Congenital nystagmus: In search of simplicity on the other side of complexity

Abstract

To the Editor: This letter is in response to a recent editorial, “Congenital Nystagmus: In Search of Simplicity on the Other Side of Complexity,“1 written about the paper, “Clinical and Ocular Motor Analysis of Congenital Nystagmus in Infancy.“2 The paper in question (1) is the first prospective study of the waveforms of congenital nystagmus (CN) in infants; (2) contains the largest population so studied; (3) is the first such study to use accurate and sensitive ocular motor recording techniques; and (4) provides the first demonstration that infants do exhibit the same CN waveforms as adults, with breaking saccades, foveating saccades, and foveation periods (ie, they are not all pendular, as others have suggested). We recognize the connection in those CN patients with sensory disorders and also provide an explanation of the mechanism by which afferent disorders can facilitate the disorder of the smooth pursuit system to properly calibrate and thereby result in CN. This was done in the text and in Figure 11. Also, despite the adequate functioning of the smooth pursuit in CN subjects (ie, the velocity gain is normal), the readers of our paper can arrive at the conclusion that CN may be directly caused by the normally damped oscillations of the smooth pursuit system. We are well aware of, and largely responsible for, the overwhelming evidence that smooth pursuit is “intact” in CN (see references 19 and 20 in our paper). We were also careful to differentiate between those conditions that mightfacilitate the development of CN and what current evidence suggests is the direct cause. We would not presume to coerce any clinicians into altering their decisions to continue to use the terms sensory CN and motor CN. We welcome Dr Hamed’s wish to “bridge the gap between basic scientists and pediatric ophthalmologists” (although that was not an issue for us-one of the authors of our paper is a pediatric ophthalmologist). For the record, (1) CN is a single motor sign reflecting the primary oscillations of smooth pursuit and the secondary attempts by the saccadic and fixation subsystems to acquire the target of interest and prolong its capture by the fovea; (2) afferent deficits are neither necessary nor sufficient for the development of CN; (3) CN can only be reliably and repeatedly diagnosed and differentiated from other forms of nystagmus with the aid of ocular motor recordings; and (4) the diagnosis of CN should always prompt the ophthalmologist to assess the afferent visual system, despite the absence of direct causality. We will continue to investigate in the hope of clarifying and to report in the hope of converting the existing confusion on this side to the simplicity of understanding on the other.