The typical adult visual system provides the brain with sensory information required for interaction with the environment. The optics of the eye produce an image that is sampled by the photoreceptor array to create potentials that are transmitted in an orderly fashion to the visual cortex and other structures. Patients with clinically significant hyperopia may have disruptions in a number of stages of processing, both optical and neural. Beyond their refractive error they may have amblyopia, strabismus, poor accommodative function, or poor visual perceptual skills.
Currently we devote significant resources to treating these disruptions with optical corrections and also significant resources to the treatment for amblyopia and strabismus. There is considerable interest and pressure, however, to undertake eye examinations and screening programs at younger and younger ages before these disruptions typically manifest. The hope is that they can be both predicted and prevented with appropriate intervention. This hope raises a number of questions that are yet to be answered. The most significant is how to manage levels of hyperopia that are associated with abnormality at later ages. For example, what would the optimal strategy be for a 9-month-old with 4 D of hyperopia? This infant may develop a strabismus and amblyopia if he or she still has the hyperopia months or years later. Should glasses be prescribed? If so, at what age? What proportion of the hyperopia should be corrected? Would glasses disrupt emmetropization?
A systematic analysis of these questions is complicated by the fact that the prognosis for an individual hyperopic child is currently unclear. There is no level of hyperopia at which every child will develop strabismus or amblyopia, and we have limited understanding of the impact of hyperopia on more subtle aspects of visual function such as those represented by tests of visual perceptual skills.
The benefit of optical correction could be tested relatively easily if glasses had no negative impact on visual development. Data from animal models, however, clearly show that the growth of the eye can be modified by the presence (or, by implication, absence) of defocus. It has been shown in a number of species that eyes will adapt their growth to compensate for the presence of anisometropic refractive corrections. Thus refractive development could actually be modified by spectacle wear—emmetropization could be prevented by full correction of hyperopia for instance. At this time the evidence related to this question in human infants is still mixed and so we must proceed with caution.
For prediction of abnormal development and preventative care to be effective, we need to understand the progression and consequences of hyperopia as a function of age during postnatal development. Only then can evidence-based guidelines for refractive correction be provided to clinicians for systematic management of these patients. The goal of this special issue is to present the latest approaches to and analyses of these questions, with a view to also stimulating further discussion and insight. The manuscripts include studies of both typical and atypical populations and a series of perspectives that discuss the literature alluded to above.
Donald O. Mutti
Ohio State University
Susan A. Cotter
Southern California College of Optometry
University of California