Amblyopia, or lazy eye, is an acquired defect in vision caused by a deficient visual experience early in life.  It is usually found in one eye only but may be affecting both eyes. Amblyopia itself produces no change in the appearance of the eye, but it nearly always develops in association with some other condition, such as a high refractive error or eye turn, which is observed through an eye examination and which is responsible for an inadequate visual experience.

The word amblyopia (derived from Greek roots meaning “blunt sight”) is occasionally still applied to other disorders in which partial loss of vision is congenital or acquired through processes unrelated to visual experience.  The degree of amblyopia is determined by observing how well the patient can read a standardized eye chart with the best visual correction.  

The visual system is sensitive to the effects of abnormal visual experience only during a limited time in infancy and childhood when it is immature and still changing. For humans, this period extends roughly from birth through the end of the first ten years of life. Vulnerability is greatest during the first few months of life and decreases gradually thereafter, with apparently considerable variation from person to person in the degree of sensitivity at a particular age.

Abnormal early visual experience can affect monocular vision through either or both of two amblyopia causing mechanisms: In the first, lack of exposure to the sharply focused images necessary for normal development disturbs and limits the development of normal vision.

In the second, marked disparity in the quality or directionality of inputs from the two eyes prevents binocular fusion and results in abnormal competitive binocular interaction, which leads to active interference with, or exclusion of, one eye’s input to higher visual centers that persists during single eye viewing. This is caused by a persistent eye turn, also called strabismus. This disturbs development of normal vision in the brain’s visual center.   

Either mechanism can contribute to one eye or two eye amblyopia: The two eyes may be affected simultaneously by the first mechanism or sequentially by the second.

Refractive amblyopia may be corrected over time by wearing glasses or contact lenses with the correct prescription full time.  The earlier a patient wears the correct prescription, the better the outcome of achieving normal vision be.  In strabismic amblyopia with large eye turns, surgery may be needed to fully correct the eye and restore normal vision.  Patching the better eye will often encourage the patient to use the amblyopic eye and stimulate the visual system to develop correctly, resulting in improved vision.      

Children’s eyes, and many adult’s eyes, often turn in when reading because they are farsighted or hyperopic.

When eyes are hyperopic, focusing on near objects, also known as accommodation, causes the eyes to converge or turn in.  Hyperopes who don’t wear their glasses must focus their eyes, or accommodate, to a large degree in order to see clearly at near. 

These focusing attempts stimulate the accommodative reflex which in turn causes the eyes to converge.  When the convergence of the eyes is greater than the amount the eyes can focus, the eyes then will turn in even more making the eyes appear crossed.  This is called accommodative esotropia.  

This is easily corrected by wearing and eyeglass or contact lens prescription with plus lenses of suitable strength.  With the patient wearing the correct prescription, the eyes will not be turning in anymore.

Your vision may be blurry for different reasons.  Blurry vision may be caused by the need for glasses, or what is called refractive error, or may be caused by medical eye problems such as cataracts, diabetes, glaucoma and macular degeneration.   Medical eye problems require a correct diagnosis and appropriate medical management to prevent permanent vision loss or blindness. 

It is very important to see an eye care professional to figure out what the problem is.  The most common cause of constant blurry vision is refractive error.  This includes being nearsighted, farsighted, having astigmatism or being unable to read after the age of forty. 

The technical terms for these conditions are myopia, hyperopia, astigmatism and presbyopia.  Myopia, the most common is where the eye is too long relative to the focusing power off the cornea and lens leading to a blurry image on the retina.  

Hyperopia is the opposite problem.  The eye is too short for the focusing ability of the cornea and lens, creating a blurry image on the retina.  In hyperopia, however, the eye can be partially correct the problem by focusing the internal lens using the ciliary muscles.  This often leads to eye strain and blurry vision that becomes clear and blurry throughout the day. 

Astigmatism creates a distinct type of blur as a result of the cornea not being exactly spherical in shape.  This results in two meridians of focus that don’t come together creating a blurry image on the retina. 

Presbyopia is the universal decreased ability of the eye to focus at near after the age of 40.  All these refractive conditions can be corrected for the most part with glasses and contact lenses after your eyes are examined and a proper diagnosis is made.