Chronic paroxysmal hemicrania (CPH), also known as Sjaastad syndrome, was first described in 1974, by Sjaastad and Dale.  In 1976, the term CPH was proposed by Sjaastad on the basis of the first 2 patients, who had daily (ie, chronic), solitary, limited attacks (ie, paroxysmal) of unilateral headache that did not shift sides (ie, hemicrania).  CPH, which has been included in the International Headache Society (IHS) classification system since 1988, is much less common than cluster headache (CH).  (See Presentation.)
The short-lasting primary headache syndromes may be divided into (1) headaches with autonomic activation and (2) headaches without autonomic activation. Headaches with autonomic activation include chronic and episodic paroxysmal hemicrania, CH, and short-lasting unilateral neuralgiform headache with conjunctival injection and tearing (SUNCT syndrome). (See Etiology and DDx.)
At least 50 attacks fulfilling the criteria mentioned in the bullet points below
Attacks of severe, unilateral orbital, supraorbital, and/or temporal pain, always on the same side, lasting 2-45 minutes
Attack frequency of more than 5 per day for more than half of the time (periods with lower frequency may occur)
Pain associated with at least 1 of the following signs/symptoms on the symptomatic side: (1) conjunctival injection, (2) lacrimation, (3) nasal congestion, (4) rhinorrhea, (5) ptosis, and (6) eyelid edema
Absolute effectiveness of indomethacin (150mg daily or less)
At least 1 of the following: (1) history, physical, and neurologic examination findings do not suggest any of several disorders that include organic headaches, headaches associated with drug withdrawal, metabolic disorders, and other conditions; (2) history and/or physical and/or neurologic examination findings do suggest such a disorder, but it is ruled out by appropriate investigations; (3) such a disorder is present, but CPH does not occur for the first time in close relation to the disorder
Chronic versus nonchronic
CPH may be present in nonchronic or chronic form, although the chronic type is 4 times more common. Nonchronic attacks are similar to chronic ones but may be less severe and less frequent. (See Prognosis.)
Before the development of chronic symptoms, many patients (42%) pass through a nonchronic stage, with attacks separated by intervals of complete remission.
The term pre-CPH stage or prechronic was preferred initially on the basis of the assumption that all patients would develop chronic symptoms. However, approximately 20% of patients appear to remain in the nonchronic stage for long periods.
Dividing CPH into unremitting and remitting forms may be appropriate. The unremitting form is the chronic form, which either is unremitting from the onset or evolves from the remitting form
The mechanisms responsible for pain in CPH remain unknown. The past medical history in patients with CPH is usually unremarkable. A history of head or neck trauma is reported in about 20% of cases, but these findings are similar to those for CH or migraine.
Occasionally, attacks may be provoked mechanically by bending or rotating the head and by applying external pressure against the transverse processes of C4-C5, C2 root, or the greater occipital nerve.
No familial disposition is known for CPH; families of affected individuals do not have higher incidences of CH or migraine than does the general population.
Discussion of important features, such as intense, unilateral headache; autonomic abnormalities; and effectiveness of indomethacin, may help in understanding the pathogenesis. 
Trigeminal, sympathetic, and parasympathetic involvement
The release of trigeminal and parasympathetic neuropeptides during headache has been described.  Activation of the ipsilateral trigeminovascular system may explain sudden, unilateral headache and may lead to miosis, increased intraocular pressure (IOP), and other autonomic abnormalities.
Increased sweating and decreased salivation during attacks and the ability of an alpha-blocking agent or a stellate ganglion blockade to inhibit an increase in IOP suggest sympathetic involvement.
Increased tearing, nasal secretion, and miosis may be due to parasympathetic stimulation. Trigeminoparasympathetic activation during CPH attacks has been suggested; increases in the vasoactive intestinal peptide (ie, parasympathetic peptide) level have been reported. levels of calcitonin gene-related peptide also are reported to be high during CPH attacks.
Pain and pressure threshold, nociceptive flexion reflex, and blink and corneal reflexes have been studied in patients with CPH. The corneal reflex thresholds have been found to be decreased bilaterally during CPH attacks. Increases in corneal temperature on the symptomatic side also have been reported; this finding may be due to increased ocular blood flow.
The effectiveness of indomethacin in CPH may be due partly to reduction of intracranial blood flow (via a nonprostaglandin mechanism) and partly to the drug’s anti-inflammatory effects.
These findings may indicate a primary central mechanism and a secondary involvement of peripheral factors, affecting the sympathetic and parasympathetic systems.
Occurrence in the United States
CPH is a rare syndrome, but the number of diagnosed cases is increasing. The prevalence of CPH is not known, but the relative frequency compared with CH is reported to be approximately 1-3%.
Many cases of CPH have been described throughout the world, in different races and in different countries, including Australia, the Czech Republic, Slovakia, Denmark, Italy, France, Mexico, Canada, Sweden, Germany, Poland, India, Spain, Brazil, South Africa, Norway, New Zealand, the United Kingdom, and the United States.
Sex- and age-related demographics
In the past, because of a female preponderance, CPH was considered a disease of women. However, CPH has been reported in increasing numbers of men. A study conducted in 1979 reported a female-to-male ratio of 7:1, but a review of 84 patients in 1989 reported a female-to-male ratio of 2.3:1.
CPH can occur at any age, with the mean age of onset being 34 years.  Patients as young as age 6 years have been described in the literature, while the oldest known patient with CPH was age 81 years. [12, 13] In one report, CPH beginning at age 3 years was described; however, the condition may have been related to ipsilateral occipital hemorrhagic infarction in this patient. [14, 15]
Patients in the nonremitting stage of CPH may need lifelong therapy, possibly with smaller doses of indomethacin.
Long-lasting remission periods usually reflect a nonchronic stage, but they may occur in patients with established chronic disease. In chronic cases, in fact, recurrence of attacks after a drug-free period of 1.5 years has been reported.
The mortality rate and morbidities associated with CPH have not been reported, although the therapy of choice for this condition, indomethacin, is known to be associated with the risk of bleeding.
Factors that may or may not affect the course of CPH include the following:
Pregnancy - CPH attacks have been reported to improve during pregnancy; however, they recur after delivery
Menstruation - Menstruation may have either a positive or negative effect on attacks
Birth control - Birth control pills do not seem to influence attack frequency
Menopause - Reliable data do not exist regarding the effects of menopause on CPH
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