The lifetime prevalence of infrequent episodic tension-type headache (ETTH) among 40-year-olds from the general population is 53%: 50% among women and 55% among men (7). The high prevalence causes a positive family history simply by chance in >92% of the families, if the proband has four first-degree relatives (parents, siblings, and children). One or both parents are affected by chance in >77% of the families. Thus, a positive family history does not necessarily suggest the importance of genetic factors. Due to the high prevalence, a genetic epidemiologic survey is not likely to elucidate the importance of genetic and environmental factors. Infrequent ETTH is most likely a heterogeneous disorder that can be caused by different mechanisms, and for that reason pathophysiologic studies may be helpful.

The lifetime prevalence of frequent ETTH headache among 40-year-olds from the general population is 20%: 31% among women and 13% among men (7). Thus, the gender ratio is different from that of infrequent ETTH and similar to that of chronic tension-type headache (3). This indicates that the subdivision of frequent ETTH and CTTH is arbitrary and not necessarily precise. The frequency cut-off point is not based on scientific evidence, so it is important to take that into consideration in future studies. A population-based twin study analyzed infrequent and frequent ETTH and found that the phenotypic variation consisted of 81% nonshared environmental effects and of 19% additive genetic effects (6). The twin population was selected among twin pairs where at least one twin had self-reported migraine or severe headache.

A genetic epidemiologic survey included 122 probands from a headache clinic (2). The first-degree relatives and spouses ages 18 years or above were interviewed by a neurologic resident. Table 68-1 shows CTTH assessed by proband report compared to the clinical interview. The observed agreement rate was 82% and kappa, the agreement rate corrected for chance agreement, was 48% (5). This indicates that a direct interview of all the participants is necessary in family studies of CTTH.