title: Rectus Extraocular Muscle Size and Pulley Location in Concomitant and Pattern Exotropia.
abstract: To determine whether rectus extraocular muscle (EOM) sizes and pulley locations contribute to exotropia, we used magnetic resonance imaging (MRI) to measure these factors in normal control participants and in patients with concomitant and pattern exotropia.
  
  
  
  Prospective case-control study.
  
  
  
  Nine patients with concomitant exotropia, 6 patients with pattern exotropia, and 21 orthotropic normal control participants.
  
  
  
  High-resolution surface-coil MRI scans were obtained in contiguous, quasicoronal planes. Rectus pulley locations were determined in oculocentric coordinates for central gaze, supraduction, and infraduction. Cross sections in 4 contiguous image planes were summed and multiplied by the 2-mm slice thickness to obtain horizontal rectus posterior partial volumes (PPVs).
  
  
  
  Rectus pulley locations and horizontal rectus PPVs.
  
  
  
  Rectus pulleys were located differently in patients with A-pattern, versus V- and Y-pattern, exotropia. The lateral rectus (LR) pulleys were displaced significantly superiorly, the medial rectus (MR) pulleys were displaced inferiorly, and the inferior rectus pulleys were displaced laterally in A-pattern exotropia. However, the array of all rectus pulleys was excyclorotated in V- and Y-pattern exotropia. The PPV of the medial rectus muscle was statistically subnormal by approximately 29% in concomitant, but not pattern, exotropia (P < 0.05). The ratio of the PPV of the LR relative to the MR muscles in concomitant exotropia was significantly greater than in control participants and those with pattern exotropia (P < 0.05).
  
  
  
  Abnormalities of EOMs and pulleys contribute differently in pattern versus concomitant exotropia. Abnormal rectus pulley locations derange EOM pulling directions that contribute to pattern exotropia, but in concomitant exotropia, pulley locations are normal, and relatively small medial rectus size reduces relative adducting force.
authors.affiliation: Department of Ophthalmology, David Geffen Medical School at University of California, Los Angeles, Los Angeles, California; Tianjin Eye Hospital, Tianjin Key Lab of Ophthalmology and Visual Science, Tianjin Eye Institute, Clinical College of Ophthalmology Tianjin Medical University, Peoples Republic of China.
authors.forename: Rui
authors.initials: R
authors.lastname: Hao
authors.affiliation: Department of Ophthalmology, David Geffen Medical School at University of California, Los Angeles, Los Angeles, California.
authors.forename: Soh Youn
authors.initials: SY
authors.lastname: Suh
authors.affiliation: Stein Eye Institute, David Geffen Medical School at University of California, Los Angeles, Los Angeles, California.
authors.forename: Alan
authors.initials: A
authors.lastname: Le
authors.affiliation: Department of Ophthalmology, David Geffen Medical School at University of California, Los Angeles, Los Angeles, California; Stein Eye Institute, David Geffen Medical School at University of California, Los Angeles, Los Angeles, California; Department of Neurology, David Geffen Medical School at University of California, Los Angeles, Los Angeles, California; Neuroscience Interdepartmental Program, David Geffen Medical School at University of California, Los Angeles, Los Angeles, California; Bioengineering Interdepartmental Program, David Geffen Medical School at University of California, Los Angeles, Los Angeles, California. Electronic address: jld@jsei.ucla.edu.
authors.forename: Joseph L
authors.initials: JL
authors.lastname: Demer
mesh_terms: D000328:Adult; D016022:Case-Control Studies; D003198:Computer Simulation; D005099:Exotropia / Q000473:pathology* / Q000503:physiopathology; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D009801:Oculomotor Muscles / Q000473:pathology* / Q000503:physiopathology; D011446:Prospective Studies; D055815:Young Adult