The congenital pinhole: a persistent pupillary membrane.

During embryonic development the lens receives nourishment from the posterior and anterior tunica vasculosa lentis. During the fourth month of gestation, macrophages play a role in the regression of the tunica vasculosa lentis, revealing the pupillary aperture posteriorly. The persistent pupillary membrane (PPM) represents a congenital remnant of the anterior tunica vasculosa lentis. As in this case, the membrane often spans the pupil from its insertion in the iris collarette. This is a particularly florid example. The PPM will often involute spontaneously in the first year of life but some denser membranes can persist and need to be considered for treatment. PPM’s are commonly seen in premature babies. The visual acuity in this patient remains good despite this significant anterior segment anomaly obstructing most of the visual axis. This is due to the pinhole effect of multiple apertures within the pupillary membrane. This is otherwise known as the stenopeic effect (from the Greek word meaning ‘little opening’). These pinholes reduce the width of the incoming ray of light producing a ‘pencil of light’ from each point of the object being observed. The more peripheral rays of light are blocked by the membrane, giving rise to a clear image. The smallest useful pinhole size is thought to be 1.5 mm in diameter, below which visual development may be compromised. The effect of multiple pinholes will reduce the overall illumination of the retinal image. In addition, because of the varying size and irregular shape of Figure 1. The iris and persistent pupillary membrane with multiple apertures in a 10-year-old boy C L I N I C A L A N D E X P E R I M E N T A L