Ankle Sprain occurs when the ligaments supporting the ankle are stretched beyond their limits leading to tearing. This can happen to people of all ages. Symptoms include pain, swelling, bruising, tenderness to touch, instability, stiffness, coldness or numbness in your foot, inability to walk or put weight on that foot.
There could be many causes for ankle sprains such as walking or exercising on an uneven surface, falling down, playing sports with rapid twisting foot movement, getting your foot stepped on by others, etc. Without proper treatment the sprain can weaken your ankle, making it likely to get sprained again. Repeated sprains can cause chronic pain, arthritis, instability, etc.
Doctors will first perform a physical examination and then they may ask you to get an X-Ray or MRI scan. Based on their observation they will assign a particular grade to the sprain.