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.