Causes of Headache

Headaches have literally hundreds of causes. In most cases, the cause of a headache is a simple condition: just the everyday common headache, a tension headache, or something similar. But there are also more chronic types of headache, and also there is a possibility that a headache is caused by a severe medical condition like stroke or brain injury.

Simple Causes: There are a number of types of headache that have simple causes. The common everyday headache is well-known, but not the only one. Some causes of headache include:

Chronic or Recurring Headache Conditions: Some people get daily headaches, called Chronic Daily Headache (CDH). There are various medical conditions with chronic headache, potentially including many of the causes of headache in general, but also some specific causes of chronic headaches include:

Infectious Causes of Headache: If you have an illness, such as a virus, it is common for a headache to develop. Usually there are other symptoms such as fever. Almost any disease that causes fever can also cause headache. Causes of headache include:

Serious Brain or Head Conditions: A headache can be due to some serious problem with the brain, the skull, or the head. Many of these conditions can be a medical emergency. Some of the possibilities include:

Head Pain Conditions: Disorders affecting parts of the head, other than the brain, that cause headache or “head pain” include:

Neck Spinal Conditions: A number of spine and spinal cord conditions may result in headache, such as:

Eye Conditions: Various eye disorders can result in headaches.

Serious Conditions. A number of severe medical conditions can cause headache, such as:

Febrile illnesses: Almost any condition causing a fever can also cause a headache. Some examples include:

Medical Care Causes: various causes from treatments and tests include:

  • Spinal fluid diagnostic test (lumbar puncture test) (side-effect)
  • Cervical traction treatment (side-effect)

Drugs and Toxins: Substance-related causes of headache include:

Additional Indirect Causes: What are the causes of the causes of headache? See also the various causes of:

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If you’re feeling stressed, depressed or anxious—or if you’re not getting enough sleep—you’re at risk for a tension headache. Most diagnosed headaches are related to tension or muscle tightness, usually in the shoulders, neck, scalp or jaw. Tension headaches can cause a sensation of painful pressure on the head and neck. They sometimes feel like the grip of a tight elastic band around your head.

Migraine headaches are the second-most common, striking about 3 times more women than men. Migraine pain is often described as intense, pulsing or throbbing. Migraine sufferers can have extreme sensitivity to light and sound. In some cases, nausea and vomiting accompany the headache. Although the exact cause of migraine pain is unknown, one theory is that it’s linked to the widening and tightening of blood vessels surrounding the brain, which irritates the nearby nerves.

A less common but more severe type of headache occurs in “clusters,” often at the same time of day for several weeks. These cluster headaches usually begin suddenly as a sharp or burning pain centered around one eye. The pain can last up to 3 hours.

Source: NIH News in Health (NIH)1

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Some examples of underlying conditions include fever, infection, medication overuse, stress or emotional conflict, high blood pressure, psychiatric disorders, head injury or trauma, stroke, tumors, and trigeminal neuralgia.

Some of the more serious causes of secondary headache include:

Brain tumor. Extremely few individuals with headache have a brain tumor. However, a tumor that is growing in the brain can press against nerve tissue and pain-sensitive blood vessel walls, disrupting communication between the brain and the nerves or restricting the supply of blood to the brain. Headaches may develop, worsen, become more frequent, or come and go, often at irregular periods. Headache pain may worsen when coughing, changing posture, or straining, and may be severe upon waking. Treatment options include surgery, radiation therapy, and chemotherapy.

Disorders of blood vessels in the brain, including stroke. Several disorders associated with blood vessel formation and activity can cause headache. Most notable among these conditions is stroke in both its hemorrhagic and ischemic forms. Headache itself can cause stroke or accompany a series of blood vessel disorders that can cause a stroke.

A hemorrhagic stroke occurs when an artery in the brain bursts, spilling blood into the surrounding tissue. Hemorrhagic stroke is typically associated with disturbed brain function and an extremely painful, sudden headache.

When the hemorrhage results from a rupture of a blood vessel located within the subarachnoid space--a fluid-filled space between layers of connective tissue (meninges) that surround the brain--the first sign of a subarachnoid hemorrhage is a severe headache with a split-second onset and no known cause. Neurologists call this a thunderclap headache. Pain may also be felt in the neck and lower back. This sudden flood of blood can contaminate the cerebrospinal fluid that flows within the spaces of the brain and cause extensive damage throughout the brain.

