Headache is a pain in the head with the pain being above the eyes or the ears, behind the head (occipital), or in the back of the upper neck.
All headaches are considered primary headaches or secondary headaches. Primary headaches are not associated with other diseases.
Headache has many causes.
Examples of primary headaches are:
- Tension headaches
- Migraine headaches
- Cluster headaches
Secondary headaches are caused by other diseases, the associated disease may be minor or major.
Tension headaches are the most common type of primary headache. As many as 90% of adults experience tension headaches. It is more common among women than men. The pain typically impacts the whole head, but may begin in the back of the head or above the eyebrows. Some people experience a band-like sensation, which encircles their skull, while others describe pain as a muscle tension in their neck or shoulder regions. The pain is constant and pressure-like. Most people who have a tension headache are able to continue their daily activities despite the pain.
Migraine headaches are the second most common type of primary headache. Migraine headaches affect children as well as adults. Before puberty, boys and girls are affected equally by, but after puberty more women than men have them. Migraine often goes undiagnosed or is misdiagnosed as tension or sinus headaches.
Migraines are defined as headaches that are moderate to severe in intensity, often on one side of head, typically associated with other symptoms such as light sensitivity, nausea, or vomiting. It is more likely to happen after a stressful event, it is called a let-down headache or weekend headache.
This might happen because when a person is under stress their body produces cortisol to get through the event. As the cortisol wears off, the person becomes more vulnerable to an attack. 
Before the onset of a migraine attack, many people notice:
- Blurring of vision
- Flashing lights, see “stars”
- Unusual smell sensations
The vision disturbances that occur prior to the onset of the head pains are referred to as the aura. This can be followed by a pounding pain on the sides or front of the head, often with a pulsating sensation. Once migraine headaches begin, there can be sensitivity to bright lights. This sensitivity is referred to as photophobia.
Cluster headaches are a rare but important type of primary headache, affecting mainly men. The average age of cluster headache sufferers is 28-30 years, although headaches may begin in childhood.
The term “cluster headache” refers to a type of headache that recurs over a period of time. People who have cluster headaches experience an episode one to three times per day during a period of time (the cluster period), which may last from two weeks to three months. People who suffer from cluster headaches get them at the same time each year, such as the spring or fall.
The affected nerve of cluster pain is the nerve trigeminus. The pain is located behind one eye or in the eye region, without changing sides. It may radiate to the forehead, temple, nose, cheek, or upper gum on the affected side. The scalp may be tender, and the pulsing in the arteries often can be felt.
The activation of the trigeminal nerve appears to come from a deeper part of the brain called the hypothalamus.
If you have a mild headache behind both eyes, that’s most likely to be tension-type headache or an “everyday” headache, and it will usually respond well to over-the-counter medications.
But if the headache is behind just one eye, and is more severe, it could be a migraine or cluster headache. Cluster headaches are a rare type of headache that is more common in men. Cluster headaches are also characterized by redness or tearing of the eye on that size or a drooping of the eyelid on that side.
The causes for secondary headaches include:
- Tight neck muscles
- Cervical spine
- Nerve problems
- Dental or jaw problems
- Eye problems
- Hormonal influences
- High blood pressure
Trigger for headaches
- Warm weather (sudden change in temperature)
- Strong scents (perfumes)
- Hair accessories (tight hair bands)
- Exercise including sex
- Poor posture (desk work)
- Aged cheese (tyramine)
- Red wine (tyramine)
- Hunger (dip in blood sugar)
- Smoking (narrowing of blood vessels in the brain)
- Lack of caffeine 
The International Headache Society defines occipital neuralgia as shooting pain in the dermatomes of the nn. occipitalis major et minor. The nerve occipitalis major is in 90% involved as compared the n. occipitalis minor in 10%, and both nerves are responsible in 8.7% for occipital neuralgia.
- occipitalis major is a branch of the ramus dorsalis of the second cervical nerve that supplies the back part of the scalp. On it’s way from the origin to the supply area the n. occipitalis major perforates the m. semispinalis capitis and the m. trapezius ascendens. In the case of tightness of one or both of those muscles the nerve gets irritated and causes an “occipital neuralgia”. This is found as the main cause for that symptom. 
Other potential causes for irritating the greater occipital nerve are:
- Irritation of the nerve roots C1/C2 by an aberrant branch of the arteria inferior posterior cerebelli
- Dural arteriovenous fistula at the cervical level
- Bleeding from a bulbovervical cavernoma
- Cervical intramedullar cavernous hemangioma
- Giant cell arteritis
- Fenestrated arteria vertebralis pressing on C1/C2 nerve roots
- Aberrant course of the arteria vertebralis
- Schwannoma of the n. occipitalis major (benign nerve sheath tumor)
- C2 myelitis
- Multiple sclerosis
- C1/C2 arthrosis, atlantodental sclerosis
- Hypermobile arcus posterior of the atlas
- Cervical osteochondroma (benign tumor)
- Osteolytic lesion of the skull
- Exuberant callus formation after fracture C1/C2 
Another form of headaches is the one-sided headaches, which occur always on the same side and in some cases are caused by an accident. The headaches appear frequently and last between one and more days. The cause is a narrowing of the plexus cervicalis, which affects one or more nerves.
Plexus cervicalis (C1-4) sensory innervations: