In population-based surveys using the Silberstein and Lipton criteria, primary CDH occurred in 4.1% of Americans,
8 4.35% of Greeks, 3.9% of elderly Chinese,
10 and 4.7% of Spaniards.
9 Scher et al
8 ascertained the prevalence of primary CDH in 13,343 individuals aged 18 to 65 years in Baltimore County, Maryland. The overall prevalence of primary CDH was 4.1% (5.0% women, 2.8% men; 1.8:1 ratio of women to men). In both men and women, prevalence was highest in the lowest educational category. More than half (52% women, 56% men) met criteria for CTTH (2.2%), almost one third (33% women, 25% men) met criteria for transformed migraine (1.3%), and the remainder (15% women, 19% men) were unclassified (0.6%). Overall, 30% of women and 25% of men who were frequent headache sufferers met IHS criteria for migraine (with or without aura). On the basis of chance, migraine and CTTH would co-occur in 0.22% of the population; the fact that transformed migraine occurred in 1.3% of this population would suggest that the co-occurrence of migraine and CTTH is more than random.