Detection and maturation of VEP albino asymmetry: an overview and a longitudinal study from birth to 54 weeks

Expanding the Spectrum of Oculocutaneous Albinism: Does Isolated Foveal Hypoplasia Really Exist?

Oculocutaneous albinism is an autosomal recessive disorder characterized by the presence of typical ocular features, such as foveal hypoplasia, iris translucency, hypopigmented fundus oculi and reduced pigmentation of skin and hair. Albino patients can show significant clinical variability; some individuals can present with only mild depigmentation and subtle ocular changes. Here, we provide a retrospective review of the standardized clinical charts of patients firstly addressed for evaluation of foveal hypoplasia and slightly subnormal visual acuity, whose diagnosis of albinism was achieved only after extensive phenotypic and genotypic characterization. Our report corroborates the pathogenicity of the two common TYR polymorphisms p.(Arg402Gln) and p.(Ser192Tyr) when both are located in trans with a pathogenic TYR variant and aims to expand the phenotypic spectrum of albinism in order to increase the detection rate of the albino phenotype. Our data also suggest that isolated foveal hypoplasia should be considered a clinical sign instead of a definitive diagnosis of an isolated clinical entity, and we recommend deep phenotypic and molecular characterization in such patients to achieve a proper diagnosis.

The changing shape of the ISCEV standard pattern onset VEP

Purpose

Pattern onset VEPs do not always show distinct C1–C2–C3 peaks and troughs. Our purpose was to study changes in pattern onset VEP with age to determine when the illustrated ISCEV standard onset VEP waveform can be reliably recorded.

Methods

We recorded pattern onset VEPs from an Oz electrode referred to mid-frontal electrode according to ISCEV standards by presenting checks of 60′ and 15′ side length in a 15° field. Twenty-four adults aged 20–63 years participated. Amplitudes and latencies were collated. Pattern onset adult VEP shapes were compared to the waveform published in the ISCEV VEP standard and to paediatric pattern onset VEP waveforms recorded from 16 infants aged 7 months.

Results

The shape of the pattern onset VEP changed gradually with age. The C1–C2–C3 morphology of the ISCEV standard pattern onset VEP becomes apparent consistently after 40 years to 60′ check stimulation. As age increases a negative trough, C2 is more frequently seen; however, the broad positive peak which characterises infant onset VEPs may still be recorded at 20 years. The group median measurements of onset VEPs to 60′ were C1 7 µV@ 88 ms (range 67–110 ms), C2 9 µV@109 ms (range 89–158 ms) and C3 13 µV@121–246 ms. To smaller 15′ checks, peak latencies were earlier and C2 became more obvious. The group median measures of onset VEPs to 15′ were C1 2 µV@69 ms (55–108 ms), C2 10 µV@90 ms (77–145 ms) and C3 14 µV@122 ms (99–200 ms).

Conclusion

The ISCEV standard onset VEP best describes the waveform configuration and latency of the onset VEP produced by 60′ checks in adults of more than 40 years of age. The onset VEP waveform produced by 15′ checks is distinguished by more prominent negative C2 and earlier C1 and C2 latencies.

Abnormale Repräsentationen im visuellen Kortex von Albinismus-Patienten

A characteristic feature of patients with albinism is the misrouting of the optic nerves, which causes the visual cortex to receive an abnormal input. This report details how the detection of misrouting using visual evoked potentials assists the clinical diagnosis of albinism. Further, it shows how the projection abnormality observed in patients with albinism provides a model for investigating the self-organisation of the human visual cortex. This is highlighted by recent findings that were obtained using functional magnetic resonance imaging, visual evoked potentials, and static visual field perimetry.

