August 1999
Volume 40, Issue 9
Letters to the Editor  |   August 1999
Hirschberg Ratio Variability and Its Correction
Author Affiliations
  • Paul E. Romano
    Dillon, Colorado
Investigative Ophthalmology & Visual Science August 1999, Vol.40, 2163-2164. doi:
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      Paul E. Romano; Hirschberg Ratio Variability and Its Correction. Invest. Ophthalmol. Vis. Sci. 1999;40(9):2163-2164.

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      © ARVO (1962-2015); The Authors (2016-present)

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Congratulations to Drs. Hasebe, Ohtsuki, Kono, and Nakahira on their fine contribution to strabology and strabometry published in the December issue (Biometric Confirmation of the Hirschberg Ratio in Strabismic Children. Invest Ophthalmol Vis Sci. 1998;39:2782–2785). We would like to make a few comments confirming and supplementing their findings. 
1. A semantic note: we suggest to the authors (and recommend strongly for our contributors to Binocular Vision & Strabismus) the use of the term “corneal light reflection test” instead of “corneal light reflex test” to describe the Hirschberg and similar tests. We do this to avoid possible confusion with the neurologic reflex phenomenon, which is not a light reflection but a true neurologic reflex arc to light stimulation. Yes, “reflex” is not wrong, but the term we use,“ reflection,” avoids any possible confusion. 
2. We were pleased to note the authors confirmed biometrically our previously reported finding 1 of the marked variability of the HR (Hirschberg ratio) from person to person. We note in their Figure 2, the markedly dispersed distribution of HR ratios, seemingly totally random, from 16Δ/mm to 24Δ/mm. It is important to note that although the average HR is close to 20Δ/mm, many patients are still close to the traditional HR of 15Δ/mm, whereas others have 50% greater HR. Equally important, there is no clinical information or finding on which the HR can be predicted for any given patient, including age and ocular dimensions. (See also 4 below.) 
3. The authors did not biometrically determine HR over a range of binocular misalignments for each of their subjects, but we urge them to add to their characterization of the HR that it also is definitely not linear, according to our prior research. 1 Rather, the HR for any given patient tends to increase as the deviation increases, up to 80Δ of esodeviation and 50Δ of exodeviation, and then decreases again. The HR for our patients ranged from 20Δ/mm for small deviation to 24Δ/mm at the maximum and then back to 19Δ/mm for esotropia of 95Δ and 16Δ/mm for 75Δ XT. (See unnumbered second and third figure in our reference). This asymmetric nonlinearity is undoubtedly due to the combination of angle kappa and corneal asphericity. 
4. There is, however, a way to eliminate all these HR problems for any single given patient. That is through photographic calibration of their own individual HR. By having pictures taken of the corneal light refection test with the subject fixating at known gaze positions, much as we did in our experiment, 1 one can provide a calibration table for the HR for that given patient. That calibration can be used to determine quite accurately the strabismic angle of binocular misalignment for their deviation, not just in primary position, but in eccentric gaze positions as well. 
5. When performing Hirschberg Corneal Light Reflection testing, we recommend that the Hirschberg measurement, whether performed by direct observation or by photography, be calculated (derived) in a manner parallel to the iris plane 1 ;i.e., as a fraction of corneal width (rather than the frontal plane) as this produces a smaller error, from the real deviation, than does using the frontal plane. (This is more easily done on a photograph.) 
6. Although the Hirschberg Corneal Light Reflection Test has long been considered a rough measurement, an estimate, a secondary, less than ideal measurement, a test to be used only when some type of prismatic measurement cannot be performed, we should remember that all these other prism measurement methods have their own set of measurement problems, 2 and all, except the simultaneous prism and cover test, do significantly disturb or interrupt whatever sort of binocular vision and binocular sensory and motor cooperation may be present and thereby or otherwise change the strabismic deviation angle. 
The most important, “most real,” measurement of a strabismic deviation still is in “free space,” i.e., under conditions of normal binocular viewing, uncontaminated by the measurement method. This may or will ultimately be by a precise calibrated photogrammetric Hirschberg Corneal Light Reflection Test. It is the only such pure method. The work of these authors is another step toward that ideal strabometric method. 
Romano PE. Iris plane parallel measurement in the Hirschberg (Corneal Light Reflection Test) for binocular misalignment (strabometry). In: Update on Strabismus and Pediatric Ophthalmolgy. Lennerstrand G, ed. Proc Joint ISA and AAPOS Meeting, Vancouver, Canada, 1994; CRC Press, Boca Raton. Chapter 111, Section 13 pp. 163–166.
Thompson JT, Guyton DL. Ophthalmic prisms: measurement errors and how to minimize them. Ophthalmology. 1983;90:204–210. [CrossRef] [PubMed]

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