Introduction

Tuberculous meningitis is the most severe form of tuberculosis (TB), associated with significant morbidity—including severe and irreversible neurologic sequelae—and mortality [1]. Following inhalation of infectious droplet nuclei containing Mycobacterium tuberculosis, hematogenous dissemination occurs, seeding oxygen-rich areas of the body, including the brain and other parts of the central nervous system (CNS) [2,3,4]. Meningitis occurs when infection spreads into the subarachnoid space, resulting in an exudative inflammatory response that can be complicated by obstruction of cerebrospinal fluid (CSF) flow and vascular compression and occlusion, with occasional involvement of the cranial nerves in the brain stem [2,3,4]. In this review, we highlight updates from the past 5 years in the epidemiology, diagnosis, and management of TB meningitis.

Epidemiology

Globally, the World Health Organization estimated that in 2015, there were 10.4 million new cases and 1.4 million deaths from tuberculosis [5]. Prior studies estimated that 10% of patients with active tuberculosis develop CNS disease, resulting in an incidence as high as 20.6 per 100,000 in high-burden countries [6]. Young children [7] and persons living with HIV (PLWH) [8,9,10] are at the highest risk for TB meningitis. The disease also disproportionately affects resource-limited countries with a high prevalence of poverty, lack of public health infrastructure, and limited access to healthcare [11]. While mortality associated with TB meningitis has decreased with anti-tuberculosis treatment [2, 11], it continues to be as high as 55–75%, especially in PLWH and those with advanced disease on presentation [12]. Up to 50% of survivors suffer from long-term neurologic sequelae [12] such as developmental delay [7], blindness [13], stroke, and seizures [11, 14].

In 2016, 9287 new cases of tuberculosis were reported in the USA, with an estimated incidence rate of 2.9 cases per 100,000 [15]. Of the incident cases, 68% were in persons born outside of the USA [15]. US surveillance data for 2015 from the Centers for Disease Control and Prevention (CDC) reported 1933 incident cases of extrapulmonary tuberculosis, of which 92 (4.5%) involved the meninges [16]. Prior CDC surveillance data from 1993 to 2006 had noted similar findings: 19% of tuberculosis patients had extrapulmonary disease, of whom 5.4% had meningeal involvement [17].

Diagnosis

Presenting Symptoms

The diagnosis of TB meningitis is often challenging, as clinical symptoms are typically subacute and nonspecific [3]. Compared to bacterial meningitis, patients with TB meningitis typically report longer symptom duration—up to 1 month in a recent study [18]. Adults with TB meningitis commonly present with stiff neck, headache, fever and vomiting—symptoms generally seen with other forms of bacterial meningitis. TB meningitis, however, is characterized by a higher likelihood of neurologic symptoms, with altered consciousness, personality changes, and coma seen in 59, 28, and 21% of TB meningitis patients, respectively [18]. Additionally, cranial nerve palsies, most frequently involving cranial nerve VI, are other common features, occurring in almost one-third of patients [19]. Unfortunately, by the time more advanced neurological symptoms are apparent (cranial nerve palsies, elevated intracranial pressure, altered level of consciousness), patients may have already suffered irreversible long-term sequelae.

Evaluation of Cerebrospinal Fluid

Several algorithms have been proposed to distinguish TB meningitis from other more common causes of meningitis [20,21,22]. Common features in the cerebrospinal fluid (CSF) analysis are included in Table 1. Compared to bacterial meningitis, patients with TB meningitis have higher relative lymphocytosis in the CSF [23].

Table 1 Characteristic CSF findings in patients with different types of meningitis [24,25,26]

Staining and Microscopy

The isolation of acid-fast bacilli in the CSF by culture is the gold standard for diagnosing TB meningitis. The Ziehl-Neelsen stain, also known as the acid-fast stain, has poor sensitivity for the diagnosis of TB meningitis, reported to be as low as 30% [27] and generally not exceeding 60% [28]. Sensitivity can be improved through serial sampling and meticulous microscopic evaluation [28]. Performing cytospin with Triton X-100 prior to acid-fast staining, the modified Ziehl-Neelsen stain [29], as well as staining of early secretory antigen target (ESAT)-6 [30], has significantly better performance than the conventional acid-fast stain.

