Changes in Extraocular Muscle Volume During Ocular Duction

PURPOSE: It has been tacitly assumed that overall extraocular muscle (EOM) volume is conserved during contraction and relaxation, yet this assumption has been untested up to now. We used high-resolution magnetic resonance imaging (MRI) to determine if total EOM volume changes during relaxation and contraction. METHODS: Surface coil MRI in quasi-coronal planes was obtained in target-controlled, maximal secondary gaze positions in 30 orbits of 15 normal subjects at 312-μm resolution. Ductions were quantified by changes in globe-optic nerve positions. Cross-sections of EOM were manually outlined in contiguous image planes so that volumes could be calculated by multiplying summed cross sections by the 2-mm slice thickness. Three-dimensional reconstruction allowed measurement of the lengths of terminal, unresolvable EOM segments, providing estimates of terminal EOM volumes to be summed with measured midorbital volumes to obtain total EOM volumes. RESULTS: Duction range averaged 44.3 ± 4.8° from relaxation to contraction. There was a significant increase in total volume in each rectus EOM from relaxation to contraction: superior rectus (SR) 92 ± 36 mm(3) (+18%, P < 10(−11)); inferior rectus (IR) 51 ± 18 mm(3) (+9%, P < 10(−11)); medial rectus (MR) 78 ± 36 mm(3) (+11%, P < 10(−5)); and lateral rectus (LR) 47 ± 45 mm(3) (+7%, P = 0.005). Because volume changes for SR and MR exceed IR and LR, total rectus EOM volume increases in supraduction 41 ± 42 mm(3) (+3.7%) and adduction 32 ± 63 mm(3) (+2.3%). CONCLUSIONS: Total EOM volume is not conserved but instead increases with contraction and decreased with relaxation. Contractile volume increases may be secondary to increased actin-myosin lattice spacing, so that density decreases. This effect is opposite that of possible hemodynamic changes.