Mitral stenosis in Africa: magnitude of the problem

Introduction

Despite declining prevalence and improved overall outcome of rheumatic heart disease (RHD) in developed countries, RHD is still a major health problem in low-income and middle-income countries where poor socio-economic status, overcrowded housing and lack of medical care still prevail. RHD has an endemic pattern in Africa with highest prevalence in sub-Saharan Africa where there is a reported prevalence of clinically detected RHD of 1-14/1,000 school children, which jumped to 7.5-51.6/1,000 when echocardiography was used to detect clinically silent RHD, also known as latent RHD [1,2]. RHD is the main underlying aetiology of mitral stenosis (MS) in Africa, which may present alone or in association with other valvular lesions with a different natural history and a more progressive course than seen in developed countries [3].

Prevalence of MS in Africa

Despite the fact that RHD is endemic in Africa and still constitutes a major health problem in most countries, estimating the true prevalence of MS in Africa is challenging and is faced with several obstacles.

Inadequate data

For the most part, Africa is composed of low- or middle-income countries with a lack of proper medical resources and adequately trained medical personnel. There is an absence of accurate medical records in most of the rural regions where RHD mainly exists. All of these factors have contributed to the poor quality and/or absence of data from some of the most affected regions in Africa. Extrapolation of available data from a limited number of countries to the whole region was the only way to overcome this paucity of data. However, this extrapolation did not take into consideration the substantial dissimilarity in disease burden that is expected to exist between countries and definitely between different regions within the same country [4].

Screening age and population

Most of the screening studies reported from Africa were carried out in schoolchildren aged between 5 and 15 years. The fact that RHD in general (and MS in particular) is a cumulative disease with a peak prevalence between the ages of 20 and 29 and there is an expected gap between school enrolment and the population as determined by the high rate of school dropout in rural areas in Africa may contribute to underestimation of the true prevalence of MS in Africa [5].

Methods of screening

In the last two decades, there has been a substantial change in the means of detection and screening for RHD - from auscultation for a murmur, to auscultation with second-line echocardiographic confirmation of suspected cases, to first-line echocardiography without auscultation [4]. Echocardiography screening has been shown to detect 3-10 times more RHD compared with clinical examination alone which drew our attention to the seemingly submerged iceberg of latent RHD [6].

In 2012, the World Heart Federation (WHF) developed evidence-based echocardiographic guidelines to improve detection of latent RHD. The WHF criteria for the diagnosis of MS secondary to RHD depend on the presence of a mean gradient of ≥4 mmHg across the mitral valve besides morphological features of RHD [7]. The use of this criterion only to define MS without the use of mitral valve area measured by planimetry may lead to overlooking many cases with true but mild MS, thus underestimating the true prevalence of the disease. However, clinical studies are needed to confirm such an observation. Wide use of WHF criteria for determining the prevalence of MS requires the existence of personnel highly trained in echocardiography (specifically RHD diagnosis) and a lot of resources which may not be affordable in many of the rural areas in Africa.

Current data on MS prevalence

Due to the paucity of data concerning the prevalence of RHD in general and MS in particular in Africa and given the fact that most reported screening studies were carried out on schoolchildren and there is lack of nationwide screening registries in areas where RHD is endemic such as sub-Saharan Africa, estimating the true prevalence of MS may be difficult. Current data show that the prevalence of MS is 0.9-16.4/1,000 [5,8,9]. The discrepancy in reported prevalence between different studies may be related to differences in methods of diagnosis, characteristics of the populations screened and expected geographic and socio-economic differences between countries and districts where screening was carried out. On the other hand, there are several screening studies that failed to detect the presence of MS among the studied population [10,11,12], which again shows the inconsistency of available data, despite the fact that these data may be reported from the same country but from different regions [8,11].

Geographically, Africa is commonly divided into five regions, north Africa, eastern, central, western and southern sub-Saharan Africa. Despite the reduction in the prevalence of RHD in all regions of Africa, central sub-Saharan Africa is still in second place worldwide concerning the prevalence of RHD [1]. Data from the VALVAFRIC study showed that the prevalence of MS in central and western sub-Saharan Africa is 7.7/1,000 [5]. Apart from the VALVAFRIC study which involved eight countries, current data may not reflect the true prevalence of MS in different regions of Africa since most data come from a single country of the region and extrapolation of such data to the whole region has its own drawbacks (Table 1).

Table 1. Prevalence of mitral stenosis in various geographic regions and countries of Africa.

 WHF: World Heart Federation

Socio-economic status and MS prevalence

RHD is usually a disease of poverty associated with overcrowding, poor sanitation, and other social determinants of poor health. The reduction in the rates of RHD in developing countries was attributed largely to improvements in socio-economic conditions, widespread use of benzathine penicillin G (BPG), and implementation of secondary prevention programmes which proved to be a cost-effective way to stop RHD progression [1].

