Analysis of Tuberculosis Meningitis Pathogenesis, Diagnosis, and Treatment

Tuberculosis (TB) is the most prevalent infectious disease in the world. In recent years there has been a significant increase in the incidence of TB due to the emergence of multidrug resistant strains of Mycobacterium tuberculosis (M. tuberculosis) and the increased numbers of highly susceptible immuno-compromised individuals. Central nervous system TB, includes TB meningitis (TBM-the most common presentation), intracranial tuberculomas, and spinal tuberculous arachnoiditis. Individuals with TBM have an initial phase of malaise, headache, fever, or personality change, followed by protracted headache, stroke, meningismus, vomiting, confusion, and focal neurologic findings in two to three weeks. If untreated, mental status deteriorates into stupor or coma. Delay in the treatment of TBM results in, either death or substantial neurological morbidity. This review provides latest developments in the biomedical research on TB meningitis mainly in the areas of host immune responses, pathogenesis, diagnosis, and treatment of this disease.


Introduction
Worldwide, Tuberculosis (TB) remains the most important infectious disease in causing morbidity and death [1]. About one-third of the population worldwide has currently contracted TB infection through Mycobacterium tuberculosis [1]. Recently, the WHO reported that there are about eight million new TB cases yearly. In addition, the incident of TB is expected to increase [1]. Notably, patients who suffer from immunosuppression are much more likely to contract extra-pulmonary tuberculosis [2][3][4], and of the extra pulmonary variants of TB, TBM shows the highest mortality rate [5][6][7][8].
TBM is characterized as a severe manifestation of TB and usually requires emergent intervention, due to the quick hematogenous dissemination of the tuberculosis bacillus. This dissemination is quickly advanced and seen clinically with focal neurological defects, altered mental status, cerebral infarcts, prolonged fever, and highly likelihood of stroke. The presence of stroke is a general indicator of basal ganglia damage and a poor prognosis at three months [3][4][5][6][7][8]. Unfortunately, TBM is difficult to diagnose due to its clinical similarity with other neurological disease manifestations. It has been concluded in several research articles that a better prognosis is expected through early treatment and diagnosis. Failure to do so is highly associated with death or severe neurological impairment [3][4][5][6][7][8].

Methodology
TBM is considered the most severe form of TB. It is our intention to better understand the host-pathogen interactions, pathogenesis, diagnostics, and treatment modalities to bridge the knowledge gap between different fields and better understand TBM as a clinical threat to humanity. To do this, our group of 8 co-authors decided to conduct an extensive PubMed search to analyze 77 articles relating to different aspects of TBM. These articles were specifically selected by relevance of the topic and credibility of the source (Table 1). While, the majority of the information is from past research and literature, we tried introducing novel concepts as can be exemplified in Section 4 adjunctive therapy. Keywords that were used to obtain these articles were as follows: TBM, extrapulmonary TB, cerebrospinal TB, HIV and TB, clinical diagnosis of TBM, laboratory diagnosis of TBM, current treatments of TBM and potential treatments of TBM. We decided to exclude the following in our searches: Bacterial meningitis, viral meningitis, fungal meningitis, TBI. To allow us to keep the information on TBM as recent as possible, we selected articles with a publish date no later than 1970 (Table 1). Finally, to maintain fluidity and uniformity within the project, any disputes that arose were solved by discussion amongst the group members initially, with a final decision from Dr. Venketaraman. Report provided information on TB epidemiology, progression in prevention, diagnosis, and treatment.

Extrapulmonary tuberculosis in the United
States [2].

Rieder
Extrapulmonary TB is largest in children and decreases with increasing age, commonly seen in females and blacks/Asians. 3. Extrapulmonary tuberculosis in patients with human immunodeficiency virus infection [3].

Shafer
HIV infected patients are more likely than control patients to have TB. Although, fever was seen in the patients infected w/TB, the diagnosis was difficult and delayed as there is co-infection w/HIV and other pathologies have to be ruled out. Acid-fast bacteria sputum results were not as accurate (+ in less than 50%). Most immediate diagnostic test is a biopsy.
4. Use of ImiD3, a thalidomide analog, as an adjunct to therapy for experimental tuberculous meningitis. Antimicrobial agents and chemotherapy [4].

Tsenova
Treatment with TB drugs along with IMiD3 limited the changes in patients' neurology and improved the survival by 73% 5. Presentation and outcome of tuberculous meningitis in a high HIV prevalence setting [5].

Marais
Six month all because mortality is lower in patients who received antiretroviral therapy during their TB treatment course with the hazard ratio at 0.30 (95% CI = 0.08 to 0.82) 6. Oxidative stress and antioxidants in tubercular meningitis [6].

Sudha
The study found that the blood antioxidant levels of TB meningitis patients were low compared to controls, and the levels improved after treatment, suggesting a role of free radicals in TB meningitis 11. Immunomodulation by vitamin D: implications for TB [11].

Chun
It has been found that Vitamin D deficiency may be linked to increased risk of TB and other immune disorders. This article details cellular and molecular mechanisms of Vitamin D and its potential role in normal and abnormal immune function.

Drennan
One of the ways that the immune system combats Mycobacterium infection is by recognition of the immune system. Specifically, in this infection, macrophages use the TLR -2 to eliminate the antigen, it is proposed that mice TLR-2 levels may be associated with mycobacterium infection and improved prognosis in human patients.  [24].

Daniel
The paper reports that children that have TB meningitis should be treated with medications against tuberculosis, as well as steroids. They also report that levels of TB medications achieve lower concentration in the CSF of children vs. than in adults. 25. Tuberculous meningitis: advances in diagnosis and treatment [25].

Torok
The paper states that human genetic polymorphism may explain the differences in response to anti-inflammatory therapies 26. Tuberculous meningitis in children is characterized by compartmentalized immune responses and neural excitotoxicity [26].

Rohlwink
The study shows that the disease processes of tuberculosis are different in the peripheral vs. the central nervous system 27. Tuberculous meningitis in adults: a review of 160 cases [27].

Pehlivanoglu
The frequency of altered mental status, change in personality, and coma were noted in 59, 28, and 21 percent of patients, respectively.
28. Incidence, predictors and prognostic value of cranial nerve involvement in patients with tuberculous meningitis: a retrospective evaluation [28].

Sharman
The frequency of cranial nerve palsy was observed in 33 percent of patients in a studying involving 158 patients.

Hinman
The paper states that the discovery of isoniazid in 1952 has decreased the mortality rate of TB meningitis from 100% to between 20% to 50%.

Tuberculous meningitis [30]. Kennedy
The paper states that in a study of 52 patients with TB meningitis, 85% recovered, 4% had residual disability, and 15.
31. Tuberculosis of the central nervous system in children: a 20-year survey [31].

Farinha
The paper examined 38 children with CNS tuberculosis and found that overall mortality was 13% and permanent neurological sequelae were seen in 47%.

Udani
Patients present case in which tuberculosis infection presents as dementia and as encephalitis instead of the classic signs and symptoms of meningitis.
34. Tuberculous meningitis in adults: review of 61 cases [34]. Sutlas Only 6 patients out of 61 reported that they were aware of previous TB infection. 43. Natural killer cells, glutathione, cytokines and innate immunity against Mycobacterium tuberculosis [43].

Millman
The study found that glutathione in combination with IL-2 and IL-12 improve NK cell functions, helping to control TB infection

Valdivia
The group found that supplementation with L-GSH in HIV patients whose CD4 + <350 help correct cytokine balance 51. Analysis of Glutathione levels in the Brain tissue samples from HIV-Positive Individuals and subject with Alzheimer's disease and its implication in the pathophysiology of the disease process [51].

Saing
The group found that the levels of many enzymes involving in the synthesis of GSH were decreased in brain tissue samples from HIV-1 patients 52. Role of glutathione in macrophage control of mycobacteria [52].

Venketaraman
Glutathione is a tripeptide and antioxidant, and it synthesizes high levels of reactive oxygen and nitrogen intermediates used in regulating antigen-processing and ultimately intracellular mycobacterial growth.

Franklin
Their findings suggest that the suppression of GSH antioxidant defenses and the depletion of intracellular GSH may play role in enhancing TGFbeta-1 induced oxidative stress and potentiating apoptotic cell death 60. Circulating markers of free radical activity in patients with pulmonary tuberculosis [60].

CI
This study aims to measure circulating free radical markers in patients with TB. Their results showed markedly elevated levels of the three radical markers tested in all patients with TB.

Marais
In an attempt to standardize criteria for the diagnosis and management of Tb meningitis, 41 international participants along with a consensus committee bound together to discuss treatment, management, pathogenesis and future of clinical research in the topic. It is the hope that by unifying the clinical diagnosis and management, decreased mortality and morbidity in Tb Meningitis should be observed. 73. Diagnostic standards and classification of tuberculosis in adults and children [73].

American Thoracic Society
This set of standards and classifications for Tb are installed in an attempt to provide a complete framework and understanding of clinical tuberculosis etiology and management.
74. Burden of tuberculosis at postmortem in inpatients at a tertiary referral center in sub-Saharan Africa: A prospective descriptive autopsy study [74].

Bates
Asymptomatic tuberculosis presents a clinical problem in that diagnosis is difficult due to the absence of symptoms and signs. This article reports on an autopsy performed in Lusaka Zambia in hopes of yielding information explaining the true burden of asymptomatic tuberculosis on affected patients. 75. The vitamin D-antimicrobial peptide pathway and its role in protection against infection [75].

Gombart
This article provides information on Vitamin D deficiency and its association with increased rates of infection. Specifically, the report provides insight on the possibility of using sunlight (environmental) and dietary vitamin D in the treatment of TB.

Host-Immune Responses against Infection
M. tuberculosis infection is usually acquired by inhalation of infectious aerosol particles containing the pathogen [9]. Most individuals in the general population who become infected with M. tuberculosis do not develop clinical disease (active pulmonary TB), due to the concerted effector mechanisms mounted by the cells of innate and adaptive immune systems, which results in M. tuberculosis becoming dormant. This condition is commonly referred to as latent tuberculosis infection (LTBI) [10]. Little is known about what happens during the early phase of immunity against M. tuberculosis infection, even before the pathogen is encountered by the phagocytic cells. Myeloid dendritic cells and macrophages are considered to provide initial first-line defense against M. tuberculosis infection. Once dendritic or alveolar macrophages encounter M. tuberculosis, the bacteria are recognized via microbe-associated molecular patterns (MAMPs) by toll-like receptors (TLRs) on the host phagocytic cells. The interaction between MAMPs and TLRs trigger cell signal transduction that induces a proinflammatory response. However, M. tuberculosis has evolved mechanisms to subvert these host responses for its own survival in the host [11,12].
TLR2 and TLR4 on the host phagocytic cells are important for recognizing M. tuberculosis MAMPs. M. tuberculosis growth can be inhibited in both mouse and human macrophages by activation of the TLR2 [12]. However, the inhibition in the growth of M. tuberculosis in the mouse macrophage was dependent on the intracellular nitric oxide pathway, whereas in the human macrophage, it was independent on the nitric oxide pathway [13].
Further characterization of the mechanism of killing of M. tuberculosis in human macrophages has shown that TLR2 activation up-regulates the expression of the vitamin D receptors as well as vitamin D-1 hydroxylase. The expression of vitamin D receptor-related genes leads to an increased expression of cathelicidin, an antimicrobial peptide, which is then responsible for inhibiting the growth of M. tuberculosis [14,15].
TNF-α seems to be involved in formation and maintenance of granuloma. In conjunction with IFN-γ, TNF-α can enhance the effector responses against M. tuberculosis infection [16].

Pathogenesis of TB Meningitis
Extrapulmonary TB begins when the bacteria disseminate from the lungs to the lymph nodes, and during this time, there is bacteremia which seed M. tuberculosis to other organs in the body for TB, specifically the meninges and the brain parenchyma [17,18]. During hematogenous dissemination, mycobacteria may be deposited adjacent to the ventricles or subarachnoid space, leading to granuloma formation at those sites of deposition [19,20]. M. tuberculosis can breach the blood brain barrier (BBB) extracellularly or intracellularly via dendritic cells or macrophages [17,21]. TBM occurs when subependymal or subpial tubercles, also known as "rich foci" seed during bacillemia of primary infection or disseminated disease [17,18,22,23]. This rupture of the granuloma into the subarachnoid space leads to an intense inflammatory response, which eventually causes meningitis [18,19,21,23]. The tissue damage seen in the brain is due to a host inflammatory response rather than over-replication in the CSF [21,23].
The inflammatory response is due to rupture which includes a collection of a tuberculous, thick gelatinous exudate (erythrocytes, mononuclear cells, neutrophils, and bacilli) at the basal brain and vasculitis within the cerebral arterial system, including branches of the middle cerebral artery, the vertebrobasilar system, and the vessels of the Circle of Willis [18,19,24]. All of this can lead to long-term neurological defects from either infarction or compression by the exudate, which can encase cranial nerves and cause nerve palsies, entrapment of blood vessels, and blocking of CSF flow in the cerebral aqueduct to cause hydrocephalus. These processes produce adhesions, obliterative vasculitis (internal carotid, middle cerebral arteries), and encephalitis [18,19,23].
On average, TBM occurs 6 to 12 months after the primary infection, and patients show a prolonged inflammatory response [24]. The risk factors for TBM include malignancy, malnutrition, alcoholism, HIV, the use of immunosuppressive agents, and cortisol deficiency [17,25].
TBM can eventually lead to intracranial tuberculomas in an immunocompromised patient [17,26]. Focal neurological signs result from formation of tuberculomas and abscess after infection, with basal ganglia the most common site of infarction [26].
Although the role of TNF-α is crucial for the formation of granuloma and enhanced killing of infected cells in the lungs during the primary infection, concentrations of TNF-α in CSF correlate with clinical correlation of TBM [16,18]. Intervention with thalidomide, an anti-TNF agent, resulted in an improvement in survival and neurological outcome due to TBM [18].
One study showed that there was also significant elevation of cathelicidin LL-37, interleukin (IL)-13 and vascular endothelial growth factor (VEGF) and reduction of IL-17 in the CSF of children with TBM, compared to children with viral and bacterial meningitis [36]. This biomarker pattern suggests a host immune response which is disease-specific and may be of diagnostic and therapeutic importance [24].

Clinical Presentation of TB Meningitis
The clinical presentations of TBM have many similar features to those of generalized bacterial meningitis, which include, but are not limited to, headache, fever, stiff neck, nausea, and vomiting. However, according to data obtained from many clinical trials, there are clinical features that are present more commonly in TBM than in generalized bacterial meningitis and may have values in distinguishing TBM in clinical practice. The presence of neurologic signs and symptoms are frequently observed. In a study involving 160 patients, the frequency of altered mental status, change in personality, and coma were noted in 59, 28, and 21 percent of patients, respectively [27]. Cranial nerve palsy is also common and most frequently involve cranial nerve II, which affects vision, and cranial nerve VI, which affects lateral movement of the eyeball. The frequency of cranial nerve palsy was observed in 33 percent of patients in a studying involving 158 patients [28].
TBM has three clinically distinct phases, which are the prodromal phase, the meningitic phase, and the paralytic phase. In the prodromal phase, which lasts from one to three weeks, patients experience nonspecific signs and symptoms, which include, but are not limited to, malaise, headache, low-grade fever, and change in personality. In the meningitic phase, patients experience more prominent neurologic signs, which include nausea, vomiting, headache, lethargy, confusion, and cranial nerve palsies. Finally, in the paralytic phase, the illness progresses quickly, and patients can deteriorate into coma, seizure, and possibly paralysis. For patients in this stage, death follows quickly if they are not treated [29][30][31][32].
Aside from the typical presentations above, TBM can also present atypically in some patients, potentially mimicking other neurologic conditions, complicating the diagnosis and treatment. Instead of an acute condition, it can present as a slowly progressing dementia over a period of years, potentially mimicking Alzheimer, and characterized by personality change, social withdrawal, memory deficits, and impaired executive functions. Alternatively, patients can present with signs of encephalitis instead of meningitis. Signs and symptoms of encephalitis include coma and convulsions [33].
TBM can appear in patients who have no previous history of signs and symptoms from M. tuberculosis infection. In one paper studying 61 patients with TBM, only 6 patients reported that they were aware of previous M. tuberculosis infection [34].

Diagnosis
The conundrum of TBM arises due to the need for rapid diagnosis for better outcomes. However, it is difficult to do so. We will analyze the following diagnosis paradigms: CSF content, acid-fast smear, lumbar puncture, NAAT, and neuroimaging (i.e., MRI) [24].
CSF findings consistent with TBM will reveal leukocytosis, with an increase in protein and decrease in glucose [24]. It is worth noting that the initial presentation will have neutrophil predominance versus chronic etiologies will show lymphocytes [23]. A study illustrated that a CSF glucose concentration of < 2.2 mmol/L had specificity 0.96 and sensitivity 0.68 in diagnosis [35]. That same study also illustrated that protein concentration > 1 g/L had specificity 0.94 sensitivity 0.78 [35].
CSF acid fast smear has shown to have very low sensitivity. However, by performing analysis on several large volume (10-15 mL) samples lumbar punctures daily can increase the sensitivity up to more than 85% [23]. Although, culturing can take several weeks, it should still be carried out to determine the drug sensitivity of the organism [23]. Drug resistant strains have many important prognostic and treatment implications; indeed, TBM due to isoniazid resistant strains have been associated with two time increase in mortality [23].
Nucleic acid simplification testing is the new tool in toolkit for diagnosing TBM [24]. When definite TB meningitis is used as a reference, the sensitivity and specificity for the Xpert MTB/RIF are 39% and 100% in children respectively. A meta-analysis of 14 studies which analyzed the accuracy of NAAT in diagnosing TBM reported a sensitivity of 0.56, specificity of 0.98, negative likelihood of 44, and positive likelihood ratio of 35.1, suggesting their role in confirmation, but not ideal for ruling out TBM [24].
The diagnosis of TBM can be supported by neuroimaging and the class neuroradiologic features of TB meningitis, such as basal meningeal enhancement and hydrocephalus [23]. The incidence of hydrocephalus is higher in children. In a computed tomography study of 60 cases of TBM in both adults and children, 87% of children was reported to have hydrocephalus on imaging, whereas only 12% of adults had hydrocephalus [18]. The CT showed infarcts in 28%, 83% of which occurring in the middle cerebral artery territory [36]. All patients showed signs of basal enhancement [24]. The MRI findings in HIV-infected children, include high frequency of ventricular dilation following cerebral atrophy, high frequency of communicating hydrocephalus, low frequency of basal meningeal enhancement and granuloma formation [24].

Treatment
The traditional treatment for pulmonary TB has been standardized to the RIPE [rifampin (RIF), isoniazid (INH), pyrazinamide, ethambutol) therapy for 2 months followed by RI (rifampin, isoniazid) for 10 months [20,23,[37][38][39]. The empirical treatment for TBM remains the same as the treatment for pulmonary TB; the empirical treatment is warranted when clinical features and CSF findings are suggestive of TBM, even before microbiologic confirmation, since timely treatment dramatically improves the outcome of TBM [20,23].
INH is considered the most critical of the first line agents due to its excellent CSF penetration and high bactericidal activity [23,24]. While, rifampin penetrates the CSF less freely, the high mortality of TBM due to RIF-resistant strains has confirmed its importance [23,24]. Pyrazinamide has excellent penetration into the CSF and is a key drug in reducing the total treatment for drug-susceptible TB. Hence, if pyrazinamide cannot be tolerated, the treatment course for TBM should be lengthened to a total of 18 months [23].
There are many supplements to the RIPE therapy leading to favorable outcomes, most notably fluoroquinolones and corticosteroids. Fluoroquinolones decrease DNA topoisomerase 2 or 4, inhibiting replication and serving as a bactericidal. Corticosteroids are anti-inflammatory and are prominently used for immunosuppression. The newer generation fluoroquinolones, for example, levofloxacin and moxifloxacin, have strong activity against most strains of M. tuberculosis and have excellent CSF penetration and safety profiles, thus fluoroquinolones would appear to have great potential as part of first-line therapy for TBM [23]. In a randomized controlled study for TBM treatment, the addition of a fluoroquinolone to a standard regimen enhanced anti-TB performance, as measured by various clinical parameters [23].
Another drug mentioned that decreased the mortality for patients would be thalidomide. Thalidomide decreases TNF-α, a cytokine that activates macrophages to form caseous granulomas in TB. The addition of thalidomide, a potent inhibitor of TNF-α, to antibiotics was superior to antibiotics alone in protecting rabbits from dying (50% reduction in their model of TBM) [4]. Thalidomide is severely teratogenic, resulting in limb defects in the newborn. Consequently, female patients should be screened before the course of thalidomide for precautionary measures. Another agent discussed would be bedaquilline, an agent that inhibits the proton pump of mycobacterial ATP synthase which is responsible for energy generation in M. tuberculosis.
It is worth mentioning that patients who present with TB with past medical history conditions are not treated similarly. For example, HIV patients are given the standard regimen of 2 months RIPE followed by 10 months of RI, but also given an ART therapy after [20,23,[37][38][39]. Patients who develop syndrome of inappropriate antidiuretic hormone (SIADH) from M. tuberculosis infection should engage in fluid restriction and take medications such as furosemide and demeclocycline. A hydrocephalus complication from TB should be treated with surgery, such a shunt to decrease the pressure. Regardless, it is definitive that treating all cases in a standard manner in a continuum can be potentially lethal, thus, a thorough medical history should be obtained prior to starting therapy.

Adjunctive Therapy
Immuno-adjunctive therapy appears to be promising in improving the outcome of clinical control of refractory mycobacterial infections. Dr. Venketaraman's research group has reported that individuals with active pulmonary TB exhibit a marked deficiency in glutathione (GSH), the principal non-protein thiol responsible for cellular homeostasis and maintenance of the intracellular redox balance. GSH levels are significantly compromised in peripheral blood mononuclear cells (PBMCs) and red blood cells (RBCs) isolated from individuals with active pulmonary TB and this decrease correlated with increased production of pro-inflammatory cytokines and enhanced growth of M. tuberculosis [40]. GSH possesses a direct antimycobacterial activity in vitro and at physiological concentrations (5 mM) [36,41]. In combination with cytokines such as IL-2 and IL-12, GSH enhances the functional activity of natural killer (NK) cells to inhibit the growth of M. tuberculosis inside human monocytes [42,43]. Similarly, GSH activates the functions of T lymphocytes to control M. tuberculosis infection inside human monocytes [44]. GSH levels have also been shown to be compromised in HIV positive subjects and in individuals with uncontrolled type 2 diabetes (T2DM) who have increased risks for susceptibility to both pulmonary and extrapulmonary TB [42,[44][45][46][47][48][49][50]. Importantly, Dr. Venketaraman's research also demonstrated in the autopsied human brain tissues that the levels of total and reduced forms of GSH were significantly compromised in HIV-1 infected individuals who have increased risks for susceptibility to TBM [51].

Conclusions
The human host serves as the natural reservoir for M tuberculosis. The ability of the organism to efficiently establish latent infection has enabled it to spread to nearly one-third of the world's population. The underlying mechanisms responsible for successful dissemination of M tuberculosis to the meninges to cause TB meningitis remains poorly understood. Given the magnitude of the health problem and the emergence of drug-resistant strains of the organism, a better understanding of the protective immunity and pathogenesis of TB meningitis, development of reliable rapid laboratory diagnosis, therapeutics and effective vaccine are highly desirable.