Richard V. Abadi

Motor and sensory characteristics of infantile nystagmus.

Background/aims: Past studies have explored some of the associations between particular motor and sensory characteristics and specific categories of non-neurological infantile nystagmus. The purpose of this case study is to extend this body of work significantly by describing the trends and associations found in a database of 224 subjects who have undergone extensive clinical and psychophysical evaluations. Methods: The records of 224 subjects with infantile nystagmus were examined, where 62% were idiopaths, 28% albinos, and 10% exhibited ocular anomalies. Recorded variables included age, mode of inheritance, birth history, nystagmus presentation, direction of the nystagmus, waveform types, spatial and temporal null zones, head postures and nodding, convergence, foveation, ocular alignment, refractive error, visual acuity, stereoacuity, and oscillopsia. Results: The age distribution of the 224 patients was between 1 month and 71 years, with the mean age and mode being 23 (SD 16) years and 16–20 years respectively. By far the most common pattern of inheritance was found to be autosomal dominant (n = 40), with the nystagmus being observed by the age of 6 months in 87% of the sample (n = 128). 139 (62%) of the 224 subjects were classified as idiopaths, 63 (28%) as albinos, and 22 (10%) exhibited ocular anomalies. Conjugate uniplanar horizontal oscillations were found in 174 (77.7%) of the sample. 32 (14.3%) had a torsional component to their nystagmus. 182 (81.2%) were classed as congenital nystagmus (CN), 32 (14.3%) as manifest latent nystagmus (MLN), and 10 (4.5%) as a CN/MLN hybrid. Neither CN nor MLN waveforms were related to any of the three subject groups (idiopaths, albinos, and ocular anomalies) MLN was found in idiopaths and albinos, but most frequently in the ocular anomaly group. The most common oscillation was a horizontal jerk with extended foveation (n = 49; 27%). The amplitudes and frequencies of the nystagmus ranged between 0.3–15.7° and 0.5–8 Hz, respectively. Periodic alternating nystagmus is commonly found in albinos. Albino subjects did not show a statistically significantly higher nystagmus intensity when compared with the idiopaths (p>0.01). 105 of 143 subjects (73%) had spatial nulls within plus or minus 10° of the primary position although 98 subjects (69%) employed a compensatory head posture. Subjects with spatial null zones at or beyond plus or minus 20° always adopted constant head postures. Head nodding was found in 38 subjects (27% of the sample). Horizontal tropias were very common (133 out of 213; 62.4%) and all but one of the 32 subjects with MLN exhibited a squint. Adult visual acuity is strongly related to the duration and accuracy of the foveation period. Visual acuity and stereoacuity were significantly better (p<0.01) in the idiopaths compared to the albino and ocular anomaly groups. 66 subjects out of a sample of 168 (39%) indicated that they had experienced oscillopsia at some time. Conclusions: There are strong ocular motor and sensory patterns and associations that can help define an infantile nystagmus. These include the nystagmus being bilateral, conjugate, horizontal uniplanar, and having an accelerating slow phase (that is, CN). Decelerating slow phases (that is, MLN) are frequently associated with strabismus and early form deprivation. Waveform shape (CN or MLN) is not pathognomonic of any of the three subject groups (idiopaths, albinos, or ocular anomalies). There is no one single stand alone ocular motor characteristic that can differentiate a benign form of infantile nystagmus (CN, MLN) from a neurological one. Rather, the clinician must consider a host of clinical features.

Waveform characteristics in congenital nystagmus

Using infra-red oculography, electro-oculography and fundus video-recordings, waveform characteristics (amplitude, frequency, waveform shape and foveation) were examined in over 150 individuals with congenital nystagmus. For many of the subjects the nystagmus exhibited marked variability in both space and time. The sources of this variability were explored and the state of attention of the observer was found to be a dominant factor. The waveform and precision of foveation were not found to be related to any one classification group (eg. albino or idiopathic).

Characteristics of saccadic intrusions.

Primary fixation is never perfectly stable, but is frequently interrupted by slow drifts, microsaccades and saccadic intrusions (SI). SI are involuntary, conjugate movements which take the form of an initial fast movement away from the desired eye position and followed after a short duration, by either a return secondary saccade or a drift. The purpose of this study was to examine the prevalence and metrics of SI in a population of 50 healthy subjects. Using both one and two dimensional recordings we find that all 50 members of the subject group exhibited SI. The SI were bilateral, conjugate and horizontal. No purely vertical SI were detected when examined in three subjects. SI amplitude mean and range was 0.6 degrees +/-0.5 degrees, 0.1 degrees -4.1 degrees; SI frequency mean and range was 18.0+/-14.3 per min, 1.0-54.8 per min; SI duration mean and range was 225+/-150, 20-870 ms. The mean SI amplitude and frequency when SI<0.5 degrees were removed was 0.97 degrees +/-0.56 degrees and 7.0+/-11.4 per min respectively. Age was positively correlated with SI amplitude (p<0.01), but there was no correlation between age and SI frequency. Three of four types of SI monophasic square wave intrusions (MSWI), biphasic square wave intrusions (BSWI) and double saccadic pulses (DSP) were found to be exclusively saccadic, whilst the fourth type, the single saccadic pulses (SSP), were confirmed to exhibit a slow secondary component. MSWI were the most frequently observed SI occurring in 47 out of 50 (94%) of the subjects with a mean amplitude, frequency and duration of 0.7 degrees +/-0.5 degrees, 11.5+/-11.6 per min, and 255+/-147 ms respectively. Mean amplitudes and frequencies for BSWI (n=20), SSP (n=11) and DSP (n=34) were found to be 0.50 degrees +/-0.2 degrees, 1.2+/-2.5 per min; 0.40 degrees +/-0.20 degrees, 0.4+/-1.0 per min and 0.3 degrees +/-0.4 degrees, 5.0+/-8.7 per min respectively. No differences in MSWI characteristics were found between binocular and monocular viewing. Possible explanations for SI occurrence include experimental viewing conditions, subject fatigue and covert shifts in attention.

