Modelling non-linearities in the vestibulo-ocular reflex (VOR) after unilateral or bilateral loss of peripheral...

We recorded the vestibulo-ocular reflex (VOR) in 18 normal subjects, 50 patients with unilateral loss of vestibular function and 18 patients with bilateral loss of vestibular function. The unilateral cases had either partial loss (i.e. vestibular neuronitis or Meniere's disease) or total loss (i.e. vestibular nerve section), whereas bilateral cases had only partial loss (i.e. due to ototoxicity or to suspected microangiopathy, secondary to severe kidney disease). Tests were performed at 1/6-Hz passive head rotation in the dark, with peak head velocities ranging from 125 to 190 degrees/s. We report on the distinct VOR non-linearities observed in unilateral versus bilateral patients: whereas unilateral patients all exhibit an asymmetric hypofunction with decreasing VOR gain at higher head velocities, bilateral patients have a more severe but symmetric hypofunction associated with increasing VOR gain at higher head velocities. We present a model study that can duplicate the nature of these characteristics, based mainly on peripheral non-linear semicircular canal characteristics and secondary central compensation. Theoretical analyses point to the importance of clinical test parameters (rotation speed and frequency) in the determination of a functional VOR and the detection of reflex non-linearities, so that test protocols can seriously bias the evaluation of adequate functional recovery.