The Immunocompromised Patient

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Chapter 183

The Immunocompromised Patient

Perspective

As increased numbers of immunocompromised persons present to the emergency department (ED), it is essential that emergency physicians possess the knowledge and skills to recognize and to treat infectious complications of cancer, organ transplantation, diabetes, renal failure, cirrhosis, asplenism, human immunodeficiency virus (HIV) infection, and other immunosuppressive conditions. These include infectious complications of immunosuppressive and immunomodulating medications used for a wide variety of disorders.

Compared with individuals with an intact immune system, infections in immunocompromised patients are more common, progressive, and severe, and they are caused by a wider variety of microorganisms. Immunocompromised persons who present with acute infections may appear deceptively benign initially, their symptoms and signs often mimicking noninfectious complications, only to deteriorate rapidly if they are not aggressively treated. Many interrelated factors cause patients to become immunocompromised and predispose them to the development of infections with potentially pathogenic microorganisms. These include disruption of the body’s protective surfaces, such as skin and mucosal barriers (oral and respiratory mucosa and intestinal and genitourinary surfaces); disorders that directly impair the function of the body’s immune system (e.g., lymphoma, asplenism, and myeloma); drugs and irradiation that suppress or alter immune function; alterations in body substances (hyperglycemia) or solid organ function (kidney and liver failure); and malnutrition, aging, and exposure to antimicrobial agents that inhibit the normal protective resident bacterial flora.1

Principles of Disease

The body’s defense mechanisms consist of surface barriers, such as skin, enzymes, and mucus, as well as innate (natural) and acquired (adaptive) responses. Innate responses occur to the same extent regardless of how often the body encounters the infectious agent, whereas acquired responses improve on repeated exposure.2 Innate immunity is activated immediately on exposure to an infecting agent, rapidly controlling replication and allowing the requisite 3 to 5 days for the adaptive component to clone sufficient T and B cells to respond more specifically.35

Non–Microbe-Specific Immunity

Physical Barriers

The first line of defense against microorganisms consists of physical barriers. These include intact skin, gastrointestinal and respiratory mucosa, cilia, biofilm, gastric acid, antibacterial substances in pancreatic and biliary secretions, antimicrobial peptides and proteins on skin and mucous membranes, and resident microflora.6

In the respiratory tract, mucociliary transport and the cough reflex remove particulate matter and microbes, but this mechanism is impaired with smoking and ineffective cough. Mechanical ventilation or tracheostomy introduces large numbers of microbes that often overwhelm natural clearance.7

Gastric acid and pancreatic enzymes have antibacterial properties that prevent overgrowth in the upper gastrointestinal tract. Normal peristalsis and mucosal shedding help maintain normal gut flora. Alterations in these factors increase susceptibility to infection. Broad-spectrum antibiotics alter normal flora, permitting overgrowth of pathogens such as Candida species, multiantibiotic-resistant bacteria, and Clostridium difficile.

Initial Inflammatory Response and Innate Immunity

The first response to microbial invasion, the initial inflammatory response (formerly called the acute-phase response), acts to promote phagocytosis and microbial killing and to activate the immune system.8 Sentry cells detect pathogens, immediately triggering inflammation. This innate immune response is not dependent on prior exposure to the pathogen. The initial inflammatory response factors, mainly produced in the liver, activate many cell types to synthesize and to release cytokines, chemokines, and “trigger molecules” that kill the invading organism.3

This response delivers humoral and cellular immune components to sites of inflammation and initiates antibody responses. Cytokines, platelet-activating factor, and hormone-like proteins, including interferons, are secreted from various immune cells and play important roles in mediation of this response.9 These cytokines result in migration and adhesion of polymorphonuclear leukocytes and monocytes to sites of bacterial invasion. These cells release granules of substances that mediate vasodilation and increased vascular permeability, leading to edema, warmth, and redness, but also allow both phagocytic cells and humoral components to be concentrated at the site of infection.

