Restricting supplemental oxygen use reduces the rate of ROP, but may raise the risk of other hypoxia-related systemic complications, including death. Supplemental oxygen exposure, while a risk factor, is not the main risk factor for development of this disease. Multiple factors can determine whether the disease progresses, including overall health, birth weight, the stage of ROP at initial diagnosis, and the presence or absence of "plus disease". Associated with the growth of these new vessels is fibrous tissue (scar tissue) that may contract to cause retinal detachment. This is growth of abnormal new vessels this may regress, but frequently progresses. The key disease element in ROP is fibrovascular proliferation. When the excess oxygen environment is removed, the blood vessels rapidly begin forming again and grow into the vitreous humor of the eye from the retina. If excess oxygen is given, normal blood vessels degrade and cease to develop. The normal growth of the blood vessels is directed to relatively low-oxygen areas of the retina, but the vessels remain in the plane of the retina and do not grow into the vitreous humor. Normally, maturation of the retina proceeds in utero, and at term, the medial portion (Nasal retina) of the retina is fully vascularized, while the lateral portion (Temporal retina) is only incompletely vascularized. When the blood and abnormal vessels are reabsorbed, it may give rise to multiple band like membranes which can pull up the retina, causing detachment of the retina and eventually blindness before 6 months. These abnormal blood vessels may grow up from the plane of the retina and may bleed inside the eye. If the vessels grow and branch abnormally the baby develops ROP. If blood vessels grow normally, ROP does not occur. However, in premature babies it is incomplete. This process is completed a few weeks before the normal time of delivery. They are:ĭuring development, blood vessels grow from the central part of the retina outwards. Various risk factors contribute to the development of ROP. In rare cases ROP has been found in some patients with a mutation in the NDP gene, which is normally associated with the more damaging Norrie disease. Such formation of blood vessels appears to be very sensitive to the amount of oxygen supplied, either naturally or artificially. By the fourth month of pregnancy, the fetal retina has begun to develop vascularization. Gaze stabilization and optokinetic reflexes are governed by a group of small. Visual orientation and eye movements are served by retinal input to the superior colliculus.
Circadian rhythms are timed by a retinal projection to the suprachiasmatic nucleus. The pretectal nuclei send their output to the Edinger-Westphal nuclei, which in turn provide parasympathetic innervation to the iris sphincter via an interneuron in the ciliary ganglion. Pupil responses are mediated by input to the pretectal olivary nuclei in the midbrain.
Ganglion cells that mediate pupillary constriction and circadian rhythms are light sensitive owing to a novel visual pigment, melanopsin. Although the lateral geniculate body is the main target of the retina, separate classes of ganglion cells project to other subcortical visual nuclei involved in different functions. This afferent retinogeniculocortical sensory pathway provides the neural substrate for visual perception. Cells in the lateral geniculate body project in turn to the primary visual cortex. The majority of fibers synapse on cells in the lateral geniculate body, a thalamic relay station. Ganglion cell axons sweep along the inner surface of the retina in the nerve fiber layer, exit the eye at the optic disc, and travel through the optic nerve, optic chiasm, and optic tract to reach targets in the brain.
In the middle of the macula a small pit termed the fovea, packed exclusively with cones, provides the best visual acuity. The majority of cones are within the macula, the portion of the retina that serves the central 10° of vision. The cone system is specialized for color perception and high spatial resolution. The cones function under daylight (photopic) conditions. The rods operate in dim (scotopic) illumination. In the human retina there are 100 million rods and 5 million cones. Light is absorbed by pigment in two types of photoreceptors: rods and cones. The retina is actually part of the brain, banished to the periphery to serve as a transducer for the conversion of patterns of light energy into neuronal signals. The act of seeing begins with the capture of images focused by the cornea and lens on a light-sensitive membrane in the back of the eye called the retina. The visual system provides a supremely efficient means for the rapid assimilation of information from the environment to aid in the guidance of behavior.