The criteria for the diagnosis of NDPH were proposed in 1994 (the Silberstein–Lipton criteria)  but not included in the International Classification of Headache Disorders (ICHD) until 2004. Signs and symptoms
The headaches can vary greatly in their clinical presentation and duration.Quality of the headache has been described as dull and/or pressure-like sensation, and throbbing and/or pulsating sensation. The pain is usually on both sides of the head (in 88–93% of people with NDPH), but may be unilateral, and may be localized to any head region. The pain can fluctuate in intensity and duration, is daily, and lasts more than 3 months.There may be accompanying photophobia, phonophobia, lightheadedness or mild nausea. Co-morbidity with mood disorders has been reported in a subset of patients.Cranial autonomic nervous symptoms occur with painful exacerbations in 21%, and cutaneous allodynia may be present in 26%.In 2002, Li and Rozen conducted a study of 56 patients at the Jefferson Headache Center in Philadelphia and published the following results:

82% of patients were able to pinpoint the exact day their headache started. 30% of the patients, the onset of the headache occurred in correlation with an infection or flu-like illness. 38% of the patients had a prior personal history of headache. 29% of the patients had a family history of headache. 68% reported nausea. 66% reported photophobia. 61% reported phonophobia. 55% reported lightheadedness.Imaging and laboratory testing were unremarkable except for an unusually high number of patients who tested positive for a past Epstein-Barr virus infection.
Diagnosis
Although NDPH is classified as a primary headache syndrome, it must be remembered that a number of important conditions can present with a new-onset persisting headache, and these must be excluded prior to making a diagnosis of a primary headache disorder.The diagnosis is one of excluding the many secondary types or NDPH mimics, which is especially critical early in the course of the disease when a secondary etiology is more likely. NDPH mimics include but are not limited to:
neoplasms
subarachnoid hemorrhage
idiopathic intracranial hypertension
temporal arteritis
chronic subdural hematoma
post-traumatic headaches
sphenoid sinusitis
hypertension
spontaneous cerebrospinal fluid leak
cervical artery dissections
pseudotumor cerebri without papilledema
cerebral venous thrombosis
Chiari malformation
NDPH with medication overuse headacheMany doctors state that the condition is best viewed as a syndrome rather than a diagnosis. Once a diagnosis of NDPH is made, clinicians argue that patients are best managed according to the more detailed pathophysiology-based diagnosis than lumped together into a single group, since a single disorder is unlikely to exist.NDPH is classified as a Primary Headache Disorder by the ICHD-2 classification system (by the IHS) using number 4.8. It is one of the types of primary headache syndromes that present as a chronic daily headache, which is a headache present for more than 15 days a month for more than 3 months.