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[2] in which eye velocity slows down (less than 4 degrees/s) while the visual target crosses the foveal region (± 0.5 degree); in this short time interval called ‘foveation window’, it is said that the subject ‘foveates’.
Visual acuity was found to be mainly dependent on the duration of the foveation periods [2,20,22], but the exact repeatability of eye position from cycle to cycle and the retinal image velocities also contribute to visual acuity [1, 35].
Numerous studies of CN in infants and children confirm an age-dependent evolution of waveforms during infancy from pendular to jerk [4,17,31,42]. This concept is consistent with the theory that jerk waveforms reflect modification of the nystagmus by growth and development of the visual system [28,29].
Accurate, uniform, and repeatable classification and diagnosis of nystagmus in infancy as CN is best accomplished by a combination of clinical investigations and motility analysis; in some cases, eye movement recording and analysis are indispensable for diagnosis. If a subject is diagnosed with CN, ocular motility study can also be helpful in determining visual status. Analysis of binocular or monocular differences in waveforms and foveation periods could be an important information in therapy planning or can be used to measure outcome of (surgical) treatment. Presence of pure pendular or jerk waveforms without foveation periods are indicators of poorer vision whereas waveforms of either type with extended periods of foveation are associated with good vision; moreover significant interocular differences in a patient reflect similar differences in vision between the two eyes. Ocular motility analysis in CN subjects is also the most accurate method to determine nystagmus changes with gaze (null and neutral zones).
Clinical Assessment
The clinical examination of a subject affected by congenital nystagmus is a complex task; eye movement recording is often one of the necessary steps, but a general physical examination and the assessment of vision are usually preliminary performed.
During the first examination, the physicians can assess the most important features of nystagmus, such as direction of eyes’ beating, presence of anomalous head positions while viewing distant or near objects. In addition an adequate fundus examination is often carried out, in order to asses eventual prechiasmal visual disorders.
Complete clinical evaluation of the ocular oscillation also includes identification of fast-phase direction, movement intensity, conjugacy, gaze effects,
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convergence effects, and effect of monocular cover. Changes in the character of the nystagmus with convergence or monocular viewing are often evaluated.
Visual acuity of the patient is habitually tested with both eyes open (binocular viewing) and with one eye covered (monocular). It is important not to forget that binocular acuity is the “person’s” acuity and monocular acuity is the “eye’s” acuity. These two are often very different in patients with nystagmus and both has to be tested in CN subjects. Among the various available tests, the best choice to assess visual acuity in an older child and cooperative adult is the ETDRS chart, since it provides LogMar evaluation of all acuities, especially those between 20/400 and 20/100 [29].
Examination Techniques: Motility
However, it is well documented that differentiating true nystagmus from saccadic oscillations and intrusions is sometimes impossible clinically. Recent advances in eye movement recording technology have increased its application in infants and children who have disturbances of the ocular motor system [1,4].
As stated above, nystagmus is caused by disorders of the mechanisms responsible in holding gaze steady: the vestibular system, the gaze-holding mechanism, the visual stabilization system, and the smooth pursuit system. Thus, evaluation of a patient’s nystagmus requires a systematic examination of each functional class of eye movements. Measurement of the nystagmus waveform, using reliable methodology, is often helpful in securing a diagnosis. Such measurements help differentiate acquired nystagmus from congenital forms of nystagmus and from other saccadic disorders that lead to instability of gaze [36].
Ocular Motility Recordings
Qualitative or quantitative analysis of eye movements was attempted since the early twentieth century, with primitive electronic technology available at that time. Nowadays more complex and less invasive methods are available, from biopotential recording up to high-speed photographic methods. Various techniques are currently in use to record eye movements: electro-oculography (EOG), infrared oculography (IROG), magneto-oculography (MOG) also known as scleral search coil system (SSCS) and video-oculography (VOG). The first technique relies on the fact that the eye has a standing electrical potential between the front and the back. Horizontal EOG is measured by placing electrodes on the nasal and temporal boundaries of the eyelids; as the eye turns a proportional
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change in electrodes potential is measured. The IROG approach relies on measuring the intensity of an infrared light reflected back from the subject’s eye. Infrared emitters and detectors are located in fixed positions around the eye. The amount of light reflected back to a fixed detector varies with eye position. The VOG approach relies on recording eye position using a video camera, often an infrared device coupled with an infrared illuminator (in order to avoid disturbing the subject), and applying image processing techniques. The scleral search coil method is based on electromagnetic interaction at radio frequencies between two coils, one (embedded in a contact lens) fixed on the eye sclera and the other external.
Bilateral temporal and nasal electrode placement is useful for gross separation of fast and slow phases but is limited by nonlinearity, drift, and noise. Infrared reflectance solves these problems and can be used in infants and children but it is limited by difficulty in calibration. IR video systems have become increasingly popular in research laboratories and in the clinical setting, hence the comparison between IR and the scleral search coil method has become an actual issue.
