title: Retinal image stability in head tremor and nystagmus: counterintuitive observations.
abstract: BACKGROUND
  Ophthalmoscopy of the fundus in a patient with head tremor is facilitated by image stability, whereas such observation of a patient with nystagmus is made difficult by image movement. Why does this occur?
  
  
  METHODS
  We offer an explanation of these observations and derive mathematical proofs for both direct and indirect ophthalmoscopes in various states of ametropia.
  
  
  RESULTS
  By means of image displacement calculations, we have created a graphical display of refractive error versus image displacement, showing zero displacement in emmetropia in head tremor and exaggerated displacement during nystagmus, regardless of the refractive status of the eye.
  
  
  CONCLUSIONS
  Simple optics and a mathematical proof explain the clinical observations. The image stability in the patient with head tremor and the image instability in an eye with nystagmus are the consequences of the optical system of the eye.
authors.affiliation: Department of Ophthalmology, Mayo Clinic, Rochester, Minnesota 55905, USA. BYounge@mayo.edu
authors.forename: Brian R
authors.initials: BR
authors.lastname: Younge
authors.forename: Jay W
authors.initials: JW
authors.lastname: McLaren
authors.forename: William L
authors.initials: WL
authors.lastname: Brown
mesh_terms: D005654:Fundus Oculi; D019416:Head Movements; D006801:Humans; D008433:Mathematics; D009759:Nystagmus, Pathologic / Q000503:physiopathology*; D019370:Observation; D009887:Ophthalmoscopy / Q000379:methods*; D012160:Retina / Q000473:pathology*; D014202:Tremor / Q000503:physiopathology*