title: Alexander's law revisited.
abstract: BACKGROUND
  It is a common occurrence in the balance function laboratory to evaluate patients in the post-acute period following unilateral vestibular system impairment. It is important to be able to differentiate spontaneous nystagmus (SN) emanating from peripheral vestibular system impairments from asymmetric gaze-evoked nystagmus (GEN) that originates from central ocular motility impairment.
  
  
  OBJECTIVE
  To describe the three elements of Alexander's Law (AL) that have been used to define SN from unilateral peripheral impairment. Additionally, a fourth element is described (i.e., augmentation of spontaneous nystagmus from unilateral peripheral vestibular system impairment) that differentiates nystagmus of peripheral vestibular system origin from nystagmus that originates from a central eye movement disorder.
  
  
  METHODS
  Case reports.
  
  
  METHODS
  Case data were obtained from two patients both showing a nystagmus that followed AL.
  
  
  METHODS
  None
  
  
  METHODS
  Videonystagmography (VNG), rotational, vestibular evoked myogenic potential (VEMP), and neuro-imaging studies were presented for each patient.
  
  
  RESULTS
  The nystagmus in Case 1 occurred as a result of a unilateral, peripheral, vestibular system impairment. The nystagmus was direction-fixed and intensified in the vision-denied condition. The nystagmus in Case 2, by appearance identical to that in Case 1, was an asymmetric gaze-evoked nystagmus originating from a space-occupying lesion in the cerebello-pontine angle. Unlike Case 1, the nystagmus did not augment in the vision-denied condition.
  
  
  CONCLUSIONS
  Although nystagmus following AL usually occurs in acute peripheral vestibular system impairment, it can occur in cases of central eye movement impairment. The key element is whether the SN that follows AL is attenuated or augmented in the vision-denied condition. The SN from a unilateral peripheral vestibular system impairment should augment in the vision denied condition. An asymmetric GEN will either not augment, decrease in magnitude, or disappear entirely, in the vision-denied condition.
authors.affiliation: Vanderbilt Bill Wilkerson Center for Otolaryngology and Communication Sciences, Department of Hearing and Speech Sciences, Vanderbilt University, USA. gary.jacobson@vanderbilt.edu
authors.forename: Gary P
authors.initials: GP
authors.lastname: Jacobson
authors.forename: Devin L
authors.initials: DL
authors.lastname: McCaslin
authors.forename: David M
authors.initials: DM
authors.lastname: Kaylie