# Seminar 04: Approach to Fever ## Year 3: Internal Medicine Clerkship --- ## Learning Objectives By the end of this seminar, students will be able to: 1. Define fever and its pathophysiology 2. Develop a systematic approach to fever evaluation 3. Differentiate infectious from non-infectious causes 4. Describe fever of unknown origin workup 5. Apply appropriate management strategies 6. Recognize fever in special populations --- ## Seminar Outline ### I. Definition and Pathophysiology Body temperature follows a predictable physiologic range with characteristic variations throughout the day. Normal core body temperature typically ranges from 36.1 to 37.2 degrees Celsius (97 to 99 degrees Fahrenheit) when measured orally, though rectal temperatures run approximately 0.5 degrees higher. Fever is defined as a temperature exceeding 38.0 degrees Celsius (100.4 degrees Fahrenheit), representing an upward adjustment of the hypothalamic thermoregulatory set point in response to pyrogenic stimuli. Hyperpyrexia describes temperatures above 41.5 degrees Celsius (106.7 degrees Fahrenheit) and constitutes a medical emergency requiring immediate intervention. Hypothermia, defined as temperature below 35 degrees Celsius (95 degrees Fahrenheit), can paradoxically occur in severe infections, particularly in elderly or immunocompromised patients. Diurnal variation causes temperatures to be lowest in the early morning and highest in the late afternoon, with variations of 0.5 to 1.0 degrees being normal. The mechanism of fever generation involves a coordinated neuroimmune response mediated through the hypothalamus. Pyrogens are substances that trigger fever and can be exogenous (such as bacterial lipopolysaccharide) or endogenous (cytokines including interleukin-1, interleukin-6, and tumor necrosis factor). These pyrogens stimulate prostaglandin E2 production in the hypothalamus, which raises the thermoregulatory set point to a higher temperature. The body then perceives its current temperature as below the new set point and initiates heat-generating mechanisms including shivering, peripheral vasoconstriction to conserve heat, and behavioral responses such as seeking warmth. Heat conservation through peripheral vasoconstriction causes the characteristic chill that patients experience as fever develops, while defervescence is accompanied by vasodilation and sweating. Fever patterns, while not diagnostically specific, may provide useful clinical clues to underlying etiology. Continuous fever shows minimal variation (less than one degree Celsius) throughout the day and is associated with typhoid fever, drug fever, and central nervous system infections. Remittent fever demonstrates daily variations exceeding one degree but without return to normal baseline, seen in most bacterial infections. Intermittent fever returns to normal at least once daily and may suggest abscess, malaria, or endocarditis when dramatic swings occur. Relapsing fever features discrete fever-free intervals of days to weeks between febrile episodes, occurring in brucellosis and lymphoma. The Pel-Ebstein pattern consists of weeks of fever alternating with weeks of normal temperature, classically described in Hodgkin lymphoma though rarely observed in clinical practice. Fever serves important host defense functions but also carries physiologic costs that must be considered. Benefits include enhanced immune function through improved leukocyte mobility, phagocytosis, and cytokine production at elevated temperatures. Bacterial replication is inhibited at febrile temperatures, and certain antibiotics demonstrate improved activity in the setting of fever. Fever also serves as a warning signal that prompts patients to seek medical attention and allows clinicians to monitor disease progression. However, fever increases metabolic demand by approximately 10% for each degree Celsius of temperature elevation, stressing patients with limited cardiac reserve. Febrile seizures can occur in children between 6 months and 5 years of age, though these are generally benign. Elderly patients may experience delirium, and prolonged hyperpyrexia can cause protein denaturation and organ damage. Panel A: Diagram of the hypothalamic thermoregulation system showing how pyrogens (exogenous and endogenous) stimulate prostaglandin E2 production, raising the thermoregulatory set point and triggering heat generation and conservation mechanisms. Panel B: Temperature curve graphs illustrating the four major fever patterns (continuous, remittent, intermittent, and relapsing) with corresponding disease associations labeled. Panel C: Timeline showing diurnal temperature variation with lowest point in early morning and peak in late afternoon, with normal range shaded. Panel D: Infographic comparing the beneficial effects of fever (enhanced immunity, decreased bacterial replication) versus risks (increased metabolic demand, seizures, delirium) with clinical implications. --- ### II. Initial Assessment History taking in the febrile patient must be comprehensive, exploring multiple dimensions that help narrow the differential diagnosis. The onset and duration of fever distinguish acute processes (typically infectious) from subacute or chronic presentations that may suggest malignancy, autoimmune disease, or indolent infection. The pattern of fever, including relationship to specific times of day and response to antipyretics, provides additional clues. Associated symptoms must be systematically reviewed to identify the source of infection or inflammation, with particular attention to localizing symptoms such as cough, dysuria, headache, or abdominal pain. Recent exposures including travel to endemic areas, sick contacts, animal contact, insect bites, and occupational hazards may identify specific pathogens. Medical history should