A family of distinct transmembrane proteins, called Toll-like receptors, are found on many cell types, including macrophages, neutrophils, dendritic cells, mucosal epithelial cells, and endothelial cells. They recognize molecular patterns associated with microorganisms even in the absence of prior exposure, alert the host to the presence of the infectious agent and rapidly initiate a cascade of processes to activate innate immune responses, and help bridge innate and adaptive immune responses.3,6

Adaptive (Microbe-Specific) Immunity

Humoral Immunity

Immunoglobulins.: IgM is the first immunoglobulin to appear in response to a new antigen. Although it has less affinity at binding antigens than IgG does, IgM provides some recognition of antigens and begins B-cell proliferation before the subsequent development of IgG.10 IgM is detectable earlier in serum than IgG and serves as a marker for a patient’s early response to acute infection.

Secretory IgA is the predominant immunoglobulin present in gastrointestinal fluids, nasal and oral secretions, tears, and other mucous fluids. IgA inhibits cell adherence of viral, bacterial, and protozoan pathogens and therefore prevents invasion by organisms through the respiratory or gastrointestinal tract.10

IgE, which is expressed in high concentration on the surface of mast cells and basophils, is responsible for immediate-type hypersensitivity responses. Mast cells and IgE are important in defense against helminthic pathogens.

IgG, which accounts for 75% of the total immunoglobulin mass, is widely distributed in tissues. It crosses the placenta and provides fetal immunity during the first 6 months of life. Congenital or acquired deficiencies of IgG lead to infection with encapsulated organisms because the predominant subtype (IgG2) has affinity for the dense polysaccharides of bacterial cell capsules, such as those of Streptococcus pneumoniae and H. influenzae.

Complement.: The complement cascade, consisting of a complex interaction of 30 proteins, is another crucial component of humoral response. Complement is important in producing inflammation and leukocytosis and in recruiting leukocytes to sites of infection by production of chemoattractants. Complement also neutralizes viruses, enhances opsonization of bacteria, and produces bacterial cell wall and membrane lysis.

Both IgG and IgM, when they are in contact with an antigen, activate the classical pathway, whereas molecules with repeating chemical structures (e.g., bacterial cell walls and capsules) activate the cascade through the alternative pathway. Components of C3, the merging point of the classical and alternative paths, provide opsonization and modulate the response of lymphocytes (cell-mediated immunity). Opsonization is important in defense against infection with S. pneumoniae, Streptococcus pyogenes, H. influenzae, and Staphylococcus aureus. The terminal leg of the cascade, C5 through C9, forms the membrane attack complex, which inserts into cell walls and membranes and leads to cell death.

Individuals with inherited complement deficiencies are predisposed to frequent and recurrent infections with S. pneumoniae, H. influenzae, and especially Neisseria meningitidis and Neisseria gonorrhoeae.11 The risk of meningococcal infection is increased several thousand-fold and most often develops in people deficient in C3 and in late complement components (C5-C8). Paradoxically, the disease is usually milder with complement deficiency, and mortality is likewise reduced fivefold to tenfold.12 This suggests that the host response may be, in part, responsible for the severity of disease in normal individuals and is attenuated in complement deficiency. People with meningococcemia should be tested for inherited complement deficiencies because they may benefit from immunization.

Acquired deficiencies of complement function may develop in people with rheumatologic diseases, especially systemic lupus erythematosus (SLE). Approximately 40% of patients with SLE have an inhibitor of C5a-derived chemotaxis in their serum that results in enhanced susceptibility to infection.13

Cell-Mediated Immunity

Cell-mediated immunity (CMI) generally includes immune responses that are mediated by T lymphocytes, natural killer (NK) cells, and mononuclear phagocytes. CMI is vitally important in the control of infections caused by microbes that survive and replicate intracellularly, including most viruses and some bacterial (obligate and facultative intracellular types), fungal, and protozoan pathogens.

