Jenny, it does get to be a vicious cycle because the anxiety about getting stiffer makes the myotonia worse.
The listing of lidocaine as a contraindicated drug only applies to the local anesthetic which contains epinephrine (trade name Xylocaine). The oral form should not be a problem since it would not contain that.
Most of the people who report side effects with Mexitil tell me it's either GI problems (you have to take it with food or an antacid and most doctors don't mention that), or heart arrhythmias (rare). You would get an EKG before starting and work up the dose slowly. It is really life-changing for many people.
You can also try the amino acid L-Taurine (1-5 grams per day) or licorice root extract if you don't have high blood pressure (500-1000 mg per day). The Taurine has an effect very similar to anti-seizure medications which are often given to treat MC. Licorice root extract binds the aldosterone receptor which causes sodium retention, an increase in blood volume, and urine potassium loss. The effect is similar to taking something like Prednisone but without the more serious side effects of diabetes, cataracts, osteoporosis, etc. Corticosteroids do really improve myotonia, but they just aren't practical for long-term use.
Let me know what you think if you try the Mexitil -