[Glucocorticoid hypofunction in myotonic dystrophy]

Myotonic dystrophy (DM1) is an autosomal dominant disorder whose genetic defect consists of the amplification of an unstable CTG trinucleotide repeat in the 3' untranslated region of the dystrophia myotonica protein kinase gene (DMPK). This is a multi-systemic disease with a well-known endocrinological repercussion. With respect to the adrenal function variable results have been described, although lately they are interpreted as indicators of a hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis.

Twenty-five patients (13 men and 12 women) with DM1 were recruited. They were analysed for: basal cortisol and ACTH, stimulus test with 0.25 mg of ACTH for cortisol and CRH test for cortisol and ACTH. Similarly, the degree of expansion of CTG was evaluated by Southern blot and PCR. Twenty-five healthy individuals, comparable by age and sex, were studied as a control group; the CRH test was carried out on 11 of them.

One patient was diagnosed with primary non-autoimmune adrenal failure. In the rest of the cases there were no differences between the basal ACTH of patients and controls, and the cortisol response to ACTH was normal. The patients showed a lower level of basal cortisol (p<0.01) and also showed, following stimulation with CRH, a lower cortisol response (p<0.05) with higher average values of ACTH.

Our data differs from the latest publications and point to an adrenal hypofunction due to lack of efficacy of the ACTH on its receptor or at the post-receptor level. We suggest that the etiology might be related to the underlying defect in the gene that codifies DMPK.