Despite their muscular appearance, these patients are inept in athletic pursuits as a result of the myotonia. When severe, myotonia affects all skeletal muscles but is especially prominent in the lower limbs. Attempts to walk and run are impeded to the extent that the patient stumbles and falls. Other limb and trunk muscles are also thrown into spasm, as are those of the face and upper limbs. One of the characteristic features is grip myotonia in which the patient is unable to release a handshake and must slowly open the fingers one at a time. Occasionally a sudden noise or fright may cause generalized stiffness and falling. Small, gentle movements such as blinking or elicitation of a tendon reflex do not initiate myotonia, whereas strong closure of the eyelids, as in a sneeze, sets up a spasm that may prevent complete opening of the eyes for many seconds. Spasms of extraocular muscles occur in some instances, leading to strabismus. If the patient has not spoken for a time, there is sometimes a striking dysarthria. Arising at night, the patient cannot walk without first moving the legs for a few minutes. After a period of rest, the patient may have difficulty in arising from a chair or climbing stairs. Loosening of one set of muscles after a succession of contractions does not prevent the appearance of myotonia in another area, nor in the same ones if used in another pattern of movement. Smooth and cardiac muscles are not affected and intelligence is normal. Lacking also are the narrow face, frontal balding, cataracts, and endocrine changes typical of myotonic dystrophy that is discussed in Chap. 45. Myotonia that is evident in infancy is far more likely to represent myotonia congenita than myotonic dystrophy, in which myotonia rarely has its onset in the first few years of life.
Myotonia can also be induced in most cases by tapping a muscle belly with a percussion hammer (percussion myotonia). Unlike the lump or ridge produced in hypothyroid or cachectic muscle (myoedema), the myotonic contraction involves an entire fasciculus or an entire muscle and, unlike the phenomenon of idiomuscular irritability (contraction of a fascicle in response to striking the muscle), it persists for several seconds. If tapped, the tongue shows a similar reaction. An electrical stimulus delivered to the motor point in a muscle induces a prolonged contraction (Erb myotonic reaction). In Thomsen disease, as in virtually all forms of myotonia, the stiffness is somewhat exaggerated in cold. On a cold day, affected individuals may have a prolonged grimace with closed eyes after a sneeze. We encountered two brothers with this disorder who described diving into a cool swimming pool on a hot summer day and having to lie nearly motionless at the bottom of the pool for several seconds until the muscle stiffness abated enough to allow them to swim to the top. However, as mentioned, prominent cold-induced myotonia, is more characteristic of paramyotonia congenita (see later).
Biopsy reveals no abnormality other than enlargement of muscle fibers, and this change occurs only in hypertrophied muscles. As often happens in fibers of increased volume, central nucleation is somewhat more frequent than it is in normal muscle. The large fibers contain increased numbers of normally structured myofibrils. In well-fixed biopsy material examined under the electron microscope, Schröder and Adams discerned no significant morphologic changes.
Myotonia levior was the name applied by DeJong to a dominantly inherited form of myotonia congenita in which the symptoms are milder and of later onset than those of Thomsen disease. In 2 patients of a myotonia levior family, Lehmann-Horn and coworkers (1995) identified a mutation of the same chloride ion channel (CLCN1) that is implicated in Thomsen’s disease. Thus it appears that myotonia levior is simply a mild form of Thomsen disease.