There is no definitive information on cold weather and it’s effects on patients with myotonic dystrophy vary from person to person. Some individuals have reported problems, and here is some information collected from various resources. Its clear that for some individuals there is a mild moderate effect, for others its more severe. If possible plan long term to move to a more temperate climate without harsh winters.
In addition, individuals with DM may be more sensitive to cold and may get numbness in their hands and feet from being in the cold.
Here is a question from MDA a couple of Doctors answering. I’m not sure there is any clear information. This is from the MDA website: http://www.mdausa.org/experts/ask_dm.html
SUBJ (02/98): MD & WINTER WEATHER I have unclassified MD. I dread winter each year because I seem to react slower than usual. Does the cold weather have any affect on MD? If I move to a warmer climate, will warmer weather have an adverse affect on my MD? REPLY  from MDA: Stanley B. Holstein, M.D., MDA Clinic Director, New Rochelle, NY
Individuals without any type of neuromuscular disease can be adversely impacted by exposure to cold weather. However, for the following diseases prolonged exposure to coldness can worsen symptoms to those of near paralysis: paramyotonia congenita, hyperkalemic periodic paralysis and often myotonic muscular dystrophies such as myotonic dystrophy and myotonia congenita. There should be no adverse effect on muscular dystrophy, or any neuromuscular disease to my knowledge, from (non-extreme) warmth exposure. REPLY  from MDA: Carl J. Crosley, M.D., MDA Clinic Director, Syracuse, NY
Winter weather should not directly affect muscular dystrophies. However, the limitations imposed by muscle weakness of any origin are magnified during the conditions of winter weather including the cold. These problems will resolve once these conditions change as do the seasons. There are a few muscle disorders that have the phenomenon of myotonia (failure to easily relax muscles) in which the myotonic phenomenon is temporarily exaggerated by many conditions in the environment including cold. This too is both transient and fluctuating.
Some Posts from bulletins boards information from those with myotonic dystrophy:
My wife is VERY effected by the cold weather … She feels it most in her hands, legs and face … The most noticable is around her eyes and chin, which freeze up constantly in cold weather …
We live in SC. I doesn’t get real cold here very often. But, recently, we had temps of 22 and it snowed. Jeffrey, age 10 dawned his parka and socks on his hands for mittens. He stayed outside in the weather for about 5 mins. He came in and said he was cold. He does not complain of any cramping though.
We live in Wisconsin and this time of year can become very cold. I notice with my daughter that she is much slower moving than in the warmer seasons. She seem more stiff and has complained of cramps in legs and hands when out in the snow. At school for this time of year she has an option to stay in if it is to cold or go out to play if she feels like it. This works well for everyone.
We live in Nebraska and the cold weather does cause problems for me and my son. We get cramps more and fatigued easier.
My daughter had the congenital form of MyD, when she was about 10 she started getting cramps in her arms and shoulders during the winter time. We did move to a warmer climate and she complained of cramps a couple of times when we first moved here but then never did. We did find in the warmer climate she could get around faster and didn’t have to worry about the snow.
As a matter of fact cold weather highlited the first real noticable symptom of DM. I am now 47. When I was snow skiing about 12 years ago I noticed that I could hardly talk while traveling up the lift. All of the other wonderful symptoms are now with me but when it gets very cold outside my speech is most affected.
My girlfriend has MMD. When the temperature gets too cold she will get muscle and severe stomach cramps frequently causing vomiting which will immobilize her for several hours. Nothing can be done about this unfortunately except to keep warm,avoid chills, and limit cold weather exposure.
I have spent most of my life in Minnesota. In the USA we proudly claim the title of “The Nation’s Icebox.” Every winter is pretty miserable. I do not enjoy outdoor winter sports because being outdoors causes me to hurt so much. I spend most of the winter with that dull kind of muscle ache where you feel like you have the flu. My doctor put me on the anti-spasm medication, baclofen, a couple of years ago. That has made a tremendous difference. Also exercising in a warm pool and sitting in a hot tub help. I just have to make sure I bundle up before leaving the health club. I have a down filled coat that goes to my ankles. My rule of thumb is, I stay inside if it is under 30 degrees farenheit, except to go to the car. If it is 30 below, I call in sick.
I’m 50 with DM and the cold makes everything worse. Muscle cramps, stiffness, dull aches all over and lots more “mytonia”. I work out in a warm pool and do physical therapy weekly-it helps. I get the speech thing too, like my mouth is frozen.
From MDA Canada http://www.mdac.ca/clients/brochures/brochure_myotonic.html
Cold foods may cause some individuals to choke
The patient may notice difficulty releasing a firm grasp especially in the cold