《Table 2 Average MD and retinal pigmentation in 928 observed eyes at the patients’first visit》

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《Visual field mean deviation and relevant factors in 928 Chinese retinitis pigmentosa patients》

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MD:Mean deviation.

MD indicates the mean difference between the normal expected retinal sensitivity in terms of age and visual acuity and the measured patient sensitivity[5].This definition has already indicated the comparation with normal eyes，so there is no need to set a normal comparative group when MD is used for clinical research.In this paper，as MD is the major research index，there is also no need to set normal comparative groups for minor indexes such as visual acuity，gender，age，and so on.Calculating according to the above linear regression equation of MD on visual acuity in 928 observed eyes at the patients’first visit，the authors expected that when visual acuity was lower than 1.0 the MD should be lower than-7.75 dB.As the R2 value of the equation was 0.6168，it was possible that the equation couldn’t well express the correlation between MD and visual acuity.Table 1 showed when average MD inobserved eyes was-9.18 dB，the visual acuity was still at 1.0and above.It indicated that in the early stage of RP the visual acuity of observed eyes would maintain normal.The socalled early stage in this paper was that when average MD was higher than or equal to-9.18 dB，but the authors suggested the clinical early stage of RP should be defined as the period from the onset of the disease to the time point when patient’s visual acuity was just under 1.0.This definition accorded with the pathologic process of RP.The progressive atrophy of the rod photoreceptor cells leads to the secondary death of the cones in RP，affected individuals often present with night blindness and constricted visual fields，but in the early stage of RP the central vision is normal or nearly normal.Eventually central vision is lost as the cone cells degenerate[6-9].Clinically，visual field becomes more and more narrow，accordingly visual field sensitivity or MD is lower and lower.When MD is low to a certain extent，visual acuity will be less than 1.0.