Abnormalities of ocular motility in myotonic dystrophy.

Anastasopoulos et al. (1996) reported a parallel degradation of smooth pursuit (SP) and vestibulo-ocular reflex suppression (VOR-S) indicating a central nervous system dysfunction in patients with myotonic dystrophy, who all had visual acuity better than 0.6. However, in our study on 13 myotonic dystrophy patients (Verhagen et al., 1992), we found SP impairment only in the two cases with the lowest visual acuity (0.3 and 0.5), but optokinetic nystagmus (OKN) responses were normal in all cases. We were therefore inclined to attribute the poor SP response in these latter cases to their poor visual acuity. Because OKN responses are less dependent on foveal vision than SP, we wrote that in view of the fact that most myotonic dystrophy patients have reduced vision due to cataract and retinal abnormalities (Miller, 1985), we would not be inclined to accept a deficit in visual following responses, unless it showed up in both the SP and the OKN responses (Verhagen et al., 1992). Unfortunately OKN was not examined by Anastasopoulos et al. (1996). In fact, OKN responses have never been included in reports on myotonic dystrophy, as far as we know, except in ours (Verhagen et al., 1992) and in a case report by Emre and Henn (1985). In our opinion, the VORS findings add nothing substantial to the SP findings in the study by Anastasopoulos et al. (1996), as the visual target (a laser spot) was identical in both cases, as was, apparently, the visual background (Ganzfeld-like structure of the cylindrical screen or darkness?) which, if not identical, must have been at least so designed that background structures could not compete with SP of the moving target by eliciting OKN responses in the opposite direction. In other words, the method was such that the emphasis was very much, in a fairly similar way, on foveal vision in both cases. Therefore an additional finding of OKN impairment might have made it more easy to accept central dysfunction as the underlying cause. Studying VOR-S