of the amount of deviation.
1. Perform Hirschberg Test as normal
2. Hold up a prism bar in front of the deviated eye and increase
the amount of prism until the reflex becomes equally centered.
Remember, if you have a light reflex that is too far nasal (an
exotropic eye), then you would need Base In prism to counteract the
deviation. For esotropia, you would need
Base Out prism.
Why won’t the penlight reflection be perfectly centered in the
pupil even in Normal Eyes?
Remember, even in normal eyes the corneal reflex isn’t at the
pupil center, but slightly nasal. This
is because of the optical properties of the eye. In the average eye, the foveal center of the
macula is not directly lined up with the pupil center, but slightly temporal to
it. This slight difference creates a
nasal offset of the corneal light reflex, a difference referred to in optics as
Angle Kappa. Angle Kappa can be greater or less in different ocular
anatomies. A common example that will
throw things off is a patient with a temporally dragged macula due to
conditions like retinopathy of prematurity.
Angle Kappa deviations will show up on Hirschberg testing as an abnormal
test finding, but won’t show up as strabismus or an eye turn on cover testing
(will link to our cover testing article). Just remember that you don’t expect
perfect centration when assessing Hirschberg, but you do expect equality
between the two eyes in the difference from the pupil to the corneal light
reflex. If you don’t see symmetry, then
you should assume the possibility for strabismus and use cover testing for
additional information.
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| Sometimes infants have the appearance of an eye turn, but their eyes are actually very normally aligned. The Hirschberg test can differentiate between actual strabismus and pseudostrabismus. via |
Psuedoesotropia in Infants:
A great practical use for Hirschberg testing is for infants
whose parents fear possible esotropia or an inward eye turn. Due to the lack of a fully formed nasal
bridge in infants and young children, the eyes may appear to look inwardly
crossed. To determine if there is true
esotropia versus just the false appearance of one (called pseudostrabismus), Hirschberg test can be
quickly deployed by your doctor to determine your child’s risk.
In very concerned parents, it is even possible to demonstrate to
them the appropriate alignment of the corneal reflexes to help put their
worries at ease!
Take Home: Much like Bruckner Testing, Hirschberg
Testing is a great way to quickly assess young children or noncompliant
patients without much effort from the patient – all they have to do is just look at the light. The results of Hirschberg testing can be
misleading however. It is possible that
you may see a deviation without there being a strabismus finding on cover
testing due to confounding factors like an abnormal angle kappa or anomalous correspondence. When your results aren’t
adding up, taking a broader look at the retinal anatomy and the visual system
as a whole can help. The brain is a
flexible organ, and it allows us to change “normal” visual behaviors to enhance
performance – this is the case in anomalies like anomalous correspondence where the
brain uses a retinal area other than the fovea for central vision. Even though the child may have overall good
alignment of the eyes on cover testing, they may still have reduced visual performance
of the eye since their brain is using unequal retinal images for fusion. These are the nuances that visual therapy
specialists help unlock in order to help you reprogram your brain for best
visual success! Screenings are great at
finding potential issues, but a skilled optometrist or pediatric
ophthalmologist will be able to use other data to determine the bigger and more complete picture
of your child’s visual system.
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| Dr. S is a COVD fellow practicing optometry in St. Louis |
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