Functional Anatomy of Human Extraocular Muscles During Fusional Divergence.

We employed magnetic resonance imaging (MRI) to quantify human extraocular muscle contractility during centered target fusion, and fusional divergence repeated with each eye viewing monocularly at 20 cm through 8Δ, and at 400 cm through 4Δ base in prism. Contractility, indicated by posterior partial volume (PPV) change, was analyzed in transverse rectus and in medial and lateral superior oblique (SO) muscle compartments, and by cross sectional area change in the inferior oblique (IO). At 20 cm, 3.1±0.5° (SEM) diverging eye abduction in 10 subjects was associated with 4.2±1.5% whole lateral rectus (LR) PPV increase (P<0.05), and 1.7±1.1% overall MR PPV decrease attributable to 3.1±1.8% reduction in the superior compartment (P<0.025), without change in its inferior compartment, or in muscles of the aligned eye. At 400 cm, 2.2±0.5° diverging eye abduction in 9 subjects was associated with 6.1±1.3% whole LR PPV increase (P<10-5) but no change in MR, with compartmentally similar relaxation in the LR and MR of the aligned eye. Unlike convergence, there were no IO or SO contractile changes for divergence to either target, nor any change in rectus pulley positions. Results confirm and extend to proximal divergence the unique role of the superior MR compartment, yet no MR role for far divergence. Co-relaxation of aligned eye LR and MR combined with failure of MR relaxation during divergence is consistent with the limited behavioral range of divergence.