The grueling 13-day cold snap that started around Christmas sent dozens of Minnesotans to the hospital with frostbite, and with temperatures hitting subzero again this week, doctors are seeing another spurt in cases.

Hennepin County Medical Center (HCMC) staff members treated three severe frostbite patients over Super Bowl weekend and another three since Monday.

Fortunately, they were ready. HCMC in Minneapolis and Regions Hospital in St. Paul have emerged as pioneers in frostbite treatment and earned national recognition for their work.

It helps that both hospitals operate sophisticated burn units: Severe heat and severe cold can cause similar damage to skin.

HCMC, where 38 patients were hospitalized during the last cold snap, had to open a new unit because the burn ward ran out of beds. At Regions, the burn unit had 20 hospitalizations and conducted dozens of consultations over the phone or through telemedicine.

Both hospitals pioneered the use of clot-busting drugs to help restore blood flow to frostbitten areas; as the blood warms up in a patient recovering from frostbite, there’s a risk that it will coagulate and form clots. The treatment also reduces the need for amputation. But the drugs need to be administered within hours of the warm-up to be most effective.

“The clock starts when they start to thaw out,” said Dr. Ryan Fey, director of the burn program at HCMC. “We want to restore the blood flow so there is at least a chance that those areas can heal.”

HCMC is also using its state-of-the-art hyperbaric chamber, which provides oxygen to the patient in a controlled-pressure environment, to see if it can restore damaged tissue, as some skin treatments for diabetics do already.

While local hospitals mostly see frostbite cases in the homeless or in people who become stranded outside, the influx of Super Bowl visitors, who might not have been accustomed to frigid temperatures, caused them to prepare for the worst.

“We kind of were hoping for better weather,” said Dr. Ryan Fey, director of the burn program at HCMC, which treats frostbite victims every winter.

But despite frigid temperatures and a horde of out-of-towners, HCMC reported just three new frostbite cases over Super Bowl weekend.

Several frostbite cases reported by local hospitals this year have been people who got locked out of their house or drove off the road and became stranded.

“A lot of people just got caught in a bad situation,” said Dr. William Mohr, a surgeon and burn specialist at Regions’ burn center.

We’re ‘fancy brine solutions’

Although 5 or 10 degrees above zero can seem plenty cold, the human body becomes especially susceptible to frostbite when the thermometer dips below zero degrees. On the Fahrenheit scale, zero represents the freezing point of brine, a solution comprised of equal parts salt and water.

Human bodies, said Fey, “are very fancy brine solutions.”

So at zero or below, the danger is that human skin and tissues will themselves freeze. The danger comes after 30 minutes of exposure, but the lower the temperature (including windchill temperatures), the faster frostbite will develop.

Nips of frostbite can develop on the face or the ears, but the cases most commonly seen in Minnesota are on the tips of the toes and fingers. Symptoms typically include numbness, hard or waxy-looking skin and blisters.

At its most serious, frostbite can lead to amputations and other complications. But even those who avoid losing a finger or a toe can suffer lifelong consequences. Survivors can go on to develop arthritis or sometimes be left with a cold sensitivity so bad that it becomes painful to touch a glass of ice water.

As the extremities begin to freeze, the body automatically takes steps to protect the vital organs.

“With frostbite in its most severe form, you make ice crystals in your tissues and things like hands and feet,” said Mohr. “Eventually the body will stop blood flow to those areas so that you don’t keep losing heat and making your internal temperature — your core temperature — drop down too low.”

Intense pain is a sign that frostbite is setting in.

“The pain gets worse as you get colder and colder,” said Mohr. “At some point your tissues will become so cold that the nerves will no longer conduct signals, and all of a sudden you go numb.”

Alcohol and drug use can mask the pain and numbness, keeping people outside longer than they should be.

“It blunts their senses, the warning signs aren’t appreciated as readily,” said Fey.

Physicians say the best way to treat frostbite on your own is to slowly rewarm the affected area in warm water, about 100 degrees, slightly above body temperature, for about 30 to 45 minutes. If numbness or pain continues, or if the skin has a waxy appearance or is discolored, seek medical attention.

“This is a very preventable injury,” said Fey. “Limit your time outside, dress appropriately and don’t drink too much alcohol.”