Hypnic Headache: Causes and Treatment reported by sufferers

Self-reported Hypnic Headache.

Results of an Internet Based Survey of Clinical Features, Investigations, Treatment, Outcome and Theories on Cause.

By Dr Raeburn B. Forbes MD(Hons) and Dr Rachael Kee MRCP

Introduction

Hypnic Headache (HH) is a rare headache disorder in which a person goes to sleep headache free and is woken at about the same time every night with a severe headache (1).

HH is recognised by the International Headache Society as a primary headache disorder in the IHC-2 (2).

Originally Hypnic Headache was regarded as a diffuse headache, but unilateral cases have been published (3). A disturbance of hypothalamic function has been proposed as a cause of Hypnic Headache (4), and even though headache and sleep are inextricably linked (5), the cause of Hypnic Headache remains obscure.

We describe results of an internet based survey to identify people with Hypnic Headache and seek their descriptions of headache symptoms, treatment options, prognosis and theories on causation. Specifically, we wanted to know what sort of research questions this population of people would wish to pursue. We compare their responses with the published literature on Hypnic Headache.

If internet based surveys are valid, then this could form a basis for creating a patient-centred research agenda. Information technology could accelerate progress in our understanding of distressing, rare, headache syndromes. There is also wider recognition that people who experience chronic illness should be engaged in generating hypotheses for future research (6).

Methods

A webpage about Hypnic Headache was written by one of the authors (RBF). After several months, it became apparent that this page was being read by several hundred people per month. One reader contacted the author and asked whether her headaches may have been caused by influenza vaccination, which led to the idea that people reading this site may be willing to suggest potential causes for their condition. A form was linked to the Hypnic Headache webpage inviting people with to answer questions about their condition, and one question specifically asked about theories on causation or antecedent events (see Appendix 1). Respondents were informed that their information would be kept confidential and that any submitted information, including email addresses would not be used for any other purpose. Data was saved on a secure server, which was password protected, and we aimed to maintain the form until approximately 100 responses were received. Data was downloaded, anonymised, and then analysed. The website is owned by a limited company which is Information Commission compliant according to UK Data Protection Laws.

We asked people with Hypnic Headache to report time from onset of symptoms to diagnosis, who made their diagnosis, how often headaches occurred, how long a headache episode lasted for, what treatments they had used and which one seemed best. When reporting outcome, people were asked to report remission (resolution of symptoms and no treatment), control of symptoms (ongoing treatment controlled symptoms and this response could either be complete or partial), or ongoing headaches in spite of treatment (refractory headache). We sought information on imaging studies and polysomnography. Lastly people were asked to submit information on their own theory on why Hypnic Headaches occurred and for suggestions for future research.

Descriptive statistics were used. Due to the nature of the data being collected we did not use statistical analysis, as this study is purely descriptive, but might allow us to generate hypotheses.

We conducted a systematic review of Hypnic Headache using the PubMed database. The search terms were: ‘hypnic’ or ‘Hypnic Headache’ OR ‘Hypnic Headaches’. We excluded articles that did not include original data on cases with Hypnic Headache, and included those that described cases, or cases series of people with Hypnic Headache. The systematic review was conducted in January 2012, and had already been initiated prior to posting the website enquiry form. Descriptive statistics were used to summarise clinical features, best treatment, secondary causes and prognosis.

Results

The Internet Survey captured 90 responses between 10th June 2012 and 3rd October 2013. Of these 80 were useable as there were 10 submissions that were either duplicate or did not contain any useful information (Table 1).

Table 1: Website visits and survey responses by month.

The systematic review identified 115 articles, of which 69 contained original data on a total of 147 cases (See Appendix 2 for full bibliography). We separated our cases into Self-reported Hypnic Headache (SRHH) and Published Hypnic Headache (PHH). Of the SRHH group there were 30 who stated that they had been diagnosed by a physician, and 45 who stated that they had self-diagnosed. 5 people with SRHH did not complete the section on who made their diagnosis.

Demographic and Clinical Features

Published cases of Hypnic Headache were older (64 years) than self-reported cases (57 years), although the self reported cases who stated that they had been diagnosed by a physician had a mean age of 62 years. Females were over-represented in the internet survey at 10 females to one male, compared to 1.5 females to 1 male in the published cases. The internet survey cases who had been diagnosed by a physician were 6 females to 1 male.