Different studies analyzed this subject reporting a good performance of video oculography compared with scleral search coils. Van der Geest and Frens [43] compared the performance of a 2D video-based eye tracker (Eyelink I; SR Research Ltd., Mississauga, Ontario, Canada) with 2D scleral search coils. They found a very good correspondence between the video and the coil output, with a high correlation of fixation positions (average discrepancy, +/-1° over a tested range of 40 by 40° of visual angle) and linear fits near one (range, 0.994 to 1.096) for saccadic properties. However, Houben, Goumans, and van der Steen, [33] found that lower time resolution, possible instability of the head device of the video system, and inherent small instabilities of pupil tracking algorithms still make the coil system the best choice when measuring eye movement responses with high precision or when high-frequency head motion is involved. For less demanding and for static tests and measurements longer than a half an hour, the latest generation infrared video system is a good alternative to scleral search coils; in addition video oculography is not at all invasive, making it suitable for children younger than 10 years old. However, the quality of torsion of the infrared video system is less compared with scleral search coils and needs further technological improvement..
CN eye movement recordings are often carried out only on the horizontal axis and display the data, by convention, during continuous periods of time. Position and velocity traces are clearly marked, with up being rightward eye movements and down being leftward eye movements. Figure 2 reports, as example, a signal tracts recorded from actual CN patients; computed eye velocity is shown
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underneath the eye movement signal. It is possible to identify some nystagmus characteristics, such as nystagmus amplitude, frequency and the fast and slow phases.
Figure 2. An example of eye movement recording, jerk left The eye velocity is also depicted with a 0 °/s threshold; the figure also shows (between 26.5 s and 27.6 s) a saccade of about ten degrees corresponding to a gaze angle voluntary shift.
Semiautomatic Analysis of Eye Movement Recordings
Congenital nystagmus is a rhythmic phenomenon and researchers have tried to analyze eye movements signals using methodologies specific for frequency analysis such as spectral and wavelet analysis (Reccia et al., 1989-1990, Clement et al., 2002; Miura et al., 2003). Fewer authors (e.g. Hosokawa, 2004) applied the Short Time Fourier Transform (STFT) to congenital nystagmus recordings, in order to highlight modifications in the principal component and in the harmonics during time. However resolution of this technique is limited by the duration of the windows in which the signal is divided [3]. Wavelet analysis seems more useful since it is able to assess how much the signal in study differs in time from a specific template adopted as a reference and it is able to localize a brief transient intrusion into a periodic waveform [37]. It has been used with success to separate
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fast and slow phases in caloric nystagmus. However, as stated by Abel [3], the outcome of this analysis is a time-frequency plot or a coefficient sequence, which are difficult to relate to a subject visual ability. Moreover, the foveation time modification in each cycle and the variability of position between successive foveations can hardly be highlighted using STFT and wavelet analysis [3].
On the contrary, time domain analysis techniques, such as velocity thresholds, region-based foveation identification, syntactic recognition or time series analysis, have been routinely employed in the last decades to analyse nystagmus, either congenital or vestibular.
Usually, visual acuity increases in people suffering from congenital nystagmus if the foveation time increases and the signal variability decreases. An analysis of the signal characteristics near the desired position (target position) can easily take place in the time domain, as demonstrated by Dell’Osso et al. who defined an analytic function to predict visual acuity (NAFX) [15] and by Cesarelli et al. who defined a similar function (NAEF) [10].
Figure 3. An example of slow phase and fast phase (bold) separation in CN eye movement recordings.
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Figure 4. The local foveation windows identified for each nystagmus cycle.
Time domain analysis of congenital nystagmus is the most used technique and, in our opinion, best option so far and it is able to estimate the visual ability of a subject at different gaze angles; however its application with semi-automatic methods still needs improvement both in performance and reliability.
The first step of each algorithm for the time analysis of rhythmic eye movements is the cycles identification: in congenital nystagmus, the most common waveforms are jerk, jerk with extended foveation followed by pendular and pseudo-cycloid [1]; however only the first waveform allows foveation time which ensure a good visual ability. The CN jerk waveforms can be described as a combination of two different actions: the slow phase taking the eye away from the desired target, followed by the fast, corrective phase; the foveation takes place when eye velocity is small, which happens after the fast phase. Hence, a local foveation window can be defined, located at the end of each fast phase and at the beginning of the slow phase, which allows to separate the effects of changes in foveation time and alteration in eye position on visual acuity [10]. The analysis oh these two separate effects is of strong importance due to the presence of a slow ‘periodic’ component in the eye movement signal, which we called baseline oscillation (BLO) [7,39].