The creatine kinase (CK) is mildly elevated.
The ECG may show bradycardia with prolonged PR interval.
The EMG shows classic features of myotonia, with waxing and waning in the amplitude and frequency of motor unit potentials, as well as myopathic changes. Muscle biopsy may be normal or show non-specific type II fibre hypertrophy. Other investigations:
- Slit lamp examination of the eyes is essential to exclude cataract.
- Plain radiology may show certain bony abnormalities, e.g. hyperostosis frontalis interna, small pituitary fossa.
- Elevated blood insulin levels may be found as may a diabetic glucose tolerance curve. Myotonic dystrophy must be distinguished from other disorders in which myotonia occurs, (see later)
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