Editing Headache (section)

== Diagnosis ==
{|border="1" class="wikitable"
|+ Differential diagnosis of headaches

![[Tension headache]]
![[New daily persistent headache]]
![[Cluster headache]]
![[Migraine]]
|-
| mild to moderate dull or aching pain||  || severe pain|| moderate to severe pain
|-
| duration of 30 minutes to several hours
|| duration of at least four hours daily
|| duration of 30 minutes to 3 hours
|| duration of 4 hours to 3 days
|-
|
|| Occur in periods of 15 days a month for three months
|| may happen multiple times in a day for months
|| periodic occurrence; several per month to several per year
|-
|located as tightness or pressure across head
|| located on one or both sides of the head
|| located one side of head focused at eye or [[Temple (anatomy)|temple]]
|| located on one or both sides of head
|-
|
|| consistent pain
|| pain describable as sharp or stabbing
|| pulsating or throbbing pain
|-
|no nausea or vomiting
||
||
||nausea, perhaps with vomiting
|-
| no [[Aura (symptom)|aura]]
||no aura
||
||auras
|-
|uncommonly, [[Photosensitivity in humans|light sensitivity]] or noise sensitivity
||
|| may be accompanied by [[running nose]], [[tears]], and [[Ptosis (eyelid)|drooping eyelid]], often only on one side
||sensitivity to movement, light, and noise
|-
|
|| exacerbated by regular use of [[acetaminophen]] or [[NSAIDS]]
||
|| may exist with tension headache<ref name="BBDtriptans">{{cite journal |journal=Consumer Reports Best Buy Drugs |date=March 2013 |title=Using the triptans to treat: Migraine headaches: Comparing effectiveness, safety, and price |page=8 |url=http://www.consumerreports.org/health/resources/pdf/best-buy-drugs/triptanFINAL.pdf |access-date=18 March 2013 |url-status=live |archive-url=https://web.archive.org/web/20130320142528/http://www.consumerreports.org/health/resources/pdf/best-buy-drugs/triptanFINAL.pdf |archive-date=20 March 2013 }}</ref>
|}

Most headaches can be diagnosed by the clinical history alone.<ref name=Goadsby /> If the symptoms described by the person sound dangerous, further testing with neuroimaging or lumbar puncture may be necessary. Electroencephalography (EEG) is not useful for headache diagnosis.<ref>{{cite journal | vauthors = Gronseth GS, Greenberg MK | title = The utility of the electroencephalogram in the evaluation of patients presenting with headache: a review of the literature | journal = Neurology | volume = 45 | issue = 7 | pages = 1263–1267 | date = July 1995 | pmid = 7617180 | doi = 10.1212/WNL.45.7.1263 | s2cid = 26022438 }}</ref>

The first step to diagnosing a headache is to determine if the headache is old or new.<ref name=Smetana /> A "new headache" can be a headache that has started recently, or a chronic headache that has changed character.<ref name=Smetana /> For example, if a person has chronic weekly headaches with pressure on both sides of his head, and then develops a sudden severe throbbing headache on one side of his head, they have a new headache.{{citation needed|date=June 2021}}

=== Red flags ===
It can be challenging to differentiate between low-risk, benign headaches and high-risk, dangerous headaches since symptoms are often similar.<ref name="Abrams Journal">{{cite journal | vauthors = Abrams BM | title = Factors that cause concern | journal = The Medical Clinics of North America | volume = 97 | issue = 2 | pages = 225–242 | date = March 2013 | pmid = 23419623 | doi = 10.1016/j.mcna.2012.11.002 }}</ref> Headaches that are possibly dangerous require further lab tests and imaging to diagnose.<ref name="Clinch" />

The American College for Emergency Physicians published criteria for low-risk headaches. They are as follows:<ref name="ACEP Criteria">{{cite journal | author = American College of Emergency Physicians | title = Clinical policy: critical issues in the evaluation and management of patients presenting to the emergency department with acute headache | journal = Annals of Emergency Medicine | volume = 39 | issue = 1 | pages = 108–122 | date = January 2002 | pmid = 11782746 | doi = 10.1067/mem.2002.120125 }}</ref>
* age younger than 30 years
* features typical of primary headache
* history of similar headache
* no abnormal findings on neurologic exam
* no concerning change in normal headache pattern
* no high-risk comorbid conditions (for example, HIV)
* no new concerning history or physical examination findings

