PubMed · 2486106
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T C Hain, D S Zee. 1989. Vergence.. https://pubmed.ncbi.nlm.nih.gov/2486106/
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When visually fixating targets on an isovergence surface, the position of each eye was constrained to a plane. Thus, Listing's law holds during vergence. The planes were, however, rotated temporally with respect to those when viewing distant targets. The effect of this rotation was to produce a torsion which depended on eye elevation; extorsion of the two eyes for downward gaze and intorsion for upward gaze. The saccadic velocity command was relatively unaffected during vergence. Computer simulations suggest that the saccadic tonic command and the vergence command interact multiplicatively in three dimensions.
We report on a patient with a combination of congenital nystagmus and intermittent exotropia. When the exotropia became manifest, a gross nystagmus appeared. When, however, the exotropia was overcome by fusinal convergence, the nystagmus was largely suppressed. The angle of squint was surgically reduced by such an amount that the exodeviation was just compensated, and the fusional convergence required to overcome the remaining angle sufficed to suppress the nystagmus.
METHODS: 1. simple Maddox rod (Franceschetti) 2. set of scaled large diameter Maddox rods 3. synoptophore 4. synoptometer (Cüppers) 5. dark red glass in front of the Harms tangent scale (Kolling). Whereas the first two methods allow to measure in primary position only, the other three also permit secondary and tertiary position measurements. Thirty normal subjects underwent comparative measurements by means of the above techniques in monocular and binocular condition. The median values indicate that all these methods are reliable but that certain specific differences influence the results. For this reason, pre- and postoperative measurements of ocular torsion should always be performed by the same method. The most appropriate techniques have shown to be: the set of scaled Maddox rods for measurements in primary position only, the dark red glass in front of the Harms tangent screen for measuring also in secondary and tertiary positions.