Only 5% of lymphocytes are in circulating blood. Most mature and are active in the marrow, thymus, spleen, and lymph nodes. The last two sites expose T cells to circulating antigen from invading microbes.6 Specialized antigen-presenting cells in the lymphoid system sequester antigen and antigen-antibody complexes and present them to T cells. This process involves internalization and processing of the antigen, followed by formation of peptides that bind to a cell surface molecule called the major histocompatibility complex (MHC). Only with this specific presentation can a T lymphocyte become activated against a particular antigen.

Two major types of T lymphocytes are CD4 (helper cell) and CD8 (suppressor cell), corresponding to type II and type I of MHC, respectively. CD4 lymphocytes provide help for other cells in the immune system, including enhanced B-cell antibody production and production of cytokines. CD8 lymphocytes are generally cytotoxic and mediate the eradication of virally infected target cells and certain tumors. A decline in the number of CD4 cells, with predominance of CD8 cells, is responsible for the increased susceptibility to infection in patients with acquired immunodeficiency syndrome (AIDS).6 Despite the cytotoxicity of CD8 cells, immunity is reduced without adequate numbers of CD4 cells.

Patients with defects in CMI are at increased risk for disseminated infection with intracellular bacteria, such as Mycobacterium tuberculosis, Listeria monocytogenes, and Salmonella species. The DNA viral infections, such as cytomegalovirus, herpes simplex, and varicella-zoster, also affect these patients more severely, as do fungal infections with Candida, Cryptococcus, Mucor, Aspergillus, and Pneumocystis. Finally, some protozoa are pathogenic without intact CMI, as infections with Toxoplasma gondii demonstrate.14,15 Some infections are seen only below a certain CD4 cell count. Pneumocystis pneumonia is seen almost exclusively in patients with counts below 200 cells/mL (2 × 105 cells/L), whereas almost all patients with toxoplasmosis or cryptococcal meningitis have counts below 100 cells/mL (1 × 105 cells/L).

NK cells, closely related to lymphocytes but neither B nor T cells, are important in the innate immune response and are found in high concentrations in blood and spleen.3,6 NK cells recognize infected cells and respond by directly killing these cells, and they secrete cytokines that activate macrophages to destroy phagocytosed microbes. These cells are important in defense against intracellular microbes, particularly viruses and intracellular bacteria such as L. monocytogenes.

Granulocytic Phagocytes

Granulocytic phagocytes are the cellular effectors of microbe killing, engulfing them and enzymatically lysing their cell membranes or walls. Two major types are polymorphonuclear leukocytes (also called neutrophils) and macrophages (the tissue version of circulating monocytes). Macrophages have surface receptors that recognize nonvertebrate carbohydrates such as mannose, which form the cell wall of some microorganisms. Hence, they can identify and attack “invaders” rather than “self.”

Two other types of granulocytes, eosinophils and basophils, are less involved in the ingestion of organisms.16 Eosinophils mediate the destruction of certain parasitic helminths through release of toxic proteins. Normally only 3% of total granulocytes, this cell type can reach 20% during times of high parasite load. Basophils (rare in circulation) and their tissue counterparts, mast cells, have high affinity for IgE. On exposure to antigens, they release granules with histamine, prostaglandins, and leukotrienes, which affect the allergic-inflammatory response with increased vascular permeability, bronchospasm, and vasodilation.2

Neutrophils constitute 90% of circulating granulocytes and spend only 6 to 8 hours of their average 4-day life in circulation (the remainder in tissues). Effective antibacterial activity depends on the ability of neutrophils to travel to sites of infection, a process known as chemotaxis. The locomotion of neutrophils along vascular endothelium is facilitated by adherence to cell surface proteins whose production is enhanced in the initial inflammatory response.16

Half of all neutrophils that leave the bone marrow circulate in the plasma. The other half become marginated, adhering to endothelium, primarily in the lungs, liver, and spleen. During periods of stress or with endogenous or exogenous catecholamines or corticosteroids, these neutrophils demarginate and enter the circulation. As long as the patient is not neutropenic, demargination causes an increased peripheral neutrophil count composed of mature cells, whereas with infection, an increased proportion of immature (band) forms is typically seen.

