Development of the optic nerve head and foveation strategies in infants and young children with infantile nystagmus

The three aims of this study were to:• Investigate the postnatal development of the optic nerve head (ONH) in infants and children with albinism, isolated infantile nystagmus (IIN) and achromatopsia against normal controls.• Investigate the asymmetry of ONH development in the four groups.• Explore the development of foveation strategy in albinism and IIN.Methods:Hand-held optical coherence (HH-OCT) tomography was used to image the ONH from 81 patients with albinism, 73 with IIN and 14 with achromatopsia and analysed using ImageJ, and statistically modelled using fractional polynomial regression. Child friendly stimuli were created to obtain eye movement traces and were correlated using LASSO regression and mixed linear models to foveal OCT parameters.Results:The disk width in albinism was wider than controls whereas IIN was consistent with controls, and in achromatopsia the disk was narrower than controls. In both albinism and IIN groups developed shallower narrower cups and larger nasal and temporal rim areas in the first 2 years of life. The optic cup in achromatopsia was shallower and narrower at birth, developing consistently with the control group. The nasal rim was larger at birth but normalised in the first 2 years of life, whereas the temporal rim was consistent with the control group throughout development. Inter ocular asymmetry of the ONH was subtle in the control group, but fewer asymmetric parameters were found in the albinism and IIN groups. In Both albinism and IIN measures of amplitude decreased with age. In IIN frequency increased with age. In both groups, the decrease in amplitude was associated to thickening of the inner segment at the fovea of the right eye. Parameters associated to frequency were positively correlated to parameters in the left eye in the photoreceptor layers. The beat direction was associated to thicker nasal outer segments of the right eye.Conclusion:The ONH in albinism and IIN has the appearance of crowded disks. This is associated to hypermetropia that is documented to be prevalent in these patient groups. In achromatopsia the appearance is similar to ONH hypoplasia. The development of these features is charted by the present study and gives important insight into the pathophysiology of these conditions. The retinal correlates of nystagmus characteristics allow for a greater understanding of why nystagmus develops. A theory pertaining to intra-retinal and inter-ocular rivalry is presented.