Ординатура / Офтальмология / Английские материалы / Handbook of Pediatric Retinal Disease_Wright, Spiegel, Thompson_2006

of individuals who appear cortically blind yet have normal flash VEPs.23,34 Other studies assessing a large number of young children with acute-onset cortical blindness report preserved F.VEPs, predicting a good recovery of vision regardless of etiology.209 There is agreement that patients with persistent and abnormally attenuated degraded F.VEPs were likely to remain behaviorally blind.174,176,210

In spite of these findings, one must always be mindful of the underlying condition. For example, a patient later diagnosed as having maple syrup urine disease presented to the authors with cortical visual impairment and absent flash VEPs. She responded to treatment with oral thiamine, with vision recovering and the white matter changes resolving on MRI. These findings were mirrored by the recovery of P.VEPs.13 Conversely, the authors have also recorded normal VEPs from an infant with apparent delayed visual maturation; however, when behavioral vision had not improved by 5 months of age, MRI studies were performed and showed lissencephaly. VEPs indicate that visual information reaches the visual cortex, but cannot yet provide information about the child’s ability to integrate and interpret this visual information.

PERINATAL HYPOXIA

Perinatal hypoxia often causes cerebral palsy with damage to visual cortical areas.170 The F.VEP during the acute stage can give an indication of the degree of visual function and eventual outcome.209 In milder cases of cerebral palsy, P.VEPs are attenuated and recordable to the larger ( 100 ) check sizes only. The authors found that some severe cases of cerebral palsy with rudimentary vision have a flash VEP with a consistent, but unusual, morphology and distribution. There is an early pronounced positivity at 80 ms, with maximal amplitude 3 to 5 cm lateral to the midocciput. Shepherd et al.189 have described a delayed N3 and absent positive P2 at term age as correlating with adverse outcomes in premature infants.

HYDROCEPHALUS

A common electrodiagnostic picture associated with hydrocephalus is normal flash ERG and attenuated, degraded, and delayed flash and pattern VEPs. Often, this picture is evident before optic atrophy is seen on fundoscopy. VEP latency shows

a close relationship with intracranial pressure (ICP) and parallels improvements after shunting and deterioration with shunt blockage.3

NEURODEGENERATIVE CONDITIONS

In Neuronal ceroid lipofuscinosis (Batten’s disease), in the late infantile form, the flash VEP is reported to be unusually large, but this appears to be an epileptic activity with a different distribution to a normal VEP.

Tay-Sachs disease (GM2 type 1) is an autosomal recessive condition with defective hexosaminidase A causing GM2 ganglioside to build up in neurous. The mixed rod/cone flash ERG is usually normal throughout the course of the disease, but the F.VEP is poorly defined during early stages and not detectable in the later stages. Leucodystrophies are characterized by deor dysmyelination. In AR metachromatic leucodystrophy and X- linked adrenoleucodystrophy, the flash ERG is normal but the VEP findings are somewhat variable. In some patients they are normal, but in others they are poorly formed and delayed, or even absent.

Pelizaus–Merzbacher disease is X-linked and associated with poor central myelination. Patients have markedly delayed VEPs.5,65

UNILATERAL OCCIPITAL HEMISPHERE DYSFUNCTION

This situation produces certain hallmark VEP features that warrant emphasizing. In young children, P.VEPs usually show a conspicuous occipital asymmetry when there is dysfunction of one hemisphere associated with hemianopia. The authors have described asymmetrical changes for both flash and P.VEP stimulation in patients with space-occupying vascular lesions and damage due to amniocentesis where clinical signs are not obvious. VEPs provided initial evidence, which subsequently led to obtaining MRI or CT studies.97,125,166 The uncrossed VEP asymmetry does not vary and is the same for monocular or binocular stimulation. As mentioned previously, the P100 of the P.VEP and the main positivity of the flash VEP (also around 100 ms) occur over the midline and over the dysfunctional hemisphere (paradoxical lateralization). A negativity with peak latency of about 100 ms is usually recorded over the opposite hemisphere.