Neutrophils (and macrophages in tissue) bind to and ingest bacteria, a process called phagocytosis. This process is enhanced by proteins called opsonins that bind to bacterial surfaces. C-reactive protein, one of the initial inflammatory response proteins, fulfills this function for certain bacteria, including S. pneumoniae. IgG and complement protein C3b also opsonize bacteria, again illustrating the interdependence of the immune system. Actual killing takes place within granulocytes when cytoplasmic granules enzymatically produce potent oxidants. Granulocytes further control bacterial proliferation at the site of infection by elaborating lactoferrin, which locally binds free iron necessary for bacterial replication.

In addition to phagocytosis, macrophages (located in the spleen, alveoli, liver, and lymph nodes) modulate the immune response by presenting antigens to lymphocytes and releasing cytokines and complement components. Activation of macrophages to ingest bacteria depends on interaction with interferon-γ, a cytokine manufactured by T cells.6 Thus the once clear demarcation between cellular and humoral immunity is breaking down as more is understood about the interdependent immune system.

Specific Immunocompromised States

Cancer

Patients with cancer frequently have multiple immune defects, such as neutropenia and impaired function of T and B cells, induced by cancer chemotherapy or by the disease process itself, which predisposes them to infection. Other factors leading to infection are defects in physical barriers (skin and mucous membranes), including cytotoxic effects of chemotherapy on cells lining the gastrointestinal tract. In addition, splenic dysfunction or splenectomy, use of long-term intravascular catheters, frequent use of complex invasive diagnostic and therapeutic procedures, toxic effects of radiation therapy, and frequent colonization with antimicrobial-resistant pathogens are predisposing factors. Cancer treatments (e.g., allogeneic bone marrow and autologous stem cell transplantation, platelet transfusion, granulocyte colony-stimulating factor, and implanted central venous catheters) increase survival during episodes of profound immunosuppression, allowing patients to receive more intense cytotoxic cancer chemotherapy regimens. This results in long survival of patients with neoplastic diseases that were formerly rapidly fatal. Despite many advances in supportive care, infections continue to result in serious morbidity and mortality. Furthermore, increasing resistance to antimicrobials is occurring among common pathogens along with the emergence of new opportunistic pathogens. Infection is much more common in patients with acute leukemia and lymphoma (75% of patients) and multiple myeloma (50% of patients) than in those with solid tumors.17 Factors predisposing to infection in immunocompromised patients are listed in Box 183-1.

BOX 183-1   The Immunocompromised Patient

Factors Predisposing to Infection and the Most Common Pathogens

Neutropenia

Principles of Disease.: Neutropenia is defined as a neutrophil count of less than 500 cells/mL (5 × 105 cells/L), including band forms, or less than 1000 cells/mL (1 × 106 cells/L) and expected to fall to less than 500 cells/mL.18,19 It usually results from cytotoxic chemotherapy or radiation therapy or the disease process, especially in hematologic malignant neoplasms. In addition, cancer chemotherapeutic agents and radiation therapy can cause functional defects in granulocytes. The risk of febrile neutropenia and mortality is higher in the first one or two cycles of multicycle cytotoxic chemotherapy regimens.20

The incidence and severity of infection in cancer patients with neutropenia are inversely proportional to the absolute neutrophil count and directly proportional to the duration of neutropenia. Although the incidence begins to rise as the neutrophil count falls below 500 cells/mL (5 × 105 cells/L), most severe infections and almost all bacteremias occur when the neutrophil count is less than 100 cells/mL.21 Fever in the neutropenic patient is defined as a single temperature of 38.3° C (101° F) or higher or a temperature of 38.0° C (100.4° F) or higher during 1 or 2 hours.19 In neutropenic patients, the temperature should be measured orally or tympanically, not rectally. Although fever can be suppressed or lessened by immunosuppressive agents such as corticosteroids and nonsteroidal anti-inflammatory drugs, most cancer patients with infection manifest fever despite the use of these agents.22 Also, although it is uncommon, immunocompromised patients can have serious local or systemic infections without fever. This is manifested by unexplained tachypnea or tachycardia, mental status changes, metabolic acidosis, increased volume requirements, rapid changes in serum glucose or sodium concentration, or acute abdominal pain. Because the onset of life-threatening infections can be rapid in cancer patients with severe neutropenia or a history of splenectomy, urgent evaluation and initiation of antimicrobial therapy are essential. A prospective multicenter observational study of febrile neutropenic cancer patients in EDs in France found that critically ill patients were poorly recognized and undertreated, low-risk patients were overtreated, and compliance with established guidelines was low.23

