Fatema Ghasia

Frequency and Severity of Visual Sensory and Motor Deficits in Children with Cerebral Palsy: Gross Motor Function Classification Scale

PURPOSE Cerebral palsy (CP) is a permanent, nonprogressive disorder of movement and posture due to a lesion of the fetal or infant brain. The goal was to determine whether children with different severities of CP, as defined using the Gross Motor Function Classification System (GMFCS), had different degrees or types of visual dysfunction. METHODS An observational, cross-sectional-design study was conducted by using neurologic and masked ophthalmic measurements on a representative cohort of 50 children with CP. Mean age was 5.6 years (range, 2-19.5 years); mean gestational age was 31 weeks. RESULTS The likelihood of debilitating visual deficits was greater in children with higher GMFCS scores, independent of gestational age. Children with level 5 disease (most severe) were at greatest risk for high myopia, absence of binocular fusion, dyskinetic strabismus, severe gaze dysfunction, and optic neuropathy or cerebral visual impairment (CVI). These deficits were rare or absent in children with the mildest disease, level 1. When categorized by anatomic or physiologic CP subtype, diplegic and spastic children were more often hyperopic and esotropic, but had the highest prevalence of fusion and stereopsis. In contrast, children with quadriplegic and mixed CP (dyskinetic, athetoid, hypotonic, and ataxic) more often had high myopia, CVI, dyskinetic strabismus, and gaze dysfunction. CONCLUSIONS Visual deficits differ in children who have mild versus severe CP. Children with GMFCS level 1 to 2 have sensorimotor deficits resembling those of neurologically normal children with strabismus and amblyopia; children at level 3 to 5 have more severe deficits, not observed in neurologically normal children.

Sensory Convergence Solves a Motion Ambiguity Problem

Our inner ear is equipped with a set of linear accelerometers, the otolith organs, that sense the inertial accelerations experienced during self-motion. However, as Einstein pointed out nearly a century ago, this signal would by itself be insufficient to detect our real movement, because gravity, another form of linear acceleration, and self-motion are sensed identically by otolith afferents. To deal with this ambiguity, it was proposed that neural populations in the pons and midline cerebellum compute an independent, internal estimate of gravity using signals arising from the vestibular rotation sensors, the semicircular canals. This hypothesis, regarding a causal relationship between firing rates and postulated sensory contributions to inertial motion estimation, has been directly tested here by recording neural activities before and after inactivation of the semicircular canals. We show that, unlike cells in normal animals, the gravity component of neural responses was nearly absent in canal-inactivated animals. We conclude that, through integration of temporally matched, multimodal information, neurons derive the mathematical signals predicted by the equations describing the physics of the outside world.

Do motoneurons encode the noncommutativity of ocular rotations

As we look around, the orientation of our eyes depends on the order of the rotations that are carried out, a mathematical feature of rotatory motions known as noncommutativity. Theorists and experimentalists continue to debate how biological systems deal with this property when generating kinematically appropriate movements. Some believe that this is always done by neural commands to a simplified eye plant. Others have postulated that noncommutativity is implemented solely by the mechanical properties of the eyeball. Here we directly examined what the brain tells the muscles, by recording motoneuron activities as monkeys made eye movements. We found that vertical recti and superior/inferior oblique motoneurons, which drive sensory-generated torsional eye movements, do not modulate their firing rates according to the noncommutative-driven torsion during pursuit. We conclude that part of the solution for kinematically appropriate eye movements is found in the mechanical properties of the eyeball, although neural computations remain necessary and become increasingly important during head movements.

Neural Correlates of Forward and Inverse Models for Eye Movements : Evidence from Three-Dimensional Kinematics

Inverse and forward dynamic models have been conceptually important in computational motor control. In particular, inverse models are thought to convert desired action into appropriate motor commands. In parallel, forward models predict the consequences of the motor command on behavior by constructing an efference copy of the actual movement. Despite theoretical appeal, their neural representation has remained elusive. Here, we provide evidence supporting the notion that a group of premotor neurons called burst-tonic (BT) cells represent the output of the inverse model for eye movements. We show that BT neurons, like extraocular motoneurons but different from the evoked eye movement, do not carry signals appropriate for the half-angle rule of ocular kinematics during smooth-pursuit eye movements from eccentric positions. Along with findings of identical response dynamics as motoneurons, these results strongly suggest that BT cells carry a replica of the motor command. In contrast, eye-head (EH) neurons, a premotor cell type that is the target of Purkinje cell inhibition from the cerebellar flocculus/ventral paraflocculus, exhibit properties that could be consistent with the half-angle rule. Therefore, EH cells may be functionally related to the output of a forward internal model thought to construct an efference copy of the actual eye movement.

