In: Binocular Vision |
ISBN: 978-1-60876-547-8 |
Editors: J. McCoun et al, pp. 107-123 |
© 2010 Nova Science Publishers, Inc. |
Chapter 4
EYE MOVEMENT ANALYSIS IN CONGENITAL
NYSTAGMUS: CONCISE PARAMETERS
ESTIMATION
Pasquariello Giulio1, Cesarelli Mario1, La Gatta Antonio2,
Bifulco Paolo1 and Fratini Antonio1
1 Dept. of Biomedical, Electronic and Telecommunication Engineering, University “Federico II” of Naples, Via Claudio, 21, 80125, Napoli, Italy 2 Math4Tech Center, University of Ferrara,
via Saragat, 1, 44100, Ferrara, Italy
Abstract
Along with other diseases that can affect binocular vision, reducing the visual quality of a subject, Congenital Nystagmus (CN) is of peculiar interest. CN is an ocular-motor disorder characterized by involuntary, conjugated ocular oscillations and, while identified more than forty years ago, its pathogenesis is still under investigation. This kind of nystagmus is termed congenital (or infantile) since it could be present at birth or it can arise in the first months of life. The majority of CN patients show a considerable decrease of their visual acuity: image fixation on the retina is disturbed by nystagmus continuous oscillations, mainly horizontal. However, the image of a given target can still be stable during short periods in which eye velocity slows down while the target image is placed onto the fovea (called foveation intervals). To quantify the extent of nystagmus, eye movement recordings are routinely employed, allowing physicians to extract and analyze nystagmus main features such as waveform
108 Pasquariello Giulio, Cesarelli Mario, La Gatta Antonio et al.
shape, amplitude and frequency. Use of eye movement recording, opportunely processed, allows computing “estimated visual acuity” predictors, which are analytical functions that estimate expected visual acuity using signal features such as foveation time and foveation position variability. Hence, it is fundamental to develop robust and accurate methods to measure both those parameters in order to obtain reliable values from the predictors. In this chapter the current methods to record eye movements in subjects with congenital nystagmus will be discussed and the present techniques to accurately compute foveation time and eye position will be presented.
This study aims to disclose new methodologies in congenital nystagmus eye movements analysis, in order to identify nystagmus cycles and to evaluate foveation time, reducing the influence of repositioning saccades and data noise on the critical parameters of the estimation functions. Use of those functions extends the information acquired with typical visual acuity measurement (e.g., Landolt C test) and could be a support for treatment planning or therapy monitoring.
Introduction
Congenital nystagmus (CN) is an ocular–motor disorder that appears at birth or during the first few months of life, characterized by involuntary, conjugated, bilateral to and fro ocular oscillations. Clinical descriptions of nystagmus are usually based on the direction of the fast phase and are termed horizontal, vertical, or rotary, or any combination of these. CN is predominantly horizontal, with some torsional and, rarely, vertical motion [1]. Nystagmus oscillations can persist also closing eyes, moreover they tend to damp in absence of visual activity. In vertebrates, eye movements are controlled by the oculomotor system in a complex manner, depending on the stimuli and viewing conditions. In the human eye, the little portion of the retina which allows the maximal acuity of vision is called the fovea. An attempt to bring the image of a target onto the fovea can involve up to five oculomotor subsystems: the saccadic, smooth pursuit, vestibular, optokinetic and vergence systems. The vestibular system is driven by non-visual signals from the semicircular canals, while the other systems are mainly driven by visual signals encoding target information. Pathogenesis of the congenital nystagmus is still unknown; dysfunctions of at least one of the ocular stabilization systems have been hypothesized, but no clear evidence was reported.
Nystagmus can be idiopathic or associated to alteration of the central nervous system and/or ocular system such as achromathopsia, aniridia and congenital cataract. Both nystagmus and associated ocular alterations can be genetically transmitted, with different modalities; estimates of the prevalence of infantile nystagmus range from 1 in 1000 to 1 in 6000 [23,26,34,38].
