Purpose: The purpose of this study is to develop and test criteria for diagnosis of an ‘overacting’ cyclovertical muscle using a modification of Parks’ Three-Step Test. Background: The Parks' Three-Step Test has become invaluable for isolation of a weak muscle in those strabismus cases in which there is a known paralysis. This test can yield confusing results in cases of non-paralytic vertical strabismus. The utricular induced ocular counter-rolling response combined with the premise that vertical strabismus is, in part, secondary to imbalances in vertical forces, are the physiologic bases for this test and the described modification. Methods: A modification of the first step of the Parks' Three-Step Test was designed which isolates an ‘overacting’ cyclovertical muscle in the same manner as a ‘paretic’ muscle. Possible ‘overacting’ muscles are isolated in primary position, lateral gaze and right and left head tilt. Fifty patients from the author's practice with non-paralytic vertical strabismus examined from July, 1990 through June, 1991 who had complete ocular motility evaluations were included in this analysis. The modified test was used during patient evaluations to corroborate the clinical impression of a predominantly overacting cyclovertical muscle.

Results: The Three-Step Test modification correctly identifies a cyclovertical muscle as ‘overacting’ when the vertical deviation is asymmetric in horizontal gaze (step 2) and when the Bielschowsky head tilt (step 3) is asymmetric.

Conclusions: This modification of the Parks' Three-Step Test assists with vertical strabismus diagnosis and can be an adjunct to surgical planning by isolating a cyclovertical muscle to weaken in order to balance vertical forces.