title: Magnetic resonance imaging of the functional anatomy of the inferior rectus muscle in superior oblique muscle palsy.
abstract: OBJECTIVE
  Biomechanical modeling consistently indicates that superior oblique (SO) muscle weakness alone is insufficient to explain the large hypertropia often observed in SO muscle palsy. Magnetic resonance imaging (MRI) was used to investigate if any size or contractility changes in the inferior rectus (IR) muscle may contribute.
  
  
  METHODS
  Prospective, case-control study.
  
  
  METHODS
  Seventeen patients with unilateral SO muscle palsy and 18 orthotropic subjects.
  
  
  METHODS
  Surface coils were used to obtain sets of contiguous, 2-mm-thick, high-resolution, coronal MRI views in different gazes. Cross-sectional areas of the IR and SO muscles were determined in supraduction and infraduction for evaluation of size and contractility. Diagnosis of SO muscle palsy was based on clinical presentations, subnormal contractility, and SO muscle size less than the normal 95% confidence limit.
  
  
  METHODS
  Cross-sectional areas of the IR and SO muscles.
  
  
  RESULTS
  Patients had 15.9+/-7.2 prism diopters (Delta; mean+/-standard deviation) of central gaze hypertropia and exhibited ipsilesional SO muscle atrophy and subnormal contractility. Mean ipsilesional, contralesional, and normal IR muscle cross-sections were 28.5+/-3.5 mm(2), 31.9+/-3.8 mm(2), and 31.8+/-5.8 mm(2), whereas mean contractility was 16.5+/-3.8 mm(2), 20.5+/-4.1 mm(2), and 16.6+/-4.8 mm(2), respectively. Ipsilesional IR muscle cross-section and contractility was significantly less than contralesional cross-section and contractility (P<0.01).
  
  
  CONCLUSIONS
  In SO muscle palsy, the contralesional IR muscle is larger and more contractile than the ipsilesional IR muscle, reflecting likely neurally mediated changes that augment the relatively small hypertropia resulting from SO muscle weakness alone. Recession of the hyperfunctioning contralesional IR muscle recession in SO muscle palsy is a physiologic therapy.
authors.affiliation: Department of Ophthalmology, David Geffen Medical School, University of California, Los Angeles, Los Angeles, California, USA.
authors.forename: Li
authors.initials: L
authors.lastname: Jiang
authors.forename: Joseph L
authors.initials: JL
authors.lastname: Demer
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D009801:Oculomotor Muscles / Q000503:physiopathology*; D009886:Ophthalmoplegia / Q000503:physiopathology*; D011446:Prospective Studies; D013285:Strabismus / Q000503:physiopathology*; D020432:Trochlear Nerve Diseases / Q000503:physiopathology*; D015348:Vision, Binocular; D055815:Young Adult