What is a headache? What causes headache?

A headache is pain felt in the head, either all over or in one specific area. Headache pain can be sharp, throbbing, or dull. Depending on the type of headache, the pain may be associated with other symptoms, such as nausea. Chronic headaches, including constant headaches and frequent headaches, can interfere with work, family, and personal life.

Headaches are one of the most common types of pain. Generally, even a severe headache does not indicate a serious medical problem. However, some headaches can signal a dangerous underlying medical condition, especially:

  • Severe headaches that come on and worsen quickly,

  • Headaches that occur days or weeks after an injury to the head, and

  • Headaches that are accompanied by seizures or other neurological symptoms .

Experts have long divided headaches into two main categories: primary and secondary. In 2005, the International Headache Society revised its classification system to add a third category, which includes cranial neuralgias , facial pain , and other causes. Several types of headache fall under each category.

  • Primary headaches occur when nerves that sense pain in the head or neck are stimulated and no other cause of the headache can be identified. Migraine, tension, and cluster headaches are included in this category.

  • Secondary headaches are symptoms of an underlying medical condition. For instance, a secondary headache might occur in someone with a high fever. Secondary headache may also result from a brain tumor, bleeding in the brain, meningitis or encephalitis.

  • Cranial neuralgias occur when nerves in the head and upper neck become inflamed, leading to headache. Facial pain can have many origins. Other causes of headache are those that do not fall under any of these other classifications.

(See further for more about Different Types of Headache

What Causes Headaches?

There are many causes for headaches. In many cases, experts cannot tell someone what causes headaches. Headache causes range from muscle tension to underlying disease. However, doctors do understand many of the physiological causes of headaches. Headaches occur when nerves that are sensitive to pain, called pain fibers, are disturbed. Pain fibers can be disturbed by inflammation, infection (such as sinusitis), injury, or other means. In turn, these fibers send pain signals to the brain, causing headache.

Headache pain does not originate in the brain. The brain has no nerve endings. But the brain is surrounded by several layers of tissues and blood vessels that do contain pain fibers. If these tissues or vessels become inflamed or compressed, pain results.

Headache pain can start in most structures of the head and neck, including:

  • The large blood vessels of the head and neck—including those within the brain (though the number of pain fibers is reduced in the branches that lie deep within the brain itself)

  • The sinuses (pain can affect the bone and/or the many veins)

  • The first layer of bone (periosteum)

  • The teeth

  • The eyes

  • The muscles of the face and the neck

  • The tendons (which connect muscle to bone)

  • The skin

  • The joints between the vertebrae of the neck (cervical spine)

Headaches can also start outside the head

Sometimes the source of headache pain is obvious. For instance, the sinuses may become infected (sinusitis) and trigger pain at the site of the infection. But not all headaches start in your head. Pain that is perceived in the head can also come from elsewhere in the body, such as the neck.

Pain signals travel complex routes before they reach the brain, the brain processes it, and you say, “Ouch!”

The route that leads to head pain from nerve stimulation in the neck (and vice versa) is known as the cervical trigeminal relay. To understand this pathway, imagine that the spinal cord is a major river—such as the Mississippi—and the other nerves are connected to it like tributaries, all the way down to the tiniest mountain brook (nerve endings and pain fibers).

Any stimulus, such as touch, heat, or cold, acts like snowmelt. The stimulus touches the immediate nerve endings (the mountain brook), which release a signal that travels to the next larger nerve and the next, in the same way that a many mountain brooks flow into larger and larger streams until the water reaches the Mississippi.

As the signal passes along this pathway, receptors in various tissues may trigger activity in those tissues. For example, if the stimulus starts in an internal organ, it might trigger a sensory nerve in the abdominal wall so that the person feels pressure on the abdominal wall, regardless of the location or “message” (heat, pain) of that first stimulus.

Conversely, a sensory nerve can “fire” backward, causing the release of a noxious substance around the blood vessels that it supplies. This could lead to inflammation around the vessels, so the signal is translated from its original form to pain in the affected vessels.

Certain nerves of the head and face are so closely associated with the nerves of the neck (cervical spine) that signals can become “crossed”—for example, a pain stimulus that originates in the neck might be “felt” behind the eye. Recent studies have shown that this network of nerves may play a key role in causing headaches.

Are Severe Headaches Dangerous?

Most headaches, even severe ones, are not dangerous or life-threatening. But some warning signs do indicate that further testing and treatment may be required.

How can you tell the difference between “just another headache” and a headache that signals a potentially serious medical problem, such as a stroke or brain tumor, or a dangerous infection such as meningitis?

The following warning signs can help you to identify headaches caused by potentially dangerous conditions.

Does the headache:

  • Start suddenly, remain constant and severe, and grow in intensity over 24 hours or less?

  • Worsen rapidly and reach a crescendo within five minutes?

  • Occur more frequently than in the past?

  • Occur after a blow or a fall that causes injury to the head, especially if the person feels drowsy or nauseous or acts strangely?

  • Occur after a change in medication, especially a blood-thinning medication such as warfarin (Coumadin) or aspirin?

  • Prevent you from going about your normal activity more than any other headache has in the past?

  • Are accompanied by unusual neurological symptoms such as seizures, temporary loss of or change in vision, memory loss, changes in speech, loss of strength in or numbness or tingling in one or both arms or legs, or other changes in sensation? Although these symptoms can occur with migraine headaches, they should be checked out if they have not occurred previously. They may indicate a stroke.

  • Are accompanied by stiff neck, fever, night sweats, chills, loss of weight, loss of appetite, muscle pain, or other signs of illness?

  • Grow worse when you cough or strain?

  • Hardly hurt at all when you are lying down, but suddenly worsen when you sit or stand?

  • Lead to personality changes, sudden changes in mood, seizures, weakness, excessive sleepiness, lethargy, or loss of consciousness?

Need to Know:

Any significant change in headache pattern should be evaluated, even in people who regularly have severe headaches.

Frequently Asked Questions

Q: I have had severe headaches for years, but my doctor says that nothing serious is wrong. I find that very hard to believe.

A: Your doctor is most likely correct. The vast majority of headaches, although frightening if frequent and/or severe, do not signify any underlying problem. Headaches that accompany serious health problems tend to come on suddenly. Your history of headache would seem to argue in favor of your doctor’s reassuring diagnosis.

Q: I’ve suffered from headaches for years and I asked my doctor for a CAT scan or an MRI. She says it isn’t necessary. Why not?

A: Once liberally used on people who complained of headaches, CAT and MRI scans and are usually now reserved for people suspected of having brain abnormalities. This means they aren’t generally used for people with a history of headaches unless they have experienced a substantial change in their symptoms, such as seizures or abnormal neurological symptoms, or unless their headache developed following a blow or other trauma to the head.

Q: My doctor says that my headache medication itself may be causing my headaches. How is this so?

A: Overuse of over the counter or prescription medication can result in dependence (addiction). Chronic overuse of these substances may interfere with the brain’s ability to modulate pain. In addition, stopping the use of pain relievers, once the body becomes accustomed to a certain amount of the drug, can lead to withdrawal headaches.

Q. I have frequent, severe headaches. Should I be taking medications to prevent my headaches?

A: It depends. Some people with frequent, severe headaches can ward off future attacks simply by identifying and avoiding headache triggers such as smoking, eating certain foods, or changes in sleep and eating patterns. But if you meet the following criteria, you should talk to your doctor about using medications to prevent future attacks.

  1. You have frequent attacks (more than one per week).

  2. Pain-relief drugs don’t work well for you during the attack.

  3. You don’t do well or, because of other reasons, can’t use pain-relief drugs.

  4. You have used pain medications for a long time, frequently, or excessively in the past, making you less responsive to acute drug therapies.

  5. You prefer to take medications.

Facts about Headache

  • Chronic, recurring headaches affect more than 45 million Americans (more than asthma, diabetes and coronary heart disease combined). (National Headache Foundation)

  • More than 29.5 million Americans suffer from migraines each year (National Headache Foundation).

  • About 20 percent of children and teens have significant headaches (National Headache Foundation)

  • About 70 percent of migraine sufferers are women (National Headache Foundation)

  • About 25 percent of women and 10 percent of men will experience migraine headaches sometime during their lives.

  • Businesses lose an estimated industry 50 billion dollars per year due to absenteeism and medical expenses caused by headache.

  • Migraine sufferers are absent from work more than 157 million days each year.

  • More than four billion dollars are spent annually on over-the-counter (non-prescription) pain relievers for headache.

  • The first clinic devoted to headache pain was founded in 1945 at Montefiore Hospital in Bronx, New York. Since then, hundreds of headache clinics have sprung up across the country.

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