The most common sites of infection in neutropenic patients are the lung (25%); mouth and pharynx (25%); gastrointestinal tract (15%); skin, soft tissue, and intravascular catheters (15%); perineum and anorectal area (10%); urinary tract (5%); and nose and sinuses (5%).24 Pneumonia and anorectal infection are more likely to be associated with bacteremia. Bacteremia may occur without an obvious source despite intensive investigation. Historically, the most important bacteria are three gram-negative bacilli—Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa—and four gram-positive cocci—Staphylococcus epidermidis, viridans group streptococci, Enterococcus species, and S. aureus. Many centers that treat large numbers of cancer patients note a decrease in these gram-negative bacilli and an increase in infections caused by others, such as Enterobacter, Citrobacter, and Serratia species, which are capable of rapidly developing resistance to cephalosporins and extended-spectrum penicillins. Anaerobes are uncommon but may be important in certain mixed infections (e.g., mouth, abdominal, and perianal).

During the past 25 years, infection with gram-positive organisms (e.g., coagulase-negative staphylococci, S. aureus, viridans streptococci, and Enterococcus species) has increased, and this is now the leading cause of bacterial infection (50-70% at some centers) in febrile neutropenic cancer patients in the United States, Canada, and western Europe. Gram-negative organisms still predominate in developing countries.25,26 With the exception of viridans streptococci, most of these gram-positive organisms do not produce immediately life-threatening infections compared with the rapid lethality of many gram-negative infections. Life-threatening bloodstream infections caused by viridans streptococci (especially Streptococcus mitis) are common in many cancer centers and often respond poorly to penicillins and cephalosporins. Risk factors for serious viridans streptococcal infections include aggressive cytoreduction therapy for acute leukemia or allogeneic bone marrow transplantation (especially after high-dose cytosine arabinoside treatment), profound neutropenia, and severe oral mucositis. Other factors include prophylactic use of trimethoprim-sulfamethoxazole or fluoroquinolones, use of antacids or H2 receptor antagonists, and childhood.27,28

Aspergillus and Candida species are the most common fungi producing infection in cancer patients with fever and neutropenia.24,29,30 Infection is most likely to develop in neutropenic patients treated with broad-spectrum antimicrobials and in those whose fever persists for more than 7 days. Aspergillus species usually produce necrotizing infections in the lung or sinuses. Pulmonary aspergillosis often is manifested with pleuritic pain, hemoptysis, and localized wheezing. The chest radiograph demonstrates pleural effusion or focal infiltrates. Computed tomography (CT) is more sensitive in detection of pulmonary infiltrates compatible with aspergillosis, and it may demonstrate a distinct halo of low attenuation surrounding a pulmonary infiltrate. This pattern is highly suggestive of invasive aspergillosis, although mucormycosis and other disorders may mimic the halo. Invasive aspergillosis originating in the paranasal sinuses may extend to the surrounding bone and brain. Often, an initial red-purple lesion on the nasal turbinate or palate turns pale and then black as vascular invasion produces infarction of the mucosa and bone. The black eschar on the nose or palate is easily misdiagnosed as dried blood. Patients presenting with head or facial pain or swelling, or proptosis, should be rapidly evaluated for invasive aspergillosis and mucormycosis. Candida species produce infections of the skin, oral cavity, and esophagus as well as fungemia. The sudden onset of generalized rash consisting of pink-purple, nontender subcutaneous nodules is characteristic of candidemia.