address immunocompromising conditions, prosthetic devices, and recent hospitalizations. A complete medication list may reveal drug fever as a potential cause. The review of systems should systematically interrogate each organ system to identify the fever source. Head, ears, eyes, nose, and throat symptoms including pharyngitis, sinusitis, and otitis are common fever sources, particularly in outpatient settings. Respiratory symptoms such as cough, sputum production, and dyspnea suggest pneumonia or bronchitis. Cardiovascular symptoms are less common but a new heart murmur raises concern for endocarditis. Gastrointestinal complaints including diarrhea, abdominal pain, nausea, and vomiting may indicate gastroenteritis, cholecystitis, or intra-abdominal abscess. Genitourinary symptoms of dysuria, frequency, and flank pain suggest urinary tract infection or pyelonephritis. Skin examination may reveal rashes, wounds, or soft tissue infections. Neurologic symptoms including headache, neck stiffness, and photophobia are critical to identify as they may indicate meningitis requiring immediate intervention. Risk factor assessment identifies patients at increased risk for specific infections based on their underlying conditions and exposures. Recent travel history should specify destinations and activities, as travel to specific regions implies risk for endemic infections including malaria, typhoid, dengue, and other tropical diseases. Immunocompromise from HIV, chemotherapy, organ transplant, or immunosuppressive medications dramatically expands the differential to include opportunistic pathogens. Intravenous drug use increases risk for endocarditis, skin and soft tissue infections, and osteomyelitis, often with unusual organisms. Recent hospitalization raises concern for healthcare-associated infections including Clostridioides difficile, catheter-associated urinary tract infections, and ventilator-associated pneumonia. Recent surgical procedures increase risk for surgical site infections, typically presenting 5-7 days postoperatively. Indwelling devices including central venous catheters, prosthetic joints, and pacemakers serve as potential foci for infection. Vital signs provide immediate severity assessment and may suggest specific diagnoses. Tachycardia typically accompanies fever, with heart rate increasing approximately 10 beats per minute for each degree Celsius of temperature elevation. Relative bradycardia, where heart rate fails to increase appropriately with fever, is a classic finding in typhoid fever, Legionella pneumonia, and drug fever, and should prompt consideration of these diagnoses. Hypotension indicates potential sepsis and mandates aggressive resuscitation and source identification. Tachypnea may indicate pneumonia as the fever source or represent compensation for metabolic acidosis in sepsis. Hypoxia requires immediate attention and often indicates pulmonary pathology. The combination of fever with hypotension and tachypnea should trigger immediate sepsis evaluation and management. Panel A: Flowchart for history-taking in the febrile patient showing the systematic approach from fever characterization through associated symptoms to exposure history and risk factor assessment. Panel B: Body diagram with organ systems labeled and associated symptoms for each system that help localize the fever source. Panel C: Risk factor assessment checklist organized by travel history, immunocompromise, device-related risks, and healthcare exposures with associated pathogens. Panel D: Vital signs interpretation guide showing expected relationships between temperature and heart rate, with relative bradycardia highlighted and associated conditions listed. --- ### III. Physical Examination General appearance immediately communicates disease severity and guides the urgency of evaluation and treatment. Toxic appearance, characterized by pallor, lethargy, diaphoresis, and distress, suggests serious systemic infection requiring immediate intervention. Rigors, which are true shaking chills with teeth chattering, strongly suggest bacteremia and warrant blood cultures before antibiotic administration. Diaphoresis occurs during defervescence as the hypothalamic set point decreases and the body attempts to shed excess heat through sweating. Mental status changes including confusion, agitation, or decreased level of consciousness may indicate meningitis, sepsis, or severe infection affecting brain perfusion. The overall assessment of whether the patient appears "sick" or "well" provides valuable prognostic information that influences disposition decisions. Systematic physical examination must thoroughly evaluate each body system to identify the source of infection. Skin examination looks for rashes, petechiae, cellulitis, wound infections, and embolic lesions that might indicate endocarditis. Head and neck examination includes assessment of sinuses for tenderness, pharynx for erythema and exudates, tympanic membranes for otitis, and fundoscopy for papilledema or Roth spots. Lymph node examination assesses for lymphadenopathy, noting the distribution (localized versus generalized), consistency, and tenderness. Cardiac auscultation listens carefully for new murmurs that might indicate endocarditis, particularly in patients with risk factors such as intravenous drug use or prosthetic valves. Lung examination assesses for consolidation signs including bronchial breath sounds, egophony, and crackles. Abdominal examination evaluates for tenderness, organomegaly, and peritoneal signs. Genitourinary examination includes costovertebral angle tenderness assessment. Joint examination looks for signs of septic arthritis. Neurologic examination assesses for meningismus. Skin findings provide specific diagnostic information based on their morphology and