Chronic daily headache (CDH) is defined as headaches that occur for 15 days or more in a month, at least for 3 months. Therefore, it is not a diagnosis per se. 4% of the population is affected by CDH.
Short-duration chronic daily headache (CDH) are brief headache syndromes and trigeminal autonomic cephalalgias. They are relatively rare and only a small proportion of CDH patients are affected by them  .
The trigeminal autonomic cephalalgias have autonomic features with unilateral trigeminal distribution, these include paroxysmal hemicrania, chronic cluster headache, and short-lasting unilateral neuralgiform attacks with autonomic cranial symptoms or conjunctival tearing and injection.
Most cases with long-duration CDH are affected by migraines or tension-type headaches. Other types include new daily persistent headache and hemicrania continua .
Medication overuse further complicates treatment in two-thirds of patients who regularly use abortive medication such as acetaminophen, caffeine, non-steroidal anti-inflammatory drugs, triptans, and narcotics.
In tension-type headaches, bilateral or occipital bandlike discomfort build up in a slow way and can last for several days. Unlike migraines, nausea and photophobia are generally absent. If a headache becomes constant in 72 hours it is new daily persistent headache instead of chronic tension-type headache which develops more slowly.
New daily persistent headache is more refractory to treatment compared to chronic tension-type headache.
Patients who progress to CDH may have symptoms of both a migraine and tension-type headaches which result in a challenging treatment planning .
If primary CDH is diagnosed, detection of the subtype is necessary to plan appropriate treatment accordingly.
Evaluation of red flags is important as they suggest a secondary etiology for a headache  .
Red flags and their suggestive etiologies:
Imaging studies must be ordered when red flags are observed . Magnetic resonance imaging (MRI) is considered the most useful method since it has a higher sensitivity to detect secondary causes of headaches compared to computed tomography (CT). In absence of contraindications, MRI with contrast should be ordered, especially when an intracranial mass is suspected. CT scans are more appropriate for acute settings to exclude intracranial mass or hemorrhage. Primary headaches do not have specific MRI findings.