Myotonia congenita
A 5 year old boy, the child of non-consanguineous parents with
no family history of neurometabolic or neurodegenerative disorders
presented with a one year history of easy fatigue and cramps since,
most prominent after sleep or rest. Pregnancy, labor and his neonatal
period, infancy and early childhood were uneventful. He walked
independently at 12 months and spoke the first meaningful word
at 13 months. He started the kindergarten the previous year and
his teacher reported him to be a clever and social child. Regarding
the neurological examination, muscle mass, strenght and tendon
reflexes were normal. However, his appearance was odd for a child
of his age, resembling a “body-builder” with a well-shaped body
(figures 1, 2). The boy himself seemed annoyed by his symptomatology;
he reported that he participated in a mini-football league and
because of his awareness of his symptoms he used to run a few
rounds of the field just before the game was due to start.
 Figure
1 
Figure 2
ANSWER
Myotonia congenita is a genetic disorder characterized
by muscle stiffness and hypertrophy. Inheritance can be autosomal
recessive or autosomal dominant, but most cases are sporadic.
The clinical picture is typical: the muscles are stiff after
rest and difficult to move, but after some activity, the stiffness
subsides and there is a normal range of movement. The neurological
examination is entirely normal, except for percussion myotonia
which can be demonstrated in various muscles, most prominent
at the thenar eminence (figures
3, 4)
or the tongue (fig.
5). Some patients have difficulty in opening their hands
after making a strong fist, while others cannot easily open
their eyes after forceful lid closure. Fenichel, in his classical
textbook, describes a patient who played Little League Baseball
and who could not sit while waiting to bat for fear that he
would be unable to get up. The Department had a family with
3 affected adolescent brothers who all worked in a bakery
and who changed shift every half minute while placing the
breads in and out of the oven, in order no to burn any of
them. Diagnosis of myotonia congenita is established by EMG
(fig.
6). The amplitude and frequency of spontaneous muscle
potentials wax and wane, producing a characteristic diagram
and a characteristic sound (like “bomb-diving”). The creatine
phosphokinase levels are normal and there is no evidence of
dystrophy in the muscle biopsy. In some patients the myotonia
can be relieved (temporarily or permanently) by phenytoin,
carbamazepin or 3,4-diaminopyridin, which act by stabilizing
the muscle membrane potential. In this patient, phenytoin
at standard anticonvulsant doses had a remarkable positive
effect on his symptoms. |
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