distribution. Petechiae, small non-blanching purpuric lesions, suggest meningococcemia, endocarditis, or thrombocytopenia and represent a medical emergency when accompanied by fever. Maculopapular rashes are non-specific and may indicate viral exanthems, drug reactions, or early presentations of more serious conditions. Erythroderma, diffuse skin erythema, suggests toxic shock syndrome or severe drug reaction. Eschars, black necrotic lesions with surrounding erythema, indicate rickettsial infections such as scrub typhus, Mediterranean spotted fever, or tularemia. Vesicular lesions suggest varicella zoster or herpes simplex infections. Osler nodes (tender nodules on finger pads) and Janeway lesions (non-tender erythematous lesions on palms and soles) are stigmata of infective endocarditis. Splinter hemorrhages in nail beds also suggest endocarditis. Lymphadenopathy distribution provides diagnostic clues to the underlying cause. Localized lymphadenopathy suggests regional infection or malignancy draining to that lymph node basin, with tender nodes favoring infection. Generalized lymphadenopathy involving multiple non-contiguous regions suggests systemic disease including viral infections (EBV, CMV, HIV), lymphoma, sarcoidosis, or autoimmune conditions. Supraclavicular lymphadenopathy is concerning for malignancy, particularly when found on the left (Virchow node) suggesting intra-abdominal malignancy. Epitrochlear lymphadenopathy is unusual and classically associated with secondary syphilis, lymphoma, or sarcoidosis. The consistency of nodes is informative: tender, mobile, soft nodes suggest infection, while firm, fixed, non-tender nodes raise concern for malignancy. Panel A: Illustration of a patient demonstrating toxic appearance with labeled features including pallor, diaphoresis, altered mental status, and rigors, contrasted with well-appearing patient. Panel B: Systematic physical examination guide showing the head-to-toe approach with specific findings to assess at each location for the febrile patient. Panel C: Photograph collection of skin findings in fever showing petechiae, maculopapular rash, erythroderma, eschar, vesicles, Osler nodes, and Janeway lesions with associated diagnoses. Panel D: Lymph node diagram showing anatomic distributions with localized versus generalized patterns and associated diagnoses for each region. --- ### IV. Infectious Causes Common infections represent the majority of fever presentations and can be organized by anatomic site of infection. Respiratory infections including community-acquired pneumonia, viral upper respiratory infections, sinusitis, and bronchitis are among the most frequent causes of fever in both outpatient and inpatient settings. Urinary tract infections ranging from simple cystitis to complicated pyelonephritis commonly cause fever, particularly in women and patients with urinary catheters. Skin and soft tissue infections including cellulitis, abscess, and wound infections present with fever accompanied by localizing signs of erythema, warmth, and tenderness. Gastrointestinal infections including viral gastroenteritis, bacterial enteritis, and cholangitis cause fever with associated abdominal symptoms. Bone and joint infections including osteomyelitis and septic arthritis present with fever and focal musculoskeletal complaints. Central nervous system infections including meningitis and encephalitis cause fever with neurologic symptoms. Life-threatening infections require immediate recognition because delayed treatment dramatically increases mortality. Bacterial meningitis presents with the classic triad of headache, neck stiffness, and altered mental status, though all three features are present in less than half of cases, so fever with any neurologic symptom warrants consideration. Sepsis represents life-threatening organ dysfunction caused by dysregulated host response to infection, presenting with fever (or hypothermia), tachycardia, hypotension, and evidence of end-organ dysfunction. Necrotizing fasciitis is a rapidly progressive soft tissue infection characterized by pain out of proportion to physical findings, crepitus, and rapid clinical deterioration requiring emergent surgical debridement. Infective endocarditis presents with fever, new or changing heart murmur, and embolic phenomena including stroke, splenic infarcts, and skin lesions. Malaria should be considered in any febrile patient returning from endemic areas, as severe falciparum malaria can progress rapidly to death. Hospital-acquired infections develop 48 hours or more after hospital admission and involve a distinct spectrum of pathogens. Central line-associated bloodstream infections (CLABSIs) occur when central venous catheters become colonized with bacteria or fungi, presenting with fever and sometimes signs of catheter site infection. Catheter-associated urinary tract infections (CAUTIs) develop in patients with indwelling urinary catheters, often with minimal symptoms beyond fever. Ventilator-associated pneumonia (VAP) occurs in mechanically ventilated patients, presenting with fever, new infiltrate, and purulent secretions. Clostridioides difficile infection follows antibiotic exposure and presents with fever and diarrhea, potentially progressing to toxic megacolon. Surgical site infections typically present 5-7 days postoperatively with fever, wound erythema, and drainage. Opportunistic infections occur in immunocompromised patients and require consideration of organisms that rarely cause disease in immunocompetent hosts. HIV-infected patients with low CD4 counts are susceptible to Pneumocystis jirovecii pneumonia, cytomegalovirus disease, Mycobacterium avium complex infection, cryptococcal meningitis, and toxoplasmosis, with specific infections correlating with degree of immunosuppression. Neutropenic