A number of characteristics make it more likely that the headache is due to potentially dangerous secondary causes which may be life-threatening or cause long-term damage. These "red flag" symptoms mean that a headache warrants further investigation with neuroimaging and lab tests.<ref name="Clinch" />

In general, people complaining of their "first" or "worst" headache warrant imaging and further workup.<ref name="Clinch" /> People with progressively worsening headache also warrant imaging, as they may have a mass or a bleed that is gradually growing, pressing on surrounding structures and causing worsening pain.<ref name="Abrams Journal" /> People with neurological findings on exam, such as weakness, also need further workup.<ref name="Abrams Journal" />

The American Headache Society recommends using "SSNOOP", a mnemonic to remember the red flags for identifying a secondary headache:<ref name="Smetana">{{cite book | chapter-url = http://accessmedicine.mhmedical.com/content.aspx?bookid=500&sectionid=41026552 | vauthors = Smetana GW | chapter = Chapter 9. Headache. | veditors = Henderson MC, Tierney Jr LM, Smetana GW | title = The Patient History: An Evidence-Based Approach to Differential Diagnosis | location = New York, NY | publisher = McGraw-Hill | date = 2012 | archive-url = https://web.archive.org/web/20150531030403/http://accessmedicine.mhmedical.com/content.aspx?bookid=500&sectionid=41026552 |archive-date = 31 May 2015 }}</ref>
* Systemic symptoms (fever or weight loss)
* Systemic disease (HIV infection, malignancy)
* Neurologic symptoms or signs
* Onset sudden (thunderclap headache)
* Onset after age 40 years
* Previous headache history (first, worst, or different headache)

Other red flag symptoms include:<ref name="Clinch" /><ref name="Smetana" /><ref name="Abrams Journal" /><ref name="Hainer">{{cite journal | vauthors = Hainer BL, Matheson EM | title = Approach to acute headache in adults | journal = American Family Physician | volume = 87 | issue = 10 | pages = 682–687 | date = May 2013 | pmid = 23939446 }}</ref>
{|class="wikitable"
|-
! Red Flag !! Possible causes !! The reason why a red flag indicates possible causes !! Diagnostic tests
|-
| New headache after age 50 || Temporal arteritis, mass in brain || Temporal arteritis is an inflammation of vessels close to the temples in older people, which decreases blood flow to the brain and causes pain. May also have tenderness in temples or jaw claudication. Some brain cancers are more common in older people. || Erythrocyte sedimentation rate (diagnostic test for temporal arteritis), neuroimaging
|-
| Very sudden onset headache ([[thunderclap headache]]) || Brain bleed ([[subarachnoid hemorrhage]], hemorrhage into mass lesion, [[vascular malformation]]), [[pituitary apoplexy]], mass (especially in [[Posterior cranial fossa|posterior fossa]]) || A bleed in the brain irritates the meninges which causes pain. Pituitary apoplexy (bleeding or impaired blood supply to the pituitary gland at the base of the brain) is often accompanied by double vision or visual field defects, since the pituitary gland is right next to the [[optic chiasm]] (eye nerves). || [[Neuroimaging]], [[lumbar puncture]] if computed tomography is negative
|-
| Headaches increasing in frequency and severity || Mass, subdural hematoma, medication overuse || As a brain mass gets larger, or a [[subdural hematoma]] (blood outside the vessels underneath the [[Dura mater|dura]]) it pushes more on surrounding structures causing pain. Medication overuse headaches worsen with more medication taken over time. || Neuroimaging, drug screen
|-
| New onset headache in a person with possible HIV or cancer || [[Meningitis]] (chronic or carcinomatous), [[brain abscess]] including [[toxoplasmosis]], [[metastasis]] || People with HIV or cancer are immunosuppressed so are likely to get infections of the meninges or infections in the brain causing abscesses. Cancer can metastasize, or travel through the blood or lymph to other sites in the body. || Neuroimaging, lumbar puncture if neuroimaging is negative
|-
| Headache with signs of total body illness (fever, stiff neck, rash) || [[Meningitis]], [[encephalitis]] (inflammation of the brain tissue), [[Lyme disease]], [[collagen vascular disease]] || A stiff neck, or inability to flex the neck due to pain, indicates inflammation of the meninges. Other signs of systemic illness indicates infection. || Neuroimaging, lumbar puncture, serology (diagnostic blood tests for infections)
|-
| [[Papilledema]] || Brain mass, [[benign intracranial hypertension]] (pseudotumor cerebri), [[meningitis]] || Increased intracranial pressure pushes on the eyes (from inside the brain) and causes papilledema. || Neuroimaging, lumbar puncture
|-
| Severe headache following head trauma || Brain bleeds ([[intracranial hemorrhage]], [[subdural hematoma]], [[epidural hematoma]]), post-traumatic headache || Trauma can cause bleeding in the brain or shake the nerves, causing a post-traumatic headache || Neuroimaging of brain, skull, and possibly cervical spine
|-
| Inability to move a limb || Arteriovenous malformation, collagen vascular disease, intracranial mass lesion || Focal neurological signs indicate something is pushing against nerves in the brain responsible for one part of the body || Neuroimaging, blood tests for collagen vascular diseases
|-
| Change in personality, consciousness, or mental status || [[Central nervous system infection]], [[Intracranial bleeding|intracranial bleed]], mass || Change in mental status indicates a global infection or inflammation of the brain, or a large bleed compressing the brainstem where the consciousness centers lie || Blood tests, lumbar puncture, neuroimaging
|-
| Headache triggered by cough, exertion or while engaged in sexual intercourse || Mass lesion, subarachnoid hemorrhage || Coughing and exertion increases the intra cranial pressure, which may cause a vessel to burst, causing a subarachnoid hemorrhage. A mass lesion already increases intracranial pressure, so an additional increase in intracranial pressure from coughing etc. will cause pain. || Neuroimaging, lumbar puncture
|-
|}