Age-related changes in the dynamics of human albino visual pathways

A deficiency of melanin in the retinal pigment epithelium, which regulates the development of neural retina, leads to chiasmal misrouting such that the uncrossed pathway (to the ipsilateral hemisphere) is reduced relative to the crossed pathway (to the contralateral hemisphere). This study examines age-related changes in the flash and pattern appearance visual evoked potentials (VEP) of human albinos. Scalp recorded cortical VEPs to flash (FVEP) and pattern appearance stimulation were recorded in 58 albino (8 months to 60 years) and 34 normal subjects (4-55 years). VEPs were analysed by amplitude and latency. The contralateral hemisphere FVEP amplitude decreased with age in albino subjects, as in both hemispheres in normals. However, the ipsilateral hemisphere FVEP amplitude was significantly lower in young albino subjects, initially giving a marked interhemispheric asymmetry, but this normalized with age. Significant interhemispheric FVEP latency asymmetries were not observed. The contralateral pattern appearance VEP latency in albino subjects decreased with age, as in both hemispheres in normals; the ipsilateral latency increased significantly with age. Significant interhemispheric pattern appearance VEP amplitude asymmetries were not observed. These novel and unexpected observations indicate significant age-related changes in the retinocortical pathways of the human albino. These changes have implications for our understanding of development and plasticity of the central visual pathways.

See-saw nystagmus and congenital nystagmus identified in the non-decussating retinal-fugal fiber syndrome

PURPOSE

The purpose of the present study was to accurately document the oculomotor misalignments and instabilities associated with the non-decussating retinal-fugal fiber syndrome, a rare inborn, isolated, achiasmatic condition. To date, the achiasmatic syndrome described has thus far been identified in three unrelated females. A comparable achiasmatic condition has also been identified in a canine breed.

MATERIALS AND METHODS

The unique, inborn visual pathway malformation in the form of an isolated absence of the optic chiasm was confirmed by metabolic assay, the visual evoked potential (VEP) misrouting paradigm and neuroradiological evaluation, including magnetic resonance imaging (MRI). Horizontal and vertical, left and right eye, oculomotor scleral search coil recordings were measured, in one of the achiasmats, at 15 years of age. Comparable oculomotor recordings were implemented in three controls, including a normal age-matched control, an adult control and an adult with idiopathic congenital nystagmus (CN). In addition to central and eccentric target fixation with binocular and monocular viewing, binocular and monocular pursuit and optokinetic nystagmus (OKN) also were recorded.

RESULTS

Following a binocular and monocular fixation paradigm, classic congenital nystagmus (CN) profiles were recorded in the achiasmat in the horizontal planes together with see-saw nystagmus (SSN) in the vertical planes; an alternating esotropia and vertical tropias also were documented. The CN showed interocular conjugacy and waveforms typically exhibited well-defined foveation periods. In general, both achiasmatic horizontal and vertical eye movements as well as corresponding interocular misalignments varied with stimu-lus and recording conditions. Moreover, horizontal eye movements showed sustained interocular conjugacy while vertical eye movements typically showed disconjugacy. Previous oculomotor studies in the achiasmatic canine breed revealed comparable oculomotor instabilities, albeit the latter were reported to present primarily with unyoked eye movements and uniocular saccades. In the present study, in addition to target fixation with binocular and monocular viewing, binocular and monocular pursuit and OKN responses also were recorded. For the achiasmat, high-gain pursuit results, as with eccentric fixation, emphasized a preference for fixation with the adducting eye. Also at eccentric gaze with monocular viewing, the eye under cover showed, relative to the fixating eye, a hypodeviation concomitant with adduction and a hyperdeviation concomitant with abduction. OKN profiles in the achiasmat with stimuli presented under both normal viewing and open-loop, retinal-field locked conditions proved rather complex; however, when an OKN response was identified, reversed OKN tracking was not apparent.

CONCLUSIONS

In general, the oculomotor results reported herein demonstrate that the non-decussating retinal-fugal fiber syndrome is an afferent visual pathway disorder associated with vertical and horizontal misalignments concurrent with congenital nystagmus and also with the rarer see-saw nystagmus.

Comparison of techniques for detecting visually evoked potential asymmetry in albinism

PURPOSE

We compared techniques for analyzing visually evoked potential (VEP) asymmetry in children with albinism to find one that could be used effectively and efficiently.

