EOM Pulleys and Sequelae: A Critical Review.

Abstract

The discovery of extraocular muscle (EOM) pulleys resolved long-standing issues in oculomotor physiology, revived interest in EOM function generally, and led to several new theories. We describe the pulley concept of Miller and Demer (M-D Pulleys) and briefly review evidence, distinguishing this well-supported notion from the Active Pulley Hypothesis (APH) and the EOM Compartments hypothesis, and critically reviewing the methodologies and evidence on which the latter are based. We analyze evidence on mechanical independence of individual EOM fibers, implications of nerve tracing for functional independence of EOM layers and compartments, validity of image-based methods of assessing EOM contraction, and data analysis issues. M-D Pulleys are well-supported by several lines of evidence from several labs. The APH, which predicts relative movements of EOM lamina sufficient to alter muscle actions, has been effectively disproved. The width-wise articulations of EOM Compartments, in contrast, might produce significant contractile oculorotary force gradients across muscle tendons, although existing evidence is unconvincing. We suggest how this hypothesis could be effectively tested.