There is no definitive information on cold weather and it's effects on patients with myotonic dystrophy. Some individuals have reported problems, and here is some information collected from various resources.

From Vtmed.net

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.

From MDA

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 [1] 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 [2] 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.

From MDA Canada  http://www.mdac.ca/clients/brochures/brochure_myotonic.html

Cold foods may cause some individuals to choke

From Yale  http://info.med.yale.edu/neurol/CNeurophysiol/myotonic.htm

The patient may notice difficulty releasing a firm grasp especially in the cold