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      Saccadic reaction time and ocular findings in phenylketonuria

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          Abstract

          Background

          Phenylketonuria (PKU) is an inherited metabolic disorder characterized by reduced activity of phenylalanine hydroxylase resulting in elevated blood phenylalanine (Phe) concentration. Despite some obvious ocular changes, the disorder has been poorly recognized by ophthalmologists. Neurophysiologic tests imply prolonged reaction time correlating with increased phenylalanine blood concentrations. We aimed to test saccadic reaction time in PKU patients in dependency of blood phenylalanine concentrations.

          Methods

          Nineteen biochemically diagnosed PKU patients and 100 controls completed comprehensive ophthalmologic and orthoptic examinations including saccadometry by infrared based video-oculography. Peak velocity, gain, and particularly latency of reflexive saccades were compared to controls, and regression analysis was performed.

          Results

          Latency of reflexive saccades was not associated with the current phenylalanine concentration. Although in 10 out of 19 patients phenylalanine concentrations were outside the age-related therapeutic range, latency differed little between PKU patients and the controls, as well as peak velocity and gain. Ocular findings occurred as partial hypopigmentation of the iris in one late diagnosed patient aged 36 years, and as bilateral cataracts (possibly due to steroid intake) with refractive amblyopia, strabismus, high myopia, and glaucoma in another late diagnosed patient aged 46 years. Visual acuity was reduced in eight PKU patients.

          Conclusions

          Saccadometry, particularly saccadic reaction time, is not useful in the monitoring of phenylketonuria. Ophthalmic examination is recommended in PKU patients, as the occurrence of ocular pathologies was relatively high.

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          Most cited references 10

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          Eye Movements in Parkinson’s Disease and Inherited Parkinsonian Syndromes

          Despite extensive research, the functions of the basal ganglia (BG) in movement control have not been fully understood. Eye movements, particularly saccades, are convenient indicators of BG function. Here, we review the main oculomotor findings reported in Parkinson’s disease (PD) and genetic parkinsonian syndromes. PD is a progressive, neurodegenerative disorder caused by dopaminergic cell loss within the substantia nigra pars compacta, resulting in depletion of striatal dopamine and subsequent increased inhibitory BG output from the internal globus pallidus and the substantia nigra pars reticulata. Eye movement abnormalities are common in PD: anomalies are more evident in voluntary than reflexive saccades in the initial stages, but visually guided saccades may also be involved at later stages. Saccadic hypometria (including abnormally fragmented saccades), reduced accuracy, and increased latency are among the most prominent deficits. PD patients show also unusually frequent and large square wave jerks and impaired inhibition of reflexive saccades when voluntary mirror saccades are required. Poor convergence ability and altered pursuit are common. Inherited parkinsonisms are a heterogeneous group of rare syndromes due to gene mutations causing symptoms resembling those of PD. Eye movement characteristics of some parkinsonisms have been studied. While sharing some PD features, each syndrome has a distinctive profile that could contribute to better define the clinical phenotype of parkinsonian disorders. Moreover, because the pathogenesis and the underlying neural circuit failure of inherited parkinsonisms are often well defined, they might offer a better prospect than idiopathic PD to understand the BG function.
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            The effect of cognitive distraction on saccadic latency.

            Saccadic initiation is increasingly being studied as a surrogate for more general neural mechanisms of decision-making. Visual 'decision-making' is thought to be controlled by higher cortical functions. Lower areas such as the superior colliculus are thought to be involved with more primitive optomotor reflexes that can generate short-latency saccades. It is now well established that imposition of fronto-executive load on subjects performing a saccadic task which, in particular, involves suppression of saccades (the no-go saccadic task), increases the number of errors made. It is theorised that a weakening of cortical control of the superior colliculus is responsible for the increase in error rate. One way to test this theory is to measure the latency of incorrect saccades made in a no-go saccadic task in relation to error rate under different conditions of fronto-executive load. A high error rate combined with an increased number of short-latency saccades in the range of express or early saccades would indicate that subjects have an inability to inhibit these short-latency more reflexive saccades, which seem to originate in the superior colliculus. Hence the normal cortical control of the superior colliculus is weakened. We used a saccadic go/no-go task under fronto-executive load and found that the proportion of short-latency saccades increased with audio-verbal interference, in conjunction with an increase in error rate. These findings provide strong empirical evidence to support the theory that maintenance of cortical functions is key to the control of saccadic responses. Under conditions of fronto-executive loading such cortical control is weakened, leaving subjects with a reduced ability to inhibit short-latency more reflexive saccades.
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              Impaired sensitivity to visual contrast in children treated early and continuously for phenylketonuria.

              Contrast sensitivity was assessed in 47 children aged 5.4-9.8 years: 12 with phenylketonuria (PKU), six unaffected siblings and 29 children from the general population. Children with PKU, despite early and continuous treatment and despite phenylalanine (Phe) levels within accepted limits, were impaired across the range of spatial frequencies [1.5-18.0 cycles per degree of visual angle (c.p.d.)]. They were most impaired at the next to the highest spatial frequency, where "group' accounted for 70% of the variance in sensitivity to contrast, controlling for acuity, sex, age and test site. Never, at any spatial frequency, was the contrast sensitivity of any PKU subject better than that of his or her sibling. All subjects were tested under conditions of 20/20 vision, with correction if needed. The mean IQ of PKU subjects was 99; IQ was not significantly related to contrast sensitivity performance. We interpret these findings as support for Diamond's hypothesis that moderately elevated plasma Phe levels (3-5 x normal), combined with reduced plasma tyrosine (Tyr), moderately reduce the levels of Tyr reaching the eye and brain, which adversely affects those dopamine neurons that fire and turn over dopamine most rapidly (the dopamine neurons in the retina and those projecting to prefrontal cortex). This would lead to the deficit in contrast sensitivity found here and to the selective deficit in prefrontal cortex cognitive functions previously reported in PKU children under moderately good dietary control.
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                Author and article information

                Contributors
                hopf101@gmail.com
                nowak.caroline@gmx.net
                julia.hennermann@unimedizin-mainz.de
                irene.schmidtmann@unimedizin-mainz.de
                norbert.pfeiffer@unimedizin-mainz.de
                s.pitz@buergerhospital-ffm.de
                Journal
                Orphanet J Rare Dis
                Orphanet J Rare Dis
                Orphanet Journal of Rare Diseases
                BioMed Central (London )
                1750-1172
                25 May 2020
                25 May 2020
                2020
                : 15
                Affiliations
                [1 ]GRID grid.410607.4, Department of Ophthalmology, , University Medical Center Mainz, ; Langenbeckstr 1, 55131 Mainz, Germany
                [2 ]GRID grid.5802.f, ISNI 0000 0001 1941 7111, Johannes Gutenberg University Mainz, ; Mainz, Germany
                [3 ]GRID grid.410607.4, Villa Metabolica, Department of Pediatric and Adolescent Medicine, , University Medical Center Mainz, ; Mainz, Germany
                [4 ]GRID grid.410607.4, Institute of Medical Biostatistics, Epidemiology and Informatics (IMBEI), , University Medical Center Mainz, ; Mainz, Germany
                [5 ]GRID grid.500078.a, ISNI 0000 0004 0619 1944, Orbital Center, Ophthalmic Clinic, , Bürgerhospital Frankfurt, ; Frankfurt, Germany
                Article
                1407
                10.1186/s13023-020-01407-7
                7249436
                32450880
                © The Author(s) 2020

                Open AccessThis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver ( http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.

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