2021 Volume 80 Issue 2 Pages 75-86
We had previously presented the case of a 67-year-old woman with Arnold-Chiari malformation (type-I), who concurrently showed three different kinds of rebound nystagmus (RN) in the dark in a single electronystagmography (ENG) study; (i) rightward (leftward) nystagmus during rightward (leftward) gaze, followed by leftward (rightward) nystagmus upon returning to the primary position, (ii) rightward (leftward) nystagmus followed by reversed nystagmus in the opposite direction while rightward (leftward) gaze was still maintained, with the reversed leftward (rightward) nystagmus persisting even after return to the primary position, and (iii) scarce or no nystagmus on leftward (rightward) gaze and marked lateral nystagmus in the direction opposite to the preexisting deviation soon after return to the primary position. There are already reports in the literature of the first two kinds of RN, that is, (i) and (ii) above; however, the third type of RN ((iii) above) had never been reported in any other case. Recently, however, we encountered two other cases with the third type of RN. One was a case of multiple lacunar infarction in the dorsal pons and another was a case of a traumatic lesion of a cerebellar hemisphere (inferior semilunar lobule). The common findings of the ENG were as follows: (1) horizontal third type of RN in the dark; (2) while the vertical (upward) pursuit was a back-up (overshoot) saccadic pursuit, the horizontal pursuit was well-preserved; (3) The peak slow phase velocity of the OKN was slightly reduced, while the frequencies of the slow phase velocity of the OKN were preserved; (4) the OKAN were within normal limits.
In general, RN is attributed to a smooth eye movement bias that is generated by both visual sources and non-visual sources, in order to oppose gaze-evoked centripetal drift of the eyes. In our cases, such a bias could have been generated only by non-visual sources, because the RN was observed in the dark, when visual sources are not operative. Although the origin of the non-visual sources are still obscure, efference copy, such as the eye velocity signal and/or eye position signal, is considered as a possible candidate.
It is assumed that apparently no nystagmus was observed during lateral gaze, which would be resulted from gaze nystagmus was equally offset by RN each other. Although gaze nystagmus had already disappeared by the time of return to the primary position, RN was still preserved, derived from signal processing error of the efference copy because of cerebellar dysfunction.
