James J. Chamberlain MD
Donald L. Granger MD
Essentials of Diagnosis
The variant gattii has been isolated most frequently from tropical and subtropical climates, most commonly in Australia, Southeast Asia, Brazil, Venezuela, Zaire, and southern California. This finding appears to be related to the distribution of the river red gum tree (Eucalyptus camaldulensis), which harbors the organism. Infectivity correlates with flowering of eucalyptus trees. The organism does not cause outbreaks or clusters of infection, and a lack of history of exposure to bird droppings or flowering Eucalyptus trees should not exclude the diagnosis of Cryptococcus infection—or “cryptococcosis.”.
AIDS and the use of immunosuppressive drug therapy have produced a dramatic rise in the number of infections due to C neoformans over the past 25 years (see Chapter 40). In fact, more cases of cryptococcosis were described in the United States in 1976 (338 cases) than had been described worldwide by 1955 (~ 300 cases). Prior to the AIDS epidemic, nearly half of the cases of Cryptococcus infection were described in patients with altered cellular immunity, including leukemias, lymphomas, sarcoidosis, and chronic corticosteroid use, and in immunosuppressed patients who had undergone organ transplantation. It is interesting that nearly every case of cryptococcosis in patients with AIDS has been caused by the variety neoformans. This phenomenon has led to a dramatic increase in the incidence of cryptococcosis caused by variety neoformans, especially in central Africa, where the majority of infections were caused by variety gattii before the AIDS epidemic. The reason for this skewed epidemiologic finding remains speculative at this time.
Cryptococcosis remains the most common life-threatening fungal infection seen in patients with AIDS and usually is seen in patients with CD4 counts of < 100 cells/mm3. Estimates of the prevalence of cryptococcosis in patients with AIDS range from 6% to 10%. C neoformans is also the third most common organism affecting the central nervous system (CNS) in AIDS cases. Considerable variation seems to exist in the frequency of cryptococcosis among AIDS patients in different regions of the world, with as few as 3% of patients being infected in parts of Europe but possibly ≤ 30% of sub-Saharan Africans being affected. Males are affected approximately threefold as often as females, regardless of the presence of AIDS.
Encapsulated yeast cells generally appear round or oval shaped in tissue samples and are usually 4 to 8 µm in size. Cryptococcus cells may be as large as 15 µm as they begin to bud. The size of the polysaccharide capsule encircling the organism varies greatly and depends mainly on surrounding growth conditions. The size of the capsule in vitro does not correlate with virulence of a particular strain.
Table 74-1. Characteristics of C neoformans varieties.
C neoformans grows at 37°C on Sabouraud's or malt extract agar, a characteristic that distinguishes this species from other nonpathogenic Cryptococcus species. The organisms are usually visible within 72 h as white or tan-colored, mucoid, smooth colonies.
The variety neoformans comprises capsular serotypes A and D, and variety gattii capsular serotypes B and C. The two varieties of C neoformans may be distinguished on canavanine-glycol-bromthymol blue agar. Variety gattii grows to produce a color change to cobalt blue; variety neoformans does not grow, and the indicator remains yellow. In addition, Cryptococcus spp. hydrolyze urea and assimilate maltose, sucrose, glucose, and galactose, but not lactose. Carbohydrates are not fermented by species in this genus. C neoformans produces melanin from catecholamines by phenoloxidase. This enzyme has recently been cloned and characterized. It is a type of laccase. Melanin production has been shown to be a virulence factor.
The main factors responsible for virulence include (a) the surrounding polysaccharide capsule, (b) the enzyme phenoloxidase, (c) ability to grow at 37°C, and (d) alpha mating type. The polysaccharide capsule has been shown to impair host phagocytosis and may impede migration of leukocytes. Phagocytic cells may be unable to engulf heavily encapsulated organisms due to the size of the pathogen. Capsular elements also activate the complement system, which probably has a significant effect on host defense, depending on the amount of capsular material surrounding the organism. In addition, the alternative complement pathway could be activated by cell wall components that might deplete host humoral factors locally. Oxidative killing mechanisms of macrophages and neutrophils are important elements of host defense and may be impeded by Cryptococcus spp. C neoformans produces the enzyme phenoloxidase, which converts hydroxybenzoic substrates to melanin. This activity may protect against oxidative host defenses. The organism has never been shown to produce toxins.
