Sensory loss, bilateral involvement, and younger age (<40) are associated with a higher risk of secondary<unk> , but their absence does not rule out secondary<unk> 
Consider CT/MRI in these patients to rule out structural etiology
<5% of patients have V1 distribution, examine carefully for zoster in these patients
Trigeminal neuralgia
Migraine headache
Tension headache
Cluster headache
Meningitis/encephalitis
Post-lumbar puncture headache
Neoplastic meningitis
Heat syncope