4.9 Fever of Unknown Origin, Neutropenic Fever & Toxic Shock

1. Fever of Unknown Origin

ABIM lists fever of unknown origin (FUO) as its own subsection. The classic definition, essentially unchanged since Petersdorf and Beeson, has three components that must all be satisfied:

1. Temperature above 38.3 degrees Celsius (101 degrees Fahrenheit) on several occasions
2. Duration of more than three weeks
3. No diagnosis after appropriate initial investigation (historically one week of inpatient workup; now defined by a specified set of ambulatory investigations)

The duration requirement is what makes the differential distinctive. Three weeks excludes essentially all common self-limited viral illness, so the remaining causes are indolent, intermittent or hidden.

The four host categories

Causes of classic FUO

Infection — the historically largest group, though its share has fallen with better imaging:

• Extrapulmonary and miliary tuberculosis
• Occult abscess: intra-abdominal, hepatic, pelvic, dental, paraspinal
• Culture-negative endocarditis
• Osteomyelitis, particularly vertebral
• Zoonoses and travel-related infection: brucellosis, Q fever, bartonellosis, typhoid, visceral leishmaniasis

Malignancy:

• Lymphoma — the single most common neoplastic cause
• Renal cell carcinoma; hepatocellular carcinoma; atrial myxoma; leukemia

Non-infectious inflammatory disease — now the largest group in older adults:

• Giant cell arteritis and polymyalgia rheumatica — the highest-yield cause of FUO in a patient over 50
• Adult-onset Still disease — quotidian spiking fever, evanescent salmon-colored rash, arthralgia, sore throat, and a markedly elevated ferritin
• Systemic lupus erythematosus, sarcoidosis, inflammatory bowel disease, vasculitis

Miscellaneous:

• Drug fever — occurs at any time after starting a drug, often with relative bradycardia and eosinophilia, and resolves within roughly 72 hours of withdrawal. Common culprits: beta-lactams, sulfonamides, anticonvulsants, allopurinol, heparin.
• Venous thromboembolism; subacute thyroiditis; familial Mediterranean fever; factitious fever.

A rational approach

The single most productive intervention is a repeat history and physical examination performed serially, with an exhaustive review of travel, occupational, animal, dietary and sexual exposures and a complete medication list including supplements. Then:

1. Discontinue every non-essential medication.
2. Basic laboratory studies including erythrocyte sedimentation rate and C-reactive protein, ferritin, and blood cultures held for fastidious organisms.
3. Cross-sectional imaging of chest, abdomen and pelvis.
4. Temporal artery biopsy in patients over 50 with elevated inflammatory markers.
5. FDG-PET/CT to localize occult inflammation when the above is unrevealing.

Empiric antibiotics are generally the wrong answer in a hemodynamically stable patient with FUO: they obscure cultures without treating the two-thirds of cases that are not infectious. The exception is suspected culture-negative endocarditis or a deteriorating patient.

2. Fever and Neutropenia

Enumerated under bacteremia and sepsis syndrome, and one of the true medical emergencies on the exam.

Definitions:

• Fever: a single oral temperature of 38.3 degrees Celsius (101 degrees Fahrenheit), or 38.0 degrees Celsius (100.4 degrees Fahrenheit) sustained over one hour
• Neutropenia: absolute neutrophil count below 500 cells per microliter, or below 1,000 with an expected decline to below 500

The governing principle is that the inflammatory signs of infection are absent because the neutrophils that produce them are absent. There will be no infiltrate on chest radiograph, no purulence, no fluctuance. Fever may be the only sign of overwhelming infection.

Management

1. Empiric antipseudomonal beta-lactam monotherapy within one hour. Cefepime, piperacillin-tazobactam or a carbapenem. Speed matters more than breadth.
2. Do not routinely add vancomycin. It is added only for specific indications: hemodynamic instability, suspected catheter-related infection, skin or soft tissue infection, known colonization with resistant gram-positive organisms, or severe mucositis.
3. Add antifungal coverage for persistent fever beyond four to seven days of appropriate antibacterial therapy.
4. Avoid digital rectal examination and rectal thermometry, which can seed bacteremia through friable mucosa.
5. Risk-stratify. Selected low-risk patients with anticipated short neutropenia and no comorbidity may be managed as outpatients on oral therapy.

Typhlitis (neutropenic enterocolitis) presents with right lower quadrant pain, fever and diarrhea in a neutropenic patient; it is managed medically, and surgery is reserved for perforation or uncontrolled bleeding.

3. Toxic Shock Syndrome

A toxin-mediated illness in which a superantigen bypasses normal antigen presentation and activates a large fraction of T cells simultaneously, producing massive cytokine release.

Shared features: fever, hypotension, and multiorgan involvement across at least three systems — renal, hepatic, hematologic, gastrointestinal, mucous membrane, muscular or central nervous system.

Management principles:

1. Aggressive fluid resuscitation — capillary leak produces enormous fluid requirements.
2. Source control — remove the tampon or packing; take necrotizing infection to the operating room. Antibiotics alone will not work.
3. Antibiotics: a beta-lactam plus clindamycin. Clindamycin is essential because it inhibits ribosomal toxin synthesis; beta-lactams are less effective against a high-density, stationary-phase inoculum (the Eagle effect).
4. Intravenous immunoglobulin is considered in severe streptococcal disease to neutralize circulating superantigen.

The desquamation of palms and soles occurs one to three weeks after onset and is therefore a retrospective clue, not a diagnostic tool at presentation.