PubMed · 13189249
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G C GUZZINATI. 1954. [Proximal convergence].. https://pubmed.ncbi.nlm.nih.gov/13189249/
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BACKGROUND, MATERIAL AND METHODS: Down's syndrome is the most common cause of mental retardation with an incidence of about 1.5/1000 live births. Life expectancy and quality of life have improved substantially for this group over the last decades. The aim of this paper is to give an updated short survey of ocular changes present in Down's syndrome based on current international literature and the clinical experience of the authors. RESULTS AND INTERPRETATION: Ocular problems are common, mostly refractive errors, poor accommodation, strabismus, cataract, and keratoconus. Accommodation deficit is present in a majority of individuals with Down's syndrome, also in children and young people. Bifocal or progressive glasses should therefore be prescribed liberally. Because of the high frequency of ocular pathology, all individuals with Down's syndrome should be enrolled in a continuous visual screening programme from birth. We suggest the following screening guidelines: first examination at one month of age, then at one year of age, at 2-3 years of age, at 5-6 years of age (school start), and thereafter every five years. In case of positive findings (e.g. refractive errors, poor accommodation, strabismus) the frequency of examination should be increased and determined individually by the responsible ophthalmologist.
There has been no comprehensive study involving each of the primary dynamic components of accommodation in the same cohort as related to age and presbyopic onset; furthermore, the current findings are equivocal. Dynamic monocular components of accommodation (latency, time constant, peak velocity/amplitude relationship, and microfluctuations) were assessed objectively using an infrared optometer within the linear region of accommodation in 30 visually-normal human subjects aged 21-50 years. The time constant and the peak velocity/amplitude relationship did not change with age. However, latency progressively increased, and microfluctuation amplitude and frequency progressively decreased, with increasing age. The invariance in time constant suggests that the gross biomechanical aspects of the lens and related structures in the remaining linear region are relatively unaffected by age. In contrast, the decrease in microfluctuation activity with age suggests more subtle alterations in the biomechanical aspects of the lens to these very small perturbations, such as a response amplitude non-linearity. With respect to neurologic control, the progressive latency increase suggests a processing delay of the blur input, and this is consistent with age-related changes in reaction time measures. The lack of any age-related changes in the peak velocity/amplitude relationship implies normalcy of central and peripheral neuromotor control, as well as grossly normal first-order lens biomechanics, in this linear response region. The results are consistent with the Hess-Gullstrand theory of presbyopia.
The aim of the experiment was to test for a contribution from short-wavelength sensitive cones to the static and step accommodation response, to compare responses from short and long- plus middle-wavelength sensitive cone types, and to examine the contribution of a signal from longitudinal chromatic aberration to the accommodation response. Accommodation was monitored continuously (eight subjects) to a square-wave grating (2.2 c/d; 0.57 contrast) in a Badal optometer. The grating stepped (1.00 D) randomly towards or away from the eye from a starting position of 2.00 D. Five illumination conditions were used to isolate cone responses, and combine them with or without longitudinal chromatic aberration. Accuracy of the response before the step, step amplitude, latencies and time-constants, were compared between conditions using single factor ANOVA and t-test comparisons. Both S-cones and LM-cones mediated static and step accommodation responses. S-cone contrast drives "static" accommodation for near, but the S-cone response is too slow to influence step dynamics when LM-cones participate.