Quantitative analysis of binocular alignment by Hess screen testing in surgery for superior oblique palsy

Conventional prism-cover strabismus measurement is compromised by nonstandard and poorly repeatable target locations. We developed a general method to quantify incomitant strabismus in repeatable, defined gazes and illustrate its application to commonly performed surgical strategies in anatomically confirmed cases of superior oblique (SO) palsy.

Hess screen alignment data were obtained before, 1 week, and 3-7 months after surgery in 19 patients with unilateral SO atrophy demonstrated by magnetic resonance imaging. Five underwent ipsilateral inferior oblique (IO) recession, four contralateral inferior rectus (IR) recession, and 10 underwent both. An automated program measured scanned Hess charts at 21 fixation targets at ±15° and 30° horizontal and vertical eccentricities for each fixating eye. Heat maps were created across the fixation field, quantifying strabismus angles, surgical effects, and secular trends. Individual heat maps were pointwise averaged for affected and fellow eyes, and according to the type of surgery: ipsilateral IO recession, contralateral IR recession, and combined.

Pre-operative alignment maps demonstrated three incomitant hypertropia patterns addressed by distinct surgical approaches. Recession of the IR alone produced immediate and stable correction of hypertropia with minimal incomitant effect. Recession of the IO corrected large lateral incomitance and had progressively improving late effects. Combining these approaches corrected lateral incomitance and hypertropia in infraversion, with progressively improving late effect.

Alignment heat maps allow intuitive visualization of selective strabismus surgical strategies treating varied incomitance in SO palsy, reflecting progressive improvement in alignment when surgery includes IO recession.