Our understanding of headache has been augmented by observations made during operations on the brain by Ray and Wolff. These observations inform us that only certain cranial structures are sensitive to noxious stimuli: (1) skin, subcutaneous tissue, muscles, extracranial arteries, and external periosteum of the skull; (2) the delicate structures of the eye, ear, nasal cavities, and paranasal sinuses; (3) intracranial venous sinuses and their large tributaries because they are intradural; (4) parts of the dura at the base of the brain and the arteries within the dura, particularly the proximal parts of the anterior and middle cerebral arteries and the intracranial segment of the internal carotid artery; (5) the middle meningeal and superficial temporal arteries; and (6) the first three cervical nerves and cranial nerves as they pass through the dura. Interestingly, pain is practically the only sensation produced by stimulation of these structures. Much of the pia-arachnoid, the parenchyma of the brain, and the ependyma and choroid plexuses lack sensitivity.
The reference sites of pain from the aforementioned structures are important in understanding the genesis of cranial pain. Pain that arises from distention of the middle meningeal artery is projected to the back of the eye and temporal area. Pain from the intracranial segment of the internal carotid artery and proximal parts of the middle and anterior cerebral arteries is felt in the eye and orbitotemporal regions. The pathways whereby cephalic sensory stimuli are transmitted to the central nervous system (CNS) are the trigeminal nerves, particularly their first and, to some extent, second divisions, which convey impulses from the forehead, orbit, anterior and middle fossae of the skull, and the upper surface of the tentorium. The sphenopalatine branches of the facial nerve convey impulses from the nasoorbital region. The ninth and tenth cranial nerves and the first three cervical nerves transmit impulses from the inferior surface of the tentorium and all of the posterior fossa. The tentorium roughly demarcates the trigeminal from the cervical–vagal–glossopharyngeal innervation zones. The central sensory connections, which ascend through the brainstem or the cervical spinal cord and brainstem to the thalamus, are described in Chaps. 7 and 8. Sympathetic fibers from the three cervical ganglia and parasympathetic fibers from the sphenopalatine and otic ganglia are mixed with the trigeminal and other sensory fibers. These assume importance in certain headache syndromes considered further on.
There may be local tenderness of the scalp at the site of the referred pain. Dental or temporomandibular joint pain impulses are carried by the second and third divisions of the trigeminal nerve. With the exception of the cervical portion of the internal carotid artery, from which pain is referred to the eyebrow and supraorbital region, and the upper cervical spine, from which pain may be referred to the occiput, pain from disease in extracranial parts of the body is not referred to the head. There are, however, rare instances of angina pectoris that may produce discomfort at the cranial vertex or adjacent sites and, of course, in the jaw.
Mechanisms of Cranial Pain