CSF mycobacterial culture provides a definitive diagnosis of TB meningitis and allows for phenotypic drug susceptibility testing. However, cultures require an incubation period of up to 2 months and have imperfect sensitivity. The microscopic observation drug susceptibility (MODS) assay provides faster results than standard culture and is based on microscopic evaluation of M. tuberculosis grown in liquid culture [31]. However, MODS is not routinely available.

Adenosine Deaminase Testing

Adenosine deaminase (ADA) testing can be used in the diagnosis of pleural, pericardial, or peritoneal tuberculosis; however, it has not been useful for the diagnosis of TB meningitis. In a meta-analysis of 13 studies evaluating ADA in the diagnosis of TB meningitis, test utility was highly dependent on the cut-off used, with a lower sensitivity and higher specificity as the cut-off value increased [32]. Similar findings were noted in PLWH [33]. An ADA cut-off value that optimizes both sensitivity and specificity has been difficult to establish.

Nucleic Acid Amplification Testing

Nucleic acid amplification tests (NAATs) are PCR-based assays that are highly specific but not sensitive for the diagnosis of TB meningitis, making them more useful as confirmatory rather than diagnostic screening tests [34]. While higher sensitivities have been reported with novel real-time PCR assays [35, 36], they are still insufficient to exclude a diagnosis of TB meningitis. The Xpert MTB/Rif assay (Cepheid, Sunnyvale, CA, USA) is increasingly being used for molecular diagnosis of pulmonary tuberculosis [37, 38]. With a 2-h turnaround time, this commercial NAAT provides a rapid, highly sensitive method for detection of tuberculosis and rifampin resistance in pulmonary disease [39]. While NAAT is highly specific for tuberculosis in patients with extrapulmonary disease, including TB meningitis, the sensitivity of NAAT in these conditions is highly variable [40,41,42]. In 2013, WHO recommended using it preferentially over conventional microscopy and culture for initial TB meningitis testing in resource-limited settings [43]. However, Xpert MTB/Rif test quality is highly dependent on the CSF volume [44] and sample processing [40]. Additionally, Xpert MTB/Rif has a low positive predictive value in countries with low tuberculosis prevalence, which could result in false positive results [45]. To overcome the imperfect sensitivity of Xpert MTB/Rif, a next-generation assay—Xpert MTB/Rif Ultra—has been developed by Cepheid [46]. A recent WHO report found Ultra to be non-inferior to Xpert MTB/Rif in the diagnosis of TB, with improved sensitivity of the new assay in smear-negative culture-positive specimens, and in specimens obtained from PLWH [47]. Accordingly, WHO plans to update their policy in 2018, recommending the use of Ultra [47].

Interferon Gamma Release Assays

Interferon gamma release assays (IGRAs) detect the immune response to synthetic mycobacterial peptides present in M. tuberculosis, through measuring the amount of interferon gamma released, and providing results that are not influenced by prior bacilli Calmette-Guérin (BCG) vaccination [48]. IGRAs are not able to distinguish between latent M. tuberculosis infection and active TB disease. Their role in the diagnosis of TB meningitis has been evaluated in several studies [49, 50]. A recent systematic review evaluated the use of IGRA on blood (8 studies) and CSF samples (6 studies) for the diagnosis of TB meningitis [51]. The sensitivities for blood and CSF IGRA tests were 77 and 78%, respectively, while the specificity was lower for blood than that for CSF (61 vs. 88%) [51]. The performance of the test is significantly improved when combined with other diagnostic tools. Combined with a low CSF ADA (< 5.8 U/L), CSF IGRA has a sensitivity of 94% [50] and a high negative predictive value. When combined with a negative CSF bacterial Gram stain and a negative cryptococcal antigen latex test, the specificity is almost 100% [49], resulting in a high positive predictive value.