give a higher resolution acuity than recognition acuity secondary to spatial distortion. VEP acuity is also a different measure, and it would be unrealistic to expect a direct relationship between objective and behavioral acuity measures. Nevertheless, P.VEP findings do provide a good benchmark and are especially useful in highlighting interocular differences associated with amblyopia. In particular, these findings include clinically unrecognized indications of iatrogenic effects of patching on the unaffected eye. The authors have noted irreversible and significant VEP amplitude loss in the fellow (patched) eye of a 3- month-old patient with unilateral congenital cataract (followed up until 7 years of age).113 Conversely, patching 50% of waking hours was stopped in a 3-month-old with a lid hemangioma because the P.VEP from the fellow eye showed degradation and attenuation. The P.VEP amplitude and definition showed clear improvement in a recording done 3 months later.

Unilateral congenital cataracts are a particular challenge for amblyopia occlusive therapy. Although the lensectomized eye shows significant improvements in acuity, overall, the authors find that the fellow (patched) eye shows smaller transient P.VEPs and reduced LogMAR acuity compared with untreated unilateral cataract patients and healthy controls.215

Transient P.VEP amplitudes demonstrate an inverted “U- shaped” spatial tuning curve. The peak of the tuning curve is broadly associated with the size of the stimulus field that, for clinical work, is greater than 10°. For these field sizes, patterns with element sizes of 10° to 20° arc give the largest P.VEPs from 5 to 6 years onward.229 In amblyopia, the P.VEP is slightly increased in latency and relatively attenuated as check size diminishes, losing the inverted U-shaped tuning curve. If the amblyopia is very profound, such that large checks and flash VEPs are affected, it is not possible to distinguish amblyopia from other causes of postretinal dysfunction.

Wright et al. have reported on the use of chloral hydrate sedation for obtaining pattern visual evoked potential in children with amblyopia. Eye-to-eye comparison of P1 amplitudes provided an indication of the presence of amblyopia and gross quantification of the amblyopia. Chloral hydrate P.VEPs can be used in infants whose diagnosis of amblyopia is equivocal, and appropriate management is dependent on knowledge of visual function. Decisions regarding surgery for partial cataracts and corneal opacities that split the visual axis may be aided by information obtained by the chloral hydrate pattern visual evoked potential.236–238

Amblyopia affects the broad spectrum of visual subsystems that constitute vision, including contrast and color, motion, and vernier discriminations. P.VEP studies show that the pursuit of high-contrast acuity does not ensure optimum performance at lower contrasts. Rather than relying on just one measure to monitor amblyopia therapy (e.g., high-contrast recognition acuity), it is likely that a battery of specific stimuli may provide an enhanced profile of amblyopia against which the results of occlusion can be better monitored.

Sweep VEPs

P.VEP amplitudes tend to fall off linearly with spatial frequency near the limit of acuity.30,31 The extrapolation of the high spatial frequency limb of the spatial tuning function to zero amplitude (or the noise level) produces an intercept that correlates with subjective visual resolution; this has been the basis of the sweep VEP techniques. Transient pattern VEP recording can take 20 to 40 s to obtain an average for one spatial frequency. When the stimulation rate is speeded up, steady-state VEPs are recorded. Sweep techniques can progressively and rapidly test (usually in 8–16 s) a range of spatial frequencies (checks or gratings) to determine a VEP threshold.174,220 Each spatial frequency is presented for a discrete interval (usually for 5–10 s), and the change in pattern size can be continuous or sampled.185 It is found that the “optimal” temporal frequency giving the largest VEPs increases with maturation;148 it is around 8 reversals/s in adults and about 4 reversal/s in young infants less than 3 months of age. There is some degree of intersubject and age-related variability in the spatial tuning function of the sweep VEP. It may monotonically decrease with increasing spatial frequency or may have two or three maximal peaks. A double peak is not uncommon in older infants and adults.208,221 Pattern onset/offset stimulation gives larger amplitude responses than P.VEPs to equivalent pattern sizes and also has a simpler tuning function. However, the phase change with spatial frequency (which is a useful indicator of signal reliability with pattern reversal stimulation) is not present for onset/offset stimulation. VEP contrast sensitivity threshold to a reversing sinusoidal grating over a range of spatial frequencies can also give an acuity estimate.4,153,203 There is good correlation between subjective perceptual judgments and P.VEP amplitude estimates of contrast threshold (Fig. 1-10).4,30

