Dawn A. Marcus, an associate professor at the University of Pittsburgh School of Medicine¿s department of anesthesiology, explains.
When experiencing a severe, throbbing headache, a person often places his hands on both sides of his head and claims, "It feels like my brain is pushing to get out, so it feels better to hold it in." This sensation gives a false impression that the brain itself is enlarging and causing the pain sensation. Interestingly, brain tissue does not feel pain in the same way skin or other organs do. Because the brain is encased in a hard, protective covering, it has not developed to respond to touch or pressure sensations like other, more exposed parts of our bodies have. Indeed, a brain surgeon can actually cut brain tissue in an awake patient without the patient feeling the knife.
Head pain instead occurs because of activation or irritation of structures that do sense pain: skin, bone or neck joints, sinuses, blood vessels or muscles. When a person has a brain tumor, pain is usually a late symptom to develop--brain tumors generally only cause pain when they have grown large enough to damage bone or stretch blood vessels or nerves. Neck problems may also result in head pain, with pain from the neck and back of the head often radiating over the top of the head to an eye. Sinus infection or inflammation (usually occurring as part of an allergy reaction), however, is an uncommon cause of recurring headaches. Interestingly, Roger Cady and Curtis Schneider of the Headache Care Center in Springfield, Mo., have shown that 25 to 30 percent of migraine sufferers report nasal symptoms during their typical migraine episodes, and nearly 98 percent of people who believed they had sinus headaches were actually experiencing a migraine.
The most common types of chronic headaches are the migraine and tension-type varieties. A migraine is an intermittent headache, usually occurring between once a month and twice a week, with each episode lasting eight to 12 hours. Migraine is often experienced as a one-sided, throbbing head pain that limits activities and may be associated with nausea and sensitivity to lights, noises and smells. Tension-type headaches may occur more frequently, and the pain-- typically a dull pressure pain on both sides of the head that does not limit activities--sometimes lasts several days. Both of these kinds of headache occur in response to exposure to internal or external triggers, such as hormonal changes, sleep deprivation, fasting or stress. Exposure to these triggers prompts the brain to signal pain centers that produce a variety of chemical messengers, including serotonin and norepinephrine, which cause expansion of meningeal blood vessels surrounding the brain. This expansion results in increased blood flow, and blood vessels on the side of the head can become more prominent and tender. As the blood vessels swell they stretch the nerves that surround them, causing these nerves to send signals to the trigeminal system, an area of the brain that relays pain messages for the head and face. Activation of the trigeminal system most commonly causes pain around the eye and cheek, creating the false perception of "sinus" pain. The trigeminal system also sends messages to the hypothalamus, an area of the brain involved in food cravings, and to the upper part of the cervical spinal cord, which may result in muscle spasms in the neck.
Once the full headache pathway is activated, it becomes more difficult for headache treatments to work effectively. Recent work led by Rami Burstein of Harvard University in both rats and humans has consistently shown that headache medications need to be taken early in a headache episode in order to be effective. Migraine patients often notice that their headaches begin with a throbbing sensation followed by increased skin sensitivity. This increased skin sensitivity, called allodynia, may take the form of scalp tenderness, "painful" hair or pain associated with hair brushing or wearing earrings or glasses. Once allodynia has occurred, headache treatments are much less effective. Carefully recording headache symptoms in a diary can provide a good estimate of when allodynia usually occurs and can help an individual determine when medications should be taken to offer the most relief.