This extension of the BASID project utilizing the epidemiology “gold standard” of door-to-door surveillance suggests that cases are truly missed using ascertainment strategies that focus only on hospitals in Mexico. However, despite the labor intensiveness of a door-to-door approach to 2437 subjects, stroke is rare enough to leave wide confidence intervals on estimates of incidence and prevalence. For more precise estimates, a much larger sample size with prohibitive expense and time would be needed. For example, using random sampling, in the case of Durango Municipality, which has a target population of 168,859 and a point estimate of stroke incidence of 232/100,000, a sample size of 20,000 would be needed to obtain a margin of error of ± 24/100,000, or 10%. However, there are additional, more sophisticated sampling techniques that could have been used to achieve smaller margins of errors without necessarily increasing costs.
One sampling technique that could increase the precision is to initially perform a screening phase to identify the location of residents at high-risk for stroke, for example by focusing on the elderly. Once high risk areas are identified, they would be over-sampled in comparison to low-risk areas (e.g. neighborhoods with younger residents), a framework known as disproportionate sampling.12
A higher precision is gained by focusing efforts on the subpopulations where more cases are likely to be found. In the BASID door-to-door surveillance, only 6% of our sample was ≥75 years and only 16% were ≥65.
Other screening methods can be employed to capture more strokes. The most cost-effective is telephone interviews. However, they may bias towards patients with higher socio-economic status (who are likely already captured in hospital based-surveillance) and towards urban residents (in Durango Municipality, ~70% of the rural population have phones compared to 94% of those in urban homes). Mailing surveys is another approach, but could lead to a lower response rate and underestimation of cases particularly in populations with limited mail services (e.g. rural areas in developing countries). Another possibility would be to simply include neighboring houses if the target house was unoccupied at the time of the visit.
Another sampling technique is network sampling, which works under the assumption that members of the stroke population know each other. In this framework, surveyors ask an identified member of the affected population (i.e. a stroke survivor) to identify other affected members. For example, stroke sufferers may know other stroke victims through stroke support groups or by knowledge of relatives and friends. This reduces the number of contacts necessary by obtaining information on multiple subjects from one case. Alternatively, the cases identified by the first case may be contacted, and again asked to identify more cases. When this process is iterated, that is, each case found is asked to identify more cases until no more cases are identified, the technique is called snowballing. Though it is more difficult to identify isolated cases using network sampling and snowballing,13
statistical approaches can be used to obtain an estimate of the missed cases by evaluating the overlap cases (those cases identified multiple times).12
Multiple sampling techniques can be used in combination to compensate for their individual limitations, though combining methods may require complex computational analysis to obtain correct standard errors.
The proportion of stroke patients admitted to hospitals reported in recent population-based studies in developing countries is variable, ranging from 66% in Georgia14
to as high as 95% in Brazil.15
The cumulative stroke incidence of 232.3/100,000 found in the current study is similar to that reported in recent population-based studies in developing countries, with the notable exception of Iran16
which report the highest incidences of stroke in developing countries.
Although the BASID cumulative stroke incidence in Mexico is consistent with most developing countries, MA’s living in the US have a higher cumulative stroke incidence compared with Mexicans living in Durango Municipality. One possibility to explain the discrepancy between stroke incidence rates in Durango Municipality and in Corpus Christi, is due to environmental or lifestyle changes that alters stroke susceptibility, as occurred in Japanese immigrants to the United States.17
An important focus of future work in both Durango Municipality and Corpus Christi will be to explore the social, environmental, biological, and genetic reasons for the discrepant stroke incidence in Mexicans on both sides of the border.
The stroke prevalence rate of 5.1–7.7/1000 in the community of Durango Municipality is within the range found in other Latin America door-to-door surveys ().18–23
Other studies have used different definitions and age groups making comparisons difficult. Prevalence rates are also comparable to those in European population-based studies, where the range of crude stroke prevalence was 5–10/1000.24
Door-to-door prevalence studies of stroke in Latin America
In summary, this work provides the first population-based study of stroke incidence and prevalence in Mexico comparing both hospital and door-to-door surveillance. Stroke cases that do not present to hospital are important in developing countries. More advanced sampling techniques are needed to maximize efficiency of community surveillance methods.