FIGURE 1-10. Example of a monocular sweep acuity VEP from a 3- month-old baby. A range of rapidly reversing, 16/s sine wave gratings steadily decreased in size every second. Amplitude is plotted versus spatial frequency, and regression analysis estimates the intersection or acuity threshold. Confidence limits on the regression are shown by the curved lines. The area of quiet phase highlighted distinguishes the response from background noise. cpd, cycles per degree.

VEP Acuity Development

Transient P.VEP estimates of acuity indicate that adult levels are reached between 6 to 10 months.48,137,200 Sweep VEP estimates indicate acuities of about 4.5 cycles per degree (cpd; a cycle is made up of one black and one white bar) in the first month of life, increasing to the adult level of 20 cpd by 8 months of age.4,154,155 During the first year of life, sweep VEP acuity is higher and has a slower developmental course compared with transient VEPs. Sweep VEP estimates exceed behavioral forced-choice

preferential looking (FCPL) acuity in the first year of life, but they become similar after this.4,82,201,215

The differences between electrophysiological and behavioral acuity assessments may be accounted for by stimulus differences (i.e., central field for VEPs versus smaller peripheral field for FCPL) or by static stimulus versus changing stimulus involving motion for pattern reversal VEPs. There is good agreement between interocular acuity estimates between the two methods. Children with good optotype acuity tend to have higher sweep acuity at lower temporal frequency, and those with poor optotype acuity tend to have better sweep acuity at higher temporal rates.69 This difference reflects the contrast sensitivity function: lower spatial frequencies are more visible when they move whereas high spatial frequencies are best seen when static or slow moving.

Hysterical (or Functional) Visual Loss

Hysterical visual loss is associated with normal flash and pattern VEP findings. It is particularly important to carefully monitor fixation performance (e.g., with closed-circuit TV) and also not to use too small a check size ( 15 ), because voluntary changes in accommodation by patients may lead to apparently significant VEP changes affecting both amplitude and latency. Larger check sizes are more resistant to such maneuvers by patients.78

Suppression and the Pattern Visual

Evoked Potential

Visual potential can be useful in investigating the mechanisms of suppression associated with amblyopia and strabismus.239,240 Central suppression associated with monofixation syndrome and anisometropic amblyopia has been shown to reduce and, essentially, extinguish the P1 of the P.VEP. These results imply that suppression inhibits neuronal activity at the level of the visual cortex.239 P.VEP evidence of suppression has also been identified in childhood-onset strabismus with suppression and adults with acquired strabismus where diplopia was ignored. Wright et al. demonstrated that patients with childhood-onset and adult-onset “ignored” diplopia similarly showed essentially no response when the nonpreferred eye was stimulated during binocular viewing. The study suggests that adults with acquired

strabismus who ignore the diplopic image actually have suppression of cortical visual activity.240

FUTURE OF VISUAL ELECTROPHYSIOLOGY

Objective measures of function are invaluable to pediatric examination. Visual electrophysiology is objective and will be used in the future for assessing the success of retinal gene therapies and visual pathway rescue. Improvements in pediatric visual electrophysiology practice will lie with techniques that minimize patient contact; for example, using smart electrodes and amplifiers that require no skin preparation for good signal-to-noise ratios. Eventually, there may be remote detection of bioelectric activity. Visual stimulation will be developed to mimic normal visual space, yet allow the systematic manipulation of individual visual parameters and independent stimulation of each eye. The functional significance and specificity of treatment will be enhanced with topographic montages that directly and immediately relate to an individual’s brain image or retinal image.

Finally, the integration of these data with measures of higher cortical function will become a very active area of study. It will help to understand how our ability to interpret the processed afferent visual information is assembled.

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