=== Old headaches ===
Old headaches are usually primary headaches and are not dangerous. They are most often caused by [[migraines]] or [[tension headaches]]. Migraines are often unilateral, pulsing headaches accompanied by nausea or vomiting. There may be an aura (visual symptoms, numbness or tingling) 30–60 minutes before the headache, warning the person of a headache. Migraines may also not have auras.<ref name=Hainer /> Tension-type headaches usually have bilateral "bandlike" pressure on both sides of the head usually without nausea or vomiting. However, some symptoms from both headache groups may overlap. It is important to distinguish between the two because the treatments are different.<ref name=Hainer />

The mnemonic 'POUND' helps distinguish between migraines and tension-type headaches. POUND stands for: {{bulleted list|Pulsatile quality of headache|One-day duration (four to 72 hours)|Unilateral location|Nausea or vomiting|Disabling intensity<ref>{{cite journal | vauthors = Gilmore B, Michael M | title = Treatment of acute migraine headache | journal = American Family Physician | volume = 83 | issue = 3 | pages = 271–280 | date = February 2011 | pmid = 21302868 | url = https://www.aafp.org/afp/2011/0201/p271.html | access-date = 15 September 2021 }}</ref> }}

One review article found that if 4–5 of the POUND characteristics are present, a migraine is 24 times as likely a diagnosis than a tension-type headache ([[Likelihood ratios in diagnostic testing|likelihood ratio]] 24). If 3 characteristics of POUND are present, migraine is 3 times more likely a diagnosis than tension type headache ([[Likelihood ratios in diagnostic testing|likelihood ratio]] 3).<ref name=Detsky /> If only 2 POUND characteristics are present, tension-type headaches are 60% more likely (likelihood ratio 0.41). Another study found the following factors independently each increase the chance of migraine over tension-type headache: nausea, photophobia, phonophobia, exacerbation by physical activity, unilateral, throbbing quality, chocolate as a headache trigger, and cheese as a headache trigger.<ref name="Smetana 2">{{cite journal | vauthors = Smetana GW | title = The diagnostic value of historical features in primary headache syndromes: a comprehensive review | journal = Archives of Internal Medicine | volume = 160 | issue = 18 | pages = 2729–2737 | date = October 2000 | pmid = 11025782 | doi = 10.1001/archinte.160.18.2729 | doi-access = free }}</ref>

