Eng analysis of 150 cases of posterior fossa disease
Authors: Parker Willard 
Published: October 2006

Analysis of the ENG records of 150 patients with well substantiated neurologic disease involving the structure of the posterior fossa revealed a high incidence of diminished optic fixation inhibition (DOFI) of caloric induced nystagmus. DOFI is defined as less than 50 percent decrease of speed of the slow phase when eyes are open and fixated. The whole group was subdivided into seven subgroups according to type and location of disease and the incidence of DOFI was found to vary significantly in the different subgroups. The highest incidence was found in the group of cere‐bellar mass lesions and in the group of degenerative diseases, most of which had cerebellar involvement.

A review of the literature reveals work done by others showing that suppression of vestibular reflexes induced by high speed turning and also habituation of caloric induced nystagmus is dependent on optic fixation inhibition (opto‐vestibular reflex). It has been shown that the flocculo‐nodular lobe of the cerebellum has an inhibitory effect on the effects of vestibular activity by both destructive and stimulating techniques. Afferent and efferent pathways between the cerebellum and primary vestibular reflex arcs have been extensively described. It has been shown that cerebellar nodulectomy severely interferes with acquisition of habituation and retention of previously acquired habituation.

Since habituation and suppression of vestibular nystamus depend on optic fixation inhibition and equally on the cerebellum, it is reasonable to infer that the opto‐vestibular inhibitory reflex is mediated by the cerebellum. The clinical data of the 150 cases were analyzed for correlation of DOFI and other cerebellar signs. This analysis gives a high incidence of correlation: 66 percent of the cases having DOFI also had other cerebellar signs, whereas only 19 percent of cases that did not have DOFI had other cerebellar signs. These data lend further support to the inference that the cerebellum is involved in mediation of the opto‐vestibular inhibitory reflex.

Under most circumstances visual perception of the environment during movement depends on an inter‐reaction between the vestibulo‐oculomotor and the opto‐vestibular reflexes. Eye movements during head movements are entirely different with eyes closed than with eyes open and fixating. In fact, it can be shown that ocular fixation is far more efficient during head movements than it is during movement of an object with head stationary. It is probable that fixation in the former case is the result of vestibulo‐oculomotor slow phase activity modified by the opto‐vestibular inhibitory reflex and elimination of the fast phase. A defect in the latter should resuit in a sense of spatial insecurity during movement by virtue of altered visual perception.

The optovestibular inhibitory reflex is a central descending reflex probably mediated through the cerebellum. It is easy to test during an ENG. It is essential to normal visual perception during head movement. Deficits of this reflex should result in subjective visual insecurity during movement (nonvertiginous dizziness). Deficits demonstrated by ENG are an indication of CNS disease