An intracerebral hemorrhage is also usually associated with severe headache. Several conditions can render blood vessels in the brain prone to rupture and hemorrhaging. Chronic hypertension can weaken the blood vessel wall. Poor blood clotting ability due to blood disorders or blood-thinning medications like warfarin further increase the risk of bleeding. And some venous strokes (caused by clots in the brain's veins) often cause bleeding into the brain. At risk for these strokes are mothers in the post-partum period and persons with dehydration, cancer, or infections.

An aneurysm is the abnormal ballooning of an artery that causes the artery wall to weaken. A ruptured cerebral aneurysm can cause subarachnoid hemorrhage and a sudden, incredibly painful headache that is generally different in severity and intensity from other headaches individuals may have experienced. Individuals usually describe the thunderclap-like headache as "the worst headache of my life." There may be loss of consciousness and other neurological features that require emergency medical attention. "Sentinel" or sudden warning headaches sometimes occur from an aneurysm that leaks prior to rupture.

Arteriovenous malformation (AVM), an abnormal tangle of arteries and veins in the brain, causes headaches that vary in frequency, duration, and intensity as vascular malformations press on and displace normal tissue or leak blood into surrounding tissue. A headache consistently affecting one side of the head may be closely linked to the site of an AVM (although most one-sided headaches are caused by primary headache disorders). Symptoms may include seizures and hearing pulsating noises. Treatment options include blocking the blood vessel in the malformation by injecting particles or glue, or through focused radiotherapy or surgery.

An ischemic stroke occurs when an artery supplying the brain with blood becomes blocked, suddenly decreasing or stopping blood flow and causing brain cells to die. Headache that accompanies ischemic stroke can be caused by several problems with the brain's vascular system. Conditions of ischemic stroke that can cause headache include:

Thunderclap headache can also be associated with reversible cerebral vasoconstriction syndrome (RCVS). It is more common in women. Blood vessels in the head show reversible narrowing sometimes leading to stroke. RCVS has been associated with pregnancy vasoconstriction drugs.

Exposure to a substance or its withdrawal. Headaches may result from toxic states such as drinking alcohol, following carbon monoxide poisoning, or from exposure to toxic chemicals and metals, cleaning products or solvents, and pesticides. In the most severe cases, rising toxin levels can cause a pulsing, throbbing headache that, if left untreated, can lead to systemic poisoning, organ failure, and permanent neurological damage. These headaches are usually treated by identifying and removing the cause of the toxic buildup. The withdrawal from certain medicines or caffeine after frequent or excessive use can also cause headaches, as can overuse of medications to treat headache (medication overuse headache).

Head injury. Headaches are often a symptom of a concussion or other head injury. The headache may develop either immediately or months after a blow to the head, with pain felt at the injury site or throughout the head. In some rare cases, post-concussion or head trauma headaches may last for weeks to years. Emotional disturbances may worsen headache pain. In most cases, the cause of post-traumatic headache is unknown. Sometimes the cause is ruptured blood vessels, which result in an accumulation of blood called a hematoma—a mass of blood that displaces brain tissue and cause headaches as well as weakness, confusion, memory loss, and seizures. Hematomas can be drained surgically to produce rapid relief of symptoms. The most serious hematomas require immediate emergency treatment.

Increased intracranial pressure. A buildup or poor absorption of cerebrospinal fluid (CSF) in the brain can raise pressure in the brain and compress nerves and blood vessels, causing headaches. Two disorders that can lead to increased intracranial pressure are hydrocephalus and idiopathic intracranial hypertension. Hydrocephalus is an excess accumulation of CSF; Idiopathic intracranial hypertension, previously known as pseudotumor cerebri (meaning "false brain tumor"), is associated with severe headache and is most commonly seen in young, overweight females.

Inflammation from meningitis, encephalitis, and other infections. Inflammation from infections can harm or destroy nerve cells and cause dull to severe headache pain, along with other life-threatening conditions Headaches may also occur with a fever or a flu-like infection but are not serious and generally do not require medical attention. A headache may accompany a bacterial infection of the upper respiratory tract that spreads to and inflames the lining of the sinus cavities.

Seizures. Migraine-like headache pain may occur after a seizure. Moderate to severe headache pain may last for several hours and worsen with sudden movements of the head or when sneezing, coughing, or bending. Other symptoms may include nausea, vomiting, fatigue, increased sensitivity to light or sound, and vision problems.

