The vast majority of stable ankle injuries in the outdoors result from rolling the ankle laterally (to the outside) while hiking or running on an uneven surface. If the patient has no obvious deformity, tenderness slightly in front of and below the lateral malleolus (ankle bone), and can bear weight shortly after the event they probably have an uncomplicated lateral ankle sprain (sprain = ligament damage).
In uncomplicated ankle sprains, swelling can be prevented with compression around both malleoli (ankle bones), elevation of the injured ankle higher than the heart, cold water baths, and pain-free ROM and light strengthening exercises 2-3 times a day. Ideally, the patient should avoid extended walking on the injured ankle for 2-5 days. If the patient must walk during this time, you will need to support their ankle and significantly reduce their pack weight. Whether a patient requires an evacuation for an ankle sprain depends of the severity of the injury, the type of activity (backpacking, cycling, canoeing, etc.), and the difficulty of the terrain they can be expected to traverse.
The photos on the left show how to use foam donuts to deliver focal compression to the soft tissue surrounding the malleoli in order to prevent excessive swelling and promote rapid recovery in uncomplicated ankle medial or lateral ankle sprains. The commercial donuts are super lightweight and come coated with a waterproof skin adhesive; simply peel off the backing and attach.