Entoptic phenomena (from Ancient Greek  ἐντός (entós) within, and  ὀπτικός (optikós) visual) are visual effects whose source is within the human eye itself. (Occasionally, these are called entopic phenomena, which is probably a typographical mistake.) In Helmholtzs words: "Under suitable conditions light falling on the eye may render visible certain objects within the eye itself. These perceptions are called entoptical." Overview
Entoptic images have a physical basis in the image cast upon the retina. Hence, they are different from optical illusions, which are caused by the visual system and characterized by a visual percept that (loosely said) appears to differ from reality. Because entoptic images are caused by phenomena within the observers own eye, they share one feature with optical illusions and hallucinations: the observer cannot share a direct and specific view of the phenomenon with others. Helmholtz[1] commented on entoptic phenomena which could be seen easily by some observers, but could not be seen at all by others. This variance is not surprising because the specific aspects of the eye that produce these images are unique to each individual. Because of the variation between individuals, and the inability for two observers to share a nearly identical stimulus, these phenomena are unlike most visual sensations. They are also unlike most optical illusions which are produced by viewing a common stimulus. Yet, there is enough commonality between the main entoptic phenomena that their physical origin is now well understood.
Persistent fetal vasculature syndrome also known as persistent hyperplastic primary vitreous that can cause a traction retinal detachment difficult to differentiate but typically unilateral. Screening
Almost all infants with ROP have a gestational age of 31 weeks or less (regardless of birth weight) or a birth weight of 1250 g (2.76 lbs) or less; these indications are generally used to decide whether a baby should be screened for ROP, but some centres, especially in developing countries [3] extend birth weight screening criteria to 1500 g (3.3 lbs).Any premature baby with severe illness in perinatal period (Respiratory distress syndrome, sepsis, blood transfusion, Intra ventricular haemorrhage, apnoeic episodes, etc.) may also be offered ROP screening. Timing
Retinal examination with scleral depression is generally recommended for patients born before 30–32 weeks gestation, or 4–6 weeks of life, whichever is later. It is then repeated every 1–3 weeks until vascularization is complete (or until disease progression mandates treatment). Procedure
Following pupillary dilation using eye drops, the retina is examined using a special lighted instrument (an indirect ophthalmoscope). The peripheral portions of the retina are sometimes pushed into view using scleral depression. Examination of the retina of a premature infant is performed to determine how far the retinal blood vessels have grown (the zone), and whether or not the vessels are growing flat along the wall of the eye (the stage). Once the vessels have grown into zone III (see below) it is usually safe to discharge the child from further screening for ROP.