title: Ocular rotation axes during dynamic Bielschowsky head-tilt testing in unilateral trochlear nerve palsy.
abstract: OBJECTIVE
  To explain the positive Bielschowsky head-tilt (BHT) sign in unilateral trochlear nerve palsy (uTNP) by the kinematics of three-dimensional eye rotations.
  
  
  METHODS
  Twelve patients with uTNP monocularly fixed on targets on a Hess screen were oscillated (+/- 35 degrees, 0.3 Hz) about the roll axis on a motorized turntable (dynamic BHT). Three-dimensional eye movements were recorded with dual search coils. Normal data were collected from 11 healthy subjects.
  
  
  RESULTS
  The rotation axis of the viewing paretic or unaffected eye was nearly parallel to the line of sight. The rotation axis of the covered fellow eye, however, was tilted inward relative to the other axis. This convergence of axes increased with gaze toward the unaffected side. Over entire cycles of head roll, the rotation axis of either eye remained relatively stable in both the viewing and covered conditions.
  
  
  CONCLUSIONS
  In patients with uTNP, circular gaze trajectories of the covered paretic or unaffected eye during dynamic BHT are a direct consequence of the nasal deviation of the rotation axis from the line of sight. This, in turn, is a geometrical result of decreased force by the superior oblique muscle (SO) of the covered paretic eye or, according to Hering's law, increased force parallel to the paretic SO in the covered unaffected eye. The horizontal incomitance of rotation axes along horizontal eye positions can be explained by the same mechanism.
authors.affiliation: Department of Neurology, Zurich University Hospital, Zurich, Switzerland.
authors.forename: Konrad P
authors.initials: KP
authors.lastname: Weber
authors.forename: Klara
authors.initials: K
authors.lastname: Landau
authors.forename: Antonella
authors.initials: A
authors.lastname: Palla
authors.forename: Thomas
authors.initials: T
authors.lastname: Haslwanter
authors.forename: Dominik
authors.initials: D
authors.lastname: Straumann
mesh_terms: D000293:Adolescent; D000328:Adult; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D003941:Diagnostic Techniques, Ophthalmological; D005133:Eye Movements / Q000502:physiology*; D005260:Female; D019416:Head Movements / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009799:Ocular Physiological Phenomena; D009801:Oculomotor Muscles / Q000503:physiopathology*; D011187:Posture; D012027:Reflex, Vestibulo-Ocular / Q000502:physiology; D012399:Rotation; D020432:Trochlear Nerve Diseases / Q000503:physiopathology*; D015348:Vision, Binocular / Q000502:physiology