Before the advent of highly active antiretroviral therapy (HAART), cryptococcosis carried a high mortality. During diagnosis and initial treatment with antifungal therapy, ≤ 25% of patients with AIDS died. At 1 year after diagnosis of C neoformans, 30–60% of patients with AIDS died from the disease. Cancer patients diagnosed with cryptococcal meningitis have a median overall survival of only 2 months. These findings serve to emphasize the importance of immunocompetence in susceptibility to cryptococcosis and its correlation with severity of either AIDS or lymphoreticular malignancy.
CNS Infection. The most common clinical manifestation of C neoformans infection is chronic infection of the brain and meninges, termed meningoencephalitis. The use of this term has become customary because cryptococcal organisms truly invade the cerebral, cerebellar, and brain stem parenchyma as well as the meninges.
The most common symptom of meningoencephalitis is headache. Signs of meningeal inflammation including stiff neck and photophobia are present in ~ 50% of patients but are usually mild. Malaise, dizziness, and nausea may be present as well. As the disease progresses over several weeks, patients may develop altered mental status, visual loss, cranial nerve palsies, ataxia, seizures, coma, and brain stem herniation. Signs and symptoms of increased intracranial pressure may predominate, including severe headache, neck stiffness, projectile vomiting, altered consciousness, and papilledema. Many patients with cryptococcosis never have fever.
Pulmonary Infection. Pulmonary disease is another potentially fatal manifestation of infection with C neoformans. However, the majority of patients probably experience subclinical disease, with half experiencing no symptoms. Still, the scope of disease may range from acute, self-limited pneumonia in otherwise healthy individuals or chronic, stable colonization in patients with underlying lung disease to a severe, progressive pneumonia in AIDS patients, with mortality approaching 50%. The most common symptoms seen in pulmonary infection include dry cough, low-grade fever, sputum production, and pleuritic chest pain. Pleural effusions are rarely seen on chest radiograph. Dissemination to the CNS should probably be regarded as the most serious complication of pulmonary cryptococcosis and may occur during any stage of pulmonary infection, even as the pulmonary disease seems to be resolving.
Other Infections. C neoformans may affect other organ systems as well. Skin lesions may be observed in ≤ 10% of patients and signify disseminated disease with a high risk of CNS involvement. Up to 5–10% of patients have skeletal involvement with cryptococcosis. The vertebrae and long bones are most often affected. Cryptococcosis may affect the eyes, adrenal cortex, genitourinary system, gastrointestinal system, and most any other organ system in rare instances.
CNS Infection. The diagnosis of C neoformans infection of the CNS is made definitively by fungal culture of the pellet of centrifuged CSF. Culture media should be maintained aerobically at 37°C for 6 weeks before a culture is deemed negative. Cryptococcal antigen is positive by latex agglutination nearly 95% of the time in the supernate of centrifuged CSF. Serial dilutions of CSF run by latex agglutination allow titers to be reported, which may be useful in assessing prognosis or monitoring response to therapy in patients who are not immunocompromised. Antigen titers do not seem to correlate with severity of infection or response to treatment in patients with AIDS. The presence of cryptococcal antigen in serum is suggestive but not diagnostic of CNS disease in non-AIDS patients. However, ≤ 95% of AIDS patients with documented CNS infection will have positive cryptococcal antigen in serum. Positive identification of cryptococcal organisms in CSF by India ink preparation occurs in ≤ 50% of cases of meningoencephalitis. Lymphocytes in the CSF are present in low numbers, especially in AIDS patients. One study of cryptococcal meningitis in patients with AIDS showed an average of four lymphocytes/mm3 in the CSF. CSF glucose is generally low but may be normal. Protein in the CSF is usually elevated, as is the opening pressure. Computed tomography and magnetic resonance imaging may detect the presence of nodules (cryptococcomas), hydrocephalus, or gyral enhancement. Nearly 50% of patients may have an abnormal computed tomography scan. Cryptococcomas may be seen in ≤ 25% of patients with meningoencephalitis.
Pulmonary Infection. Diagnosing the presence of C neoformans in the lung is less satisfying and more difficult to interpret. Colonization with cryptococcal organisms has often been noted on repeated cultures of sputum from patients with chronic pulmonary disease. Culture of sputum or bronchoalveolar lavage should be considered the gold standard for diagnosis of pulmonary cryptococcosis. Patients with symptoms of pulmonary disease and cryptococcal antigen titers ≥ 8 on bronchoalveolar lavage specimens should be considered to have pulmonary cryptococcosis. The chest radiograph may not necessarily be helpful in the diagnosis of suspected pulmonary disease. Most commonly, chest radiographs are normal or show a nonspecific pattern of interstitial infiltrates and lymphadenopathy. Occasionally, patients have multiple large, dense nodules that are subsequently found to be caused by cryptococcosis. Pleural effusions are rarely noted on chest radiographs.
