Headaches are a type of pain referred to the surface of the head from deep head structures.
Some headaches result from pain stimuli arising inside the cranium, but others result from pain arising outside the cranium, such as from the nasal sinuses.
Pain-Sensitive Areas in Cranial Vault. The brain tissuesthemselves are almost totally insensitive to pain. Even cutting or electrically stimulating the sensory areas of the cerebral cortex only occasionally causes pain; instead, it causes prickly types of paresthesias on the area of the body represented by the portion of the sensory cortex stimulated. Therefore, it is likely that much or most of the pain of headache is not caused by damage within the brain itself.
Conversely, tugging on the venous sinuses around thebrain, damaging the tentorium, or stretching the dura at the base of the brain can cause intense pain that is rec-ognized as headache. Also, almost any type of trauma-tizing, crushing, or stretching stimulus to the bloodvessels of the meninges can cause headache. An espe-cially sensitive structure is the middle meningeal artery, and neurosurgeons are careful to anesthetize this artery specifically when performing brain operations under local anesthesia.
Areas of the Head to Which Intracranial Headache Is Referred.
Stimulation of pain receptors in the cerebral vault above the tentorium, including the upper surface of the tentorium itself, initiates pain impulses in the cerebral portion of the fifth nerve and, therefore, causes referred headache to the front half of the head in the surface areas supplied by this somatosensory portion of the fifth cranial nerve, as shown in Figure 48–9.
Conversely, pain impulses from beneath the tentorium enter the central nervous system mainly through the glossopharyngeal, vagal, and second cervi-cal nerves, which also supply the scalp above, behind, and slightly below the ear. Subtentorial pain stimuli cause “occipital headache” referred to the posterior part of the head.
Headache of Meningitis. One of the most severeheadaches of all is that resulting from meningitis, which causes inflammation of all the meninges, including the sensitive areas of the dura and the sensitive areas around the venous sinuses. Such intense damage can cause extreme headache pain referred over the entire head.
Headache Caused by Low Cerebrospinal Fluid Pressure.
Removing as little as 20 milliliters of fluid from the spinal canal, particularly if the person remains in an upright position, often causes intense intracranial headache. Removing this quantity of fluid removes part of the flotation for the brain that is normally provided by the cerebrospinal fluid. The weight of the brain stretches and otherwise distorts the various dural surfaces and thereby elicits the pain that causes the headache.
Migraine Headache. Migraine headache is a special typeof headache that is thought to result from abnormal vas-cular phenomena, although the exact mechanism is unknown. Migraine headaches often begin with various prodromal sensations, such as nausea, loss of vision in part of the field of vision, visual aura, and other types of sensory hallucinations. Ordinarily, the prodromal symptoms begin 30 minutes to 1 hour before the beginning of the headache. Any theory that explains migraine headache must also explain the prodromal symptoms.
One of the theories of the cause of migraine headaches is that prolonged emotion or tension causes reflex vasospasm of some of the arteries of the head, including arteries that supply the brain. The vasospasm theoretically produces ischemia of portions of the brain, and this is responsible for the prodromal symptoms. Then, as a result of the intense ischemia, something happens to the vascular walls, perhaps exhaustion of smooth muscle contraction, to allow the blood vessels to become flaccid and incapable of maintaining vascu-lar tone for 24 to 48 hours. The blood pressure in the vessels causes them to dilate and pulsate intensely, and it is postulated that the excessive stretching of the walls of the arteries—including some extracranial arteries, such as the temporal artery—causes the actual pain of migraine headaches. Other theories of the cause of migraine headaches include spreading cortical de-pression, psychological abnormalities, and vasospasm caused by excess local potassium in the cerebral extra-cellular fluid.
There may be a genetic predisposition to migraine headaches, because a positive family history for migraine has been reported in 65 to 90 per cent of cases. Migraine headaches also occur about twice as fre-quently in women as in men.
Alcoholic Headache. As many people have experi-enced, a headache usually follows an alcoholic binge. It is most likely that alcohol, because it is toxic to tissues, directly irritates the meninges and causes the intracra-nial pain.
Headache Caused by Constipation. Constipation causesheadache in many people. Because it has been shown that constipation headache can occur in people whose pain sensory tracts in the spinal cord have been cut, we know that this headache is not caused by nervous impulses from the colon. Therefore, it may result from absorbed toxic products or from changes in the circula-tory system resulting from loss of fluid into the gut.
Headache Resulting from Muscle Spasm. Emotional tensionoften causes many of the muscles of the head, especially those muscles attached to the scalp and the neck muscles attached to the occiput, to become spastic, and it is postulated that this is one of the common causes of headache. The pain of the spastic head muscles suppos-edly is referred to the overlying areas of the head and gives one the same type of headache as intracranial lesions do.
Headache Caused by Irritation of Nasal and Accessory Nasal Structures. The mucous membranes of the nose andnasal sinuses are sensitive to pain, but not intensely so. Nevertheless, infection or other irritative processes in widespread areas of the nasal structures often summate and cause headache that is referred behind the eyes or, in the case of frontal sinus infection, to the frontal sur-faces of the forehead and scalp, as shown in Figure 48–9. Also, pain from the lower sinuses, such as from the max-illary sinuses, can be felt in the face.
Headache Caused by Eye Disorders. Difficulty in focusingone’s eyes clearly may cause excessive contraction of the eye ciliary muscles in an attempt to gain clear vision. Even though these muscles are extremely small, it is believed that tonic contraction of them can cause retro-orbital headache. Also, excessive attempts to focus the eyes can result in reflex spasm in various facial and extraocular muscles, which is a possible cause of headache.
A second type of headache that originates in the eyes occurs when the eyes are exposed to excessive irradia-tion by light rays, especially ultraviolet light. Looking at the sun or the arc of an arc-welder for even a few seconds may result in headache that lasts from 24 to 48 hours. The headache sometimes results from “actinic” irritation of the conjunctivae, and the pain is referred to the surface of the head or retro-orbitally. However, focusing intense light from an arc or the sun on the retina can also burn the retina, and this could be the cause of the headache.
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