Migraine: This is often characterized by sudden onset headache, dizziness, nausea, and photophobia. Migraines can be severe and debilitating, matching the symptoms described.

Tension Headache: While typically less severe than migraines, tension headaches can cause significant discomfort, nausea, and sensitivity to light, especially if severe.
Viral Meningitis: Early stages can present with headache, nausea, and photophobia, though fever and neck stiffness are more specific signs that may develop.
Benign Paroxysmal Positional Vertigo (BPPV): Dizziness is a key feature, and while headache and nausea can occur, photophobia is less common. However, the sudden onset of dizziness could initially present with these accompanying symptoms.

Subarachnoid Hemorrhage (SAH): A sudden, severe headache (often described as "the worst headache of my life") with nausea, vomiting, and sometimes photophobia, necessitates immediate medical attention due to its high mortality rate if not promptly treated.
Stroke or Transient Ischemic Attack (TIA): Though more commonly associated with focal neurological deficits, sudden onset headache and dizziness can be presenting features, especially in posterior circulation strokes.
Meningitis (Bacterial): Similar to viral meningitis but with a more severe and rapid progression, including high fever, stiff neck, and potential for severe complications or death if not treated promptly.
Encephalitis: Inflammation of the brain tissue, which can present with headache, fever, confusion, and sometimes photophobia, requiring urgent diagnosis and treatment.

Thunderclap Headache due to other causes (e.g., Reversible Cerebral Vasoconstriction Syndrome, RCVS): These are rare conditions that can mimic SAH in severity and sudden onset but have different underlying pathologies.
Cervicogenic Headache: Originates from the cervical spine and can cause headache, dizziness, and nausea, though photophobia is less common.
Chiari Malformation: A structural defect in the cerebellum that can cause sudden onset headache, especially with coughing or straining, and may be associated with dizziness and nausea.

CT or MRI of the Head: To rule out structural causes like hemorrhage, stroke, or mass lesions.
Lumbar Puncture: If meningitis or subarachnoid hemorrhage is suspected and not visible on CT.
Blood Tests: To check for infection or inflammation markers.
Neurological Examination: To assess for focal neurological deficits or signs of meningeal irritation.

Stabilization: Ensure the patient is stable, with particular attention to airway, breathing, and circulation (ABCs).
Pain Management: Depending on the diagnosis, manage headache and other symptoms appropriately.
Specific Treatments: Based on the diagnosis, this could include antibiotics for bacterial meningitis, antivirals for encephalitis, or surgical intervention for certain structural issues.
Monitoring: Close observation for worsening symptoms or development of new signs that could indicate a more serious condition.
Referral: To a specialist (e.g., neurologist, neurosurgeon) if the diagnosis suggests a need for specialized care.