During normal pregnancy, hormonal, metabolic, hematological, vascular, and immunological changes can be observed (9), which can be shown to be associated with ocular changes during pregnancy. These changes include increased curvature as well as steepness of the cornea that begin during pregnancy, continue during breastfeeding, and returns to the original status with the end of breastfeeding (13). A slight increase in the thickness of the cornea at the end of the pregnancy period associated with changes in refraction and compliance can continue during breastfeeding (14, 15). Another change is the reversible reduction in tear film as a result of the prolactin secretion increase during pregnancy and breastfeeding (16). Therefore, refractive surgery is not usually recommended during pregnancy and a year after childbirth (17,18).
However, pregnancy and breastfeeding have some differences in physiological changes, which could affect results of the laser vision correction (LVC). One of these differences is the physiological changes associated with the role of placenta during pregnancy. During pregnancy, as an endocrine system, the placenta is independently capable of producing cytokines and hormones. The placenta cytokines are tumor necrosis factor alpha-TNFa, Resistin, and leptin. The important hormones of the placenta are human chorionic somatomammotropin-HCS, cortisol, estrogen, progesterone, and human placental growth hormone –HPG (19-21). TNFα potentially plays a role in physiological dosages in causing corneal scars followed by corneal opacity after PRK (22). Meanwhile, in the normal people (non-pregnant and non-breastfeeding women), a natural increase in TNFa is observed at physiological levels on the first two days after PRK, which plays an important role in improving the corneal ulcer followed by laser ablation (23). In a study by Jiang, it was suggested that Resistin can increase the local cytokines and exacerbate inflammatory reaction by inducing extracellular leukocytosis (24). Prolactin effects has been examined on the production of tears and it has been specified that there is a negative relationship between serum prolactin and tear secretion, but a positive relationship has been observed between estradiol and tear secretion among women aged 30-39 years (25). It can be noted that during breastfeeding, the most important effective factor for the visual organ, which probably affects the results of PRK surgery is the prolactin hormone. Hence, the hormonal profile of pregnancy period is distinguished from breastfeeding with placenta and further production of prolactin and all types of cytokines as well as other hormonal changes in other glands of the body; thus, classifying pregnancy and breastfeeding in one group regarding the possibility of PRK surgery is not very reasonably justifiable.
In our study, postoperatively, comparison of the two groups showed no statistically significant differences in terms of corneal opacity, dry eye, UCVA, BCVA, dry and cycloplegic refraction (except for cylinder). In this regard, the coefficient of surgical effect in the two groups was in the range of the previous studies (26).
In comparing the uncorrected visual acuity, the two groups of breastfeeding and non-breastfeeding had no statistically significant differences (Table 1). Ghoreishi et al showed that the UCVA of 20/20 and 20/40 and more was observed in 92.1% and 99.2% of patients and, after a year, 69.4% and 91% of the patients had the entropy of 0.5 and 1.0 diopter (27). Similar results were obtained in the study by Hashemi et al. using MMC during PRK surgery, in which 77.1% of the eyes had uncorrected visual acuity of 20/20 or better (28).
In the study by Hashemi et al. (2015), after performing PRK using MMC on 30 eyes, the surgery impact factor was 1.01 (26). In the present study, although the MMC was not used in all of the patients, the results of the surgery impact factor were lower than the previous results.
In most of the studies, the values of manifest refraction spherical equivalent (MRSE) are in the range of mild myopia after PRK surgery (29, 30), but the mean of MRSE can also be found in the range of mild hyperopia which, for instance, can be seen in the report by Randelman et al. (2009) (31). In the upcoming study, in both groups, the general overview of the average post-operative results of MRSE was shifted towards hyperopia; nevertheless, their comparison showed no statistically significant difference.
Dry eye naturally occurs during pregnancy. It is improved by the end of breastfeeding as the level of prolactin decreases (8, 20). Besides, the incidence of dry eye could remain on the mild to moderate myopic eyes for 12 years after PRK surgery (32).
In our study, the absence of statistically significant difference in the incidence of dry eye among breastfeeding and non-breastfeeding women was not consistent with the comments of Umti et al. (8). Therefore, along with the effect of prolactin on dry eye, the other cases including meibomian gland dysfunction (MGD) have been also probably effective.
A slight increase in surface regularity index (SRI) and surface asymmetry index (SAI), higher level of best spherical anterior and posterior surfaces, more severe astigmatism in 5 mm, and a slight increase in HOAs in 5.0 mm were found before surgery in the breastfeeding group. These changes can be justified despite the edema remaining in the cornea from the pregnancy period (14,15) one result of which can be an increase in the curvature of the cornea (13). However, edema occurs further in marginal areas with thicker stroma (36). The comparison of breastfeeding and non-breastfeeding groups showed no statistically significant difference in terms of the above mentioned items after surgery. Since the laser ablation was performed in the corneal center for myopia correction in which it had smaller diameter than the margin and, consequently, less edema, therefore, physiological changes after pregnancy might have a lower effect on the post-operative results. Furthermore, during the ablation of the corneal center, the laser beams collided at the corneal surface at the 90° angle, which had a greater penetration impact, while the marginal rays that reached the marginal points of the cornea collided the points with a greater angle to cornea surface and would have less tissue evaporation when the optic area was about 6.5 mm (37). Probably, fewer changes in the cornea caused by ablation in these areas associated with further edema had less inappropriate results over time with the improvement of edema after pregnancy that continued up to one year later (9). The absence of a significant difference in the comparison of the mean of the lowest corneal thickness between the two groups of breastfeeding and non-breastfeeding women after surgery could justify these cases. Anyway, it was in the range of previous studies (29).
Comparing subjective results and other evaluations including lowest corneal thickness and best anterior spherical surface to the best posterior spherical surface ratio between the two groups three years after the surgery demonstrated no statistically significant difference and were in an optimal level. From the ectasia risk factors mentioned in the previous study (38) and according to the results of Orbscan, there was probably an increase in the slope of cornea in the breastfeeding group in a stage before surgery, but these changes did not result in an even mild keratoconus.
Other factors include the ectasia risk, severe myopia correction, remaining post-ablation thin stromal thickness (39), extensive surface of ablation, irregular corneal thickness, and abnormal corneal topography (40,41). The surgery was carried out in a time interval of 11 months and 1 week after childbirth, and the average duration of breastfeeding was 8 months and 1 week. This can be associated with a decline in the natural complications of pregnancy in the visual organ, especially the cornea, (13-15), which is among other reasons that can be effective in preventing ectasia in the breastfeeding women.