Preschool vision screening.

Glossary and list of abbreviationsTechnical terms and abbreviations are used throughout this report.The meaning is usually clear from the context but a glossary is provided for the non-specialist reader.In some cases usage differs in the literature but the term has a constant meaning throughout this review. Terms relating to visionAmblyopia Reduced visual acuity in the absence of organic disease, which cannot be improved by spectacles.It is usually uniocular.Amblyopia is held to be reversible up to the age of about 8 years.Children presenting with amblyopia will be treated with occlusion or other therapies in order to reverse the visual loss.It is thought to be caused by hypermetropia and/or anisometropia as well as by the various types of squint.Some practitioners treat these refractive errors in preschool children to prevent the development of amblyopia; others follow-up these children and intervene as soon as the amblyopia appears.Anisometropia A difference in refractive error between the two eyes. Binocular single visionThe simultaneous use of both eyes so that each eye contributes to a common singular perception.There are grades of binocular single vision.In the highest form the object is fixated at the centre of the retina in both eyes and fusion of the two images allows depth perception (stereopsis).Cover-uncover test A test used to detect squint, in which each eye is covered in turn while the child fixes on a specified target, and the tester observes the movements of the eyes. Cycloplegic drugsThese drugs block the action of the ciliary muscle, preventing accommodation.In addition, pupillary dilation occurs. Dioptre (D)Unit of measurement of the power of a lens.Diplopia Double vision or seeing two images of one object simultaneously. Hypermetropia Refractive error where the principal focus is behind the eye (so-called 'long sight').Intermittent squint There is a manifest squint at some times or distances but the visual axes are aligned at others.Children with intermittent squints may respond to spectacle correction alone if they are also hypermetropic.They may be followed-up and undergo surgery if the squint becomes less well controlled, in order to prevent the loss of binocular vision and the development of a cosmetically obvious squint. Latent squint (heterophoria)With both eyes open the visual axes are aligned.When one eye is covered, the eye under cover deviates; when the cover is removed, it comes back into alignment.A small heterophoria is present in the majority of people without ocular symptoms.Small latent divergent squints are regarded as common in children aged 3-4.5 years and are not thought to be associated with any adverse effects.No intervention is recommended.Small latent convergent squints are often accompanied by hypermetropia, for which spectacle correction is prescribed with the aim of preventing further deterioration of the squint. LogMAR scale Scale used to measure visual acuity (see below). Manifest squint (heterotropia)With both eyes open the visual axis of one eye is deviated from the point of fixation.It may be constant or intermittent.