There is a reason we don't perform pupillary reactions to light in preterm babies, and that is that they don't react. They also tend to be large. This post is in response to my trying to find out what had been published about this phenomenon, when a case in our NICU raised a teaching point. I decided to go back through the literature to find out what was actually documented in peer-reviewed publications, and found, for a change, some good quality data.
The first of these, chronologically, is from 1989 Isenberg SJ, et al. The pupils of term and preterm infants. Am J Ophthalmol. 1989;108(1):75-9. This group from UCLA studied prospectively pupils of 100 babies from 26 weeks through to 46 weeks, with a standardized methodology; pupils were larger in mm for more immature infants, even though their eyes are a bit smaller. Pupillary light responses were usually absent before 30 weeks PMA, and only reliable after 32 weeks.
Robinson J, Fielder AR. Pupillary diameter and reaction to light in preterm neonates. Arch Dis Child. 1990;65(1 Spec No):35-8. A similar study from Birmingham, England followed 50 babies, and found similar things, the few babies who responded before 31 weeks all had a slow reaction.
Isenberg SJ, et al. The fixed and dilated pupils of premature neonates. Am J Ophthalmol. 1990;110(2):168-71. The UCLA group again, this time with a cohort of 30 babies who were examined every week, they were all less than 31 weeks at the first examination and none of them reacted at that time. The proportion having a detectable response to light increased, and most were reacting by 32 weeks.
Isenberg SJ, Vazquez M. Are the pupils of premature infants affected by intraventricular hemorrhage? J Child Neurol. 1994;9(4):440-2. The answer to the question in the title is no. The pupils of babies with any grade of hemorrhage (1 to 4) are not larger and do not have different reactions to light than those without hemorrhage.
There are a couple of more recent publications, but they don't really add anything. Until the baby has passed 32 weeks post-menstrual age, there is no point looking for pupillary light reflexes, and the pupils will look fixed and dilated.