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Allison M McKendrick, Jonathan Denniss, Andrew Turpin; Structure-function mapping for individuals - why individuals not populations are needed to determine the utility of customising structure-function mapping. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):2059.
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© ARVO (1962-2015); The Authors (2016-present)
Population variance in optic nerve head (ONH) position, axial length and the temporal raphe, suggest that individual mapping between visual field locations and sectors on the ONH may be clinically useful. However, several studies have found minimal differences between customised and population-based maps for group relationships between structure and function when mapping to large disc sectors (Danthurebandara et al, IOVS in press, 2015; Ganeshrao et al, ARVO, 2014). In any population, most people will be close to “average”, hence the ability to test the utility of customised mapping is frustrated when pooling across individuals if many have similar mapping for both schema. Here we use recent population data on the likelihood of particular anatomical features to determine the expected frequency of marked shifts in the mapping between structure and function from population average.
The frequency of encountering a particular angle between the fovea and the disc was modelled using data from Figure 4 of Chauhan & Burgoyne, Am J Ophthalmol, 2013). Axial length data was from Atchison et al, IOVS, 2004. Customised mapping was determined using our published model (Denniss et al, IOVS, 2012) with variables being axial length, the position of the ONH relative to the fovea, and the angle between the temporal raphe and horizontal. We determined for each 24-2 location, the probability of the mapped ONH sector being more than either 30 or 60 degrees from that predicted by a standard mapping schema (Garway-Heath et al, IOVS, 2002).
The largest inter-individual differences in mapping are in the nasal step region where the same visual field location can map to either the superior or inferior ONH. For these points, the probability of non-traditional mapping was 14%. Four superior arcade locations and 5 inferiorly had a probability of greater than 50% of a 30 degree shift but never shifted by more than 60 degrees.
Anatomically customised mapping shifts the map markedly in approximately 14% of the general population in the nasal step region where visual field locations can map to the opposite pole of the ONH than traditionally considered. Early glaucoma can affect the nasal step, hence matching structure to function in this region may prove clinically useful for the diagnosis and monitoring progression within individuals.
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