Background

Nontuberculous mycobacteria (NTM) are ubiquitous environmental bacteria, present in soil and water sources [1]. NTM are thought of as opportunistic pathogens, with disseminated NTM disease being seen in patients with systemic impaired immunity (e.g. HIV) [2, 3]. Interest in NTM pulmonary disease (NTM-PD) is increasing due to its growing prevalence in non-HIV populations [2]. It can occur in the context of lung disease caused by, for example, bronchiectasis, chronic obstructive pulmonary disease (COPD) or cystic fibrosis (CF), and also in people with apparently normal lungs [2, 3].

NTM-PD symptoms are nonspecific and variable; patients may present with both respiratory and systemic complaints, which may relate to underlying lung disease [2]. NTM-PD usually manifests radiologically with fibrocavitary or nodular/bronchiectatic forms [2]. NTM-PD diagnosis is generally made when the American Thoracic Society/Infectious Diseases Society of America (ATS/IDSA) diagnostic criteria are met [2].

MAC is considered to be the most common cause of NTM-PD [4]. It comprises various mycobacterial species, including M. intracellulare, M. avium (which has four subspecies), and several other less frequently isolated species including M. chimaera [5, 6]. The decision to treat MAC infections depends on the patient’s health status and risk of disease progression. According to published recommendations, patients with nodular/bronchiectatic MAC disease should be offered a combination of macrolide (clarithromycin or azithromycin), rifampin or rifabutin, and ethambutol [2, 4]. In patients with fibrocavitary or severe nodular/bronchiectatic disease, addition of parenteral aminoglycosides may be considered [2]. Many are, however, refractory to first-line therapy and do not achieve sustained culture conversion [7]. Effective treatment choices for these people are few, essentially limited to intensification or modification of the first-line regimen or surgical resection of infected lung tissue [7].

MAC lung disease natural history and long-term outcomes are poorly documented, particularly at the population level [8]. A retrospective chart review of patients from Oregon, USA with respiratory NTM isolates found that the median time to death was 3.6 (range 0–7.7) years for cases meeting ATS/IDSA diagnostic criteria [2] and 3.7 (range 0.0–8.6) years for those who did not (p = 0.63). Here, 55% of the cases and 61% of the non-cases died during the follow-up period (2007–2014), with no statistically significant difference in five-year mortality between cases and non-cases [8].

A previous systematic review of reported treatment outcomes in patients with MAC lung disease, based on a pooled analysis of 28 studies carried out between 1977 and 2004, found overall mortality to be 17% (95% confidence interval [CI] 15–18%) [9]. However, this mainly included studies of short duration, and the calculated mortality rates did not account for different patient follow up-times within the studies [9]. Thus, it is not possible to draw firm conclusions regarding longer-term mortality from this report. Another recent systematic review sought to examine comorbidities, health-related quality of life and mortality associated with NTM disease in various patient populations [10]. Again, variable follow-up times in the included studies (30 days to over 10 years) limited the understanding of long-term mortality. Moreover, no differentiation was made between NTM-PD and NTM-non-PD, or different NTM species [10].

We therefore sought to systematically review the published literature for data on long-term mortality in patients with MAC lung disease, pool five-year mortality results to gain an estimate of overall five-year all-cause mortality in these patients, and explore study characteristics that may have contributed to variability in mortality reports or predict patient outcome.

Methods

Data sources

Database searches were carried out in Medline® and Embase®, using PubMed® and ProQuest Dialog™ search tools, respectively, with a cut-off of 1st August 2017, according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [11]. English language studies were selected. The search strategy applied to each database is described in the Additional file 1. Duplicates, case reports, nonclinical and animal studies were excluded, as were conference abstracts, newspaper articles, notes, news, biography, conference reviews, errata and lectures.

Study selection

Relevant studies were independently selected by two reviewing authors (WH and RD), who screened the article title and abstract initially, and then went on to review the article full text as needed. Studies were included if they reported five-year all-cause mortality in cohorts of patients with MAC lung disease, or NTM-PD cohorts where the majority of patients (≥75%) had MAC lung disease. No restrictions were made regarding study design, patient subpopulation, or data collection (prospective or retrospective). Studies with fewer than ten patients were excluded because of uncertainty about validity of the presented data and outcome in smaller studies.

