Early birth with a very low birth weight (VLBW, birth weight <1500 g) increases the risk of various neurological difficulties in later life, including visual problems. Observational studies on individuals born with VLBW have shown reduced visual acuity compared to their peers from childhood to adulthood, even without a history of neurological or ocular injury at birth. Moreover, decreased contrast sensitivity (CS) has been found at low and high spatial frequencies in children and adolescents born with VLBW without neurological complications. Visual stimuli information is transmitted through ganglion cells in the retinal nerve fiber layer (RNFL) via the optic nerve, where information from both eyes converges at the optic chiasm. Thick layers of ganglion cells and fast signal conduction through the optic nerve are therefore necessary for proper visual functioning. Earlier studies have found thicker central RNFL in young adults born extremely prematurely compared to the control group. Additionally, preschoolers born prematurely with a birth weight <2500 g showed slower reaction times in pattern-reversal visual evoked potentials (PR-VEPs) compared to their peers with normal birth weight, indicating slower conduction of visual signals. Hence, reduced visual outcomes in individuals born prematurely may not solely result from vascular complications due to retinopathy of prematurity (ROP). Vision of preterm infants (VOP), a general inflammatory response involving brain injury in the perinatal period and retinal vessel formation, may provide a more explanatory perspective. Indeed, individuals born prematurely with VLBW have a significantly increased risk of perinatal brain injury, particularly white matter microstructure.
The microstructure of both gray and white matter can be examined using diffusion tensor imaging (DTI), a magnetic resonance imaging technique. Perinatal brain injury is typically associated with higher mean diffusivity (MD) and lower fractional anisotropy (FA) values in the white matter. A higher MD value characterizes increased overall diffusion, reflecting a lower amount of cell membranes and/or packing within a specific voxel. A lower FA is considered a measure of disorganization or decreased integrity of white matter microstructure. However, FA varies depending on the dataset in the white matter and fiber tract passing through a specific area. Therefore, differences in FA between groups only express differences in the orientation of microstructure in the voxel(s) or region(s). To further investigate whether the difference in FA between groups is related to degree of myelination or axon density, radial diffusivity (RD) and axial diffusivity (AD) should be considered. AD is considered a measure of diffusion parallel to the white matter tracts, while RD is a measure of diffusion perpendicular to the white matter tracts. These additional biomarkers obtained from DTI help interpret whether a lower FA value and higher MD value are associated with axonal injury and abnormal fiber organization (lower AD value), or improper myelination and axon packing (higher RD value) in individuals born with VLBW.
This study is part of the NTNU Longitudinal Birth Weight in Life (NTNU LBW Life) study, a long-term cohort study of individuals born prematurely with VLBW and a control group born full-term in Central Norway, which have been observed from infancy to adulthood. A characteristic pattern of white matter microstructure based on DTI measures, with lower FA values and higher MD values, has previously been identified in this group during adolescence, young adulthood (18-22 years), and adulthood (25-28 years). In adolescence, Lindqvist and colleagues found an association between reduced visual acuity and lower FA values in the optic chiasm and corpus callosum (CC) area, with reduced visual acuity attributed to higher RD values and several smaller foci in the frontal areas of the white matter microstructure in individuals with VLBW based on DTI data obtained at a 1.5 T field strength. Only information regarding the CC and frontal areas with shorter visual acuity and contrast sensitivity measures were included in this study. Whether the same association between altered white matter microstructure pattern and visual problems extends to other visual pathways and measures of visual function has yet to be thoroughly investigated.
FAQ:
The source of the article is from the blog regiozottegem.be