There was variability in the time to diagnosis – published cases had a shorter median time to diagnosis – 2.2 years compared to a median of 3 years for self reported cases. The median duration of an actual Hypnic Headache attack in published cases was 61 minutes, compared to a median of 105 minutes in self-reported cases. Self reported cases who stated that a physician had diagnosed their condition had a median attack duration of 90 minutes.

Self reported cases of Hypnic Headache report a mean 1.4 attacks of headache per night, with a median attack number of 1.

We tried to estimate the number of nights per month people experienced Hypnic Headaches for and found that published cases experienced Hypnic Headache on a median of 24 (Interquartile range 20 – 30) nights per month, compared a median of 17 (Interquartile range 11-25) nights for self-reported cases. Self reported cases who saw a physician experienced pain on a median of 25 nights per month (Interquartile range 17-28).

Table 2: Demographic and Clinical Features of Self-reported Hypnic Headache compared to Published Cases

Pain Severity

In the self-reported Hypnic Headache group, 79 people completed a HIT-6 score, with an mean score of 59 (range 32 to 78) and a median score of 60 (Interquartile range 53 to 66). In 133 published cases Visual Analogue scores were often used (0-10 ratings where 0 is no pain and 10 is worst imaginable pain), and the median score was 7/10 (Interquartile range 6 to 8), and a mean score of 7 (range 2 to 10).

Previous and Family Headache History

Of the 147 published Hypnic Headache cases 38 had a history of migraine (26%) and 12 (8%) had a previous history of Tension-type headache, and 94 (64%) had no prior headache history.
In our self-reported Hypnic Headache group, 10 (12%) cases reported previous migraine headache when asked about what they thought caused their headaches. We identified 6 cases who reported Hypnic Headache in first degree relative (2 in fathers, 2 in mothers, 1 in a sister and 1 in a daughter). When asked about a family history of any headache, there were 42/80 who said that they had a first degree relative with headache, but we were not able to specify which headache disorder.

Events Prior to Onset of Hypnic Headache

When asked if there was a preceding event, 21/80 reported one or more preceding events (Table 3), including Influenza Vaccination (14 instances), Severe Respiratory Infection (5 instances), Head Injury (3 instances), Diarrhoeal Illness (3 instances), (Stroke (one instance)).

Table 3: Antecedent Events in people with Self-Reported Hypnic Headache

Behaviour During a Hypnic Headache Attack

When asked about behaviour during a Hypnic Headache we found a total of 197 reported behaviours from our 80 self reported cases: 43 took painkillers, 20 stayed lying in bed, 35 rubbed their head or neck, 54 reported having to get up from bed, 12 reported agitation, 23 wanted to drink coffee and 10 wanted to eat.

In the published Hypnic Headache cases, 60 cases had behaviours reported, including 51 who got up, 5 who had to walk and 2 who reported symptoms feeling worse if lying down, and 2 who wanted to sit up.

Investigations

145/147 published cases had a neuro-imaging study performed of which 129/145 were either normal or unremarkable. 6 cases reported atrophy or changes of cerebral ischaemia, and 13 cases reported an abnormality felt to be significant by the authors (Table 4). Only one malignant tumour – a cerebellar haemangioblastoma – causing Hypnic Headache has ever been published to date (7). Imaging results were submitted by 68/80 respondents, none of whom had a malignant tumour diagnosed.

Table 4: Neuroimaging Results in Published and Self-Reported Cases of Hypnic Headache

Polysomnography and Sleep Phase

Polysomnography (PSG) has been reported in 17/147 published cases, of whom 11/17 were recorded as having Hypnic Headache during REM Sleep and 4/17 had onset during non-REM Sleep.
In the Self-reported cases, we identified a further 17 people with Hypnic Headache who had undergone PSG, of whom 3/17 reported onset during REM Sleep, 1/17 in non-REM sleep and 5/17 who were uncertain which phase of sleep. 7/17 self-reported cases did not get a headache during their PSG test.
Forty self-reported cases entered data on dreaming, of which 25/40 said they are dreaming at onset of Hypnic Headache, and 5/40 said that they were not, and the remaining 10 were uncertain.