patients, particularly those receiving chemotherapy, are at high risk for bacterial infections including gram-negative bacteremia and fungal infections including invasive aspergillosis and candidiasis. Solid organ transplant recipients are susceptible to cytomegalovirus, BK virus, and fungal infections, with risk varying by time post-transplant. Patients receiving biologic immunosuppressive agents are at increased risk for tuberculosis reactivation, fungal infections, and progressive multifocal leukoencephalopathy from JC virus. Panel A: Body diagram showing common infection sites with labeled anatomic locations and associated typical pathogens (respiratory, urinary, skin/soft tissue, intra-abdominal, musculoskeletal, CNS). Panel B: Emergency presentation photographs and clinical features of life-threatening infections including meningitis (Kernig and Brudzinski signs), sepsis (mottled skin, altered mental status), and necrotizing fasciitis (wound progression). Panel C: Timeline of hospital-acquired infections showing typical onset after admission with CLABSI, CAUTI, VAP, and C. difficile at appropriate timepoints. Panel D: CD4 count stratification chart for HIV patients showing opportunistic infections that occur at each level of immunosuppression. --- ### V. Non-Infectious Causes Malignancy causes fever through several mechanisms and should be considered when infectious workup is unrevealing. Lymphoma, particularly Hodgkin lymphoma, classically presents with fever as part of "B symptoms" along with night sweats and unintentional weight loss. Leukemia, especially acute leukemia, causes fever from the disease itself and from infectious complications of neutropenia. Renal cell carcinoma has been termed "the internist's tumor" because of its frequent presentation with fever and other paraneoplastic manifestations. Hepatocellular carcinoma may cause fever, particularly in patients with underlying cirrhosis who are also at risk for spontaneous bacterial peritonitis. Metastatic disease, particularly with extensive hepatic involvement, can cause fever without evident infection. The mechanism of malignancy-associated fever involves tumor production of pyrogenic cytokines including interleukin-1 and interleukin-6. Autoimmune and inflammatory diseases frequently present with fever as a prominent feature. Systemic lupus erythematosus causes fever as part of its multisystem involvement, though distinguishing lupus flare from infection can be challenging in these immunocompromised patients. Adult-onset Still disease classically presents with quotidian fever (returning to normal or subnormal daily), evanescent salmon-colored rash, arthritis, and markedly elevated serum ferritin. Vasculitis syndromes including giant cell arteritis, polyarteritis nodosa, and ANCA-associated vasculitis commonly cause fever along with elevated inflammatory markers. Inflammatory bowel disease, both Crohn disease and ulcerative colitis, may present with fever accompanying gastrointestinal symptoms. Sarcoidosis can cause fever as part of its systemic granulomatous inflammation. Drug fever represents an important and often overlooked cause of fever in hospitalized patients. The typical timing is 7-10 days after starting a new medication, though fever can occur earlier with re-exposure to a previously sensitizing agent. The fever pattern is often continuous and high-grade, yet patients characteristically appear well despite impressive temperatures. Relative bradycardia, where heart rate is lower than expected for the degree of fever, is a classic clue. Eosinophilia is present in approximately one-third of cases and supports the diagnosis when present. The diagnosis is confirmed by resolution of fever within 48-72 hours of discontinuing the offending medication, with rapid recurrence if rechallenged. Common culprits include beta-lactam antibiotics, sulfonamides, anticonvulsants (particularly phenytoin and carbamazepine), allopurinol, and procainamide. Other non-infectious causes of fever should be considered when initial evaluation does not reveal infection. Deep vein thrombosis and pulmonary embolism can cause low-grade fever, usually less than 38.5 degrees Celsius, and should be considered in patients with risk factors for venous thromboembolism. Myocardial infarction may cause fever in the days following the event, typically low-grade and self-limited. Thyroid storm causes high fever along with tachycardia, agitation, and other signs of severe hyperthyroidism. Adrenal insufficiency can present with fever, hypotension, and electrolyte abnormalities. Hematoma from recent procedures or trauma can cause fever as the blood is reabsorbed. Transfusion reactions, both hemolytic and febrile non-hemolytic reactions, cause fever during or shortly after blood product administration. Panel A: Malignancy-associated fever diagram showing tumor cells producing pyrogenic cytokines (IL-1, IL-6) with common malignancies listed including lymphoma, leukemia, renal cell carcinoma, and hepatocellular carcinoma. Panel B: Clinical photographs and features of autoimmune/inflammatory fever causes including Still disease rash, SLE manifestations, and vasculitis presentations. Panel C: Drug fever timeline showing typical onset at 7-10 days with rapid resolution after drug discontinuation, characteristic features listed (high fever, well appearance, relative bradycardia). Panel D: Differential diagnosis checklist of other non-infectious fever causes organized by organ system with distinguishing clinical features. --- ### VI. Fever of Unknown Origin The classic definition of fever of unknown origin (FUO) establishes specific criteria that distinguish it from common acute febrile illnesses. Temperature must exceed 38.3 degrees Celsius (101 degrees Fahrenheit) documented on multiple occasions. Duration