title: Comprehensive profile of bilateral astigmatism: rule similarity and symmetry patterns of the axes in the fellow eyes.
abstract: To study rule similarity (isorule or anisorule) and symmetry patterns of axes (direct and mirror) in bilateral astigmatism.
  
  
  
  Six years of data were collected from four ophthalmology clinics in Tehran. After applying the exclusion criteria (having eye disease or history of ocular surgery), final analyses were performed on data of 160 608 cases whose mean age was 39.2 ± 15.4 years (10-80 years) and 53.7% female. All subjects had autorefraction, retinoscopy, and subjective refraction data. Rule similarity was categorised as isorule (with, against or oblique astigmatism) if the orientation type in fellow eyes was the same and anisorule if they were different.
  
  
  
  The prevalence of isorule astigmatism was 82.9% and was similar in men and women (p = 0.44). The prevalence of isorule astigmatism decreased with age and increased with greater spherical ametropia (p < 0.001). The median of the absolute inter-ocular axis difference from exact symmetry was 10° under the mirror symmetry model (mirror axes). Under the direct symmetry model (equal axes), the median of the absolute inter-ocular axis difference was significantly higher, at 20° (p < 0.001). 16.4% and 10.8% had exact mirror and direct symmetry, respectively. In addition, 68.0% of cases with mirror symmetry and 47.1% with direct symmetry were within ±15 degrees of exact symmetry.
  
  
  
  The results of the present study showed that bilateral astigmatism was mostly isorule. Bilateral symmetry of the astigmatism axes was highly prevalent and mostly of the mirror pattern. High astigmatic cases showed better symmetry.
authors.affiliation: Noor Research Center for Ophthalmic Epidemiology, Noor Eye Hospital, Tehran, Iran.|Department of Optometry, School of Paramedical Sciences, Mashhad University of Medical Sciences, Mashhad, Iran.
authors.forename: Amir
authors.initials: A
authors.lastname: Asharlous
authors.affiliation: Department of Medical Surgical Nursing, School of Nursing and Midwifery, Shahid Beheshti University of Medical Sciences, Tehran, Iran.|Noor Ophthalmology Research Center, Noor Eye Hospital, Tehran, Iran.
authors.forename: Mehdi
authors.initials: M
authors.lastname: Khabazkhoob
authors.affiliation: Department of Optometry, School of Paramedical Sciences, Mashhad University of Medical Sciences, Mashhad, Iran.
authors.forename: Abbasali
authors.initials: A
authors.lastname: Yekta
authors.affiliation: Noor Ophthalmology Research Center, Noor Eye Hospital, Tehran, Iran.
authors.forename: Hassan
authors.initials: H
authors.lastname: Hashemi
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001251:Astigmatism / Q000175:diagnosis / Q000453:epidemiology* / Q000503:physiopathology; D002648:Child; D003315:Cornea / Q000503:physiopathology*; D005260:Female; D006801:Humans; D007492:Iran / Q000453:epidemiology; D008297:Male; D008875:Middle Aged; D015995:Prevalence; D012029:Refraction, Ocular / Q000502:physiology*; D014787:Vision Tests / Q000379:methods; D055815:Young Adult
keywords: anisorule; astigmatism axis; direct symmetry; isorule; mirror symmetry; rule similarity