Radiographic Imaging

While there are no definitive radiographic findings that are pathognomonic for TB meningitis, brain imaging commonly shows basilar meningeal enhancement, hydrocephalus, basal ganglion enhancement, and infarcts. While several of these findings can be detected on computerized tomography (CT), magnetic resonance imaging (MRI) is a superior imaging modality for TB meningitis [52]. MRI also allows for better detection of early infarcts [53], identification of basal ganglion enhancement [54], enhancing exudates in basal cisterns, as well as carotid and cerebral vascular involvement [55].

Treatment

Treatment Regimens

The treatment of TB meningitis is based on the standard anti-tuberculosis regimen of a 2-month induction phase of isoniazid, rifampin, pyrazinamide, and ethambutol, followed by a continuation phase with isoniazid and rifampin [56,57,58]. Guidelines recommend a longer continuation phase for TB meningitis than pulmonary TB, with a total treatment duration of 9–12 months for TB meningitis [56,57,58,59]. Table 2 summarizes recommended doses and expected CSF penetration for common anti-tuberculosis medications.

Table 2 Recommended use, doses, and CSF penetration of anti-tuberculosis medications in treatment of TB meningitis [56,57,58, 60]

Recent studies have focused on the role of higher rifampin doses as well as the addition of fluoroquinolones in the treatment of TB meningitis [61•, 62, 63]. A phase 2 trial conducted in Indonesia evaluating the role of intensified TB meningitis regimens (increased rifampin dose and/or adding moxifloxacin) for the first 2 weeks of treatment (together with isoniazid, pyrazinamide, and corticosteroids) showed that the higher rifampin dose (13 mg/kg intravenous vs. 10 mg/kg oral) was associated with an almost 50% decrease in 6-month mortality, more rapid resolution of coma, and higher likelihood of complete neurologic recovery after 6 months of treatment [62]. Moxifloxacin did not appear to have any survival benefit at either the 400 or 800-mg dose [62].

One study evaluating the efficacy and pharmacokinetics of adding fluoroquinolones to the standard TB meningitis regimen suggested that fluoroquinolones improve TB meningitis outcomes if they are started early in the disease course [63]. However, these results [62, 63] were not replicated in a larger randomized trial involving both PLWH- and HIV-uninfected patients that evaluated the effect of intensified anti-tuberculosis therapy on mortality in patients with TB meningitis [61•]. Compared to standard anti-tuberculosis regimens, researchers concluded that neither the addition of levofloxacin (20 mg/kg/day) nor the increase in the dose of rifampin (15 mg/kg compared to the traditional 10 mg/kg) reduced mortality by 9 months from initiating therapy. However, this intensified regimen was associated with an increased risk of seizures and visual impairment [61•].

A recent study involving PLWH diagnosed with TB meningitis found that the standard dosing regimen for rifampin results in sub-optimal CSF and systemic rifampin exposure. In addition, standard pyrazinamide dosing resulted in elevated CSF concentrations that were independently associated with neurological toxicity and increased mortality [64].

Duration of Treatment

The optimal duration of treatment for TB meningitis is not clearly established. Current guidelines from the World Health Organization [57, 59], American Thoracic Society, Centers for Disease Control and Prevention, Infectious Diseases Society of America [56], and British Infection Society [58] recommend 12 months of treatment. To date, no randomized trials have directly compared 6-month vs. longer (9–12 months) treatment durations for TB meningitis. While an earlier systematic review suggested that shorter treatment duration for TB meningitis (6 months compared to 9–12 months) is not associated with increased mortality, residual neurological deficits, or relapse [65], the findings were based on observational studies that had highly variable results. In contrast, a more recent Cochrane review evaluating the same question noted that mortality risk was higher among patients in cohorts treated for more than 6 months [66]. However, most deaths occurred in the first 6 months, indicating that the difference is likely a reflection of other confounding factors and not the duration of therapy [66].