must exceed three weeks of illness, ensuring that self-limited viral infections have been excluded. No diagnosis is established despite at least one week of intensive inpatient investigation, though modern definitions may include equivalent outpatient workup. These stringent criteria identify a population with complex underlying diseases that require systematic, often exhaustive evaluation. The modern approach emphasizes thorough outpatient workup before hospitalization, utilizing the interval to observe the fever pattern, repeat examinations, and pursue specialized testing. The causes of FUO can be categorized into four major groups with varying frequencies depending on the patient population and geographic location. Infections account for 25-30% of FUO cases and include tuberculosis, infective endocarditis, intra-abdominal abscess, osteomyelitis, and occult urinary tract infections. Malignancy represents 20-25% of cases, with lymphoma being most common followed by leukemia and solid tumors with hepatic involvement. Autoimmune and inflammatory conditions cause 15-20% of cases, including adult-onset Still disease, vasculitis syndromes, and systemic lupus erythematosus. Miscellaneous causes including drug fever, pulmonary embolism, and factitious fever account for approximately 15% of cases. Importantly, 20-25% of FUO cases remain undiagnosed despite thorough evaluation, and many of these patients recover spontaneously. The workup for FUO proceeds in a phased approach, beginning with comprehensive initial evaluation and advancing to more invasive testing as needed. Initial phase testing includes complete blood count with differential, comprehensive metabolic panel, liver function tests, erythrocyte sedimentation rate, C-reactive protein, blood cultures (at least three sets), urinalysis and urine culture, chest radiograph, and tuberculin skin test or interferon-gamma release assay. Secondary phase testing, guided by initial results, may include antinuclear antibody, rheumatoid factor, CT of chest, abdomen, and pelvis to evaluate for lymphadenopathy, abscess, or malignancy. Tertiary phase testing includes PET scan, which is increasingly valuable for localizing occult infection or malignancy, temporal artery biopsy in patients over 50 with elevated inflammatory markers, and bone marrow biopsy to evaluate for hematologic malignancy or granulomatous disease. The key principle is that testing should be guided by clinical clues rather than performed indiscriminately. Historical clues often provide the critical information needed to diagnose FUO. Travel history may reveal exposure to endemic infections such as brucellosis, Q fever, leishmaniasis, or histoplasmosis, with specific destinations determining the relevant pathogens. Pet and animal exposures suggest zoonoses including psittacosis from birds, brucellosis from unpasteurized dairy products, and Q fever from livestock contact. Occupational exposures may indicate specific infection risks such as brucellosis in veterinarians and farmers. Immigration from areas with high tuberculosis prevalence should prompt aggressive evaluation for active TB. Complete medication review may identify drug fever as the cause. Recent dental work or procedures in patients with valvular heart disease raises concern for endocarditis. Panel A: FUO definition criteria displayed as checklist with temperature, duration, and workup requirements clearly labeled. Panel B: Pie chart showing proportional distribution of FUO causes by category (infections, malignancy, autoimmune, miscellaneous, undiagnosed) with specific conditions listed in each segment. Panel C: Phased FUO workup algorithm showing initial tests, secondary tests guided by results, and tertiary invasive testing with decision points. Panel D: Geographic map highlighting endemic infections by region that should be considered based on travel history with representative pathogens labeled. --- ### VII. Diagnostic Workup Initial laboratory testing establishes baseline parameters and may identify the cause of fever or guide further investigation. Complete blood count with differential reveals leukocytosis suggesting bacterial infection, leukopenia indicating viral infection or overwhelming sepsis, or bandemia (left shift) suggesting significant bacterial infection. The basic metabolic panel assesses renal function, identifies electrolyte abnormalities that may accompany specific infections, and provides baseline values for patients who may receive potentially nephrotoxic treatments. Liver function tests may be elevated with hepatic infections, cholangitis, or disseminated disease affecting the liver. Urinalysis screening identifies urinary tract infection as a potential source. Blood cultures should be obtained before antibiotics whenever possible, with at least two sets drawn from separate venipuncture sites. Chest radiograph evaluates for pneumonia, mediastinal adenopathy, or other thoracic pathology. Inflammatory markers provide complementary information about the presence and severity of inflammation. Erythrocyte sedimentation rate (ESR) is a non-specific marker that rises in infection, malignancy, and autoimmune conditions, with very high values (greater than 100 mm/hour) suggesting conditions such as multiple myeloma, giant cell arteritis, or tuberculosis. C-reactive protein (CRP) is an acute phase reactant that rises rapidly with infection or inflammation and falls quickly with resolution, making it useful for monitoring treatment response. Procalcitonin is more specific for bacterial infection than ESR or CRP and can help distinguish bacterial from viral causes of fever, though its utility in guiding antibiotic decisions remains debated. Serum ferritin, when markedly elevated (typically greater than 10,000 ng/mL), suggests adult-onset Still disease or