Diagnosis In patients who complain of spasms, cramping, and stiffness, myotonia must be distinguished from several of the disorders of persistent muscle activity described further on and in Chap. 45. None of these disorders displays percussion myotonia or the typical electromyogram (EMG) abnormality of myotonic discharge. The only possible exceptions are the Schwartz-Jampel syndrome of hereditary stiffness combined with short stature and muscle hypertrophy, and stiff man syndrome which are discussed further on and in Chap. 45.
Uncertainty in diagnosis arises in patients who have only myotonia in early life and later prove to have classic (type 1) myotonic dystrophy or who notice myotonia in adulthood with mild proximal weakness and are found to have type 2 myotonic dystrophy (see later). The myotonia in myotonic dystrophy is usually mild and in several families that we have followed, some degree of weakness and the typical facies of myotonic dystrophy could be appreciated even in early childhood. This is not the case in the less-frequent type 2 myotonic dystrophy in which there are no dysmorphic features (also called proximal myotonic myopathy [PROMM]; see Chap. 45 and further on). In paramyotonia congenita there is also myotonia of early onset, but, again, it tends to be mild, involving mainly the orbicularis oculi, levator palpebrae, and tongue; the diagnosis of paramyotonia is seldom in doubt because of the worsening with continued activity and prominent cold-induced episodes of myotonia and paralysis.
In patients with very large muscles, one must consider not only myotonia congenita but also familial hyperdevelopment, hypothyroid myopathy, the Bruck-de Lange syndrome (congenital hypertrophy of muscles, mental retardation, and extrapyramidal movement disorder), Becker myotonia (see later), Duchenne dystrophy, and most of all, hypertrophic myopathy (hypertrophia musculorum vera); this last disease is of interest because the aberrant protein (myostatin) and gene defect have been characterized. Muscle hypertrophy is not, of course, a feature of myotonic dystrophy. The demonstration of myotonia by percussion and EMG study usually resolves the problem, although in exceptional cases of Thomsen disease, the persistence of contraction may be difficult to demonstrate. In hypothyroidism, the EMG may show bizarre high-frequency (pseudomyotonic) discharges; however, true myotonia does not occur, myoedema is prominent, and, along with other signs of thyroid deficiency, there is slowing of contraction and relaxation of tendon reflexes not seen in myotonia congenita.
Treatment Quinine is effective in reducing myotonia but is now used infrequently because of the (low) risk of causing torsade de pointes. Procainamide, 250 to 500 mg qid, and mexiletine, 100 to 300 mg tid, are beneficial in relieving the myotonia but, they too, have fallen out of favor for various reasons. Phenytoin, 100 mg tid, is useful in some cases. The cardiac antiarrhythmic drug tocainide (1,200 mg daily) has also proved effective, but it sometimes causes agranulocytosis and is no longer recommended. A trial has shown that mexilintie may be useful in a group of rare non-dystrophic myotonias (Statland et al).