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Retinopathy and myopia of prematurity
  1. University Eye Clinic of Rigshospitalet, 2100 Copenhagen Ø, Denmark

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    Editor,—I have some comments on the recently published article by Choi et al 1 dealing with long term refractive outcome and oculometry variables in Korean children of very preterm delivery. As for the sample under study (n=65) there are certain points to suggest a marked deviation in composition from the usually analysed preterm cohorts. The material appears highly selective; over a 6 year period, from two university clinics, only 10–11 preterm infants have been included per year. Screening limits were 1500 g birth weight and 28 weeks' gestational age. Exclusion of a great number of preterms appears likely, but criteria are not specified or discussed.

    Eighty three per cent acquired active ROP of at least stage 3. If unselected, this is the highest figure of advanced (and of any) ROP ever reported in developed countries. Apparently 54% of all in the series had threshold ROP according to US standards2 and were given retinal ablation therapy by cryotechnique. Again, this represents a cryotherapy top score in ROP literature.

    Sixty seven eyes (out of the 125 under study) “survived” their ROP and the subsequent cryotherapy. The eventual full sample myopia frequency of 67% is very high. No account is given of visual acuity or blindness data. Mention is made only that 17 eyes developed macular dragging after the cryotherapy (and apparently there were no cases of more advanced retinal detachment). The remaining 50 eyes with cryotherapy were even recorded as having no cicatricial ROP at follow up at the ages of 3 months, 3 years, and 6 years.

    With the overall ROP severity recorded, it is impressive that only 27% of the ROP cases had cicatricial sequelae of the retina, the narrow definition apparently being dragging of the macula.

    In this context one may wonder why the authors preferred Reese's classification3 of the early 1950s, and not its acknowledged successor regarding cicatricial ROP.4 It even appears as if the Reese classification was not quite followed to the letter.

    For comparison, in the same issue of BJO in the US university clinic material published by Saunderset al,5 143 preterm subjects were collected over 13 months and 12% acquired threshold or prethreshold ROP. To my knowledge there is no reason to assume that the Korean university clinics are not on quite such a developed level, nor that the infant susceptibility regarding ROP should markedly differ from what is known from nearby Asian metropolises. The authors further state that there are no previous longitudinal reports in the field. Depending on how “longitudinal” is defined, however, there are several studies of a rather similar set up, and with emphasis on subsequent refraction and oculometry/keratometry results.6-12 It is from these studies that our present knowledge is compiled.

    This knowledge may be summarised as follows: In ordinary myopia the correlations between the “minor” refractive factors (corneal power, anterior chamber depth, lens thickness) all tend to reduce the myopia otherwise induced by the established main factor—the axial length elongation. Contrarily, as regards myopia of prematurity: the corneal curvature is steeper, anterior chambers are more shallow, and lenses thicker; axial lengths therefore appear as “relatively short for their myopia”. Myopia is still mainly axial, but not so axial as usual. Though emphasising anterior segment features in high myopia the authors ignore or discard their own higher corneal powers compared with presumed norm values. Apparently the generally steeper corneas may have contributed 1–1.5 D to the myopia.

    Finally, it was interesting to see the split up according to +/− cryotherapy for the 29 eyes with cicatricial ROP.With cryotherapy their 6 year myopia averaged −2.97 D. In contrast, thosewithout cryotherapy had −6.18 D. This might be interpreted as some protection exerted by the cryotherapy against the relative developmental involution that myopia of prematurity seems to represent. Otherwise, the cryotherapy itself has been blamed for generating myopia,13 but here it seemed to be subordinate to the severity of the eye disease for which the ablation therapy was applied.


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