Spinal fluid leak/low CSF pressure headache. About 10 to 25 percent of people who undergo a lumbar puncture (which involves a small sampling of the spinal fluid being removed for testing) develop a headache due to a leak of cerebrospinal fluid (CSF) following the procedure. In other individuals, an internal spinal fluid leak also can cause the “Low CSF pressure headache.” Since the headache occurs only when the individual stands up, the "cure" is to lie down until the headache runs its course-anywhere from a few hours to several days. Persistent post-dural headaches may be treated by injecting a small amount of the individual's own blood into the low back to stop the leak (called an epidural blood patch). Occasionally spinal fluid leaks spontaneously, causing this "low pressure headache."

Structural abnormalities of the head, neck and spine. Headache pain and loss of function may be triggered by structural abnormalities in the head or spine, restricted blood flow through the neck, irritation to nerves anywhere along the path from the spinal cord to the brain, or stressful or awkward positions of the head and neck. Medications may ease the pain. Cervicogenic headaches are caused by structural irregularities in either the head or neck. In the Arnold-Chiari malformation, the back of the skull is too small for the brain. This forces a part of the brain to block the normal flow of spinal fluid and press on the brain stem. Chiari malformations are present at birth but may not cause symptoms until later in life, including headaches that worsen with coughing or straining. Chiari malformations may be associated with syringomyelia, a fluid-filled cyst within the spinal cord, can cause pain, numbness, weakness, and headaches.

Trigeminal neuralgia. The trigeminal nerve conducts sensations to the brain from the upper, middle, and lower portions of the face, as well as inside the mouth. A potential cause of trigeminal neuralgia is a blood vessel pressing on the nerve as it exits the brain stem, but other causes have been described. Symptoms include headache and intense shock-like or stabbing pain that comes on suddenly and is typically felt on one side of the jaw or cheek.

Source: NINDS (NIH)2

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Headaches in children can be caused by a number of factors, including emotional problems such as tension between family members, stress from school activities, weather changes, irregular eating and sleep, dehydration, and certain foods and drinks. Of special concern among children are headaches that occur after head injury or those accompanied by rash, fever, or sleepiness.

Source: NINDS (NIH)3

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Headache and Sleep Disorders

Headaches are often a secondary symptom of a sleep disorder but may also contribute to sleep disorders. For example, tension-type headache is regularly seen in persons with insomnia or sleep-wake cycle disorders. Nearly three-fourths of individuals who suffer from narcolepsy complain of either migraine or cluster headache. Migraines and cluster headaches appear to be related to the number of and transition between rapid eye movement (REM) and other sleep periods an individual has during sleep. Hypnic headache awakens individuals mainly at night but may also interrupt daytime naps. Reduced oxygen levels in people with sleep apnea may trigger early morning headaches.

Getting the proper amount of sleep can ease headache pain. Generally, too little or too much sleep can precipitate headaches, as can overuse of sleep medicines. Daytime naps often reduce deep sleep at night and can produce headaches in some adults. Some sleep disorders and secondary headache are treated using antidepressants. Check with a doctor before using over-the-counter medicines to ease sleep-associated headaches.

Source: NINDS (NIH)4

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Like other types of pain, headaches can serve as warning signals of more serious disorders. This is particularly true for headaches caused by inflammation, including those related to meningitis as well as those resulting from diseases of the sinuses, spine, neck, ears, and teeth.

Source: NINDS (NIH)5

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A headache can affect several areas of the head, including a network of nerves that extends over the scalp and certain nerves in the face, mouth, and throat. The sensation of pain can be "referred’, which means the irritation in one area can transmit the feeling of pain via associated nerves to another area. A good example is neck pain leading to headache. There are different types of headache and many different causes including stress and muscular tension, diet and food sensitivities, jaw problems, hormone problems, eye problems and ear, nose and throat problems.

Source: Queensland Health6

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Headaches can be triggered by stress, tiredness, hunger, eyestrain, noise or bright light.

Causes of headaches

Most headaches are not a cause for worry, but occasionally they are a symptom of something serious.

Source: New Zealand Health7

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10 headache triggers

It's not just stress and nasty colds that cause headaches. Cleaning your home or sleeping in late can cause them too. We reveal 10 headache triggers and how to fix them.

  1. Relaxing after stress

You put in 10-hour days from Monday to Friday and you feel fine, only to wake up after a lie-in on Saturday with a pounding headache. Why is that?