Treatment

In published cases, 70 cases reported a total of 75 treatments, of which 21/70 stated that Lithium was best, followed by Indomethacin (14/70), Caffeine (5/70) and Amitryptiline (4/70). Triptans were not often quoted as successful treatments in published cases.
In self-reported cases, we had 89 different treatments reported by 78 of our 80 cases. Caffeine was the most widely reported treatment (22/80 cases) with a further 10 cases using Caffeine containing over-the-counter preparations like Excedrin. Of the 32 self-reported cases using Caffeine or Caffeine containing preparations, 10 reported remission of symptoms, 9 reported ongoing symptoms (i.e.no control), 8 reported partial control of symptoms, 2 reported that they could cope without using caffeine, and in 3 cases the effect was not stated.
Although widely recommended, Lithium was only used in 2/80 self-reported cases. Outcomes reported for other drugs included: Paracetamol (5 cases, of whom 3 reported remission), Indomethacin (4 cases, 2 remission and 2 with control of symptoms), and Triptans (7 cases, 1 remission, 3 control of symptoms, 2 ongoing and 1 unknown effect). Table 5 lists the different drugs and combinations reported as effective in managing Hypnic Headache.

14/80 self-reported cases said that nothing works, so did not report a best treatment and are likely to have refractory Hypnic Headache.

Table 5: Best Treatment as reported in Published or Self-Reported Hypnic Headache Cases 1986-2013.

* Excedrin, Sumatriptan; Frovatriptan, Butalbital and Botulinum Toxin; Maxalt, Topiramate, Vivactal, Magnesium, Vitamin B2; Petadolor or Migraine Drop; Sumatriptan, Excedrin, Chi Gong.

** Acetazolamide, Indomethacin; Acetylsalicylic acid, Amitriptyline, Verapamil; Atenolol & Amitriptyline; Cinnarizine plus caffeine; Doxylamine, Tylenol, Eszopiclone; Indometacin and Caffeine; Indomethacin and caffeinated soda; Indomethacin, Melatonin; Lithium & Amitriptyline; Lithium and cup of coffee; Lithium and Venlafaxine; Lorazepam, Lithium;

Prognosis

Of the 147 published cases, outcomes were stated for 61 cases, of whom 56 (92%) either had remission or control of symptoms compared to 49/80 (61%) of self reported cases (Table 6). Self reported cases were therefore more likely to report refractory symptoms, and less likely to report control of symptoms than the published cases.

Table 6: Outcome from Published and Self-reported Hypnic Headache

 

Theories on causation

Personal theories on causation were sought, and more than one causation theory could be proposed in a free-text box on our questionnaire. 16 people did not make any suggestions, but the most common theory was a mechanical or neck-related problem (13 suggestions – Table 7a).

Table 7a: Theories on Causation proposed by people with self-reported Hypnic Headache

 

Another common theme was that Hypnic Headache is a manifestation of another health problem (Table 7b).

Table 7b: Hypnic Headache as a Symptom of another Health Problem

Hormonal and dietary factors, feature in most theories of headache causation and psychological factors were also volunteered. Unsurprisingly sleep disorders are suspected, including shift work, time-zone travel and insomnia (Table 7c).

Table 7c: Hypnic Headache as a Symptom of Stress, Sleep Disturbance or Time Zone

Discussion

The current world literature on Hypnic Headache is based upon less than 150 cases, and our internet survey has managed to gain information on a further 80 potential cases. We do not know if any of these self-reported cases were already published, but none of the self-reported cases made reference to being in a publication, but we cannot rule out the possibility that some of the internet based cases were already published.

Validity of Internet Survey

There are significant demographic differences between published cases and self-reported cases, the most striking of which is the over-representation of females in the self-reported cases (10:1 from survey versus 1.5:1 in published cases), and that self-reported cases tended to be younger than published cases (57 years versus 64 years). Published cases had shorter headache attack duration, shorter time to diagnosis and more frequent headache episodes per month than self-reported cases. This would make sense as self-reported cases may be tolerating milder symptoms, and may not have sought a diagnosis from a physician. In addition women are more likely to engage in health seeking behaviour than men (8), and women are more likely to search for health information on the Internet than men (9).

