Aspartame causes problems in the eyes and ears, including possible blindness in one or both eyes, decreased vision (blurring, bright flashes, tunnel vision), pain in the eyes, tinnitus (ringing or buzzing in the ears), and reduced tolerance of loud noises. In general, your eyes and ears will become more sensitive.
There have been a number of reports about drinkers of diet soda having neurological issues and episodes because of aspartame. Seizures, migraines, dizziness, confusion, and hyperactivity are just a few of the common reported side effects associated with the ingestion of this ingredient of diet soda.
Why is Diet Soda Bad for You? 
Aside from the side effects and long term risks of artificial sweeteners, there is a more important reason why diet drinks are bad for you. Oftentimes, soda and diet soda will be the main choice at every meal for those who drink them. Sodium and lack of hydrating effects can make you feel unsatisfied and bloated when you are finished with your meal.
Choosing water is a much better route if you are trying to make the right choices about what you are putting into your body.
By choosing diet soda over water, you will age quicker and notice more blemishes in your skin. Water helps maintain a young, healthy skin tone by keeping it hydrated and naturally plumping it up. Another benefit of choosing water over soda is that it makes your immune system work better.