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J. Felius, V. L. N. Fu, L. M. Wyatt, R. W. Hertle, E. E. Birch; Foveation and Acuity Deficits in Infants and Children With Infantile Nystagmus Syndrome. Invest. Ophthalmol. Vis. Sci. 2008;49(13):144.
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© ARVO (1962-2015); The Authors (2016-present)
Treatment of infantile nystagmus syndrome (INS) has led to improvements in foveation properties as documented in adults and older children by the eXpanded Nystagmus Acuity Function (NAFX). The NAFX is an appealing outcome measure because it relates waveform features indicative of foveation to visual acuity. However, infants and young children often do not provide the reliable data needed to calculate the NAFX. Because evidence suggests that treatment of INS during the critical period of visual development may yield additional improvements, it would be helpful to have an objective ocular motor outcome measure for this age group.
Horizontal eye movements were recorded in 45 infants and children with INS (idiopathic, N=23, age 5 months - 8 yrs; albinism, N=22, age 6 months - 10 yrs) with an infrared reflection goggle system. Data were calibrated individually and by using group means. A novel method was developed to calculate foveation fraction, using elements of the NAFX but with freedom in positional zero, thus allowing for head and body movement during testing. Typical position and velocity constraints were applied to a brief time window. The Nystagmus Optimal Foveation Fraction (NOFF), expressed as percentage of window-time, was found by moving the window through the record while adjusting for changes in median position. Test-retest variability was assessed in 15 patients. Deficits in binocular visual acuity were determined using Teller cards and age-corrected norms.
The NOFF was measurable in all patients and ranged from 0.6% to 100% (median 11%). Test and retest values correlated well (r=0.85, P=0.001). Cross-sectional acuity deficit-vs.-NOFF data were fit by an exponential function, yielding a mean acuity deficit of 0.39 logMAR for NOFF approaching zero, and smaller deficits with increasing NOFF. For NOFF >12%, acuity deficit leveled off to asymptote at 0.04 logMAR. Increasing acuity deficits with age (P=0.001) in the albinism group were mirrored by decreasing values for NOFF (P=0.03). In the idiopathic group, both acuity deficit and the NOFF remained approximately constant with age (P>0.3).
The NOFF appears to be a useful alternative to the NAFX in infants and children, showing a meaningful relationship with visual acuity deficit. Its feasibility makes the NOFF a promising outcome measure for treatment of nystagmus during early childhood.
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