REPLY

To the Editor: Drs. Tarczy-Hornoch and Guyton are cor-rect, and we thank them for pointing out our oversight. Aswas recently explained to us by Dr. Steven Archer, ourmodel of the reversed fixation test overlooked a confound-ing variable when concluding that a positive reversed fix-ation test necessarily reveals the underlying dissociateddeviation in the observed eye.1 Consider the patient inwhom routine prism uncover testing reveals 12� of DVDin the left eye. This hyperdeviation is neutralized with a12� base-down prism. When the cover is switched to theright eye, forcing the left eye to fixate through the prism,the left eye now has to fixate in 12� upgaze. Therefore,even if there is no right DVD, the right eye will be drivenupward 12� above the neutral position. Swinging the oc-cluder from the right to the left eye (the reversed fixationtest) will then cause the right eye to move down 12� totake up fixation. Therefore, even with no DVD in theright eye, the DVD in the left eye will cause the right eyeto move down 12�, thereby producing a positive reversedfixation test in the right eye.

If there is an additional 12� DVD in the right eye, thenthe reversed fixation test would cause the right eye to movedown 24� because the 12� of right DVD is superimposedon the 12� of right hyperdeviation induced by left eyefixation. Therefore, the reversed fixation test reveals thesum of the DVD in both eyes. For this reason, the reversedfixation test should disclose the same measurement whenperformed during fixation with either eye.

We also agree that the reversed fixation test cannoteduce whatever contribution a nondissociated hyperdevia-tion is making to the measured hyperdeviation in each eye.Consider the patient with a true left hypertropia that isneutralized by a 12� base-down prism. When the left eyefixates in its hypertropic position through the prism, thehypotropic right eye will remain in primary position, andno movement of the right eye will be observed when theright eye is uncovered and the occluder is shifted to the lefteye. When superimposed upon a DVD, only the dissoci-ated component would produce a positive reversed fixationtest. Consequently, the reversed fixation test cannot pro-vide information about any coexistent hypodeviation in thesetting of DVD. As noted in our editorial, however, tradi-tional methods of measuring the nondissociated componentalso fail in this regard.1 The bilaterality of DVD precludesclinical measurement of any nondissociated component.

The model proposed by Drs. Tarczy-Hornoch andGuyton unifies the outcome of the reversed fixation testfor dissociated horizontal and vertical deviations. In bothcases, the reversed fixation test elicits the sum of thedissociated deviations in the 2 eyes. In DVD, both non-

fixating eyes are driven dorsally (ie, in the same direction),so this sum is additive. In DHD, both nonfixating eyes aredriven nasally (ie, in opposite directions), so this sum isequal to the difference in dissociated esotonus that is gener-ated by fixation with each eye.2 For this reason, the reversedfixation test is useful in identifying but not in quantifying thedissociated esotonus that characterizes DHD.2

Michael C. BrodskyDepartment of Ophthalmology

Mayo ClinicRochester, MN

References1. Brodsky MC, Fray KJ. New observations on the reversed fixation test.

J AAPOS 2007;11:421-3.2. Brodsky MC. Dissociated horizontal deviation: Clinical spectrum,

pathogenesis, evolutionary underpinnings, diagnosis, treatment, andpotential role in the development of infantile esotropia. Trans AmOph Soc, in press.

doi:10.1016/j.jaapos.2007.12.005

VALIDITY OF A COMPUTERIZEDHESS CHART

To the Editor: Lim and colleagues compared the Assaf OcularMotility Analyzer (OMA) with the Lee Screen test, publish-ing an abstract of their study in the Journal of AAPOS (LimMK, Ashcroft A, Al-Madfai H, Watts PO. Validity of acomputerized Hess chart. J AAPOS 2007;11:96 [Abst]). Theauthors did not state the testing distance used for each test.The version of the OMA used in their study is an olderversion, which uses a testing distance of approximately 26 cm,versus approximately 50 cm for the Lee, hence the compar-atively higher figure for the OMA particularly in the direc-tion of exodeviation. Since then, the OMA was upgraded toa testing distance, of around 35 cm. The OMA with thesenew settings, has comparable deviation measurements tothose obtained by cover test carried out at 1/3 m distance,while the Lee underestimates these measurements (HorwoodA. New Adaptation to the Assaf Ocular Motility Analyser.British Isles Strabismological Association 10th Annual Meet-ing, Brighton, UK; October 2007).

Ahmed Assaf, FRCS, MDMilton Keynes Hospital NHS Foundation Trust

Buckinghamshire, United Kingdom

doi:10.1016/j.jaapos.2007.11.003

J AAPOS 2008;12:107.Copyright © 2008 by the American Association for Pediatric Ophthalmology and

Strabismus.1091-8531/2008/$35.00 � 0

Editor’s note: This letter refers to an AAPOS meeting poster abstract published inJ AAPOS in 2007. This presentation was withdrawn prior to the 2007 meeting, andthe authors have chosen not to respond to this letter. Note that in this issue of theJournal, an abstract of the same title by the same authors is published in thee-supplement to the 2008 AAPOS meeting announcement.J AAPOS 2008;12:107.

Copyright © 2008 by the American Association for Pediatric Ophthalmology andStrabismus.

1091-8531/2008/$35.00 � 0

Volume 12 Number 1 / February 2008 Letters to the Editor 107

Journal of AAPOS