Hirschberg Testing

8:17 AM

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.
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.
 
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.

Dr. S is a COVD fellow practicing  optometry in St. Louis

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