Differential Diagnosis for Severe Headache, Stiff Neck, and Fever

Single Most Likely Diagnosis

• Meningitis: This is the most likely diagnosis given the combination of severe headache, stiff neck (indicative of nuchal rigidity), and fever. Meningitis is an inflammation of the protective membranes covering the brain and spinal cord, known as the meninges, and can be caused by viruses, bacteria, or other microorganisms.

Other Likely Diagnoses

• Encephalitis: An inflammation of the brain tissue, usually caused by viral infections. Symptoms can include headache, fever, confusion, and sometimes stiff neck, making it a plausible diagnosis.
• Subarachnoid Hemorrhage: Although typically presenting with a "thunderclap" headache, it can sometimes be accompanied by fever if there's an associated infection or inflammation. Stiff neck may occur due to blood in the subarachnoid space irritating the meninges.
• Cervical Spine Infection or Abscess: Infections or abscesses in the cervical spine can cause severe neck pain and stiffness, along with fever. While less common, these conditions can mimic meningitis in their presentation.

Do Not Miss Diagnoses

• Brain Abscess: A rare but potentially life-threatening condition where an abscess forms in the brain. It can present with headache, fever, and sometimes neck stiffness, especially if the abscess is located near the brainstem or meninges.
• Subdural or Epidural Empyema: Collections of pus between the brain and the skull, which can present similarly to meningitis with headache, fever, and stiff neck.
• Ruptured Aneurysm or Arteriovenous Malformation (AVM): While these conditions typically present with sudden, severe headache, they can occasionally be accompanied by fever if there's an associated infection or inflammation and may cause stiff neck due to irritation of the meninges.

Rare Diagnoses

• Neurosyphilis: A complication of untreated syphilis infection that can cause a variety of neurological symptoms, including headache, fever, and stiff neck, although it's much less common than other causes.
• CNS Vasculitis: Inflammation of the blood vessels in the central nervous system, which can present with a wide range of symptoms including headache, fever, and sometimes stiff neck, depending on the areas of the brain affected.
• CNS Lymphoma: A type of cancer that can cause neurological symptoms, including headache, fever, and stiff neck, especially if it involves the meninges or spinal cord.