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What is Hemicrania continua and explain in detail?
Hemicrania continua (HC) is a persistent unilateral headache that responds to indomethacin. It is usually unremitting, but rare cases of remission have been documented. Hemicrania continua is considered a primary headache disorder, meaning that it is not caused by another condition.

Symptoms
In hemicrania continua, basal pain is a dull aching pressure similar to that of TTHs that occurs nearly always on the same side of the head and face. Pain ranges from mild to severe and is characterized by fluctuations where it increases in intensity up to three to five times per 24-hour cycle. The range of duration of exacerbations has no boundaries and varies from a few seconds to up to two weeks. While attacks tend to be more frequent at night, no circadian periodicity such as in cluster headache can be observed.
The nature of pain changes during the exacerbation phase, becoming more piercing, throbbing, and intense, generally paired with other highly debilitating symptoms such as nausea, vomiting, dizziness, and sensitivity to light and sounds. During these exacerbation phases, hemicrania continua may mimic other primary and secondary headache disorders, with up to 70% of patients fulfilling the diagnostic criteria for migraine. Physical exertion,changes in sleep patterns, stress, or alcohol consumption can make the headache pain more severe in some patients.In addition to persistent daily headache of HC, which is usually mild to moderate (and frequently severe), HC can present other symptoms. These additional symptoms of HC can be divided into three main categories:

Autonomic symptoms:
conjunctival injection
tearing
rhinorrhea
nasal stuffiness
eyelid edema
forehead sweating
Stabbing headaches:
Short, "jabbing" headaches superimposed over the persistent daily headache.
Usually lasting less than one minute.
Migrainous features:
throbbing pain
nausea and/or vomiting
phonophobia
photophobia

Cause
The cause of hemicrania continua is unknown.

Diagnosis
The following diagnostic criteria are given for hemicrania continua:
Headache for more than 3 months fulfilling other 3 criteria:
All of the following characteristics:
Unilateral pain without side-shift
Daily and continuous, without pain-free periods
Moderate intensity, but with exacerbations of severe pain
At least one of the following autonomic features occurs during exacerbations and ipsilateral to the side of pain:
Conjunctival injection and/or lacrimation
Nasal congestion and/or rhinorrhea
Ptosis and/or miosis
Complete response to therapeutic doses of indomethacin, although cases of hemicrania continua that do not resolve with indomethacin treatment have been documented.A variant on hemicrania continua has also been described, in which the attacks may shift sides, although meeting the above criteria in all other respects.There is no definitive diagnostic test for hemicrania continua. Diagnostic tests such as imaging studies may be ordered to rule out other causes for the headache. When the symptoms of hemicrania continua are present, its considered "diagnostic" if they respond completely to indomethacin