RT Book, Section
A1 Duncan, Jacque L.
A1 Parikh, Neeti B.
A1 Seitzman, Gerami D.
A2 Papadakis, Maxine A.
A2 McPhee, Stephen J.
A2 Rabow, Michael W.
A2 McQuaid, Kenneth R.
SR Print(0)
ID 1184191990
T1 Refractive Errors
T2 Current Medical Diagnosis & Treatment 2022
YR 2022
FD 2022
PB McGraw-Hill Education
PP New York, NY
SN 9781264269389
LK accessmedicine.mhmedical.com/content.aspx?aid=1184191990
RD 2024/04/24
AB Refractive  error  is  the  most  common  cause  of  reduced  clarity  of  vision  (visual  acuity).  In  the  normal  state  (emmetropia)  objects  at  infinity  are  seen  clearly.  Focusing  on  objects  nearer  than  infinity  requires  an  increase  in  the  refractive  power  of  the  crystalline  lens  by  the  process  of  accommodation.  In  farsightedness  (hyperopia)  objects  at  infinity  are  not  seen  clearly  unless  accommodation  is  used  and  near  objects  may  not  be  seen  because  accommodative  capacity  is  finite.  Farsightedness  is  corrected  with  plus  (convex)  lenses.  In  nearsightedness  (myopia)  the  unaccommodated  eye  focuses  on  objects  closer  than  infinity.  The  markedly  nearsighted  eye  is  able  to  focus  on  very  near  objects  without  glasses.  Objects  beyond  this  distance  cannot  be  seen  without  the  aid  of  corrective  (minus,  concave)  lenses.  Astigmatism,  in  which  the  refractive  errors  in  the  horizontal  and  vertical  axes  differ,  is  corrected  with  cylindrical  lenses.  Presbyopia  is  the  natural  loss  of  accommodative  capacity  with  age.  Persons  with  emmetropia  usually  notice  inability  to  focus  on  objects  at  a  normal  reading  distance  at  about  age  45.  Farsighted  individuals  experience  symptoms  at  an  earlier  age.  Presbyopia  is  corrected  with  plus  lenses  for  near  work.