Extracranial pain-sensitive structures include the skin, fascia, subcutaneous fat, head and neck muscles, and arteries and veins. Among intracranial pain-sensitive structures are parts of the dura at the base of the skull; dural and cerebral arteries at the base of the brain; and Cranial Nerves V, VII, IX, and XII. The brain itself does not feel pain.

In spite of the frequency with which patients complain of headaches, the amount of time devoted to headaches in medical school classrooms is relatively small (see chart below).

From: www.medscape.com (source: Headache © 2005 Blackwell Publishing)

Attempts to organize headaches into categories are numerous. To classify all of the etiologies presents an ongoing challenge. Well over 100 classifications of headaches exist. A popular classification is The International Classification of Headache Disorders (second edition, 2004) also known as the ICHD-II. It is produced by the International Headache Society (IHS). The ICHD-II, like the first edition, is intended for use by both researchers and clinicians. With researchers and practitioners using the same classification system, the transfer of information from research to practice is much simpler than if separate classifications were used. The ICHD-II places headaches into three main categories: Part I. The primary headaches Part II. The secondary headaches Part III: Cranial neuralgias, facial pain, and other headaches This classification is hierarchical, so it allows practitioners to decide how detailed they want to make their diagnosis. To facilitate headache diagnosis in daily practice, the ICHD-II provides the corresponding WHO ICD-10NA codes for each IHS code. An online version of the ICHD-II is available.