Also known as Amblyopia. Reduced vision in one or both eyes where the eye and visual system appear to be anatomically normal. Lazy eye cannot usually be corrected with glasses or contact lenses once the patient has reached 5 to 8 years old, but can be prevented by ensuring children receive eye examinations regularly from an early age. It is usually caused by a squint (turn in the eye), uncorrected refractive error (especially hypermetropia) or a large difference in prescription between the two eyes.

Wearing any necessary glasses helps in two ways, firstly it may correct any squint (turn in the eye) that is present and stop the child seeing double (though sometimes surgery to shorten a muscle that controls the movement of the eye is necessary), and secondly it will help the child see as clearly as possible. It is only when a clear image is present on the retina at the back of the eye, and the two eyes are aligned so that there is no longer a double image, that the nerve connections to the brain that actually enable us to see can fully develop. Otherwise if there is a double image the brain will suppress (ignore) the most blurred image and that eye will fail to develop normally – i.e. become lazy or amblyopic.

It is therefore important that children under eight wear their glasses exactly as advised by their optician or the hospital eye department. Sometimes the good eye is patched to force the lazy eye to work, and the child may be prescribed exercises. Again it is essential that the advice is adhered to.

Because lazy eye cannot be helped after the critical period it presents serious issues in later life. Firstly the patient does not have a "spare" eye should they be involved in an accident or lose the sight of their good eye to diabetes or some other condition. Secondly people with a lazy eye do not have good binocular vision and have difficulty judging distance and speed. This can make activities such as driving and catching a ball difficult and they are restricted in the type of work they are allowed to do where clear corrected vision is a requirement of the job. For example an amblyopic person is unlikely to pass the medical for entrance into any of the armed forces, the police or fire service. They would also be unable to pass the visual requirements to drive public transport or heavy goods vehicles.

For the sake of an eye test before starting nursery school a great many people have had their options in adult life severely limited. Additionally some parents think that their pride and joy looks better without glasses, or think it's Ok not to make children wear them, perhaps because they are picked on or teased. However it is a small price to pay to ensure their eyes develop normally.