hemophagocytic lymphohistiocytosis, both important causes of fever that may be otherwise difficult to diagnose. Imaging studies localize and characterize potential sources of infection or inflammation. Chest radiography is the initial imaging study in most febrile patients, evaluating for pneumonia, pleural effusion, mediastinal masses, and other thoracic pathology. Computed tomography of the abdomen and pelvis is valuable when intra-abdominal pathology is suspected, identifying abscesses, lymphadenopathy, organomegaly, and masses that might not be apparent on physical examination. Echocardiography, both transthoracic and transesophageal, evaluates for endocarditis when clinical suspicion exists based on positive blood cultures, new murmur, or embolic phenomena. PET scan has emerged as a valuable tool in FUO, with FDG uptake localizing sites of infection, inflammation, or malignancy that may guide biopsy. MRI provides detailed soft tissue evaluation and is particularly useful for suspected osteomyelitis or soft tissue infection. Invasive procedures may be necessary when non-invasive testing fails to establish a diagnosis. Lumbar puncture with cerebrospinal fluid analysis is essential when meningitis or encephalitis is suspected, evaluating cell count, protein, glucose, gram stain, culture, and specific pathogen testing. Bone marrow biopsy evaluates for hematologic malignancy, granulomatous disease, and disseminated infections including tuberculosis, histoplasmosis, and leishmaniasis. Tissue biopsy of enlarged lymph nodes may reveal malignancy, granulomatous disease, or specific infections. Bronchoscopy with bronchoalveolar lavage is indicated for pulmonary infiltrates when the diagnosis is unclear, particularly in immunocompromised patients. The decision to proceed with invasive testing must balance the diagnostic yield against procedural risks, guided by the clinical probability of treatable conditions. Panel A: Laboratory testing algorithm showing first-line tests (CBC, BMP, LFTs, UA, blood cultures, CXR) with arrows to second-line tests based on abnormal findings. Panel B: Inflammatory markers comparison chart showing ESR, CRP, procalcitonin, and ferritin with typical values, time course, and associated conditions. Panel C: Imaging modality selection guide showing when to use chest X-ray, CT scan, echocardiography, PET scan, and MRI with representative images of positive findings. Panel D: Invasive procedure decision tree showing indications for lumbar puncture, bone marrow biopsy, lymph node biopsy, and bronchoscopy with expected yields. --- ### VIII. Management Supportive care addresses patient comfort and prevents complications while the underlying cause of fever is identified and treated. Maintaining adequate hydration is essential because fever increases insensible fluid losses through sweating and increased respiratory rate, with fluid requirements increasing approximately 200 mL per degree Celsius of temperature elevation. Antipyretic medications including acetaminophen (650-1000 mg every 6 hours) and NSAIDs (ibuprofen 400-800 mg every 8 hours) reduce temperature by inhibiting prostaglandin synthesis in the hypothalamus. Cooling measures including removal of excess blankets, cool compresses, and lukewarm water sponging may provide comfort but should not be so aggressive as to cause shivering, which generates heat. Monitoring vital signs and mental status allows early detection of deterioration that might indicate sepsis or other serious complications. The decision to treat fever must balance potential benefits against the diagnostic value of observing the fever pattern. Treatment of fever is generally indicated when patients are uncomfortable, at risk for febrile seizures (children), or when fever poses cardiovascular stress in patients with limited reserve. Withholding treatment may be considered when observation of the fever pattern provides diagnostic information, when fever might enhance immune response to infection, or when treatment might obscure important clinical data. Hyperpyrexia (temperature exceeding 41.5 degrees Celsius) always requires treatment because of the risk of protein denaturation and organ damage. In critically ill patients, benefits of treating fever are less clear, and routine antipyretic use is not recommended for all ICU patients. Empiric antibiotic therapy may be appropriate before culture results return in specific clinical scenarios. Sepsis or septic shock requires immediate broad-spectrum antibiotics within one hour of recognition, as each hour of delay increases mortality by approximately 7%. Neutropenic fever in patients with absolute neutrophil count below 500 cells/microliter requires immediate anti-pseudomonal antibiotic coverage because of the high risk of rapid clinical deterioration. Suspected meningitis requires empiric antibiotics immediately after lumbar puncture (or before LP if significant delays are anticipated), with ceftriaxone plus vancomycin plus ampicillin providing coverage for common pathogens. For stable patients without high-risk features, it is often appropriate to await culture results and diagnostic workup before initiating antibiotic therapy, avoiding unnecessary treatment and allowing accurate microbiologic diagnosis. Antibiotic selection should be guided by the suspected source of infection and local resistance patterns. Community-acquired pneumonia in hospitalized patients typically requires a beta-lactam (ceftriaxone) plus macrolide (azithromycin) or a respiratory fluoroquinolone (levofloxacin or moxifloxacin) to cover typical and atypical pathogens. Urinary tract infection and pyelonephritis may be treated with ceftriaxone, fluoroquinolone, or aminoglycoside depending on local resistance patterns and patient risk factors for resistant organisms. Sepsis without