Phakic intraocular lens correction of high ametropia in children with neurobehavioral disorders

PURPOSE A subset of children with high ametropia and neurobehavioral disorders have chronic difficulties with spectacle or contact lens wear. We report the results of refractive surgery in a series of these children treated using bilateral or unilateral phakic intraocular lenses (IOLs) for ametropia >10 D. METHODS Clinical course and outcome data were collated prospectively in a group of 12 children (mean age, 10.1 years; range, 4-17 years) with neurobehavioral disorders exacerbated by poor compliance with spectacles. Myopia in 18 eyes ranged from -10.0 to -22.75 D (mean, -15.2 D) and in 2 hyperopic eyes from +10.25 to +10.75 (mean, +10.5 D). Goal refraction was approximately 0 to +1 D. Correction was achieved by implantation of an iris-enclaved phakic IOL under general anesthesia. Mean follow-up was 9.1 months (range, 3-15 months). RESULTS Myopia correction averaged 14.5 D and hyperopia correction 9.4 D. Eighty-six percent of eyes (17/20 eyes) were corrected to within +/- 1 D of emmetropia and the remaining 14% (3 eyes) to within +/- 2 D. Uncorrected visual acuity improved substantially in all 20 eyes (60-fold; from a mean of 20/3400 to a mean of 20/57). Ocular comorbidities in each child accounted for residual postoperative, subnormal visual acuity (eg, amblyopia, nystagmus, albinism, regressed retinopathy of prematurity). Visual functions (measured using a 23-item validated survey) improved. One eye required IOL exchange; no other clinically significant complications have been encountered. CONCLUSIONS Phakic IOL implantation improves visual function substantially in neurobehaviorally impaired children who have high ametropia and difficulties wearing glasses.

Visual acuity and visually evoked responses in children with cerebral palsy: Gross Motor Function Classification Scale

Background/aims: Measurement of visual acuity can be difficult in children with cerebral palsy (CP). The purpose of this study was to determine the probability of obtaining quantitative (logMAR) visual acuities in CP children with different severities of motor dysfunction. Methods: An observational, cross-sectional design study was conducted in 76 children (mean age 5.9 years) who had CP severity graded using the Gross Motor Function Classification Scale (GMFCS). Visual testing was performed using optotypes, spatial-sweep (SSVEP) or flash (FVEP) visually evoked potentials. Results: LogMAR acuity was obtainable in 88% of CP children, using either optotypes or SSVEPs. The success rate decreased with increasing severity of GMFCS Levels 1–5; nevertheless, logMAR acuities were obtainable in more than one-half (56%) of children with the most severe disease: Level 5. Similar results were obtained for CP severity assessed using physiological-anatomical subtype (hemiplegic; diplegic; quadriplegic) rather than GMFCS. The mean logMAR acuity (0.37; 20/48) in the CP cohort as a whole was ∼2 Snellen-equivalent lines worse than that recorded in age-matched, normal children (0.22; 20/33). Conclusion: Quantitative visual acuities can be obtained in the majority of CP children, including those with severe motor dysfunction. Difficulties in testing CP children lead often to the misconception that the children have immeasurably low vision.

Repair of strabismus and binocular fusion in children with cerebral palsy: gross motor function classification scale.

PURPOSE Children with cerebral palsy (CP) tend to be either excluded from studies of strabismus repair or pooled with children who have other neurologic disorders. The authors limited this study to children with defined CP to determine the success or failure of restoring eye alignment and fusion. METHODS An observational, cross-sectional, prospective study was conducted on a representative cohort of 50 children. CP severity ranged from Gross Motor Function Classification System (GMFCS) level 1 (least severe) to 5 (most severe). Mean age at entrance and surgery was 3.5 years, and mean follow-up was 4.1 years (minimum 1 year). RESULTS The predominant form of strabismus was infantile-onset: esotropia in 54%, exotropia in 26%, and dyskinesia in 10%. Sixty-six percent of esotropic children and 61% of exotropic children achieved optimal (microtropic) alignment after an average of 2 and 1.8 surgical procedures, respectively. The likelihood of optimal alignment was similar in children with mild (GMFCS level 1-2) versus severe (GMFCS level 3-5) CP (P = 0.7; χ(2)). Irrespective of GMFCS severity, 46% of children gained binocular fusion/stereopsis, but the quality of fusion gained was greater in children with mild CP (P < 0.05). Earlier surgery was more likely to be successful (P < 0.05). CONCLUSIONS Restoration of binocular alignment and a degree of fusion is a realistic goal in the majority of strabismic CP children. Repair may be achieved in children at both the mild and the severe ends of the GMFCS spectrum, without undue concern about treatment futility or excessive reoperation.

