Differential Diagnosis for a 66-year-old Patient with Recent Onset Fever

Single Most Likely Diagnosis

• Exertional Heat Stroke: Given the patient's recent increase in physical activity by about 60% over the last two weeks and the presentation of high fever (39 Celsius), exertional heat stroke is a plausible diagnosis. This condition occurs when the body's temperature regulation fails due to excessive physical exertion in high temperatures, leading to a rapid rise in body temperature.

Other Likely Diagnoses

• Viral Infections: The sudden onset of high fever could be indicative of a viral infection. Given the age of the patient and the lack of other symptoms, common viral infections such as influenza or a viral gastroenteritis could be considered.
• Bacterial Infections: Bacterial infections, including pneumonia, urinary tract infections, or skin infections, could also present with fever as the primary symptom, especially in an elderly patient.
• Dehydration: Increased physical activity, especially in warmer environments, can lead to dehydration, which might exacerbate or contribute to the fever.

Do Not Miss Diagnoses

• Septicemia: Although less likely given the lack of other symptoms, septicemia (blood infection) is a potentially life-threatening condition that requires immediate attention. Elderly patients may not always present with typical symptoms of sepsis.
• Malignant Hyperthermia: A rare but life-threatening medical emergency that can be triggered by certain medications used in general anesthesia and by strenuous exercise in susceptible individuals.
• Endocarditis: Infective endocarditis, an infection of the heart valves, can present with fever as a primary symptom and is critical to diagnose early due to its high morbidity and mortality.

Rare Diagnoses

• Neuroleptic Malignant Syndrome (NMS): A life-threatening neurological disorder most often caused by an adverse reaction to neuroleptic or antipsychotic medication. It presents with fever, muscle rigidity, and altered mental status.
• Thyroid Storm: A rare but life-threatening complication of untreated or undertreated hyperthyroidism, characterized by an extreme exacerbation of thyrotoxicosis symptoms, including high fever.
• Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodes of high blood pressure, fever, and other symptoms due to excessive catecholamine release.