The results of this study showed that there is less pain and stress following completion of the exam in premature children when screening for ROP is performed with WFDRI compared with examination with BIO. Several studies have already shown that examination with BIO produces pain in premature infants [6, 7, 24]. Few reports compared the effects produced by both examination techniques (WFDRI/BIO) [18–20].
Mukherjee et al. , in a population of premature infants with similar weight and gestational age to that of our study, also encountered less signs of stress/pain when the examination was performed with WFDRI. The examination with BIO was performed on some infants and with WFDRI on others. In our study, use of both examination techniques was an eligibility criterion that provided a more robust comparison. In the previous report , the main outcome measure was variation in cardiorespiratory parameters. In addition to cardiorespiratory variables (Heart rate, blood pressure, SaO2), we used specific pain scales validated for the neonatal setting that included facial expression, crying, sleeplessness, and gestational age, which provided a more comprehensive assessment of the pain and stress experience in neonates. Baseline assessment values were taken prior to administering the drops so as not to interfere with the possible reported side effects . Using the assessment of CRIES and PIPP scales in our study setting, it was not feasible to obtain the parameters of the scales during the exam, but only after the end of the exam. Facial expression could not be assessed while speculum was still in the eye or while WFDRI was being used. We decided to homogenously assess these parameters at 30 seconds after the end of all exams. The assessment at 24 hours after the procedure was included because higher values were previously reported in extremely premature infants .
Mehta et al.  compared WFDRI with speculum and BIO with and without speculum in a series that included 12 infants. These authors found no differences in cardiorespiratory indices and facial assessment between WFDRI and BIO examinations with speculum. They showed less pain with BIO when the examination was performed without speculum, and concluded that it might be appropriate not to use a speculum in particularly ill infants. In our study, the standard technique with eyelid speculum was used, with WFDRI and BIO, to allow for full visualization of the retina and for valid comparisons. Rush et al. and Laws et al.  concluded that adrenergic manifestations of stress and pain, and modifications in SaO2 following examination with BIO appeared after handling of the eye and use of the speculum. The only effect which could be attributed to the mydriatic drug used was increase in mean blood pressure.
Recently, Dhaliwal et al.  assessed PIPP scores recorded in the first minute of examination with WFDRI or BIO in 76 un-swaddled, non-nested infants, with no use of pacifier or oral sucrose. Exceedingly high pain score values at baseline and no differences between techniques were observed. Current recommendations  and recent evidence  should minimize the adverse effects of examinations in the future.
Regarding time employed during the examinations procedure, no differences were found between the two techniques. In this study, WFDRI preparation time was not included, only examination time. Mukherjee et al.  observed longer examination times with WFDRI when including the time to set up the instrument.
Pain is a multifactorial phenomenon with physiological and behavioral aspects, modified in infants by factors such as gestational age, state of health (sepsis and hypovolemia may lead to tachycardia and changes in blood pressure), and maturity . In our study, none of the determinants analyzed showed significant differences in pain measurements for both techniques and both pain scales. Rush et al.  and Mukherjee et al.  did not report higher systemic manifestations or pain in responses to mydriasis and fundoscopy in lower gestational age infants. The significantly higher difference observed in CRIES score between BIO and WFDRI, in the gestational age group >26 weeks might be explained by higher baseline pain values in more immature infants. Pain and stress responses for successive examinations did not increase when compared with the first examination, with intermediate care vs. NICU, or with higher respiratory support vs. none. These determinants were not considered in the aforementioned studies [18–20].
A potential drawback to this study is that, per protocol the technique with WFDRI was performed first, followed by the BIO technique after 3 to 5 days. The time interval between both examinations was sufficiently long enough to minimize any interference between them. Similar baseline pain assessment values were observed before WFDRI and BIO examinations. The two scales used in our study for outcome assessment provide more consistency to the results of the study.
The fact that scleral indentation with the WFDRI was not necessary to visualize the retina is of paramount importance because this factor is significantly related to pain and stress. The lower light intensity used with WFDRI might cause less photophobia and less discomfort for the infant.