Fever of unknown origin

Indicator ruling out functional pathology

In therapeutic and pediatric practice, it is crucial to differentiate functional disorders from organic lesions. The appearance of a fever of unknown origin is an absolute "warning symptom" ("red flag"). In the presence of unexplained temperature elevation, diagnoses such as irritable bowel syndrome (IBS), functional dyspepsia, intestinal colic, or biliary dyskinesia become invalid.

Functional pathology is always a diagnosis of exclusion. If a patient with abdominal pain presents with a temperature reaction of unclear origin, the physician is obligated to urgently revise their approach. Symptoms can no longer be explained by motility disorders, and a targeted search for occult inflammation, acute surgical pathology, celiac disease, or parasitic infestations is required.

FUO in pseudo-abdominal syndrome

Often, unexplained fever accompanied by acute abdominal pain mimics surgical pathology, presenting as pseudo-abdominal syndrome. In such cases, emergency surgery is not required, though the patient's condition remains severe.

• Epidemic myalgia (Bornholm disease): high fever combined with pronounced muscle tension mimicking an "acute abdomen" clinical picture, anorexia, and a forced body position.
• Infections and complications: a similar picture with a temperature reaction can occur in basal pneumonia in children, mesadenitis against the background of acute respiratory viral infections (ARVI), urinary tract infections, or as the onset of diabetic ketoacidosis.

Systemic manifestation of bowel diseases

In gastroenterology, prolonged fever of unknown origin frequently acts as a systemic extraintestinal manifestation of inflammatory bowel disease (IBD). The classic local presentation of ulcerative colitis (UC) includes diarrhea with mucus, hematochezia (blood in stool), and abdominal pain.

However, during a severe attack of the disease, systemic disorders come to the forefront: fever, pronounced tachycardia, and symptoms of anemia due to chronic blood loss. In children, systemic manifestations of IBD are often combined with progressive weight loss, delayed linear growth and puberty, as well as peripheral arthritis.

Preliminary diagnosis in immunodeficiencies

In infectious diseases and pulmonology, the term "Fever of unknown etiology" is often used as a legitimate syndromic diagnosis at the medical evacuation stage and in the emergency department. This is particularly significant for patients with severe respiratory syndrome.

If an immunosuppressed patient develops severe pneumonia with respiratory failure, the patient is transferred to an infectious diseases hospital, where an urgent differential diagnostic workup is performed against the background of FUO. Key areas of examination:

1. Ruling out HIV infection: two-stage laboratory diagnostics (screening by ELISA and verification by immunoblotting).
2. Assessment of immune status: determination of viral load by PCR and counting of CD4+ T-lymphocyte levels.
3. Search for comorbidities: targeted exclusion of pulmonary tuberculosis and markers of opportunistic infections.

Class and exam questions

What else to study on this topic

• Differential diagnosis of functional and organic gastrointestinal diseases
• Clinical manifestations and severity criteria of ulcerative colitis in children
• Pseudo-abdominal syndrome: conditions mimicking an "acute abdomen"
• Algorithm for two-stage laboratory diagnosis of HIV infection
• Opportunistic infections in acquired immunodeficiency syndrome

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.