It’s because as the tension of the week subsides, your levels of stress hormones drop, which causes a rapid release of neurotransmitters (the brain’s chemical messengers). These send out impulses to blood vessels to constrict and then dilate, which causes a headache.

How to fix it: Avoid the temptation to sleep in at weekends. More than eight hours' sleep at a time can bring on a headache. Introduce some relaxation time, such as a yoga class, into your working week, rather than squeezing it all into the weekend.

  1. Pent-up anger

When you’re angry, muscles in the back of your neck and scalp tense up, causing a tight band-like sensation around your head. This is a sign of a tension headache.

How to fix it: When you start feeling angry, breathe deeply and slowly. Breathe in through your nose and out through your mouth, This should relax your head and neck muscles.

Read more about how to control your anger.

  1. Poor posture

Poor posture causes tension in your upper back, neck and shoulders, which can lead to a headache. Typically, the pain throbs in the base of the skull and sometimes flashes into the face, especially the forehead.

How to fix it: Avoid sitting or standing in one position for a long period of time. Sit up straight and support your lower back. Consider using a special headset if you spend a lot of time on the phone, as holding a handset between your head and shoulder can strain muscles and cause headaches.

You could also see a physical therapist, such as an osteopath or Alexander technique practitioner. They may be able to help you identify and correct any posture problems.

Read more about osteopathy and the Alexander technique.

  1. Perfume

If you think housework is giving you a headache, you could be right. Household cleaners, along with perfumes and fragranced air fresheners, contain chemicals that can bring on headaches.

How to fix it: If you’re susceptible to headaches brought on by certain smells, avoid heavy perfumes and strong-smelling soaps, shampoos and conditioners. Use fragrance-free air fresheners and household cleaners, and keep your doors and windows open as much as possible at home. If a colleague’s perfume is bothering you, put a fan on your desk at work.

  1. Bad weather

If you’re prone to getting headaches, you could find that grey skies, high humidity, rising temperatures and storms can all bring on head pain.

Pressure changes that cause weather changes are thought to trigger chemical and electrical changes in the brain. This irritates nerves, leading to a headache.

How to fix it: There’s not much you can do to change the weather. However, by looking at the forecast, you can predict when you’re likely to have a headache and take a preventative painkiller a day or two in advance.

Check the weather forecast here.

  1. Grinding teeth

Grinding your teeth at night (the medical name is bruxism) makes your jaw muscle contract, causing a dull headache.

How to fix it: Your dentist can fit you with a mouth guard to protect your teeth while you sleep. They cost around £50.

Read more about teeth grinding.

  1. Bright lights

Bright lights and glare, especially if flickering, can induce migraines. This is because bright and flickering lights boost the levels of certain chemicals in the brain, which then activate the migraine centre.

How to fix it: Sunglasses are great at reducing light intensity, and you can wear them inside and outside. Polarised lenses can also help to reduce glare.

At work, adjust your computer monitor or attach a glare screen. You may be able to turn off certain lights or move them. If you can't, change where you sit in the office. Fluorescent lighting tends to flicker, so if you’re able to, substitute it with some other form of lighting.

  1. Food triggers

Your turkey and cheese sandwich and small bar of dark chocolate might be a tasty lunch, but beware of the headache that could follow it. All these foods contain chemicals that can bring on a migraine. Other culprits include aged cheeses like stilton and brie, diet fizzy drinks, and processed meats and fish.

How to fix it: Keep a migraine trigger diary and once you suspect a certain food may be the cause of your headaches, eliminate it from your diet for a couple of months to see if you get fewer headaches.

If you're concerned about avoiding any food-related trigger factor, see your GP or practice nurse or ask to be referred to a dietician for specialist advice.

Remember to eat regularly, because skipping meals can bring on a headache.

The Migraine Trust offers an online migraine trigger diary here.

  1. Sex headaches

It's a standing joke that headaches are used as an excuse to avoid sex, but for many men and women coital headaches that come on at the height of passion are a real and distressing problem.

Doctors think sex headaches are due to pressure building up in the head and neck muscles. The headaches can happen during foreplay or just before orgasm, and can last for a few minutes or up to an hour.

How to fix it: They’re inconvenient, but these headaches are usually harmless and don't mean you have to avoid sex. Take a painkiller a few hours beforehand to block the headache.

  1. Ice cream

Do you get a sharp, stabbing pain in your forehead when you bite into an ice cream cone? Then you’re susceptible to ice cream headaches, caused by cold material moving across the roof of your mouth and the back of your throat. Ice lollies and slushy frozen drinks have the same effect.