As there are clear differences between the population described in the published literature and the self-reported cases we are unable to confirm the validity of the diagnosis in our online sample, but if they are in any way representative they do have interesting observations on causation, treatment or potential research ideas. However, published case reports are by definition a selection of the more extreme cases. If we were to conduct a population-based epidemiologic survey of Hypnic Headache we may identify less severe cases in younger people that differ from published literature. Until a population based epidemiologic survey is completed, we will never be able to establish the true demographics of people with Hypnic Headache.

However, there are many similarities between the published cases and our self reported series e.g. the frequency of serious underlying causes was low in both groups (13/147 published cases v 4/80 internet cases), and pain severity was rated as severe in both groups, albeit using different methods. Each group had refractory cases – albeit more frequently in self-reported cases – which would reflect a bias towards not publishing cases where there was no success with treatment. These observations support an argument that our self-reported cases are likely to have true Hypnic Headache.

New findings

Our survey respondents made some observations that may be worth validating with original research. Firstly, the possibility that Hypnic Headache goes unreported to medical services, even though published literature suggests that the risk of an underlying disorder is potentially 9%. In the published cases the range of time from first symptom to diagnosis was wide, for example a case where 15 years lapsed until diagnosis (case 8 in reference (10)).

We also managed to identify a further 17 cases of Hypnic Headache who had Polysomnography – but still cannot confirm whether a majority of Hypnic Headache cases are related to REM or non-REM sleep (11)(12).

In published cases, Lithium is the most commonly reported treatment providing relief of symptoms (13), but in self-reported cases we found a strong preference for using caffeine, and a significant number felt able to cope with resorting to any medication. Caffeine is universally available and relatively safe, and in the absence of randomised trial data, our survey makes an argument for using caffeine as an initial pharmacologic intervention rather than Lithium when Hypnic Headache is diagnosed.

Prior Events to Onset of Hypnic Headache

Our data is not strong evidence that there are provoking factors prior to the onset of Hypnic Headache, as only 25% gave information on antecedent events, and influenza vaccination is commonly used in people over 60 years, and infectious illnesses are also commonplace. It is not clear why an infectious or vaccine related immune stimulus would lead to an exclusively nocturnal headache, although influenza vaccination has been linked, inconclusively, to the development of narcolepsy in children.

Theories on Causation

It is interesting to note that to date, no one has proposed that Hypnic Headache is a referred pain from the cervical spine, yet this was the most commonly reported theme. Formal examinations of the cervical spine have not been reported in any of the 147 previously published cases. Referred cervical facet joint or atlanto-axial joint pain can cause pain in the head that is diffuse (14) or unilateral (15), and these pains could mimic Hypnic Headache. Abnormal sleeping posture could, in theory, induce a nocturnal headache that arises from an upper cervical spine structure.

9/80 self-reported cases felt that their Hypnic Headache may be a manifestation of migraine, which is similar to the published literature where migraine was reported in the past medical history of 26% of cases. There are mechanisms that connect disturbed sleep and headaches (5), but the precise mechanisms connecting sleep and Hypnic Headache remain obscure.

The range of theories of causation are in keeping with what people tell doctors about what they think he causes of headache are – namely a combination of social, environmental, physical, emotional, hormonal, psychological, pathological and pharmacological factors (16).

Conclusions

Hypnic Headache is a rare headache syndrome of unknown aetiology. An internet based survey identified another 80 potential cases to add to the 147 in the published literature, although these self-reported cases were much more likely to be female and younger than published cases. Self-reported cases most commonly used caffeine, and very few reported using Lithium successfully – in contradiction to the experience of published cases.

Secondary causes of Hypnic Headache are rare, and the cause of Hypnic Headache remains obscure.

When asked about potential causes of Hypnic Headache, the most commonly proposed cause was a mechanical or neck related problem. Hypotheses generated by patients with Hypnic Headache may be worth pursuing in a formal epidemiological study.

