This study is the first to report data on hypertension and its associated factors in a school environment in Gabon. The mean age of participants (20.3 years) was higher than that reported by others in sub-Saharan Africa in which it varied between 11 and 18 years [10,15-17]. The fact that the selected schools were professional technical institutions could partly explain this difference. However, this study offers preliminary data in two target populations for hypertension prevention: adolescents and young adults who represent 7.3% and 88.1% of the study population, respectively. Male predominance (69.9%) is also linked to the choice of the establishment. It differs from other studies in which girls and young women predominates [10, 15, 17-19,].
Hypertension was found in 19.1% of students. Data on high school hypertension prevalence are highly variable in sub-Saharan Africa, from 1.2 % to 21.2 % [10, 15, 17-20]. Indeed, these disparities could be due to methodological differences, especially the type of measurement (oscillometric or auscultatory), the number of measures, the norms admitted, but also the age range of the study population (including or not subjects aged above 18 years). Even though American and European institutions recommend the auscultatory method, automatic measurement of blood pressure was chosen for this work as in other studies [11, 17, 19, 21]. This technique presents advantages among which ease of use and the minimization of the “white coat effect”, especially in young students.
A 10% hypertension prevalence was found in adolescents (less than 18 years of age), which is higher than the values reported by Rao (4.5%) in the USA and N’goran in Côte d’Ivoire (1.2%) [15, 22]. It is however close to data reported in Central Africa, such as in Congo (10.1%) and Cameroon (17.9%) [17, 23]. Indeed, low prevalence (1.2 to 3.5%) are frequently reported in young populations (mean age between 11.8 to 14.4 years) [10, 15, 18]. Hypertension in adolescents is known to be associated with an increase of cardiovascular mortality in adulthood, especially by cerebrovascular strokes with a risk multiplied by 3.12 . In Libreville, strokes are the main cardiovascular emergency at the emergency unit . Patients are often young and the main described etiology (52%) is neglected or unknown hypertension . The present results show the importance of leading early hypertension screenings during adolescence or even childhood in Gabon. Taking blood pressure during the clinical exams of children and adolescent should become a reflex among pediatricians. This study also reports hypertension prevalence in the 18-24 year age group. It was 18.3% and young adults were more frequently at risk. These data corroborate the link between age and the increased risk of hypertension previously described . This young adult population should definitely be also considered as a target in the fight against hypertension. Indeed, in this age group, hypertension is often associated with an irregular treatment and a lower control rate than in middle-aged adults, this contributes to the early occurrence of cardiovascular complications . Implementing early treatments and therapeutic education for these young adults is, therefore, a priority. In the absence of data on the real prevalence of hypertension in the adult population in Gabon, the prevalence obtained for students aged more than 25 years old (32.2%) is a good indication of the extent of this public health problem in the capital city.
Hypertension was twice as likely to be diagnosed in male students compared to young women who were found to be less at risk of hypertension as also observed with adolescents. These results corroborate those of other studies, with the exception of young ladies for whom the low susceptibility to hypertension has been rarely described [15, 22, 26].
Even though family history did not increase the risk of hypertension, the relatioship between them is well established . The development of hypertension in children and adolescents depends on genetic and environmental factors [27-29]. Many studies report a higher frequency of hypertension in African and Hispanic children, as is the case for adults [22, 27]. In a study performed in Côte d’Ivoire, nearly two thirds of students (64%) reported a family history of hypertension . Early lifestyle and dietary changes and the monitoring of students would diminish their risk of developing ulterior hypertension .
Obesity and overweight are two modifiable risk factors frequently associated with hypertension in this study. This association was stronger in Elenga’s study in Congo, in which the risk of hypertension was increased by 6.67 in obese students and 5.65 in overweight ones . Overweight was found to be an independent risk factor for hypertension. The association between excess weight and hypertension is already well established. Hypertension frequency correlatively increases with BMI, in both children and adults as observed in many studies performed in developing countries as well as in sub-Saharan Africa [10,18-20, 22,29-32]. The absence of an independent relationship between obesity and increased blood pressure is probably due to the low prevalence of obesity. Furthermore, obesity is responsible for a hypersensitivity to salt which increases the risk of hypertension . A genetic hypersensitivity to salt already described in African subjects . All these data show that the expected risk of hypertension in children and adolescents in sub-Saharan Africa rises with obesity. According to a WHO 2016 report, the prevalence of obesity has increased by nearly 50% in Africa since 2000 . Reducing the risk and the frequency of excess weight must be one of the targets in hypertension prevention and control in children and adolescents .
