Editor—We report the genetic counselling approaches used in a series of 72 presymptomatic genetic tests for myotonic dystrophy undertaken in our centre over an eight year period. The study has identified factors which influenced the counsellor's approach, and which can provide a basis for further, more systematic research.

Genetic counselling in myotonic dystrophy has always been difficult and complex, owing to the extreme variability of the disorder, in both severity and age at onset, with anticipation between generations and influence of the sex of the affected parent.

The identification of a CTG repeat expansion within the 3′ untranslated region of the myotonic dystrophy protein kinase gene on chromosome 191 as the primary molecular defect has transformed our understanding of the genetic aspects of this disorder and provides the basis for an accurate and specific diagnostic and presymptomatic test. The broad correlation of the size of the CTG expansion with age at onset and severity of the phenotype allows a limited degree of prognosis to be given to those found to have the mutation, particularly for very large or minimal gene expansions. Subjects carrying a minimal expansion (less than 100 repeats) usually show few or no muscle symptoms,2 but may develop cataract in later life; they contribute to a pool of mutation carriers who may transmit clinically significant disease to their offspring as a result of anticipation.

Presymptomatic genetic testing for late onset dominantly inherited disorders first became possible for Huntington's disease (HD),3 for which extensive experience has resulted in widely accepted guidelines for genetic counselling protocols; these comprise two pre-test sessions for information and preparation combined with post-test support.4 With appropriate adaptation, this has become a model for other late onset genetic disorders of the nervous system and to some extent also for the …