Drug-Resistant Tuberculous Meningitis

Isoniazid (INH) is a key component of anti-tuberculosis regimens and has high CSF penetration [67]. Unfortunately, INH resistance has been reported in 6% [68]—18% [69] of TB meningitis cases. Prior studies showed similar treatment outcome in patients with isoniazid-resistant pulmonary tuberculosis compared to those with drug-susceptible infection. However, CDC surveillance data from 1993 to 2005 demonstrated that isoniazid resistance on initial susceptibility testing is associated with a significant increase in mortality risk among patients with CSF culture-positive TB meningitis [68]. The most effective treatment for isoniazid-resistant TB meningitis remains unclear [67], although several guidelines recommend treatment with daily rifampin, ethambutol, and pyrazinamide with or without adding a fluoroquinolone to the regimen [58, 70].

Adjunctive Interventions in Management of TB Meningitis

Steroids

A 2016 Cochrane review of nine randomized trials, including 1337 patients with TB meningitis, evaluated the effects of adding corticosteroids to anti-tuberculosis medications [71]. The mortality of TB meningitis in patients who were treated with an anti-tuberculosis regimen alone was 40%; the addition of corticosteroids reduced the risk of death by one-fourth. The survival benefit of corticosteroids was mostly seen in patients with mild disease; there was no significant advantage in patients with advanced neurologic symptoms [12]. Additionally, improvement in survival was not associated with a decrease in disabling neurological deficits [71]. However, there was no increased risk of adverse events in patients receiving steroids. Similar results were found in a 2013 systematic review evaluating the role of steroids in all patients with tuberculosis [72], with pooled results from trials evaluating the role of steroids in TB meningitis showing a lower mortality and no increased risk of adverse events.

Surgery

Surgical management is infrequently warranted in the treatment of TB meningitis. Indications for surgery include persistent hydrocephalus, in which a ventriculoperitoneal shunt is placed to alleviate the increased intracranial pressure; this intervention has been associated with improved clinical outcomes [73]. Cerebral abscesses and extradural lesions with resultant cerebral or vertebral compression and neurologic deficits are other indications for surgical intervention.

Aspirin

Patients with TB meningitis are at risk for ischemic strokes secondary to vascular occlusion, and hemorrhagic strokes associated with venous sinus thrombosis. Two randomized controlled trials evaluated the role of aspirin in decreasing the risk of stroke in adults [74] and children [75] with TB meningitis. Both trials failed to show a significant association between aspirin and stroke prevention.

Tuberculous Meningitis in People Living with HIV

Prevalence of TB Meningitis in PLWH

HIV infection is associated with a significant increase in TB risk [8, 9] and particularly an increase in the risk of extrapulmonary TB. Globally, PLWH are 20–30 times more likely to develop TB disease compared to HIV-uninfected persons, with 12% of patients with TB worldwide being co-infected with HIV [5]. In the USA, HIV co-infection in patients with TB was as high as 48% in 1993, dropping to 5.5% in 2015 [16]. The prevalence of HIV co-infection in patients with TB is the highest in the 25–44-year age group [16]. Among patients with TB, 40% of all cases in PLWH are extrapulmonary, compared to 10% in HIV-uninfected counterparts [9, 10, 17, 76]. Similarly, for patients with TB, PLWH have a five-fold increase in the likelihood of having CNS involvement [11, 58] and disseminated TB [10] compared to HIV-uninfected persons [8, 60]. This risk significantly increases among patients with CD4+ lymphocyte cell count < 100 cells/mL [10]. In a prospective cohort study conducted in Uganda involving PLWH not on antiretroviral therapy (ART) with suspected meningitis, M. tuberculosis was the third most commonly identified CSF pathogen after Cryptococcus and Epstein Barr virus [77].

Disease Presentation in PLWH

The symptoms of TB meningitis are similar regardless of HIV serostatus [60]. However, it has been noted that PLWH commonly present with extra meningeal involvement [78] and are more likely to suffer from altered mental status [60]. Previous studies have reported lower lumbar puncture opening pressure, lower pleocytosis in cerebrospinal fluid [79, 80], and higher AFB culture yield among PLWH patients with TB meningitis compared to those of HIV-uninfected persons [81].