Data extraction

The following data were extracted from the selected publications: five-year all-cause mortality, proportion of MAC-attributable deaths, factors predicting all-cause mortality, all-cause mortality in patients with fibrocavitary or nodular/bronchiectatic disease, and MAC-related mortality in patients with fibrocavitary or nodular/bronchiectatic disease.

Statistical analysis

Heterogeneity in reported mortality rates was quantified in terms of the Q- and I2-statistics. The Q-statistic is based on the chi-squared test and assesses deviation between individual study effect and the pooled effect across studies. A large Q-value relative to its degree of freedom provides evidence of heterogeneity of the measured outcome (variation in outcome estimates beyond chance). The I2-statistic describes the percentage of the variability in outcome estimates due to heterogeneity rather than sampling error (chance). Five-year mortality rates were pooled across the studies using a random-effects model. The analysis was performed using Review Manager (RevMan version 5.3. Copenhagen: The Nordic Cochrane Centre, The Cochrane Collaboration, 2014 software).

Results

Study selection and identified studies

The PubMed® search of Medline® returned 845 studies, and the ProQuest Dialog search (using Embase® and Medline® databases) returned 1311 studies. Following comparison of the results and de-duplication, 1035 publications remained. This selection was screened by title, abstract content and full text if needed. Following the exclusion criteria stated in the Methods section, 14 studies comprising 17 data sets with data from 9035 patients remained for analysis. A flowchart depicting this selection process is shown in Fig. 1.

Fig. 1
figure 1

Flow chart describing the selection of studies and data sets included in the analysis.*Three of the identified publications contained data sets for two cohorts of patients, and these are considered separately here. MAC, Mycobacterium avium complex; NTM, nontuberculous mycobacterium

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The identified studies and their key characteristics are listed in Table 1 [12,13,14,15,16,17,18,19,20,21,22,23,24,25]. Among these 17 data sets, nine were retrospective medical chart review studies [12,13,14,15,16,17,18,19], five were retrospective population registry analyses [20,21,22,23] and three were from prospective, randomised studies [24, 25]. Three studies included data from two cohorts of patients with MAC lung disease, and these are considered separately for the purposes of this analysis [19, 23, 25]. The number of patients with MAC in the studies ranged from 45 to 5543. Two studies examined patients with other NTM infections (with the data for the MAC subgroup considered for this analysis) [20, 21], and one investigated nodular/bronchiectatic MAC lung disease [18]. Three studies focused on newly-diagnosed MAC lung disease [13, 14, 21]. Two of the included studies covered NTM-PD, however the majority of the patients in these studies were diagnosed with MAC lung disease [22, 23].

Table 1 Characteristics of the identified studies
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Mortality rates in the identified studies

The five-year all-cause mortality data from each study, including the ranges and pooled estimate, are shown in Fig. 2a. The mortality in the studies ranged from 10.0% (95% CI 21.5–58.4%) to 48.0% (95% CI 33.1–62.9%). Pooling data from all 17 data sets using a random effects model, the overall estimate of five-year all-cause mortality was 27% (95% CI 21.3–33.0%). The I2 statistic was 96% and the Q-statistic was 365.1, indicating a high level of study heterogeneity. This is also demonstrated in a funnel plot of data from the selected studies (Fig. 2b).

Fig. 2
figure 2

Analysis of five-year mortality in selected data sets. a Forest plot of five-year all-cause mortality rates in the identified data sets. Results are plotted ± 95% confidence interval (CI). b Funnel plot of five-year all-cause mortality versus standard error from selected data sets. BTS; The Research Committee of the British Thoracic Society

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MAC-related and all-cause mortality

The proportion of all MAC-attributable deaths was reported by nine studies, and these data are shown in Fig. 3a. MAC-related five-year all-cause mortality varied between 5% [25] and 42% [16]. Predictors of all-cause mortality are listed in Table 2. Several factors appeared to be consistent across studies. These include male sex [13, 16, 18,19,20,21, 24], presence of comorbidities [13,14,15,16,17, 19,20,21, 23], and advanced patient age [13, 16, 18, 20,21,22, 24]. Predictors of better outcome include surgical treatment [12] and nodular or bronchiectatic disease [15].

