Frostbite

Frostbite is caused by exposure of exposed areas of skin and other tissues that are separated by distance from the heart to cold or high wind. In order to preserve core body temperature, the body reacts by restricting blood flow to the extremities such as the skin, fingers and toes. Over short periods of time, this results in "frostnip", a temporary condition which causes numbness, itching and pain, but no permanent damage. However, as exposure increases, exterior tissues may harden, although interior tissues remain normal. This can cause damage to the skin that resembles that of a burn, such as peeling and ulceration. However in severe cases, the inner tissues lose blood flow resulting in damage to the muscles, nerves, tendons and blood vessels. This may require amputation.
Mild cases respond well to natural rewarming - returning the patient to a warm environment and letting the temperature of the affected tissues return to normal. Blankets are usually helpful. However, frostbite often accompanies hypothermia and in such cases intensive care is required no matter what the course of treatment. Care must be taken with rapid rewarming as blood may flow quickly out of the core to the peripheries and actually result in core body temperature dropping even as the extremities warm up.
In Perils of Paranoia, Ethan comes to the clinic thinking he has frostbite when in fact his fingertips have been discolored by silver nitrate.
Frostbite at Wikipedia
Frostbite at Mayo Clinic