clear source requires broad coverage with vancomycin for gram-positive organisms including MRSA plus piperacillin-tazobactam or carbapenem for gram-negative coverage. Neutropenic fever requires anti-pseudomonal beta-lactam coverage with cefepime, piperacillin-tazobactam, or carbapenem, adding vancomycin for specific indications such as catheter-related infection or hemodynamic instability. Panel A: Supportive care measures infographic showing fluid management calculations, antipyretic options with dosing, and cooling measures with appropriate and inappropriate techniques labeled. Panel B: Decision algorithm for when to treat versus observe fever with clinical scenarios listed for each approach. Panel C: Empiric antibiotic indications flowchart showing immediate treatment scenarios (sepsis, neutropenic fever, meningitis) versus watchful waiting situations. Panel D: Antibiotic selection guide organized by infection site (respiratory, urinary, sepsis unknown source, neutropenic) with first-line regimens and alternatives. --- ### IX. Special Populations Neutropenic fever represents a medical emergency because patients lack the normal immune response to contain infection. The definition requires fever exceeding 38.3 degrees Celsius (or two readings exceeding 38.0 degrees Celsius one hour apart) in a patient with absolute neutrophil count below 500 cells/microliter or expected to fall below 500 within 48 hours. The mortality risk is substantial, with rapid progression to septic shock possible within hours if treatment is delayed. Cultures should be obtained from blood (including through any central venous catheter), urine, and any other suspected sites of infection. Antibiotic therapy with anti-pseudomonal coverage (cefepime, piperacillin-tazobactam, or meropenem) must begin within one hour of presentation. Antifungal coverage should be added if fever persists beyond 4-7 days of antibacterial therapy. Duration of treatment continues until neutrophil recovery, with persistent fever prompting investigation for fungal infection or resistant organisms. Patients with HIV/AIDS are susceptible to a spectrum of infections that varies with their degree of immunosuppression as measured by CD4 count. At CD4 counts above 500 cells/microliter, patients experience similar infections to the general population. Between 200 and 500 cells/microliter, tuberculosis and bacterial pneumonia become more common. Below 200 cells/microliter, Pneumocystis jirovecii pneumonia and toxoplasmosis emerge as risks. Below 100 cells/microliter, cryptococcal meningitis and disseminated histoplasmosis occur. Below 50 cells/microliter, Mycobacterium avium complex (MAC) infection and cytomegalovirus disease become concerns. The diagnostic approach must consider this broad differential while focusing on epidemiologically relevant organisms. Prophylaxis against Pneumocystis is indicated when CD4 count falls below 200, and MAC prophylaxis when below 50. Post-surgical fever follows characteristic timing patterns that help guide evaluation, classically summarized by the mnemonic "5 W's." On postoperative days 1-2, "Wind" refers to atelectasis, which is the most common cause of early post-surgical fever and results from hypoventilation and collapse of dependent lung segments. On days 3-5, "Water" represents urinary tract infection, especially in patients with indwelling catheters. Days 5-7 bring concern for "Wound" infections at the surgical site. "Walking" refers to deep vein thrombosis and pulmonary embolism, which may present with fever anytime postoperatively but especially after day 7. "Wonder drugs" reminds clinicians that drug fever can occur at any time and should be considered when other causes are excluded. This framework guides evaluation but should not delay assessment for more serious causes when clinical features warrant. Fever in the returning traveler requires consideration of endemic infections based on the specific travel destination and timing since return. Infections with incubation periods under 10 days include dengue fever, chikungunya, and traveler's diarrhea (bacterial enteritis). Those with incubation periods of 10-21 days include malaria, typhoid fever, and leptospirosis. Infections with incubation periods exceeding 21 days include tuberculosis, viral hepatitis, and parasitic infections. Malaria must always be considered in febrile travelers returning from endemic areas, as delayed diagnosis can result in severe complications or death. Initial evaluation should include malaria smears (thick and thin) repeated if initially negative, blood cultures, stool studies if diarrhea is present, and serologic testing guided by destination and clinical presentation. Panel A: Neutropenic fever protocol showing definition criteria, culture requirements, antibiotic selection algorithm, and duration guidelines with red flags for escalation. Panel B: CD4 count stratification chart for HIV patients with opportunistic infections organized by threshold level and corresponding prophylaxis recommendations. Panel C: Post-surgical fever timeline showing the "5 W's" (Wind, Water, Wound, Walking, Wonder drugs) at appropriate postoperative days with evaluation approach for each. Panel D: Returning traveler fever algorithm with incubation period stratification, destination-based testing recommendations, and emphasis on malaria evaluation. --- ### X. Drug Fever and Hyperthermia Drug fever has characteristic features that distinguish it from infectious causes when clinicians maintain appropriate suspicion. Relative bradycardia, where heart rate is inappropriately low for the degree of temperature elevation, is a classic finding that should prompt consideration of drug fever as well as other causes such as typhoid and Legionella. Despite often impressive temperatures, patients typically