Fig. 3
figure 3

MAC-related five-year mortality and cavitary disease in selected data sets. a The proportion of all deaths related to MAC lung disease in the identified data sets. b Fibrocavitary disease and MAC-related five-year mortality. Black bars indicate fibrocavitary disease, grey bars indicate nodular/bronchiectatic disease. c Fibrocavitary disease and all-cause five-year mortality. Black bars indicate fibrocavitary disease, grey bars indicate nodular/bronchiectatic disease

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Table 2 Predictors of mortality in the identified studies, if any
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Two studies examined the relationship between nodular/bronchiectatic and fibrocavitary MAC lung disease and MAC-related mortality [13, 18]. Both found that patients with fibrocavitary disease had increased five-year MAC-related mortality compared with patients with nodular/bronchiectatic disease (Fig. 3b). One study also analysed the relationship between radiologic types of MAC lung disease and all-cause five-year mortality [13]. This demonstrated that patients with fibrocavitary disease have a substantially greater risk of death compared with nodular disease (Fig. 3c).

Effect of study region on five-year mortality

We performed a sensitivity analysis using the geographic region in which the selected studies were conducted (Additional file 1: Table S1). The analysis demonstrated that patients in Asian studies tended to have a lower five-year mortality (19, 95% CI 14–23%) compared with Europe (35, 95% CI 27–43%) and North America (33, 95% CI 32–35%).

Discussion

The studies identified in this systematic review show that, in general, patients with MAC lung disease are at a high risk of death following their diagnosis, with a pooled estimate of five-year all-cause mortality of 27%. In line with previous reports [9], we found there to be considerable heterogeneity between studies, with an I2 value of 96% and Q-statistic of 365.1.

Several publications have demonstrated the incremental impact of NTM infection on patient mortality. Adjemian and colleagues found that US patients aged over 65 with NTM-PD within a nationally-representative sample were 40% more likely to die during the study period (1997–2007) than patients without NTM-PD [26]. Recent work from Ontario, Canada, also reported an increased mortality in patients with MAC lung disease compared with a matched control group (HR = 1.57, 95% CI 1.48–1.66, P < 0.0001) [21]. Here, the mortality was 33.3% in cases versus 21.5% in controls. Diel et al. identified an even greater mortality risk (HR 3.64, 95% CI 2.28–5.77) and a mortality after 39 months follow-up of 22.4% for NTM-PD patients versus 6.0% for control patients [27]. These studies indicate that NTM-PD increases mortality risk at a population level, independent of underlying comorbidities.

Although predictors of mortality varied between studies, some common features were observed. A worse prognosis was noted with male sex, comorbidities (e.g. coexisting lung disease) and the presence of fibrocavitary disease. The latter was found to be associated with increased MAC-related mortality rate in two studies [13, 18], and in one, all-cause mortality [13]. This is in line with results from Fleshner and colleagues who identified fibrocavitary disease as a predictor of mortality in NTM-PD after controlling for possible confounders (adjusted hazard ratio [aHR] 3.3, 95% CI 1.3–8.3) [28]. Fleshner and colleagues also documented pulmonary hypertension as a risk factor for mortality (aHR 2.1, 95% CI 0.9–5.1), although this was not significant following adjustment for fibrocavitary disease; importantly, individual NTM species were not significantly associated with mortality, suggesting similar risks for each NTM species identified in the study [28].

Relatively few studies have explored differences in mortality between cases with confirmed ATS/IDSA disease criteria against those with NTM isolation only. From our list of identified studies, Marras and colleagues found that mortality rates were higher among patients from Ontario who fulfilled the ATS/IDSA disease criteria compared with those who had NTM isolation only (HR = 1.16, 95% CI 1.09–1.24) [21]. Similarly, five-year age-adjusted mortality rates were slightly higher for patients meeting (28.7/1000) versus not meeting (23.4/1000) ATS/IDSA criteria, respectively, in the report by Novosad identified in our analysis [23]. Andréjak and colleagues noted a similar prognosis in Danish patients with confirmed NTM-PD (57% of whom had MAC isolation) compared with those with NTM isolation only (HR 1.15, 95% CI 0.90–1.51) [20]. Thus, MAC lung disease fulfilling ATS/IDSA criteria is associated with a worse outcome. However, all patients with disease considered bad enough to be recorded by investigators, and hence included in studies, are at some increased risk of death.

Furthermore, whereas all-cause mortality is an objective measure, the proportion of deaths attributed to MAC lung infection depends largely on how clinicians determine the cause of death. This may be unclear, particularly in long-term follow up studies where underlying comorbidities may progress; it a pertinent issue in MAC lung disease as many patients are elderly and often have a number of comorbidities [2, 29]. Thus, the impact of MAC lung disease on mortality at a population level is more appropriately reflected in studies using matched control groups. As shown above, the three studies where MAC lung disease cases were matched with appropriate controls consistently showed an increased risk of mortality for patients with NTM-PD or MAC-PD [21, 26, 27].

Our sensitivity analysis identified a lower mortality rate in Asian studies, particularly those from Japan. A similar trend has previously been observed [30]. This may be driven, in part, by the relatively high proportion of nodular/bronchiectatic disease in Japanese studies [15], which most reports suggest has a better outcome.

The present study has several limitations. We were keen to include a range of studies reflecting the published literature and so did not use a complex set of stringent-pre-specified criteria. Thus, our analysis is influenced by the design of the selected studies. Specifically, only two prospective studies (including three data sets) are included in our analysis [24, 25]. This is challenging for the field as a whole, and further prospective studies of mortality in MAC-PD patients, which could support identification of additional prognostic factors, are warranted.

Furthermore, we could not account for the potential effects of patient immunosuppression (e.g. HIV) or heterogeneity of treatment regimens between studies as the selected studies did not report outcomes for different subgroups. The studies we have identified cover a wide time period (1973–2017) and thus may be influenced by potential variability in NTM diagnosis and treatment over the 44 year period.

A previous meta-analysis of treatment success rates in patients with MAC-PD and MAC-non-PD (the latter including disseminated disease) also found considerable treatment outcome heterogeneity for patients with MAC (I2 > 70%, p < 0.05 for all treatment outcomes) [9]. The authors suggested that this may be due to inconsistency among treatment protocols and in the reporting of key patient and study characteristics [9], preventing identification of clear factors related to treatment success. However, unlike our study, the authors did not distinguish between MAC-PD and MAC-non-PD.

It is important to note that, as many reported studies are frequently based on population-level data (for example, [21]), they can contain limited clinical information. This inevitably means that one must be careful to not over-interpret their findings.

Most of our selected studies do not explicitly identify patients with macrolide-resistant pulmonary disease. This is a concern, as recent work from Korea reported a five-year mortality of 47.1% (95% CI 24.0–70.1) in patients with macrolide-resistant MAC [31]. This is much higher than the pooled estimate from our analysis indicating that macrolide resistance increases mortality risk, and should be specifically identified in future studies.

Conclusions

In conclusion, our structured literature review has identified 17 data sets reporting five-year mortality in patients with NTM-PD caused by MAC. Most (i.e. ten) document a five-year mortality rate greater than 25% [12, 14, 15, 20, 21, 23,24,25], indicating a poor prognosis for patients with MAC lung disease and a need for more effective management of the condition. Substantial heterogeneity in study characteristics was found, with male sex, presence of cavitary disease and high comorbidity levels predicting worse survival outcomes. Further prospective studies using appropriately matched controls may contribute to a better understanding of long-term survival in MAC-related pulmonary disease.