Survey 

An Amsler grid is a useful tool for monitoring your central visual field. It is an important way to detect early and sometimes subtle visual changes in a variety of macular diseases such as ARMD (age-related macular degeneration) and diabetic macular oedema. It is also helpful in monitoring changes in vision once they have been detected. With the Amsler grid, each eye is tested separately by you. This helps you to recognize visual symptoms which are in one eye only
 

• Test your vision with adequate lighting.
• Wear your reading glasses or look through the reading portion of your bifocals (if you normally read with spectacles).
• Hold the Amsler grid at normal reading distance (about  35cm).
• Cover one eye at a time with the palm of your hand.
• Stare at the center dot of the chart at all times.
• Do not let your eye drift from the center dot.

If the answer to any of these questions is "yes" (and this has not been noticed by you before), you should contact your optometrist or doctor as soon as possible for an examination. 

Sometimes these changes may mean that there is leakage or bleeding in the back of the eye causing swelling of the retina. 

These images are copied from the Wikipedia website