METHOD

Subjects included 21 child volunteers, ages 10 months to 6 years (control group) and 21 children with albinism, ages 2 months to 6 years (albinism group). Five-channel flash VEP was performed on all subjects. Electrodes were positioned at Oz, O1, O2, O3, and O4 (10/20 system). Data were analyzed by use of techniques previously described. These included inspection of the VEP waveforms, measurement of hemispheric waveform parameters, calculation of an asymmetry index, and use of a bipolar derivation between left and right hemispheric responses (interhemispheric difference potential). In addition, we quantified the interhemispheric difference potential by use of Pearson's correlation coefficient. Measurements of sensitivity and specificity determined the success of the 5 analysis paradigms. The accuracy of each paradigm represented the ability to classify the data according to volunteer or albinism group and is derived from both sensitivity and specificity measures.

RESULTS

Measurement of hemispheric differences in VEP waveform parameters was the least sensitive measure method for detecting multichannel VEP asymmetry in albinism. Comparison of left and right eye interhemispheric difference potential increased accuracy to 67%. Nonquantitative inspection of waveform demonstrated an accuracy of 76%. The asymmetry index and Pearson's correlate measure yielded accuracy rates of 79% and 83%, respectively.

CONCLUSION

The efficiency and capability of Pearson's correlate measure in quantifying interhemispheric difference potentials to detect albinotic misrouting makes this a useful and practical technique in a pediatric clinic.

Unilateral alpha reactivity: an electroencephalographic finding in albinism

Persons with various forms of albinism frequently have misrouting of their optic nerve fibers. Visual evoked potential (VEP) studies can confirm this because monocular stimulation tends to result in asymmetrical rather than symmetrical occipital response. This type of decussation abnormality is associated only with albinism. In the electroencephalogram (EEG) of normal subjects, unilateral eye opening or closing does not affect the frequency, reactivity, regulation, and symmetry of alpha rhythm. In contrast, in our study, 6 of 19 persons with oculocutaneous and ocular albinism responded to unilateral eye opening with contralateral disappearance or significant attenuation of their alpha rhythm. The most likely explanation of this phenomenon is that the visual pathway misrouting is complete or nearly complete in some children with albinism so that one rather than both occipital lobes receive visual information from each eye. This type of alpha behavior, like VEP studies, supports the diagnosis of albinism, and such testing can be easily performed during a routine EEG recording.

Effects of bilirubin on visual evoked potentials in term infants

To determine bilirubin-induced neurotoxicity, serial visual evoked potentials (VEPs) of 72 infants with neonatal hyperbilirubinaemia and 22 controls were evaluated and compared in four sessions for 8 weeks after birth. The levels of maximal serum bilirubin were found positively related to the wave latencies of first VEP. Within 8 weeks after birth, the wave latencies were significantly prolonged in infants in the severe and moderate groups than in the controls. The amplitudes of VEPs were apparently lower in severe and moderate groups than in the control group only in the 1st week after birth. At 1 year, 4 of the 18 infants in the severe group had poor motor skills and one had general hypotonia.

CONCLUSION

These results suggest that bilirubin may affect the visual pathways, and that VEP is a useful adjunct to the neurological assessment of neonatal hyperbilirubinaemia.

Ocular motor behaviour of monozygotic twins with tyrosinase negative oculocutaneous albinism

The involuntary nystagmus movements of 16-year-old monozygotic twins with tyrosinase negative oculocutaneous albinism were examined. On primary gaze both girls exhibited bilateral conjugate horizontal nystagmus, a jerk with extended foveation waveform, and similar frequencies (2.0 Hz:1.9 Hz), although the fast phases were in opposite directions. The mean amplitudes differed markedly (6.8 degrees:3.7 degrees), as did the position of the null zones (+20 degrees to +30 degrees:-25 degrees to -35 degrees) and the widths of the neutral zones (-25 degrees to +20 degrees:-25 degrees to -35 degrees). Since the twins have identical genotypes these differences must have arisen from other sources.