Pattern Strabismus: Where Does the Brain's Role End and the Muscle's Begin?

Vertically incomitant pattern strabismus comprises 50% of infantile horizontal strabismus. The oblique muscle dysfunction has been associated with pattern strabismus. High-resolution orbit imaging and contemporary neurophysiology studies in non-human primate models of strabismus have shed light into the mechanisms of pattern strabismus. In this review, we will examine our current understanding of etiologies of pattern strabismus. Speculated pathophysiology includes oblique muscle dysfunction, loss of fusion with altered recti muscle pull, displacements and instability in connective tissue pulleys of the recti muscles, vestibular hypofunction, and abnormal neural connections. Orbital mechanical factors, such as abnormal pulleys, were reported as a cause of pattern strabismus in patients with craniofacial anomalies, connective tissue disorders, and late-onset strabismus. In contrast, abnormal neural connections could be responsible for the development of a pattern in infantile-onset strabismus. Pattern strabismus is likely multifactorial. Understanding the mechanisms of pattern strabismus is pivotal to determine an appropriate surgical treatment strategy for these patients.

Abnormal fixational eye movements in strabismus

Introduction Fixational saccades are miniature eye movements that constantly change the gaze during attempted visual fixation. Visually guided saccades and fixational saccades represent an oculomotor continuum and are produced by common neural machinery. Patients with strabismus have disconjugate binocular horizontal saccades. We examined the stability and variability of eye position during fixation in patients with strabismus and correlated the severity of fixational instability with strabismus angle and binocular vision. Methods Eye movements were measured in 13 patients with strabismus and 16 controls during fixation and visually guided saccades under monocular viewing conditions. Fixational saccades and intersaccadic drifts were analysed in the viewing and non-viewing eye of patients with strabismus and controls. Results We found an increase in fixational instability in patients with strabismus compared with controls. We also found an increase in the disconjugacy of fixational saccades and intrasaccadic ocular drift in patients with strabismus compared with controls. The disconjugacy was worse in patients with large-angle strabismus and absent stereopsis. There was an increase in eye position variance during drifts in patients with strabismus. Our findings suggest that both fixational saccades and intersaccadic drifts are abnormal and likely contribute to the fixational instability in patients with strabismus. Discussion Fixational instability could be a useful tool for mass screenings of children to diagnose strabismus in the absence of amblyopia and latent nystagmus. The increased disconjugacy of fixational eye movements and visually guided saccades in patients with strabismus reflects the disruption of the fine-tuning of the motor and visual systems responsible for achieving binocular fusion in these patients.

Fixational saccades are more disconjugate in adults than in children

Purpose Fixational eye movements are of particular interest for three reasons. They are critical for preventing visual fading and enhancing visual perception; their disconjugacy allows scanning in three dimensions, and their neural correlates span through the cortico-striatal, striato-collicular and brainstem networks. Fixational eye movements are altered in various pediatric ophthalmologic and neurologic disorders. The goal of this study was to compare the dynamics of fixational eye movements in normal children and adults. Methods We measured the fixational saccades and inter-saccadic drifts in eye positions using infrared video-oculography in children and adults. We assessed the frequency, amplitude, main-sequence, and disconjugacy of fixational saccades as well as the intra-saccadic drift velocity and variance between these two groups. Results We found a similar frequency but an increase in the amplitude of fixational saccades in children compared to adults. We also found that the fixational saccades were more conjugate in children than in adults. The inter-saccadic drifts were comparable between the two groups. Discussion This study provides normative values of dynamics of fixational eye movement in children and adults. The greater disconjugacy of fixational saccades in adults suggests the existence of neural mechanisms that can independently regulate the movements of two eyes. The differences between adult and pediatric populations could be due to completion of the development of binocularly independent regulation of fixational saccades nearing adulthood. The alternate possibility is that the increased disconjugacy between the two eyes may represent a deficiency in the eye movement performance as a function of increasing age.