Lazy eye (amblyopia) is a condition in which there is a problem in how the brain and the eye work together, and the brain is unable to properly register the sight from one eye resulting in poor vision in one eye. This causes the brain to rely more on the stronger eye, and the vision in the weaker eye gets worse.
Lazy eye usually starts in childhood and affects about 3% of children.
Causes of lazy eye include:
- Crossed eyes (strabismus)
- Refractive errors
- Nearsightedness (difficulty seeing things far away)
- Farsightedness (difficulty seeing things up close)
- Astigmatism (which can cause blurred vision)
- Cataracts (uncommon in children)
Risk factors for developing lazy eye include:
- Premature birth
- Smaller-than-average size at birth
- Family history of lazy eye, childhood cataracts, or other eye conditions
- Developmental disabilities
What Are Symptoms of Lazy Eyes?
Symptoms of lazy eye include:
- Double vision
- Problems with depth perception
- Appearing to strain to see clearly
- Squinting
- Shutting one eye
- Tilting the head
Parents or caregivers frequently don’t notice these problems, which can be subtle. Lazy eye is usually diagnosed through routine vision screening during a doctor's check-up or at school.
How Are Lazy Eyes Diagnosed?
Lazy eye is usually diagnosed with a routine vision screening. All children ages 3 to 5 should have their vision checked.
If the doctor suspects a child has a lazy eye, an ophthalmologist (an eye specialist) may be consulted for a more detailed eye exam.
What Is the Treatment for Lazy Eyes?
Early treatment is key in improving lazy eye and preventing long-term vision problems.
Treatment for lazy eye includes:
- Glasses or contact lenses (for children who are nearsighted, farsighted, or have astigmatism)
- Surgery
- Training the weaker eye by making it work harder so it can get stronger
- An eye patch worn over the stronger eye so that eye can't see
- Eye drops to put in the stronger eye to blur vision in that eye
Vision may start to improve within a few weeks but it typically takes months for the best results. Children may need to continue to use the treatments on occasion to prevent the lazy eye from returning.
From 
https://www.uptodate.com/contents/crossed-eyes-and-lazy-eye-the-basics?search=lazy%20eye&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1
https://www.ncbi.nlm.nih.gov/books/NBK430890/