Less common fungi producing infection in these patients include Mucor and Rhizopus species (necrotizing pneumonia and sinusitis), Trichosporon species (pneumonia and fungemia), and Pseudallescheria boydii (soft tissue infection). Other than Candida species, these fungi are rarely found in blood cultures. Specific diagnosis requires biopsy.

Clinical Features.: Certain clinical findings are characteristic of specific pathogens (Table 183-1). Noninfectious causes of fever also need to be considered, such as drug toxicity, drug allergy, transfusion reactions, and pulmonary emboli.17 Fever is frequently the only sign of infection because these patients are unable to mount a full inflammatory response at a site of infection.24 Usual symptoms and signs of infection may not be present, especially when the neutrophil count is less than 100 cells/mL (1 × 105 cells/L). When pneumonia develops, purulent sputum may be absent, and the initial chest radiograph may not show an infiltrate. Pyuria may be absent in the presence of urinary tract infection. Areas of cellulitis may have diminished or absent induration and redness and no purulent drainage. Tenderness may be the only finding in perineal and anal infections. The neutropenic patient with a documented infectious cause of fever may be difficult to distinguish from the patient with fever not caused by infection. The performance of a procedure before the onset of fever, presence of chills, “toxic appearance,” and lack of localized findings do not help determine whether the patient is bacteremic.31 Only 20% of febrile neutropenic patients have a clinical focus of infection identified at presentation, and only 30% of patients have positive blood cultures.

Table 183-1

Characteristic Clinical Findings in Neutropenia That May Be Associated with Infection with Specific Pathogens

CHARACTERISTIC CLINICAL FINDINGS SUSPECT PATHOGENS
Ulcerative lesions in the mouth Viridans streptococci, herpes simplex, Candida, anaerobes
Necrotizing skin lesions Pseudomonas aeruginosa, Aeromonas hydrophila, Aspergillus, Mucor
Nontender subcutaneous nodules Nocardia, Cryptococcus
Nontender pink skin papules Candida
Black eschar of nose or palate Aspergillus, Mucor
Generalized macular red rash Viridans streptococci
Right lower quadrant abdominal pain, tenderness, distention, bloody diarrhea Typhlitis (neutropenic enterocolitis) caused by Pseudomonas aeruginosa, Escherichia coli, Clostridium septicum
Perineal pain and tenderness without inflammation or abscess Gram-negative bacilli, anaerobes
Redness or pain at vascular catheter sites Coagulase-negative staphylococci, Corynebacterium, Bacillus species

Mucositis involving the mouth and other mucous membranes is a painful and debilitating condition that commonly occurs in cancer patients receiving intense chemotherapy. It is a frequent prelude to viridans streptococcal bacteremia, which can produce sudden onset of acute respiratory distress syndrome, a toxic shock–like syndrome, rash, and pneumonia.

Diagnostic Strategies.: The evaluation of the cancer patient with fever and neutropenia should include a meticulous search for subtle symptoms and signs of inflammation at common sites: oral cavity and pharynx, lower esophagus, lung, skin, perineum including anus, bone marrow aspiration sites, vascular catheter sites, and tissue around the nails.21 In nearly two thirds of patients, the initial evaluation does not identify a focus of infection.22 Two sets of blood culture specimens should be obtained. If the patient has a central venous catheter, culture specimens of blood should be obtained from each lumen of a multilumen catheter and from at least one peripheral site.19 Specimens for culture should also be obtained from any site of inflammation, including inflamed or draining catheter exit sites. Patients with severe mucositis should have herpes simplex cultures performed if they are not receiving antiherpes prophylaxis, and they should have a smear for Candida pseudohyphae. Complete blood count, electrolyte values, transaminase levels, blood urea nitrogen concentration, and creatinine concentration should be determined to plan management and to monitor the occurrence of drug toxicity.

Urine culture should be performed if symptoms or signs of a urinary tract infection are present, if a urinary catheter is in place, or if the urinalysis is abnormal. Examination of the cerebrospinal fluid is not recommended as a routine procedure unless subtle symptoms or signs of meningitis are present.

A chest radiograph should be obtained in patients with respiratory symptoms and signs.19

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