References

1. Raskin NH. The hypnic headache syndrome. Headache. 1988;28(8):534-6.
2. The International Classification of Headache Disorders: 2nd edition. Cephalalgia. 2004;24 Suppl 1:9-160.
3. Gould JD, Silberstein SD. Unilateral hypnic headache: a case study. Neurology. 1997;49(6):1749-51.
4. Holle D, Naegel S, Krebs S, Gaul C, Gizewski E, Diener HC, et al. Hypothalamic gray matter volume loss in hypnic headache. Ann Neurol. 2011;69(3):533-9.
5. Holland PR. Headache and sleep: Shared pathophysiological mechanisms. Cephalalgia. 2014;34(10):725-44.
6. Guise JM, O’Haire C, McPheeters M, Most C, Labrant L, Lee K, et al. A practice-based tool for engaging stakeholders in future research: a synthesis of current practices. J Clin Epidemiol. 2013;66(6):666-74.
7. Mullally WJ, Hall KE. Hypnic headache secondary to haemangioblastoma of the cerebellum. Cephalalgia. 2010;30(7):887-9.
8. Galdas PM, Cheater F, Marshall P. Men and health help-seeking behaviour: literature review. J Adv Nurs. 2005;49(6):616-23.
9. Cohen RA, PF. A. Use of the Internet for health information: United States, 2009. . Hyattsville, MD National Center for Health Statistics, 2011 2011. Report No.
10. Pinto CA, Fragoso YD, Souza Carvalho D, Gabbai AA. Hypnic headache syndrome: clinical aspects of eight patients in Brazil. Cephalalgia. 2002;22(10):824-7.
11. Manni R, Sances G, Terzaghi M, Ghiotto N, Nappi G. Hypnic headache: PSG evidence of both REM- and NREM-related attacks. Neurology. 2004;62(8):1411-3.
12. Dodick DW. Polysomnography in hypnic headache syndrome. Headache. 2000;40(9):748-52.
13. Evers S, Goadsby PJ. Hypnic headache: clinical features, pathophysiology, and treatment. Neurology. 2003;60(6):905-9.
14. Aprill C, Axinn MJ, Bogduk N. Occipital headaches stemming from the lateral atlanto-axial (C1-2) joint. Cephalalgia. 2002;22(1):15-22.
15. Cooper G, Bailey B, Bogduk N. Cervical zygapophysial joint pain maps. Pain Med. 2007;8(4):344-53.
16. Kelman L. The triggers or precipitants of the acute migraine attack. Cephalalgia. 2007;27(5):394-402.

Appendix 1: Hypnic Headache Survey Form

1 First Name
2 Country
3 Male or Female?
4 Your age today please (Years)
5 How many years did you have Hypnic Headaches before you were diagnosed?
6 When you have a Hypnic Headache how often is the pain severe?
7 How often do Hypnic Headaches limit your ability to do usual daily activities including household work, work, or social activities?
8 When you have a Hypnic Headache, how often do you wish you could lie down?
9 In the past 4 weeks, how often have you felt too tired to do work or daily activities because of your Hypnic Headaches?
10 In the past 4 weeks, how often have you felt fed up or irritated because of your Hypnic Headaches?
11 In the past 4 weeks, how often did Hypnic Headaches limit your ability to concentrate on work or daily activities?
12 When you have headaches, how often is the pain severe?
13 How often do headaches limit your ability to do usual daily activities including household work, work, school, or social activities?
14 When you have a headache, how often do you wish you could lie down?
15 In the past 4 weeks, how often have you felt too tired to do work or daily activities because of your headaches?
16 In the past 4 weeks, how often have you felt fed up or irritated because of your headaches?
17 In the past 4 weeks, how often did headaches limit your ability to concentrate on work or daily activities?
18 Have you been given a diagnosis of Hypnic Headache?
19 Who made your diagnosis?
20 Before you developed Hypnic Headache, did you have any of the following?
21 Do any of the following relatives get headaches?
22 Did any of your family members ever describe Hypnic Headaches?
23 How long does a Hypnic Headache episode last?
24 If a Hypnic Headache lasted more than 180 minutes, how long does it last for?
25 On average, how many Hypnic Headaches can you get per night?
26 Before you were treated, how many nights per month would you have had Hypnic Headaches?
27 When you waken with your Hypnic Headache, would any of the following apply?
28 Which Treatment Works Best for your Hypnic Headache?
29 If you said “other medication” please enter name here:
30 What happened when you treated Hypnic Headaches?
31 Did you have any Scans performed?
32 Was there any of the following on your brain scan?
33 Please make any other comments about any brain scans here:
34 Have you ever had Polysomnography (a detailed sleep study)?
35 If you had polysomnography, what phase of sleep did you headaches appear?
36 Are you dreaming when woken with Hypnic Headache?
37 Do you have any theories on why you get Hypnic Headaches?
38 Is there any research question you think we should try to answer?
39 Please make any other comments here:

