Functional Anatomy of Muscle Mechanisms: Compensating Vertical Heterophoria

Abstract

Magnetic resonance imaging (MRI) of extraocular muscle function was used to evaluate the role of newly recognized mechanisms underlying compensation of large heterophoria by vertical fusional vergence (VFV). Prospective case series. At one academic center, 8 adults with large hyperphoria and supernormal VFV underwent MRI during monocular and binocular fixation of a centered, near target. Contractility of the rectus and superior oblique (SO) extraocular muscles in hypertropic and hypotropic eyes was determined from changes in posterior partial volume (PPV). Five of 8 patients could sustain binocular fusion in the scanner. In those patients, VFV corrected approximately 5-degree misalignment, approximately 5-fold greater than normal VFV. Vertical strabismus was compensated mainly by significant contractility of the lateral more than the medial compartment of the inferior rectus (IR) in both eyes (P < .005). The superior rectus (SR) and inferior oblique muscles had no significant contractile contribution, although the hypotropic SO relaxed significantly. The IR lateral compartment and SR medial compartment significantly co-relaxed when binocular fusion was attained from monocular target fixation (P < .01). Although VFV protects patients from small muscle imbalances over the lifespan, even enhanced VFV may be inadequate to avert diplopia. Compensation of hyperphoria by VFV is accomplished mainly by IR muscle relaxation in the hypotropic eye, principally in its selectively innervated lateral compartment, whereas the SO contributes little. Fusion involves compartmentally selective co-relaxation in hypotropic eye vertical rectus muscles. Taken together, these overall findings suggest a physiologic basis to prefer therapeutic surgical weakening of the medial IR in the hypotropic eye.