Patients with myotonic dystrophy develop weakness of muscles, in a characteristic pattern.
The muscles that are affected first, are those of the forearms, hands, lower legs and feet. Over time, these muscles get smaller, so the lower legs and arms may appear thinner than the upper legs and arms. The grip becomes weaker, leading to trouble using the wrist or hand muscles. The muscles that pick up the foot when walking may weaken, allowing the foot to flop down and cause tripping and falling (foot drop).

The muscles of the neck, jaw and parts of the head and face may weaken, leading to a characteristic appearance. The eyelids may droop (ptosis), and the chewing muscles often are affected, which makes the temples appear hollow. Speech and swallowing may be affected.

Myotonia is another characteristic aspect of myotonic dystrophy. Myotonia is the inability to voluntarily relax muscles after contraction. Grip myotonia (not being able to release one’s grip after for example making a firm fist or shaking hands) frequently is the first noticeable symptom, and may precede other complaints for years. Myotonia can also occur in tongue or chewing muscles. Myotonia may worsen by cold temperature. Repeated contractions (warming up) may temporarily improve myotonia.