Immune Reconstitution Inflammatory Syndrome

The management of TB meningitis in PLWH is usually complicated by multiple drug-drug interactions as well as the risk of immune reconstitution [82]. Immune reconstitution inflammatory syndrome (IRIS) is a phenomenon that results from an excessive immune response against Mycobacterium tuberculosis after initiation of antiretroviral therapy. Extrapulmonary tuberculosis, low CD4+ counts, and high HIV viral load prior to initiating ART are factors associated with significant increase in risk of tuberculosis-associated IRIS [83]. TB IRIS is seen in nearly half of PLWH with TB meningitis [84] and is associated with high morbidity and mortality rates [85]. Manifestations of TB meningitis IRIS include worsening headaches, decreased level of consciousness, new onset seizures, and paresis [84]. Studies suggest a higher risk of IRIS in patients with positive CSF cultures, indicating that high baseline tuberculosis antigen load drives an inflammatory response manifesting as TB IRIS [84, 86]. Additionally, it was noted that higher neutrophil counts and percentage as well as higher levels of inflammatory cytokines such as interferon (IFN)-γ and tumor necrosis factor (TNF)-α at the time of TB meningitis presentation were associated with higher likelihood of developing TB IRIS [84, 86,87,88].

A similar phenomenon referred to as a “paradoxical worsening” is also seen among HIV-uninfected patients with TB meningitis, who can experience worsening of symptoms after TB treatment initiation [89,90,91,92,93•].

Initiating Antiretroviral Therapy

Since ART initiation and secondary IRIS can unmask previously undiagnosed tuberculosis, or cause paradoxical worsening of known tuberculosis disease [94], it is critical to gauge the timing of ART initiation in relation to anti-tuberculosis treatment.

The concomitant treatment of tuberculosis and HIV is associated with improved survival compared to starting ART after completion of anti-tuberculosis treatment [95]. However, it has been noted that the initiation of ART within 2–3 weeks of starting anti-tuberculosis therapy is associated with an increased risk of IRIS [96,97,98,99], particularly in persons with < 50 CD4+ lymphocytes/mm3. Despite the IRIS risk, initiating ART within 2–3 weeks in persons with < 50 CD4+ lymphocytes/mm3 is associated with improved survival. However, this does not appear to be true for TB meningitis. In a recent trial involving PLWH diagnosed with TB meningitis, no improvement in 9- or 12-month mortality was seen in those who started ART soon after anti-tuberculosis treatment [100••]. Further, patients with TB meningitis who were immediately started on ART had significantly higher rate of adverse events. Therefore, among persons with advanced HIV and TB meningitis, the control of the opportunistic infection maybe necessary prior to starting ART and reconstituting the immune system. This finding is similar to that seen with cryptococcal meningitis among persons with advanced HIV, in which early initiation of ART is associated with worse outcomes than delaying ART initiation until after at least 5 weeks of treatment of cryptococcal meningitis [101, 102].

Disease Prognosis in PLWH

The prognosis of TB meningitis is worse, and the risk of death is higher for PLWH than that of HIV-uninfected persons. [78]. Additionally, while the rate of TB meningitis relapse is similar in PLWH compared to that of HIV-uninfected persons, the mortality risk after relapse is significantly higher for PLWH (83 vs. 51%) [78].

Conclusion

TB meningitis, while relatively rare in the USA, continues to be a significant cause of meningitis worldwide, especially among children and PLWH. It presents with non-specific signs and symptoms and can progress to severe and irreversible neurologic sequelae, including death. The diagnosis of TB meningitis requires a high level of clinical suspicion, supported by available diagnostic tests: acid-fast staining, nucleic acid amplification testing, and cultures. Treatment involves a 2-month induction phase of isoniazid, rifampin, pyrazinamide, and ethambutol with adjunctive steroids followed by 10 months of isoniazid and rifampin.