How to fix it: The good news is that ice cream headaches don’t need treatment. In fact, they’re over in a flash, rarely lasting more than a minute or two.

Source: NHS Choices UK8

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What causes tension headaches?

The exact cause of tension-type headaches isn't clear, but certain things have been known to trigger them, including:

Tension-type headaches are known as primary headaches, which means they're not caused by an underlying condition. Other primary headaches include cluster headaches and migraines.

Source: NHS Choices UK9

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Headaches might arise because of another medical condition, such as swollen sinuses or head injury. In these cases, treating the underlying problem usually relieves headache pain as well. But most headaches—including tension headaches and migraines—aren’t caused by a separate illness.

Source: NIH News in Health (NIH)10

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In rare cases, a headache may warn of a serious illness. Get medical help right away if you have a headache after a blow to your head, or if you have a headache along with fever, confusion, loss of consciousness, or pain in the eye or ear.

“Know what kind of headache you have and, if you can’t manage it yourself, seek help,” Porter says. “Remember there are preventive behavioral steps and medicines that can help manage headaches. But if the pain is severe or lasting, get medical care.”

Source: NIH News in Health (NIH)11

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Rebound headache: Rebound headaches can occur with both over-the-counter and prescription pain-relief medicines. They can also occur whether you take them for headache or for another type of pain. Talk to your doctor if you're caught in a rebound cycle.

Source: OWH (DHHS)12

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Tension headache: Ninety percent of all headaches are tension headaches.

Source: MedLinePlus Magazine (NIH)13

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Acquired hydrocephalus: Hydrocephalus that develops in adults or children (acquired) can cause headaches.

The headache may be worse when you wake up in the morning because the fluid in your brain doesn't drain so well while you're lying down and may have built up overnight.

Sitting up for a while may improve your headache. However, as the condition progresses, the headaches may become continuous.

Source: NHS Choices UK14

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Lyme disease: Not everyone with Lyme disease gets the rash. Some people also have flu-like symptoms in the early stages.

Source: NHS Choices UK15

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Malaria: The initial symptoms of malaria are flu-like.

These symptoms are often mild and can sometimes be difficult to identify as malaria.

Source: NHS Choices UK16

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Dural ectasia: As the membrane expands, it can press on the vertebrae in your lower back, which can cause:

Source: NHS Choices UK17

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Cluster headaches: Cluster headaches

Cluster headaches are excruciating attacks of pain on one side of the head, often felt around the eye.

They begin quickly and without warning. The pain is very severe, and is often described as a sharp, burning or piercing sensation.

People often feel restless and agitated during an attack because the pain is so intense, and they may react by rocking, pacing, or banging their head against the wall.

Read more about cluster headaches.

Source: NHS Choices UK18

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Migraine: Migraine

A migraine typically feels like an intense headache on one side of the head. The pain is usually a moderate or severe throbbing sensation that gets worse when you move and prevents you from carrying out normal activities.

In some cases, the pain can occur on both sides of your head, and may affect your face or neck.

Migraines can cause vomiting and extreme sensitivity to light and sound. Sometimes, in cases of severe migraines, the best thing to do is find a dark, quiet place to lie down until the pain passes.

Find out more about migraine and read a true-life account of what it's like to have migraine.

Find your local migraine clinic.

Source: NHS Choices UK19

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Painkiller headaches: Painkiller, or rebound, headaches are frequent or daily headaches that develop after taking painkillers for tension headaches or migraines over several months.

Source: NHS Choices UK20

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Postural tachycardia syndrome: Postural tachycardia syndrome (PoTS) is an abnormal increase in heart rate that occurs after sitting up or standing. It typically causes dizziness, fainting and other symptoms.

Source: NHS Choices UK21

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Upper respiratory tract infections: Upper respiratory tract infections

Common upper respiratory tract infections include:

Source: NHS Choices UK22

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Subarachnoid hemorrhage: The main symptoms of a subarachnoid haemorrhage include:

  • a sudden agonising headache - which is often described as being similar to a sudden hit on the head, resulting in a blinding pain unlike anything experienced before

Source: NHS Choices UK23

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Post-stroke syndrome: Headaches are common after a subarachnoid haemorrhage, but they tend to ease over time.

They're not as painful as when you had your haemorrhage, and you should be able to control them with over-the-counter painkillers, such as paracetamol.