The diagnosis of disseminated infection in the bloodstream—or “cryptococcemia”—is best made based on the presence of cryptococcal antigen, as detected by latex agglutination or by cultivating the organisms using “isolator” blood cultures. The capsular polysaccharide becomes soluble in the serum of infected patients and may be detected with rabbit anti-C neoformans antiserum with ~ 95% sensitivity. Antigen titers ≥ 8 are thought to signal active disease. This method of detecting C neoformans infection is more sensitive than culture or India ink stain, which are roughly 75% and 50% sensitive, respectively. False-positive antigen detection may occur in patients with rheumatoid factor in their blood or by cross-reaction with the polysaccharide antigen of Trichosporon beigelii.
Other Infections. Skin and mucosal lesions, which may be secondary to disseminated cryptococcosis, should be biopsied for both histologic diagnosis and culture. Skeletal lesions should be biopsied as well if the diagnosis is in doubt.
The treatment of patients with cryptococcosis is determined by whether they are HIV positive or negative and whether they have localized pulmonary or disseminated (usually meningitis) involvement.
Amphotericin B is limited by significant adverse effects and complications including infusion-related toxicity, nephrotoxicity, and hypokalemia. 5-Flucytosine causes myelosuppression, most notably thrombocytopenia and neutropenia, as well as gastrointestinal complaints.
Treatment of CNS with severe pulmonary or disseminated involvement of other body sites consists of two phases. First, induction/consolidation therapy is given to control the infection and reduce viable cryptococci in CSF and tissues to an undetectable level. This is followed by maintenance therapy, which prevents relapse. It is currently unclear whether maintenance therapy may be discontinued in patients with prolonged successful HAART. Induction/consolidation therapy is with amphotericin B plus 5-flucytosine for 2 weeks followed by fluconazole for 10 weeks. Maintenance therapy consists of daily fluconazole for life (Box 74-2).
Drug interactions are an important consideration when azole antifungal drugs are used. In one case, a coadministered drug (eg, omeprazole) may decrease gastrointestinal absorption (eg, of itraconazole) or increase metabolism (eg, of fluconazole by rifampin), rendering antifungal therapy less active. Conversely, azoles may lead to unexpected toxicity of the coadministered drug (eg, warfarin) by altering hepatic metabolism via a cytochrome P-450 system. Drug interactions in HIV-infected patients taking complex therapeutic regimens require careful review when therapy with an antifungal drug, especially an azole, is initiated.
Another important consideration in the treatment of cryptococcal meningitis is that of increased intracranial pressure. A large proportion of deaths, especially early in the course of disease, are attributable to elevated intracranial pressure. Patients who present with or develop coma or other signs of increased intracranial pressure should undergo daily lumbar puncture with removal of 30 cc of CSF until symptoms resolve. Treatment with acetazolamide and ventriculoperitoneal shunting has been tried as well. The effect of these interventions on mortality is not known.
BOX 74-1 Treatment of Cryptococcal Disease in HIV-Negative Patients
BOX 74-2 Treatment of Cryptococcal Disease in HIV-Positive Patients
Cure of CNS cryptococcosis in AIDS patients is rare. Consequently, management is aimed at long-term suppression therapy. The effect of HAART on ability to cure cryptococcosis is unknown. Close follow-up throughout the course of initial therapy and maintenance therapy is imperative in AIDS-associated CNS cryptococcosis to monitor for relapse. Immunosuppressed patients with neoplastic disease may also be difficult to cure and require long-term suppressive therapy. In others the mortality rate of treated CNS infection may be 25–30%, although this varies in different series. Often prognosis is largely influenced by the underlying immunosuppressive condition, for example, in lymphoreticular malignancies.
Factors signaling a poor prognosis include abnormal mental status, advanced age (> 60 years), cryptococcemia, high titers of cryptococcal antigen in CSF or blood, and CSF measurements of high opening pressure, low white blood cell count (< 20 cells/mm3), low glucose, and a positive India ink preparation. Serious morbid complications include chronic brain syndrome with dementia and acute and chronic hydrocephalus.
Prevention & Control
Prophylaxis for cryptococcosis is not currently recommended. However, in AIDS patients with CD4 counts of < 200 cells/mm3, the incidence of cryptococcosis has been reduced from 7% to 1% in patients taking fluconazole (200 mg PO daily). Having AIDS patients or other immunocompromised patients avoid areas that are heavily populated with pigeons may be appropriate. The use of active immunization with capsular polysaccharide has not been successful.
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