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Impaired sensitivity to visual contrast in children treated early and continuously for phenylketonuria
1Department of Psychology, University of Pennsylvania, Philadelphia, USA.
Insights
Children with phenylketonuria (PKU) show impaired contrast sensitivity, even with treatment. This visual deficit may stem from altered dopamine neuron function due to phenylalanine and tyrosine levels.
Area of Science:
- Neuroscience
- Ophthalmology
- Genetics
Background:
- Phenylketonuria (PKU) is a genetic disorder requiring dietary management.
- Early treatment aims to normalize phenylalanine (Phe) levels.
- Potential long-term neurological and visual effects persist despite treatment.
Purpose of the Study:
- To assess contrast sensitivity in children with PKU.
- To investigate the relationship between PKU and visual processing.
- To explore potential underlying mechanisms for visual deficits.
Main Methods:
- Contrast sensitivity was measured in 47 children (5.4-9.8 years).
- Groups included 12 children with PKU, 6 unaffected siblings, and 29 controls.
- Testing occurred under 20/20 vision conditions, with corrections if needed.
Main Results:
- Children with PKU exhibited impaired contrast sensitivity across spatial frequencies (1.5-18.0 c.p.d.).
- Deficits were most pronounced at higher spatial frequencies.
- PKU subjects' contrast sensitivity never exceeded that of their unaffected siblings.
Conclusions:
- Early and continuous PKU treatment does not fully restore contrast sensitivity.
- Findings support the hypothesis that altered dopamine neuron function impacts visual processing in PKU.
- Visual deficits may be linked to reduced tyrosine availability in the brain and retina.
Abstract:
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.