Source: NHS Choices UK24

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Subdural hematoma: Symptoms of a subdural haematoma can include:

  • a headache that keeps getting worse

Source: NHS Choices UK25

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Temporomandibular disorder: Signs of TMD include:

  • a headache around your temples

Source: NHS Choices UK26

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Tick-borne encephalitis: Initial symptoms of TBE are similar to flu and can include:

Source: NHS Choices UK27

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Stroke: “Some people get a severe headache that’s immediate and strong, different from any kind you’ve ever had,” says Dr. Salina Waddy, an NIH stroke expert.

Source: NIH News in Health (NIH)28

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Some causes may include:29 Causes of Headache:

Causes List for Headache

List of possible causes of Headache or similar symptoms may include:30

... Full Causes List for Headache »

Physiologic Causes of Headache

Why Headaches Hurt

Information about touch, pain, temperature, and vibration in the head and neck is sent to the brain by the trigeminal nerve, one of the 12 cranial nerves that start at the base of the brain.

The nerve has three branches that relay sensations from the scalp, the blood vessels inside and outside of the skull, the lining around the brain (the meninges), and the face, mouth, neck, ears, eyes, and throat.

Brain tissue itself lacks pain-sensitive nerves and does not feel pain. Headaches occur when pain-sensitive nerve endings called nociceptors [1] are activated in response to things that may bring on a headache (such as stress, certain foods or odors, or use of medicines). The nociceptors send messages through the trigeminal nerve to the brain stem and then to the thalamus, the brain's "relay station" for pain sensation from all over the body, and then onto the parts of the >cortex (the brain’s outer layer) involved in sensory and emotional processing. Other parts of the brain may also be part of the process, causing nausea, vomiting, diarrhea, trouble concentrating, and other neurological symptoms.

Source: NINDS (NIH)31

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  1. Source: NIH News in Health (NIH): 2008/ June/ docs/ 01features_02.htm
  2. Source: NINDS (NIH): disorders/ headache/ detail_headache.htm
  3. ibid.
  4. ibid.
  5. Source: NINDS (NIH): disorders/ headache/ headache.htm
  6. Source: Queensland Health: HealthCondition/ condition/ 6/ 102/ 349/ headache
  7. Source: New Zealand Health: your-health/ conditions-and-treatments/ diseases-and-illnesses/ headache
  8. Source: NHS Choices UK: livewell/ headaches/ pages/ headachetriggers.aspx
  9. Source: NHS Choices UK: conditions/ tension-headaches/ 
  10. Source: NIH News in Health (NIH): issue/ mar2014/ feature2
  11. ibid.
  12. Source: OWH (DHHS): a-z-topics/ migraine
  13. Source: MedLinePlus Magazine (NIH): magazine/ issues/ spring09/ articles/ spring09pg18-19.html
  14. Source: NHS Choices UK: conditions/ hydrocephalus/ symptoms/ 
  15. Source: NHS Choices UK: conditions/ Lyme-disease/ 
  16. Source: NHS Choices UK: conditions/ malaria/ symptoms/ 
  17. Source: NHS Choices UK: conditions/ marfan-syndrome/ symptoms/ 
  18. Source: NHS Choices UK: Livewell/ Pain/ Pages/ 20-painful-conditions.aspx
  19. ibid.
  20. Source: NHS Choices UK: Livewell/ headaches/ Pages/ Painkillerheadaches.aspx
  21. Source: NHS Choices UK: conditions/ postural-tachycardia-syndrome/ 
  22. Source: NHS Choices UK: conditions/ Respiratory-tract-infection/ 
  23. Source: NHS Choices UK: conditions/ Subarachnoid-haemorrhage/ 
  24. Source: NHS Choices UK: conditions/ subarachnoid-haemorrhage/ recovery/ 
  25. Source: NHS Choices UK: conditions/ Subdural-haematoma/ 
  26. Source: NHS Choices UK: conditions/ temporomandibular-disorder-tmd/ 
  27. Source: NHS Choices UK: conditions/ tick-borne-encephalitis/ 
  28. Source: NIH News in Health (NIH): issue/ aug2014/ feature1
  29. Source: Human Phenotype Ontology
  30. Source: Algorithmically Generated List
  31. Source: NINDS (NIH): disorders/ headache/ detail_headache.htm

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Note: This site is for informational purposes only and is not medical advice. See your doctor or other qualified medical professional for all your medical needs.