[[Cluster headaches]] are relatively rare (1 in 1000 people) and are more common in men than women.<ref>{{cite book|title=Harrison's Principles of Internal Medicine|date=8 April 2015|publisher=McGraw-Hill Education|isbn=978-0-07-180215-4 |pages=2594–2595|edition=19th}}</ref> They present with sudden onset explosive pain around one eye and are accompanied by autonomic symptoms (tearing, runny nose and red eye).<ref name=Goadsby />

[[Temporomandibular joint dysfunction|Temporomandibular jaw pain]] (chronic pain in the jaw joint), and [[cervicogenic headache]] (headache caused by pain in muscles of the neck) are also possible diagnoses.<ref name=Smetana />

For chronic, unexplained headaches, keeping a headache diary can be useful for tracking symptoms and identifying triggers, such as association with menstrual cycle, exercise and food. While mobile electronic diaries for smartphones are becoming increasingly common, a recent review found most are developed with a lack of evidence base and scientific expertise.<ref name="pmid25138438">{{cite journal | vauthors = Hundert AS, Huguet A, McGrath PJ, Stinson JN, Wheaton M | title = Commercially available mobile phone headache diary apps: a systematic review | journal = JMIR mHealth and uHealth | volume = 2 | issue = 3 | pages = e36 | date = August 2014 | pmid = 25138438 | pmc = 4147710 | doi = 10.2196/mhealth.3452 | doi-access = free }}</ref>

[[Cephalalgiaphobia]] is fear of headaches or getting a headache.

=== New headaches ===
New headaches are more likely to be dangerous [[#Secondary headaches|secondary headaches]]. They can, however, simply be the first presentation of a chronic headache syndrome, like [[migraine]] or a [[tension headache]].

One recommended diagnostic approach is as follows.<ref>{{cite book | chapter-url = http://accessmedicine.mhmedical.com/content.aspx?bookid=383&sectionid=41676347 | chapter = Chapter 18: I Have a Patient with Headache. How Do I Determine the Cause? | veditors = Stern SC, Cifu AS, Altkorn D | title = Symptom to Diagnosis: An Evidence-Based Guide | edition = 2nd | location = New York, NY | publisher = McGraw-Hill | date = 2010 | archive-url = https://web.archive.org/web/20150531030350/http://accessmedicine.mhmedical.com/content.aspx?bookid=383&sectionid=41676347 | archive-date=31 May 2015 }}</ref> If any urgent [[#Red flags|red flags]] are present such as [[visual impairment]], new [[Seizure|seizures]], new [[weakness]], or new [[confusion]], further workup with imaging and possibly a [[lumbar puncture]] should be done (see red flags section for more details). If the headache is sudden onset (thunderclap headache), a [[CT scan|computed tomography scan]] (CT scan) to look for a brain bleed ([[subarachnoid hemorrhage]]) should be done. If the CT scan does not show a bleed, a lumbar puncture should be done to look for blood in the [[cerebrospinal fluid]] (CSF), as the CT scan can be [[False positives and false negatives|falsely negative]] and [[Subarachnoid hemorrhage|subarachnoid hemorrhages]] can be fatal. If there are signs of infection such as [[fever]], [[rash]], or [[Neck stiffness|stiff neck]], a lumbar puncture to look for [[meningitis]] should be considered. In an older person, if there is [[jaw claudication]] and scalp [[Tenderness (medicine)|tenderness]], a temporal artery biopsy should be performed to look for temporal [[arteritis]], immediate treatment should be started, if results of the biopsy are positive.{{citation needed|date=June 2021}}

