Bechterew decompensation

Relationship of static and dynamic mechanisms in vestibuloocular reflex compensation

OBJECTIVES/HYPOTHESIS

We tested the hypothesis that recovery of dynamic oculomotor behavior (specifically the vestibuloocular reflex [VOR]) after a unilateral labyrinthectomy (UL) is independent from static deficit recovery (e.g., spontaneous nystagmus). This hypothesis predicts that VOR recovery from peripheral vestibular lesions that do not cause static symptoms, such as unilateral semicircular canal plugging (UCP), would have a similar time course and magnitude as recovery from a lesion that creates both a static and dynamic imbalance, such as UL. Furthermore, animals compensated after UCP would be expected to retain their compensated VOR response after the additional insult of ipsilateral labyrinthectomy.

STUDY DESIGN

An experimental study in the Mongolian gerbil animal model.

METHODS

The horizontal VOR was measured from both eyes using infrared video-oculography on gerbils before and after UCP, UL, or ipsilateral labyrinthectomy after a previous UCP. Eye movements were recorded during yaw rotation in the dark.

RESULTS

UL resulted in a more severe acute deficit than the UCP. Over several weeks, the UCP animals compensated their horizontal VOR, particularly on rotation toward the intact side, quicker and more completely than the UL animals. Animals that underwent ipsilateral labyrinthectomy 8 to 11 weeks after UCP demonstrated preservation of the improved gain, particularly on rotation toward the intact labyrinth. However, the difference between the UL groups with or without precedent UCP was not retained after 72 hours, and long-term compensation was poorest in the UL after UCP group.

CONCLUSIONS

Plasticity in dynamic vestibular reflexes induced by UCP is preserved after a subsequent UL. However, neurologic events during the first and second day after UL appear to limit, change, or suppress the long-term dynamic compensation of the VOR, regardless of whether the animal had a previous UCP.

Compensation of horizontal canal related activity in the medial vestibular nucleus following unilateral labyrinth ablation in the decerebrate gerbil. I. Type I neurons

The spontaneous activity and dynamic responses to two frequencies (1.3 and 0.13 Hz) of sinusoidal angular horizontal head acceleration of type I neurons in the medial vestibular nucleus were recorded bilaterally in decerebrate Mongolian gerbils (Meriones unguiculatus) under three experimental conditions; normal labyrinth intact, acutely following unilateral labyrinthine lesion, and four to seven weeks following labyrinthine lesion. The mean spontaneous activity and number of detected type I neurons decreased immediately ipsilateral to the lesion but recovered significantly with time. In contrast, spontaneous activity on the contralateral side increased during compensation following hemilabyrinthectomy. The mean response gains at both frequencies of head oscillation were depressed bilaterally and asymmetrically acutely following the lesion such that the response gain of cells on the intact side exceeded that of the neurons recorded on the injured side. After compensation the number of detected type I neurons on the side ipsilateral to the injury increased but remained below normal levels. The mean gains remained depressed but became symmetric with compensation as a result of improvement in the response of ipsilateral neurons. The phase of responses were significantly advanced in the compensated animals. Although response gain is not fully restored, the linearity of the dynamic modulation in compensated animals is improved as evidenced by a continuous modulation of the increased spontaneous activity of neurons contralateral to the hemilabyrinthectomy. It is proposed that this effect is related to the concurrent improvement in the linearity of the horizontal vestibulo-ocular response. Electrical cathodal polarization of the vestibular nerve ipsilateral to the ablated labyrinth was utilized to investigate the relationship between recovery of spontaneous activity and dynamic function. Acutely following hemilabyrinthectomy, cathodal polarization restored activity in second-order type I neurons to near normal levels but their response gain to head rotation remained depressed. Similar galvanic stimulation in compensated animals also elevated ipsilateral spontaneous activity. As in the acute preparation, such stimulation did not modify the response gain or phase. Thus, the improvement in response of type I neurons in the compensated gerbil was not a direct consequence of restoration of spontaneous activity on the side of the injury.