A headache (cephalalgia in medical terminology) is a condition of pain in the head; sometimes neck or upper back pain may also be interpreted as a headache. It ranks amongst the most common local pain complaints and may be frequent for many people. The vast majority of headaches are benign and self-limiting. Common causes are tension, migraine, eye strain, dehydration, low blood sugar, hypermastication and sinusitis. Much rarer are headaches due to life-threatening conditions such as meningitis, encephalitis, cerebral aneurysms, extremely high blood pressure, and brain tumors. When the headache occurs in conjunction with a head injury the cause is usually quite evident. A large percentage of headaches among women are caused by ever-fluctuating estrogen during menstrual years. This can occur prior to, or even during midcycle menstruation. Treatment of an uncomplicated headache is usually symptomatic with over-the-counter painkillers such as aspirin, paracetamol (acetaminophen), or ibuprofen, although some specific forms of headaches (e.g., migraines) may demand other, more suitable treatment. It may be possible to relate the occurrence of a headache to other particular triggers (such as stress or particular foods), which can then be avoided. The brain in itself is not sensitive to pain, because it lacks nociceptors. Several areas of the head can hurt, including a network of nerves which extend over the scalp and certain nerves in the face, mouth, and throat. The meninges and the blood vessels do have pain perception. Headaches often result from traction to or irritation of the meninges and blood vessels. The membrane surrounding the brain and spinal cord, called the dura mater, is innervated with nociceptors. Stimulation of these dural nociceptors is thought to be involved in producing headaches. Similarly the muscles of the head may be sensitive to pain. The most common type of vascular headache is migraine. Migraine headaches are usually characterized by severe pain on one or both sides of the head, an upset stomach, and, for some people, disturbed vision. It is more common in women. While vascular changes are evident during a migraine, the cause of the headache is neurological, not vascular. After migraine, the most common type of vascular headache is the "toxic" headache produced by fever. Other kinds of vascular headaches include cluster headaches, which are very severe recurrent short lasting headaches, often located through or around either eye and often wake the sufferers up at the same time every night. Unlike migraines, these headaches are more common in men than in women. Muscular/myogenic Muscular (or myogenic) headaches appear to involve the tightening or tensing of facial and neck muscles; they may radiate to the forehead. Tension headache is the most common form of myogenic headache. Cervicogenic Cervicogenic headaches originate from disorders of the neck, including the anatomical structures innervated by the cervical roots C1–C3. Cervical headache is often precipitated by neck movement and/or sustained awkward head positioning. It is often accompanied by restricted cervical range of motion, ipsilateral neck, shoulder, or arm pain of a rather vague non-radicular nature or, occasionally, arm pain of a radicular nature.