appear well without the toxic appearance seen in serious infections. Resolution occurs within 48-72 hours of discontinuing the offending medication, with this response serving as confirmation of the diagnosis. Rechallenge with the causative drug produces rapid return of fever, though intentional rechallenge is rarely necessary or appropriate for diagnosis. Common medications causing drug fever include beta-lactam antibiotics, sulfonamides, anticonvulsants (phenytoin, carbamazepine), allopurinol, procainamide, and quinidine. Malignant hyperthermia is a pharmacogenetic disorder triggered by inhaled anesthetic agents and succinylcholine. The underlying mechanism involves abnormal calcium release from skeletal muscle sarcoplasmic reticulum, causing sustained muscle contraction and heat generation. Clinical features include rapidly rising temperature, muscle rigidity, metabolic acidosis from accelerated muscle metabolism, and hyperkalemia from muscle cell breakdown. The mortality rate without treatment approaches 80% but has decreased to approximately 5% with prompt recognition and therapy. Treatment centers on immediate discontinuation of triggering agents and administration of dantrolene, a muscle relaxant that blocks calcium release from the sarcoplasmic reticulum. Dantrolene dosing begins at 2.5 mg/kg IV and is repeated every 5-10 minutes until symptoms resolve, with typical total doses of 1-10 mg/kg. Supportive care including active cooling, treatment of hyperkalemia, and hemodynamic support is also essential. Neuroleptic malignant syndrome (NMS) occurs as an idiosyncratic reaction to dopamine-blocking medications, most commonly antipsychotics. The classic tetrad includes fever (often exceeding 40 degrees Celsius), muscle rigidity (lead-pipe rigidity), altered mental status (ranging from confusion to coma), and autonomic instability (tachycardia, labile blood pressure, diaphoresis). Laboratory findings include markedly elevated creatine kinase from muscle breakdown, leukocytosis, and elevated liver enzymes. The time course typically involves symptom development over 1-3 days following exposure, though it may occur at any point during treatment. Treatment requires immediate discontinuation of the causative agent and supportive care including cooling, hydration, and hemodynamic support. Specific therapies include dantrolene for muscle rigidity and bromocriptine, a dopamine agonist that counteracts the dopaminergic blockade. Serotonin syndrome results from excess serotonergic activity in the central nervous system, typically from drug interactions involving multiple serotonergic agents. Clinical features include hyperthermia, neuromuscular hyperactivity (clonus, hyperreflexia, tremor, myoclonus), and altered mental status (agitation, confusion). The distinguishing feature from neuroleptic malignant syndrome is the presence of clonus and hyperreflexia, whereas NMS features lead-pipe rigidity without hyperreflexia. Common precipitants include combinations of SSRIs with MAO inhibitors, triptans, tramadol, linezolid, or illicit drugs such as MDMA (ecstasy). Treatment involves discontinuation of all serotonergic agents, supportive care, and cyproheptadine, a serotonin antagonist given as 12 mg initially followed by 4 mg every 6 hours until symptoms resolve. Severe cases may require mechanical ventilation and neuromuscular paralysis for temperature control. Panel A: Drug fever diagnostic features chart showing relative bradycardia calculation, typical patient appearance, timeline of resolution after drug discontinuation, and list of common causative medications. Panel B: Malignant hyperthermia pathophysiology diagram showing abnormal calcium release from sarcoplasmic reticulum, clinical features, and treatment with dantrolene mechanism illustrated. Panel C: NMS clinical tetrad illustrated (fever, rigidity, altered mental status, autonomic instability) with laboratory findings and treatment algorithm including bromocriptine and dantrolene. Panel D: Serotonin syndrome versus NMS comparison showing distinguishing features (clonus/hyperreflexia versus rigidity), common drug triggers, and treatment differences. --- ## Summary - Fever is defined as temperature exceeding 38.0 degrees Celsius, representing elevated hypothalamic set point mediated by prostaglandin E2 - Fever patterns (continuous, remittent, intermittent, relapsing) provide diagnostic clues but are not definitive - Life-threatening causes requiring immediate attention include meningitis, sepsis, and necrotizing fasciitis - Systematic approach localizes source through comprehensive history and physical examination - Non-infectious causes include malignancy, autoimmune conditions, and drug fever - FUO is defined as fever exceeding 38.3 degrees Celsius for more than 3 weeks without diagnosis after evaluation - FUO categories include infection (25-30%), malignancy (20-25%), autoimmune (15-20%), and undiagnosed (25%) - Neutropenic fever is a medical emergency requiring immediate anti-pseudomonal antibiotics - Post-surgical fever evaluation follows the 5 W's: Wind, Water, Wound, Walking, Wonder drugs - Drug fever features relative bradycardia, patient appearing well, and resolution 48-72 hours after drug discontinuation --- ## Key Terms | Term | Definition | |------|------------| | Pyrogen | Substance causing fever by raising hypothalamic set point | | Rigors | Shaking chills suggesting bacteremia | | FUO | Fever of unknown origin | | Relative bradycardia | Pulse lower than expected for temperature elevation | | Neutropenic fever | ANC below 500 cells/microliter with fever | | Drug fever | Fever caused by medication reaction | | NMS | Neuroleptic malignant syndrome | | Serotonin syndrome | Hyperserotonergic state with hyperthermia | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*