Abstract
Dear Sir We appreciate the observations of Dr Bigal and we agree with him on the importance of family aggregation of chronic daily headache (CDH). However, our study has methodological limitations, and the findings from our survey should be confirmed. First, family aggregation and the strength of family aggregation of CDH should be confirmed; future studies should determine the familial recurrence risk ratio (λR) of CDH, a measure that estimates the grade of family aggregation (1). Second, it should be determined if family aggregation is caused by genetic or environmental factors. Generally, it is accepted that a greater recurrence risk ratio is related with a stronger genetic role in the disease aetiology, and the reduction in recurrence risk by degree of familiar relation is related to the mode of inheritance (1). The best way to demonstrate the genetic component of CDH is the study of twins, with a model of multiple threshold where you can consider the liability of episodic headache and CDH simultaneously (2). Once the family aggregation of CDH is proven and that this family aggregation is due to genetic factors, you would advance in the search of genes committed to the process of chronification of episodic headache.
These proposals have important conceptual implications, especially in the progression or chronification paradigm of episodic headache (migraine and episodic tension-type headache), since it would imply that only individuals with a special genetic background would be at risk of chronification and that individuals without this genetic susceptibility would require stronger environmental stimuli to cause chronification of episodic headache.