Retinal slip velocities in congenital nystagmus

An analysis of waveform velocities was performed on 16 individuals with congenital nystagmus. The fast phases were shown to be saccadic in nature and their main sequence peak velocity was significantly slower than that of the normal control group (P less than 0.01). Peak slow phase velocities were found to reach values as high as 180 deg/sec; a relationship between the peak slow phase velocity and the amplitude of the slow phase was demonstrated. The range of the velocities that comprise the slow phase was generally in excess of 100 deg/sec. The correlation between the period of time spent at low retinal slip velocities (less than or equal to 10 deg/sec) and visual acuity was found to be significant (P less than 0.05). Implications of high slow phase velocities are discussed with reference to future experimental design.

Advances in the management of congenital and infantile cataract

Congenital and infantile cataracts produce deprivation amblyopia and can thus cause lifelong visual impairment. Successful management is dependent on early diagnosis and referral for surgery when indicated. Accurate optical rehabilitation and postoperative supervision are essential.The timing of surgery and its relationship to the duration of deprivation is important. Unilateral congenital cataract surgery within 6 weeks of birth produces the best outcomes. The equivalent ‘latent’ period for bilateral visual deprivation may be longer at around 10 weeks.Visual deprivation has a significant impact on the development of fixation stability. Major form deprivation, even after early surgery, leads to nystagmus. This is mostly manifest latent nystagmus (MLN). The latent period for fixation stability may be as short as 3 weeks. Preoperative congenital nystagmus (CN) can convert to more benign MLN after surgery.Infantile IOL implantation is becoming increasingly accepted. A satisfactory long-term refractive result requires that allowance be made for childhood axial growth and myopic shift. In a series of 25 infants (33 eyes) implanted before 12 months of age, the mean myopic shift at 12 months was 4.83 D. This increased to 5.3 D in infants implanted before 10 weeks. The initial desired refractive outcome following IOL implantation is thus hypermetropia, with the degree dependent on the age of the child.Glaucoma or ocular hypertension is a common complication following paediatric cataract surgery. Microphthalmia and surgery in early infancy are risk factors. Tonometry results may be influenced by the increased corneal thickness seen in aphakic and pseudophakic children. The long-term prognosis of eyes with aphakic glaucoma is not necessarily poor but intraocular pressure control may require three or more medications. Surgical intervention appears to be necessary in over a quarter of eyes.Posterior capsule opacification (PCO) is common in infants undergoing primary lens implantation. Primary capsulotomy and anterior vitrectomy reduce the risk of PCO. In the absence of anterior vitrectomy, primary posterior capsulotomy does not prevent visual axis opacification.Further developments will continue to be driven by clinical research. The prevention of capsule opacification and cellular proliferation may in future be achieved by the use of devices to specifically target epithelial cells at surgery.

Orientational selectivity of grating and line detectors in human vision

Abstract The orientation selectivity of grating and line detectors is determined by subthreshold summation; that is, the reduction in contrast threshold of a grating or line test stimulus due to the presence of a subthreshold background grating is measured. For a 0.6 min line and also 5 c/deg and 10 c/deg test gratings the effect of the background grating is reduced to one half maximum by a tilt of about 3 deg; this orientation selectivity is more precise than that derived by other psychophysical methods. It is suggested that lateral inhibition between detectors contributes to the orientation selectivity derived by other methods but not by the present subthreshold method. The two-dimensional spatial extents of grating and line detectors are estimated.

A new treatment for congenital nystagmus.

Congenital nystagmus is a disorder of eye movement with an associated reduction in visual acuity. The latter is mainly due to the intensity of the nystagmus (amplitude times frequency) allowing the object of regard to spend only a short time on the fovea. Training patients by an auditory feedback technique to control the nystagmus enables visual sensitivity to improve. This method is compared with present alternatives, and further studies are suggested.

The influence of nystagmoid oscillation on contrast sensitivity in normal observers

Normal observers subjected to retinal image motion equivalent to that experienced by congenital nystagmats were found to display a contrast sensitivity function with characteristics similar to those shown by nystagmat observers. The amplitude, frequency and waveform (and hence foveation time) of nystagmic oscillation were varied, with alteration of the waveform producing the most significant effect on the contrast sensitivity function. The pseudocycloid waveform with the longest percentage foveation time per cycle was commensurate with the best visual resolution of medium-to-high spatial frequency targets. For all waveforms the length of the foveation period showed good correlation with visual resolution of the targets.

Induction masking—a study of some inhibitory interactions during dichoptic viewing

Abstract Two independent signals are fed into each eye. By altering the contrast of one to suppress the other a measure of the inhibition is derived in terms of an equation which is called the relative sensitivity. This sensitivity function is shown to become more potent for differing parameters and subsequently the discussion relates to recent neurophysiological studies of orientational columns and their inhibitory connections. It is concluded that cortical columns can only inhibit their adjacent neighbours.

Visual resolution limits in human albinism

The effects of the involuntary ocular oscillations on visual resolution was examined in 22 albinos and 11 idiopaths with congenital nystagmus. The idiopaths showed a linear relationship between the proportion of the slow phase spent at low velocities (less than or equal to deg/sec) and the log of the minimum angle of resolution; such that long dwell times were compatible with good resolution. For the albinos there appeared to be a critical duration of low retinal slip velocities above which there was no improvement in acuity. This supports the contention that factors other than the congenital nystagmus limit visual resolution in the albino.