=== Neuroimaging ===

==== Old headaches ====
The US Headache Consortium has guidelines for neuroimaging of non-acute headaches.<ref>{{cite book | vauthors = Frishberg BM, Rosenberg JH, Matchar DB, McCrory DC, Pietrzak MP, Rozen TD, Silberstein SD | title = Evidence-based guidelines in the primary care setting: neuroimaging in patients with nonacute headache. | location = St Paul, MN | publisher = US Headache Consortium | date = April 2000 | pages = 1–25 | citeseerx = 10.1.1.565.1524 }}</ref> Most old, chronic headaches do not require neuroimaging. If a person has the characteristic symptoms of a migraine, neuroimaging is not needed as it is very unlikely the person has an intracranial abnormality.<ref name="AHSfive">{{cite web |author1=American Headache Society |author1-link=American Headache Society |date=September 2013 |title=Five Things Physicians and Patients Should Question |publisher=[[American Headache Society]] |work=[[Choosing Wisely]] |url=http://www.choosingwisely.org/doctor-patient-lists/american-headache-society/ |access-date=10 December 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131206060123/http://www.choosingwisely.org/doctor-patient-lists/american-headache-society/ |archive-date=6 December 2013 }}, which cites
* {{cite journal | vauthors = Lewis DW, Dorbad D | title = The utility of neuroimaging in the evaluation of children with migraine or chronic daily headache who have normal neurological examinations | journal = Headache | volume = 40 | issue = 8 | pages = 629–632 | date = September 2000 | pmid = 10971658 | doi = 10.1046/j.1526-4610.2000.040008629.x | s2cid = 14443890 }}
* {{cite journal | vauthors = Silberstein SD | title = Practice parameter: evidence-based guidelines for migraine headache (an evidence-based review): report of the Quality Standards Subcommittee of the American Academy of Neurology | journal = Neurology | volume = 55 | issue = 6 | pages = 754–762 | date = September 2000 | pmid = 10993991 | doi = 10.1212/WNL.55.6.754 | doi-access = free }}
* {{cite journal | author = Medical Advisory Secretariat | title = Neuroimaging for the evaluation of chronic headaches: an evidence-based analysis | journal = Ontario Health Technology Assessment Series | volume = 10 | issue = 26 | pages = 1–57 | year = 2010 | pmid = 23074404 | pmc = 3377587 }}</ref> If the person has neurological findings, such as weakness, on exam, neuroimaging may be considered.{{citation needed|date=June 2021}}

==== New headaches ====
All people who present with [[#Red flags|red flags]] indicating a dangerous secondary headache should receive neuroimaging.<ref name=Hainer /> The best form of neuroimaging for these headaches is controversial.<ref name=Clinch /> Non-contrast computerized tomography (CT) scan is usually the first step in head imaging as it is readily available in Emergency Departments and hospitals and is cheaper than MRI. Non-contrast CT is best for identifying an acute head bleed. Magnetic Resonance Imaging (MRI) is best for brain tumors and [[Posterior fossa malformations–hemangiomas–arterial anomalies–cardiac defects–eye abnormalities–sternal cleft and supraumbilical raphe syndrome|problems in the posterior fossa]], or back of the brain.<ref name=Clinch /> MRI is more sensitive for identifying intracranial problems, however it can pick up brain abnormalities that are not relevant to the person's headaches.<ref name=Clinch />

The American College of Radiology recommends the following imaging tests for different specific situations:<ref name="Strain_2000">{{cite journal | vauthors = Strain JD, Strife JL, Kushner DC, Babcock DS, Cohen HL, Gelfand MJ, Hernandez RJ, McAlister WH, Parker BR, Royal SA, Slovis TL, Smith WL, Rothner AD | title = Headache. American College of Radiology. ACR Appropriateness Criteria | journal = Radiology | volume = 215 | issue = Suppl | pages = 855–60 | date = June 2000 | pmid = 11037510 | doi = | url = }}</ref>

{|class="wikitable"
|-
! Clinical Features !! Recommended neuroimaging test
|-
| Headache in immunocompromised people (cancer, HIV) || [[MRI]] of head with or without contrast
|-
| Headache in people older than 60 with suspected temporal arteritis || MRI of head with or without contrast
|-
| Headache with suspected meningitis || CT or MRI without contrast
|-
| Severe headache in pregnancy || CT or MRI without contrast
|-
| Severe unilateral headache caused by possible dissection of carotid or arterial arteries || MRI of head with or without contrast, [[magnetic resonance angiography]] or [[Computed tomography angiography|Computed Tomography Angiography]] of head and neck.
|-
| Sudden onset headache or worst headache of life || CT of head without contrast, [[Computed tomography angiography|Computed Tomography Angiography]] of head and neck with contrast, [[magnetic resonance angiography]] of head and neck with and without contrast, MRI of head without contrast
|}

==== Lumbar puncture ====
A [[lumbar puncture]] is a procedure in which cerebral spinal fluid is removed from the spine with a needle. A lumbar puncture is necessary to look for infection or blood in the spinal fluid. A lumbar puncture can also evaluate the pressure in the spinal column, which can be useful for people with [[idiopathic intracranial hypertension]] (usually young, obese women who have increased intracranial pressure), or other causes of increased intracranial pressure. In most cases, a CT scan should be done first.<ref name=Goadsby />

=== Classification ===
Headaches are most thoroughly classified by the [[International Headache Society]]'s International Classification of Headache Disorders (ICHD), which published the second edition in 2004.<ref name="IHS_ICHD2">{{cite book |url=http://ihs-classification.org/en/ |title=IHS Classification ICHD-2 |publisher=International Headache Society |edition=Online |url-status=dead |archive-url=https://web.archive.org/web/20131103104037/http://ihs-classification.org/en/ |archive-date=3 November 2013 |access-date=18 November 2013 }}</ref> The third edition of the International Headache Classification was published in 2013 in a beta version ahead of the final version.<ref>[//www.ichd-3.org/ ''Website The International Headache Classification (ICHD-3 Beta)'']. Retrieved 29. August 2016.</ref> This classification is accepted by the [[WHO]].{{sfn|Olesen et al.|2005|pages=9–11}}

Other classification systems exist.  One of the first published attempts was in 1951.<ref>{{cite journal | vauthors = Brown MR | title = The classification and treatment of headache | journal = The Medical Clinics of North America | volume = 35 | issue = 5 | pages = 1485–1493 | date = September 1951 | pmid = 14862569 | doi = 10.1016/S0025-7125(16)35236-1 }}</ref> The US [[National Institutes of Health]] developed a classification system in 1962.<ref>{{cite journal | journal = JAMA | author = Ad Hoc Committee on Classification of Headache | title = Classification of Headache | volume = 179 | pages = 717–8 | year = 1962 | doi = 10.1001/jama.1962.03050090045008 | issue = 9 }}</ref>

==== ICHD-2 ====
{{Main|International Classification of Headache Disorders}}
The [[International Classification of Headache Disorders]] (ICHD) is an in-depth [[hierarchical]] classification of headaches published by the [[International Headache Society]]. It contains explicit (operational) [[diagnostic criteria]] for headache disorders. The first version of the classification, ICHD-1, was published in 1988. The current revision, ICHD-2, was published in 2004.<ref>{{cite book |title=The Headaches |edition=3 |publisher=[[Lippincott Williams & Wilkins]] |year=2005  | vauthors = Olesen PJ, Goadsby NM, Ramadan P, Tfelt-Hansen KM, Welch |ref={{Harvid|Olesen et al.|2005}}}}</ref> The classification uses numeric codes. The top, one-digit diagnostic level includes 14 headache groups. The first four of these are classified as primary headaches, groups 5-12 as secondary headaches, [[Human cranium|cranial]] [[neuralgia]], central and primary facial pain and other headaches for the last two groups.<ref>{{cite book |title=Comprehensive Review of Headache Medicine |publisher=[[Oxford University Press]] |year=2008| isbn=978-0-19-536673-0 | vauthors = Levin M, Baskin SM, Bigal ME |ref={{Harvid|Levin et al.|2008}}}}</ref>

The ICHD-2 classification defines [[migraine]]s, tension-types headaches, cluster headache and other [[trigeminal]] autonomic headache as the main types of primary headaches.<ref name="IHS_ICHD2" /> Also, according to the same classification, stabbing headaches and headaches due to [[cough]], exertion and sexual activity ([[sexual headache]]) are classified as primary headaches. The daily-persistent headaches along with the hypnic headache and thunderclap headaches are considered primary headaches as well.<ref>{{cite book | vauthors = Linn FH | title = Headache | chapter = Primary thunderclap headache | volume = 97 | pages = 473–481 | date = 2010 | pmid = 20816448 | doi = 10.1016/s0072-9752(10)97042-5 | publisher = Elsevier | isbn = 978-0-444-52139-2 | series = Handbook of Clinical Neurology }}</ref><ref>{{cite journal | vauthors = Obermann M, Holle D | title = Hypnic headache | journal = Expert Review of Neurotherapeutics | volume = 10 | issue = 9 | pages = 1391–1397 | date = September 2010 | pmid = 20839413 | doi = 10.1586/ern.10.112 | s2cid = 19141493 }}</ref>

Secondary headaches are classified based on their cause and not on their [[symptoms]].<ref name="IHS_ICHD2" /> According to the ICHD-2 classification, the main types of secondary headaches include those that are due to head or neck trauma such as [[Whiplash (medicine)|whiplash injury]], [[intracranial hematoma]], post [[craniotomy]] or other head or neck injury. Headaches caused by cranial or cervical vascular disorders such as [[Stroke#Ischemic stroke|ischemic stroke]] and [[transient ischemic attack]], non-traumatic intracranial hemorrhage, [[Cerebral arteriovenous malformation|vascular malformations]] or [[arteritis]] are also defined as secondary headaches. This type of headache may also be caused by [[cerebral venous thrombosis]] or different intracranial vascular disorders. Other secondary headaches are those due to intracranial disorders that are not vascular such as low or high pressure of the cerebrospinal fluid pressure, non-infectious inflammatory disease, intracranial neoplasm, [[epileptic seizure]] or other types of disorders or diseases that are intracranial but that are not associated with the vasculature of the [[central nervous system]].{{citation needed|date=June 2021}}

ICHD-2 classifies headaches that are caused by the ingestion of a certain substance or by its withdrawal as secondary headaches as well. This type of headache may result from the overuse of some medications or exposure to some substances. [[HIV]]/[[AIDS]], intracranial [[infections]] and systemic infections may also cause secondary headaches. The ICHD-2 system of classification includes the headaches associated with homeostasis disorders in the category of secondary headaches. This means that headaches caused by [[Kidney dialysis|dialysis]], [[high blood pressure]], [[hypothyroidism]], cephalalgia and even [[fasting]] are considered secondary headaches. Secondary headaches, according to the same classification system, can also be due to the injury of any of the facial structures including [[teeth]], jaws, or [[temporomandibular joint]]. Headaches caused by psychiatric disorders such as [[somatization]] or [[psychotic disorders]] are also classified as secondary headaches.{{citation needed|date=June 2021}}

The ICHD-2 classification puts cranial neuralgias and other types of [[neuralgia]] in a different category. According to this system, there are 19 types of neuralgias and headaches due to different central causes of facial pain. Moreover, the ICHD-2 includes a category that contains all the headaches that cannot be classified.{{citation needed|date=June 2021}}

Although the ICHD-2 is the most complete headache classification there is and it includes frequency in the diagnostic criteria of some types of headaches (primarily primary headaches), it does not specifically code frequency or severity which are left at the discretion of the examiner.<ref name="IHS_ICHD2" />

==== NIH ====
{{Main|NIH classification of headaches}}
The NIH classification consists of brief definitions of a limited number of headaches.{{sfn|Levin et al.|2008|page=60}}

The NIH system of classification is more succinct and only describes five categories of headaches. In this case, primary headaches are those that do not show organic or structural causes. According to this classification, primary headaches can only be vascular, [[myogenic]], cervicogenic, traction, and inflammatory.<ref>{{Cite web|title=Headache Information Page {{!}} National Institute of Neurological Disorders and Stroke|url=https://www.ninds.nih.gov/Disorders/All-Disorders/Headache-Information-Page|access-date=21 July 2021|website=www.ninds.nih.gov}}</ref>