Alcohol consumption was not associated with the risk of hypertension, even though it slightly predominated in a study in Cameroon . The relatioship between alcohol and the risk of hypertension has been the subject of many controversies. The effect of alcohol would be dependent on dosage; several genetic, socioeconomic, racial and ethnical factors might influence the risk of cardiovascular diseases in regular consumers [27, 36, 37]. A moderate reduction in alcohol consumption has been shown to decrease the level of blood pressure . The high rate of regular consumers (49.1%) requires a monitoring of their cardiovascular risk. Awareness campaigns on the dangers of alcoholism must be realized at the national level and should target children and adolescents as well.
Isolated diastolic hypertension was found to be the most frequent form of hypertension (9.6%) as reported elsewhere in sub-Saharan Africa [15,26]. This type of hypertension is associated with an increased risk of cardiovascular events . Greater attention must be paid to even slight increase of diastolic pressure in young adults, especially in the 8.9% of students aged 18 to 24 years old and the 14.3% of those aged more than 24 years old. . An early start in drug treatment is sometimes necessary .
Isolated systolic hypertension, found in 5.4% of students, also deserves to be noted. Its mechanism is complex in young subjects and many hypotheses such as sympathetic hyperactivity and the increase in arterial rigidity were formulated . This form is often neglected and linked to a “white coat effect” . Even though its negative prognostic is controversial in young subjects, its management is necessary, especially through early lifestyle and dietary measures [11, 40, 41]. It is however sometimes associated with an under-diagnosis and a lack of appropriate treatment . Systolic hypertension was frequent among study participants who had regularly a fat diet. It has been reported to correlate with BMI and waist size . Frequent awareness campaigns must be implemented in school establishments in order to fight against therapeutic inertia and begin early care for these students.
Prehypertension was found in 23% of students (Table 2). It was significantly more frequent in male students. A comparable prevalence was reported in Congo (20.7%) while lower rates were noted in South-Africa (12.3%), Nigeria (2.5 à 5%) and Algeria (12.4%) [10, 18-19, 26]. Nevertheless, different study designs, particularly regarding the definition of prehypertension and the number of blood pressure measurements (one to three) could be the cause. In this study, diagnostic was established after three measurements separated each by one month measurements as recommended by the NHBP, taking into account the important variation in blood pressure during childhood and adolescence [11, 9]. However, data on prehypertension prevalence in African populations are also controversial. A possible increase in the risk of hypertension in teenagers was mentioned in the USA but in other studies [30, 31, 43, 44].
Obesity and overweight were found associated with the risk of prehypertension. This association is well known and seems more important in female adolescents and young ladies [10, 18-19, 31, 32, 45] . Waist size, which was not assessed, is also linked to the prehypertension risk, even in the absence of excess weight [31-42]. Weight loss is recommended by the NHBP to reduce the risk of hypertension.
Even though family history of hypertension and a regular consumption of tobacco and/or alcohol did not increase the risk of prehypertension among the study participants, other studies report a positive correlation between these different factors [32, 39, 45]. Thus, the public health policy makers should emphasize the fight against tobacco and alcohol consumption, which remains a priority in the youth population.
Approximately 0.5 to 1.1% prehypertensive adolescents per year develop hypertension, while prehypertension management reduces the probability of this evolution [9, 31]. The treatment of prehypertension consists in modifying the lifestyle habits of overweight subjects to induce weight loss, adopt a diet poor in salt and engage in regular physical activities [11, 44]. Collected data regarding eating habits, tobacco and alcohol consumption and excess weight of these students, warrant their monitoring and the implementation of awareness campaigns within schools. Moreover, many studies report the role of chronic inflammation in the pathogenesis of prehypertension and its complications . Chronic parasitism, usually asymptomatic in sub-Saharan Africa, is responsible for chronic inflammation which could have been involved in this study’s prehypertension data. This hypothesis warrants the collaborative nature of this study with the parasitology department of the Faculty of Medicine of Libreville.
This study had some limits. First, it was performed in professional technical schools which are not representative of the whole youth and adolescent population. However, over 60% of Libreville youth have the same living conditions as the participants in this survey. Other risk factors such as low birth weight, hips-size ratio, socioeconomic level, and physical activity could not be recorded. Similarly, salt consumption could not be accurately quantified. An additional study including these data and other mixed school establishments of Libreville should be performed. However, the results obtained on cardiovascular disease risk factors and lifestyle give insight on the expected increase of cardiovascular diseases but also other non-communicable diseases (cancer, chronic respiratory diseases, and diabetes) in Libreville in the absence of a true prevention policy. National control programs adapted to all cardiovascular disease risk factors are essential. New communication techniques, which are widely used by the target population of children and adolescents, could be an excellent awareness tool.