In general, there is a higher prevalence of RHD in low-income African countries, with the odds of detecting RHD 1.5 times greater than in upper middle-income countries, with more severe RHD lesions in low-income countries [5,11]. Even within the same country there is a higher prevalence of RHD in rural areas and low socio-economic communities where overcrowding and poor hygiene broadly exist [11,13].

The development of MS secondary to RHD requires exposure to recurrent attacks of rheumatic fever, resulting in ongoing inflammation and fibrosis with consequent diffuse thickening of leaflets, fusion of the commissures, shortening and fusion of the chordae tendineae, resulting in the full pathological features of MS. People living in low-income countries, especially inhabitants of rural areas with low socio-economic status, lower income and little access to medical care leading to less access to penicillin to treat streptococcal throat infection and little or no access to BPG for secondary RHD prevention, are thus more prone to recurrent attacks of rheumatic fever and development of MS [12].

Data from the VALVAFRIC studies showed that the lowest prevalence of RHD disease including MS was observed in countries with the highest gross domestic product (GDP) per inhabitant compared with other countries with a lower high GDP [5]. Ethiopia, which has one of the lowest GDP in Africa, reported a higher prevalence of MS compared with other low-income countries but with higher GDP [6,14].

Regional variation in clinical characteristics of MS

MS in Africa shows female predominance with early presentation in life. The prevalence reaches 26% among children between 6 and 10 years, with only 24.9% of patients recalling symptomatic episodes consistent with acute rheumatic fever [6]. MS usually presents with other valvular lesions; 4.7-9.5% of cases present only with pure mitral stenosis [5,6]. Patients usually present in New York Heart Association (NYHA) functional Class II-III; atrial fibrillation (AF) is present in 28% of patients [15], with thromboembolic events reported in 3.2% of patients [6].

RHD is endemic in south Asia and patients with MS have clinical characteristics close to those seen in Africa [1]. MS in this area shows female predominance with a higher mean age of presentation similar to that seen in western countries. Twenty-nine percent (29%) of patients report a previous history of rheumatic fever. AF is present in 32% of patients, complicated by thromboembolic events in 12.3% of patients. Paroxysmal nocturnal dyspnoea and a history of pulmonary oedema is reported in 30.5% and 16.7% of patients, respectively [16].

MS in developed countries again shows female predominance. It usually presents late in life with only 4.7% of patients being under the age of 40 years and 40% of patients giving a past history of rheumatic fever. Mean NYHA symptom score at presentation is 1.5, with worsening of functional capacity with ageing. AF is present in 72.3% of patients. The incidence increases with age and the presence of associated comorbidities. However, the incidence of AF drops to 21% in patients under the age of 40 years [17].

Diagnosis of MS

Clinical examination and auscultation have been the gold standard for the diagnosis of rheumatic MS for a long time. However, echocardiograms have consistently proved to be superior to auscultation for RHD diagnosis with auscultation sensitivity to detect RHD being only 30% in recent reports. The implementation of the WHF echocardiographic criteria for RHD improved the accuracy of RHD diagnosis with satisfactory levels of agreement in diagnoses made by experienced physicians [18].

Despite the fact that the WHF echocardiographic criteria standardised diagnostic criteria for RHD diagnosis and detection worldwide, it has led to the recognition of a large number of patients with latent RHD with no history of acute rheumatic fever. Unfortunately, the natural history of latent RHD is ambiguous with discrepancy of data regarding its course and with no current evidence as to whether or not the early institution of secondary prophylactic penicillin will prevent its progression to overt clinical disease [19]. 

Early and accurate diagnosis of RHD (including MS) is a key factor for optimal management of patients with RHD. In order to achieve this, the implementation of appropriate technical expertise in RHD diagnosis, widespread use of echocardiograms and adoption of WHF echocardiographic criteria for diagnosis of RHD have been encouraged by the Addis Ababa communiqué aiming for RHD eradication in Africa by the year 2025. All of this will improve the accurate diagnosis of MS in Africa and help to detect its true prevalence [20].  

Conclusion

Data concerning the true prevalence of MS in Africa are scant. In order to detect the real burden of the disease, community-based surveillance of different age groups is needed.

MS mainly affects populations with low socio-economic status. It has a very progressive nature with presentation early in life which requires early and proper diagnosis and use of secondary prophylaxis antibiotics in order to halt its vicious course.

There is regional disparity in clinical features of MS which is largely determined by the socio-economic status and quality of medical services provided.       

For general information on the management of valvular heart disease including MS, please refer to the “2017 ESC/EACTS Guidelines for the management of valvular heart disease” [21].