title: Predicting dysthyroid optic neuropathy using computed tomography volumetric analyses of orbital structures.
abstract: OBJECTIVE
  To evaluate the ability of orbital apex crowding volume measurements calculated with multidetector-computed tomography to detect dysthyroid optic neuropathy.
  
  
  METHODS
  Ninety-three patients with Graves' orbitopathy were studied prospectively. All of the patients underwent a complete neuro-ophthalmic examination and computed tomography scanning. Volumetric measurements were calculated from axial and coronal contiguous sections using a dedicated workstation. Orbital fat and muscle volume were estimated on the basis of their attenuation values (in Hounsfield units) using measurements from the anterior orbital rim to the optic foramen. Two indexes of orbital muscle crowding were calculated: i) the volumetric crowding index, which is the ratio between soft tissue (mainly extraocular muscles) and orbital fat volume and is based on axial scans of the entire orbit; and ii) the volumetric orbital apex crowding index, which is the ratio between the extraocular muscles and orbital fat volume and is based on coronal scans of the orbital apex. Two groups of orbits (with and without dysthyroid optic neuropathy) were compared.
  
  
  RESULTS
  One hundred and two orbits of 61 patients with Graves' orbitopathy met the inclusion criteria and were analyzed. Forty-one orbits were diagnosed with Graves' orbitopathy, and 61 orbits did not have optic neuropathy. The two groups of orbits differed significantly with regard to both of the volumetric indexes (p<0.001). Although both indexes had good discrimination ability, the volumetric orbital apex crowding index yielded the best results with 92% sensitivity, 86% specificity, 81%/94% positive/negative predictive value and 88% accuracy at a cutoff of 4.14.
  
  
  CONCLUSIONS
  This study found that the orbital volumetric crowding index was a more effective predictor of dysthyroid optic neuropathy than previously described computed tomography indexes were.
authors.affiliation: Division of Ophthalmology, Faculdade de Medicina, Universidade de São Paulo, São Paulo, SP, Brazil.
authors.forename: Allan C Pieroni
authors.initials: AC
authors.lastname: Gonçalves
authors.forename: Lucas Nunes
authors.initials: LN
authors.lastname: Silva
authors.forename: Eloísa M M S
authors.initials: EM
authors.lastname: Gebrim
authors.forename: Suzana
authors.initials: S
authors.lastname: Matayoshi
authors.forename: Mário Luiz Ribeiro
authors.initials: ML
authors.lastname: Monteiro
mesh_terms: D000328:Adult; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D005260:Female; D049970:Graves Ophthalmopathy / Q000000981:diagnostic imaging*; D006801:Humans; D008297:Male; D008875:Middle Aged; D061330:Multidetector Computed Tomography; D009901:Optic Nerve Diseases / Q000000981:diagnostic imaging*; D009915:Orbit / Q000000981:diagnostic imaging*; D009929:Organ Size; D011237:Predictive Value of Tests; D011446:Prospective Studies; D012372:ROC Curve; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods*