1.About Cryptococcus The genus Cryptococcus includes 17 species and 7 variants, of which only Cryptococcus novelis and its variants are pathogenic. Cryptococcus novelis is a kind of yeast with pods, which is divided into four serotypes A, B, C and D according to the polysaccharide on its pods, corresponding to the Grubby variant (A) and the Gott variant (B and C) of Cryptococcus novelis and the neoplastic variant (D) respectively. Cryptococcus is widely distributed worldwide, and HIV carriers are basically infected by Cryptococcus neoformans variant, which is present in high levels in bird droppings, especially in pigeon and chicken droppings. However, C. gothicus variant infections are predominantly found in tropical and subtropical regions, in relatively immunocompetent populations, and are associated with exposure to decaying plants, particularly rubber trees in the Red River Valley. an outbreak of cryptococcosis on Vancouver Island from 1999-2004 was associated with the importation of eucalyptus trees from Australia [6]. Cryptococcosis is recognized as a common opportunistic infection in populations with impaired cellular immunity, such as HIV-infected patients and patients with organ transplants and rheumatic immune diseases requiring immunosuppressive therapy are susceptible populations. In HIV-infected patients, cryptococcal meningitis is classified as one of the index diseases for HIV. Despite HAART treatment, nearly one million cases of cryptococcosis develop worldwide each year [1]. Currently, cases are concentrated in Africa, and in the United States, also reaching 5-10% of AIDS patients eventually develop cryptococcal meningitis, while the percentage of incidence is higher in developing countries [5]. 2, pathogenesis and clinical characteristics of cryptococci to germinate reproduction, does not form mycelium and spores. The generated budding spores are only about 3um in size and can pass through the respiratory tract and infect through the alveoli. However, most hosts are clinically asymptomatic after infection. However, in people with severe cellular immune deficiency, Cryptococcus can enter the circulatory system through the respiratory tract and then spread the disease by asexual proliferation. After entering the body, Cryptococcus produces a thick podococcal membrane, and podococcal polysaccharide is the main pathogenic factor of Cryptococcus, which helps the organism to evade phagocytosis by host immune cells. In addition, melanin is another consistent pathogenic factor of Cryptococcus: phenol oxidase can use catecholamines as substrates to produce melanin, which accumulates in the cell wall of Cryptococcus and protects it from host cell destruction, while allowing the organism to resist drugs that act on the cell wall, such as amphotericin B. The effect of Cryptococcus on catecholamines may be the reason for its central nervous system. Clinically, cryptococci most often cause central nervous system and pulmonary infections, and other skin and mucous membrane cryptococcosis, bone and joint cryptococcosis, etc. In severe cases, it can cause disseminated or systemic cryptococcosis. The site of infection varies depending on the serotype of the organism and the immune capacity of the host. Cryptococcal CNS infection can manifest as meningitis or meningoencephalitis, which usually presents as insidious headache that lasts for several weeks, with progressive exacerbation and manifestations of cranial hypertension and blurred vision. A small number of patients will present with convulsions and impaired consciousness due to brain parenchymal damage, and often fever with or without cervical tonicity does not appear until later in life. Very few cases will develop granulomatous lesions or cystic lesions around the meninges on the basis of meningitis or meningoencephalitis, thus showing some signs and symptoms of focal occupying lesions, making the diagnosis and treatment difficult. It is also worth noting that in HIV-infected patients, cryptococcal meningitis occurs in 50% of cases with concomitant involvement of other sites, including the lungs, bone marrow, skin, and urinary tract. Pulmonary lesions manifest primarily as acute pneumonia or non-calcified granulomas, which are often difficult to detect on imaging. Some patients may present with persistent cough and dyspnea due to pulmonary cryptococcal infection; about 10% of patients have visible skin lesions, mainly in the form of infectious molluscum contagiosum-like lesions; and roughly 5% of cases present with skeletal involvement. [This disease has many similarities with tuberculous meningitis in terms of clinical manifestations and routine biochemical tests of cerebrospinal fluid, and is easily misdiagnosed as tuberculous meningitis clinically. Even in patients without neurologic symptoms, if the body is immunocompromised and there is evidence of cryptococcal infection, a lumbar puncture should be performed to rule out underlying meningitis. In addition to clinical manifestations, the diagnosis of cryptococcal meningitis mainly includes general tests such as cerebrospinal fluid routine and biochemical tests, pathogenetic tests and imaging tests. 3.1 General examination of cerebrospinal fluid The cerebrospinal fluid pressure in patients with cryptococcal meningitis is often significantly increased, usually 200-400 mmH2O, with a mild increase in cerebrospinal fluid protein, a decrease in sugar, and only a mild increase in white blood cell count, usually less than 20 cells/mm3. 3.2 Pathogenic tests Pathogenic tests for cryptococcal meningitis include the following methods: ① Latex agglutination of cryptococcal podococcal polysaccharide antigen Test: This antigen can be detected in both cerebrospinal fluid and serum, and is the main means of pathogenic detection. Among them, a positive cerebrospinal fluid cryptococcal antigen has a sensitivity and specificity greater than 90% for the diagnosis of cryptococcal meningitis [7]. ②Cerebrospinal fluid ink staining: cryptococci have a wide and thick podococcal membrane, which is not easily stained by common staining methods. After ink staining, the microscopic bacilli are 4-6 um in diameter with a translucent thick wall in the outer ring. The positive rate of ink staining in early meningitis is more than 85%. The method is simple and effective and should be used as a routine item in cerebrospinal fluid examination, but its sensitivity has been reported abroad to be lower than that of cryptococcal culture and podococcal antigen testing. In a study of 157 HIV-negative adult patients with cryptococcal meningitis, 51% were positive for ink staining compared with 89% positive for cerebrospinal fluid culture. In comparison, the sensitivity of antigen positivity in cerebrospinal fluid and blood was 97% and 87%, respectively [1]. (iii) Cryptococcal culture: specimens such as blood or cerebrospinal fluid are inoculated on Shah’s glucose agar medium, and colonies can grow at room temperature or 37°C for several days, which are milky white and become mucus-like after a long time. If Cryptococcus is isolated from the culture, it will appear as a single, narrow budding yeast that is negative for urease. A positive cryptococcal culture can be used as evidence of a confirmed diagnosis with a high sensitivity, but the examination period is relatively long. 3.3 Cephalometric CT and MRI can show diffuse meningeal enhancement, cerebral edema, parenchymal hypodense lesions, hydrocephalus, etc. In a few patients, granulomas, cysts or calcified lesions can be seen, which need to be differentiated from tumors in patients with occupying lesions. In many patients, hydrocephalus is the only manifestation of cryptococcal meningitis, but more than 25%-50% of patients with cryptococcal meningitis have no abnormalities in CT [2]. Treatment Treatment is divided into two parts: antifungal treatment and other treatment. Antifungal drugs that can be used in the treatment of cryptococcal meningoencephalitis include amphotericin and its liposomes, 5-fluorocytosine, fluconazole, itraconazole, etc. Echinocandins such as micafungin are not effective in the treatment of cryptococci. The antifungal treatment of cryptococcal meningoencephalitis is divided into induction, consolidation and maintenance phases (see Table 1 for specific treatment protocols). 4.1 Antifungal treatment 4.1.1 Induction phase treatment The US IDSA guidelines for HIV-positive patients with cryptococcal meningitis recommend an induction phase regimen of the rapid fungicide amphotericin B (0.7-1 mg/kg/day) plus flucytosine (100 mg/kg/day) for a 2-week course of treatment. The study showed that this regimen can be fungicidal faster and the treatment effect is better than that of amphotericin B alone and plus fluconazole, and the failure rate is lower. To explore the optimal dose of amphotericin B, the efficacy of two treatment doses (amphotericin B 0.7 or 1 mg/kg/day combined with flucytosine for 14 days) was compared in patients with HIV infection complicated by cryptococcal meningitis in Africa. Although there was no significant difference in mortality between the two groups in the end, the results of the study showed that the amphotericin B1mg/kg/day group had a faster bactericidal effect and a more pronounced improvement in clinical condition [ ], suggesting that a higher dose of amphotericin B may achieve a better therapeutic effect. The common side effects of amphotericin B in clinical use include nausea, vomiting, loss of appetite, fever, chills, headache, and causing electrolyte disorders; in addition, thrombophlebitis and nephrotoxicity are also more common, and proteinuria and tubuluria may occur; bone marrow suppression and neurotoxicity can lead to anemia, white blood cell decline and peripheral neuritis. Liposomal amphotericin has fewer adverse effects than amphotericin B and is often used as an alternative treatment to amphotericin B, especially in people with renal impairment. Although there is no evidence that amphotericin liposomes have better clinical outcomes, they appear to have a faster bactericidal rate. Unfortunately, amphotericin liposomes are very expensive and are still difficult to obtain in many parts of the country. To mitigate the side effects of amphotericin, it is important to hydrate before and after administration to protect renal function, and to closely monitor electrolyte and renal function changes during use. Patients receiving amphotericin therapy generally require potassium supplementation and sometimes magnesium and phosphorus supplementation. There is no definitive conclusion as to what treatment regimen should be used in the absence of amphotericin B and its liposomes. A recent study has shown that fluconazole at high doses (1200 mg/d) reduces the cryptococcal antigen titer in the cerebrospinal fluid more rapidly than at low doses (800 mg/d). In addition, a randomized controlled trial in Malawi showed that oral fluconazole (1200 mg/d) plus flucytosine (100 mg/kg/d) resulted in earlier clearance of the fungus and reduced mortality than fluconazole alone (1200 mg/d). Therefore, fluconazole alone (1200 mg/d) can also be used as an alternative to current guidelines when flucytosine is also not available. However, it is important to note that treatment with fluconazole alone has been reported to be resistant and may lead to relapse, and should only be used when no other options are available. Similarly, if flucytosine is not available, amphotericin B plus fluconazole is preferred over treatment with amphotericin B or fluconazole alone. 4.1.2 Consolidation phase treatment After completion of the 2-week induction phase treatment, lumbar puncture is repeated and cerebrospinal fluid culture is performed. If the culture is negative, start consolidation therapy with fluconazole 400 mg/day for 8-10 weeks. If the cerebrospinal fluid culture is positive, a restart of 2 weeks of induction therapy should be considered depending on the patient’s clinical status. Notably, recurrence of cryptococci in the cerebrospinal fluid often occurs within 1-2 weeks after the induction period. Extended induction therapy is recommended for patients with persistent coma, worsening symptoms or persistent high cranial pressure and poor clinical outcome. 4.1.3 Maintenance treatment In HIV-positive patients with cryptococcal meningitis, lifelong maintenance therapy is often required. After induction and consolidation therapy, lifelong maintenance therapy with fluconazole 200 mg/d for bacterial suppression should be started as soon as the cerebrospinal fluid tests negative for cryptococci. Itraconazole (400 mg/d) can also be used, but is less clinically effective than fluconazole. If immune reconstitution is achieved after HAART therapy, CD4+ T-cell levels remain above 100 cells/mm3 for >3 months, and viral load is very low or undetectable, then temporary discontinuation of therapy may be considered. The results of a prospective randomized controlled trial showed that there was no recurrence of meningitis at 48 weeks of follow-up after discontinuation of maintenance therapy in patients who had achieved good immune reconstitution. However, if CD4+ T cells dropped below 100 cells/mm3 again during follow-up, fluconazole therapy needed to be restarted. It is worth noting that the total course of antifungal therapy for cryptococcal meningoencephalitis should be at least 12 months. 4.1.4 Detection of treatment efficacy Clinical failure should be expected after 2 weeks of appropriate treatment without clinical improvement or relapse after clinical improvement, at which point cerebrospinal fluid cryptococcal cultures and cryptococcal antigen titers should be promptly rechecked. Although antigen testing is valuable in the diagnosis of cryptococcosis, it has limitations in monitoring the effectiveness of treatment. This is because serum antigen titers decrease over time in most patients, so the effectiveness of treatment cannot be judged on the basis of a decrease in serum antigen titers. However, if the antigen titer is found to be unchanged or increased during the course of treatment, especially in the acute phase, this may indicate treatment failure. The clinical prognosis of patients with cryptococcal meningitis is determined not only by the choice of antifungal agents but also by the degree of control of intracranial hypertension. Increased intracranial pressure can lead to clinical deterioration [8], and 93% of deaths occur in the first 2 weeks of treatment and are associated with increased intracranial pressure. If the patient’s cerebrospinal fluid pressure is higher than 625 pxH2O, continuous lumbar punctures (even daily) are required until the pressure drops below 500 pxH2O or decreases by 50%. U.S. guidelines recommend 20-30 ml of cerebral fluid per session, and cerebrospinal fluid shunts should be considered in those who cannot tolerate daily lumbar punctures or whose signs and symptoms of cerebral edema have not resolved. Other medications to control intracranial pressure, including mannitol or acetazolamide to lower cranial pressure are not recommended by the US guidelines. Glucocorticoids may be used in cases of immune reconstitution syndrome, persistent high cranial pressure, solid occupancy or tissue edema due to cryptococcal tumors, and acute respiratory distress syndrome. 4.3 Treatment of patients with intracranial sarcoidosis US guidelines recommend that induction phase therapy (amphotericin B 0.7-1 mg /kg/day plus flucytosine 100 mg/kg/day) should be extended beyond 6 weeks and consolidation phase therapy (fluconazole 400-800 mg/day) to 6-18 months in patients with intracranial cryptococcal sarcoidosis. Corticosteroids can be used in patients with significant occupant effects and severe cerebral edema, and surgical resection can be considered for lesions larger than 75px. 4.4 Immune reconstitution syndrome In patients with HIV combined with cryptococcal meningitis, immune reconstitution syndrome is likely to develop after initiation of HAART, mainly in the form of symptoms of meningitis and increased high cranial pressure. One study showed that up to 30% of patients who started HAART within 1 month of the diagnosis of cryptococcal meningitis developed immune reconstitution syndrome. It is important to monitor the clinical manifestations of hypercranial pressure (e.g., impaired consciousness, optic papilloedema, convulsions, etc.) and also to perform periodic lumbar punctures, especially in patients with intracranial pressure greater than 625 px H2O. It is important to identify whether these CNS symptoms are due to an immune reconstitution syndrome or disease progression or relapse by performing repeated cerebrospinal fluid cultures and observing changes in cerebrospinal fluid antigen titers. If immune reconstitution syndrome is considered, the current antifungal regimen may be continued or antifungal therapy may be initiated, and hormones (approximately prednisone 0.5-1 mg/kg/d) may be considered in critical cases. Recent guidelines recommend that HAART therapy be started after 2-10 weeks of antifungal therapy with a preference for 10 weeks in order to reduce the occurrence of immune reconstitution syndrome. Also, some undiagnosed patients after HAART treatment initiation may develop clinical manifestations of cryptococcosis due to immune reconstitution. In a retrospective study conducted in South Africa, a cryptococcal antigen titer of 1:8 was used to predict the development of cryptococcal meningitis within the first year of HAART treatment with a sensitivity of 100% and specificity of 96% in patients with CD4+ T-cell levels below 100 cells/mm3. Therefore, the need for prophylactic treatment of these populations to reduce the chance of meningitis or immune reconstitution syndrome in hyperendemic areas awaits further study. In the United States, such prophylactic treatment is not recommended at this time. 4.5 Drug Interaction Issues There are a number of drug-drug interactions between antifungals for cryptococcosis and antivirals for HIV that are worth noting. Firstly fluconazole decreases the plasma clearance of nevirapine by a factor of two. In addition, fluconazole increases the area under the plasma drug concentration-time curve of zidovudine, and therefore the toxicity of zidovudine needs to be monitored during concomitant use. Both zidovudine and flucytosine have myelosuppressive effects and therefore need to be closely monitored. If conditions permit, blood concentrations of flucytosine should be tested, but in developing countries, although drug concentrations are not tested, side effects of the drugs rarely occur. 5 Conclusion In conclusion, cryptococcal meningitis remains an important cause of morbidity or mortality in HIV-infected patients. Cryptococcal infection in HIV-infected patients who develop central nervous system infection is one of the diseases to be identified. Note: Patients with intracranial sarcoidosis should have an induction period of at least 6 weeks and a consolidation period of at least 6-18 months.