Myopia of prematurity: nature, nurture, or disease?

Myopia of prematurity: nature, nurture, or disease?Recent clinical research has shown that the degree of myopia is significantly less following laser therapy when compared with cryotherapy for severe retinopathy of prematurity (ROP). 1 2 This important finding is confirmed by Laws et al in this issue of the BJO (p 12).While the associations of prematurity, ROP, and myopia are well known, they are not precisely defined and mechanisms are even less well understood-an ideal opportunity for us to delve briefly into the evidence.Myopia is probably the normal refractive state in infants before full term 3 4 with the eye becoming more hypermetropic in early infancy.Compared with the eye of the full term baby the features of this myopia are shorter axial length, flatter anterior chamber, and more spherical lens.4 The term myopia of prematurity is not applied to this physiological and temporary type of myopia.Over three decades ago Fledelius studied a cohort of preterm babies and observed a disproportionate number of myopes; he found that this refractive state persisted even to 18 years of age. 5 This type of myopia, myopia of prematurity (MOP), has an early onset and compared with full term and juvenile onset myopes the MOP eye exhibits a relatively highly curved cornea, shallow anterior chamber, and thick lens.Axial lengths are shorter than expected for the dioptric value.6 The hallmark of MOP is arrested development of ocular anterior segment.With refreshing honesty Fledelius, in 1996, 7 stated that owing to a paucity of neonatal data in his early study it was not categorically known whether the MOP in this cohort was, or was not, associated with previous ROP.It is now confirmed that MOP without previous ROP does occur, 7-11 and at higher frequencies than in the full term population, and with the characteristics described above.12 Few reports on the refractive outcome contain detailed information of the neonatal period.Laws et al 13 studied ROP stage and refractive outcome at 6 months' corrected age and found that while there was a trend for increasing myopia with ROP presence and severity, this only reached significance with stage 3. Several authors have reported that there is a dramatic jump in the prevalence of myopia when stage 3 ROP is reached.7 9 11 14 To cite one study 9 the incidence of myopia by ROP stage was as follows: none 13%; mild ROP <20%, stage 3 ROP >44%.Of particular interest is the high incidence of myopia in both cryotherapy treated and non-treated eyes with severe ROP. 8 15 16 This is quite distinct from the outcome after laser therapy as shown by Laws et al and others. 1 2 Acknowledging the large data spread, these diVerences are not trivial, with refractions at 1 year after laser of -0.50 and -0.37 dioptres (right and left eyes), compared with -5.25 and -6.00 (right and left eyes) following cryotherapy (Laws et al, this issue).Many theories have been put forward to explain how myopia develops in premature babies.These include bone deficiency, temperature, light, visual deprivation, and retinal dysfunction.Pohlandt 17 speculated that MOP could be attributed to postnatal bone mineral deficiency, an idea which unsurprisingly generated a sharp critique.18 Secondly, preterm neonates experience a temperature deficit 19 at a time when corneal growth is especially active.While we can speculate that this deficit might impede corneal growth, it is most unlikely that it fully explains MOP.Furthermore, while this temperature deficit aVects all such babies myopia is not an invariable sequela of preterm birth.This last point also mitigates against light exposure as a myopiagenic factor.While light is known to influence eye growth in on August 18,