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CN occurrence associated with total bilateral congenital cataract is of 50– 75%, while this percentage decreases in case of partial or monolateral congenital cataract. CN is present in most cases of albinism.
The cause or causes and pathophysiological mechanisms of CN have not been clarified. Children with this condition frequently present with a head turn, which is used to maintain the eyes in the position of gaze in which the nystagmus is minimum. This happens more often when the child is concentrating on a distant object, as this form of nystagmus tends to worsen with attempted fixation. The head turn is an attempt to stabilize the image under these conditions.
CN may result from a primary defect (e.g., familial X-linked) of ocular motor calibration. Some authors (e.g., Hertle, 2006) hypothesized that CN may also result from abnormal cross-talk from a defective sensory system to the developing motor system at any time during the motor system’s sensitive period; this can occur from conception due to a primary defect (e.g., retinal dystrophy), during embryogenesis due to a developmental abnormality (e.g., optic nerve hypoplasia), or after birth during infancy (e.g., congenital cataracts). This theory of the genesis of CN incorporates a pathophysiological role for the sensory system in its genesis and modification. Although the set of physiological circumstances may differ, the final common pathway is abnormal calibration of the ocular motor system during its sensitive period.
Terminology (Definitions)
Efforts are being made to add precision and uniformity to nystagmus terminology. The terms congenital nystagmus (CN), infantile nystagmus and idiopathic motor nystagmus have become synonymous with the most common form of neonatal nystagmus [4,17,30,31,42]. However, the term infantile nystagmus syndrome (INS) is a broader and more inclusive term that we prefer not to use since it refers to the broad range of neonatal nystagmus types, including those with identifiable causes. According to the bibliography, idiopathic nystagmus can be classified in different categories depending on the characteristics of the oscillations [2]; typically in CN eye movement recordings are possible to identify, for each nystagmus cycle, the slow phase, taking the target away from the fovea, the fast (or slow) return phase. According to the nystagmus waveform characterization by Dell’Osso [20], in case the return phase is slow then the nystagmus cycle is pendular or pseudo-cycloid; if the return phase is fast then the waveform is defined as jerk (unidirectional or bidirectional). In general, CN waveform has an increasing velocity exponential slow phase [2].
110 Pasquariello Giulio, Cesarelli Mario, La Gatta Antonio et al.
A schematic illustration of a unidirectional jerk nystagmus waveform (pointing to the left) is presented in figure 1.
Figure 1. A schematic illustration of a jerk nystagmus waveform (bold line) with fast phase pointing to the left; on the picture are depicted various nystagmus features, such as: fast and slow phase components, nystagmus period and amplitude; the grey box on each cycle represents the foveation window. The baseline oscillation is shown as a dashed line, and its amplitude is also shown.
In general, CN patients show a considerable decrease of the visual acuity, since image fixation on the fovea is reduced by nystagmus continuous oscillations. CN patient visual acuity reach a maximum when eyes are in the position of least ocular instability, hence, in many cases, a compensatory head malposition is commonly achieved, in order to bring the zone of best vision into the straight-ahead position. Such so-called ‘null zones’ (or null positions) correspond to a particular gaze angle, in which a smaller nystagmus amplitude and a longer foveation time can be obtained, thus reaching a better fixation of the visual target onto the retina. Abnormal head posture could be alleviated by surgery (mainly translating the null zone to straight-ahead position).
Other clinical characteristics, not always present, include increased intensity with fixation and decreased intensity with sleep or inattention; variable intensity in different positions of gaze; decreased intensity (damping) with convergence; changing direction in different positions of gaze (about a so-called neutral position); strabismus and an increased incidence of significant refractive errors.
In normal subjects, i.e., not affected by nystagmus, when the velocity of the image projected on the retina increases by a few degrees per second, visual acuity and contrast sensitivity decrease. In CN patients, fixation is disrupted by nystagmus rhythmical oscillations, which result in rapid movements of the target image onto the retina [6]. Ocular stabilization is achieved during foveation periods