Appendix 2:

Hypnic Headache References Identified with Systematic Review

1. Neau JP, Paquereau J, Bailbe M, Meurice JC, Ingrand P, Gil R. Relationship between sleep apnoea syndrome, snoring and headaches. Cephalalgia. 2002;22(5):333-9.
2. Evans RW, Dodick DW, Schwedt TJ. The headaches that awaken us. Headache. 2006;46(4):678-81.
3. Vieira-Dias M, Esperanca P. Hypnic Headache: report of two cases. Headache. 2001;41(7):726-7.
4. Vieira Dias M, Esperanca P. [Hypnic Headache: a report of four cases]. Rev Neurol. 2002;34(10):950-1.
5. Valentinis L, Tuniz F, Mucchiut M, Vindigni M, Skrap M, Bergonzi P, et al. Hypnic Headache secondary to a growth hormone-secreting pituitary tumour. Cephalalgia. 2009;29(1):82-4.
6. Valade D, El Amrani M. [Idiopathic facial pain other than vascular pain]. Rev Neurol (Paris). 2000;156 Suppl 4:4s57-61.
7. Ulrich K, Gunreben B, Lang E, Sittl R, Griessinger N. Pregabalin in the therapy of Hypnic Headache. Cephalalgia. 2006;26(8):1031-2.
8. Son BC, Yang SH, Hong JT, Lee SW. Occipital nerve stimulation for medically refractory Hypnic Headache. Neuromodulation : journal of the International Neuromodulation Society. 2012;15(4):381-6.
9. Sibon I, Ghorayeb I, Henry P. Successful treatment of Hypnic Headache syndrome with acetazolamide. Neurology. 2003;61(8):1157-8.
10. Seidel S, Zeitlhofer J, Wober C. First Austrian case of Hypnic Headache: serial polysomnography and blood pressure monitoring in treatment with indomethacin. Cephalalgia. 2008;28(10):1086-90.
11. Schurks M, Kastrup O, Diener HC. Triptan responsive Hypnic Headache? Eur J Neurol. 2006;13(6):666-7.
12. Scagni P, Pagliero R. Hypnic Headache in childhood: a new case report. J Paediatr Child Health. 2008;44(1-2):83-4.
13. Relja G, Zorzon M, Locatelli L, Carraro N, Antonello RM, Cazzato G. Hypnic Headache: rapid and long-lasting response to prednisone in two new cases. Cephalalgia. 2002;22(2):157-9.
14. Ravishankar K. Hypnic Headache syndrome. Cephalalgia. 1998;18(6):358-9.
15. Raskin NH. The Hypnic Headache syndrome. Headache. 1988;28(8):534-6.
16. Prakash S, Dabhi AS. Relapsing remitting Hypnic Headache responsive to indomethacin in an adolescent: a case report. J Headache Pain. 2008;9(6):393-5.
17. Porta-Etessam J, Garcia-Morales I, Di Capua D, Garcia-Cobos R. A patient with primary sexual headache associated with Hypnic Headaches. J Headache Pain. 2009;10:135.
18. Pinto CA, Fragoso YD, Souza Carvalho D, Gabbai AA. Hypnic Headache syndrome: clinical aspects of eight patients in Brazil. Cephalalgia. 2002;22(10):824-7.
19. Pinessi L, Rainero I, Cicolin A, Zibetti M, Gentile S, Mutani R. Hypnic Headache syndrome: association of the attacks with REM sleep. Cephalalgia. 2003;23(2):150-4.
20. Peters N, Lorenzl S, Fischereder J, Botzel K, Straube A. Hypnic Headache: a case presentation including polysomnography. Cephalalgia. 2006;26(1):84-6.
21. Perez-Martinez DA, Berbel-Garcia A, Puente-Munoz AI, Saiz-Diaz RA, de Toledo-Heras M, Porta-Etessam J, et al. [Hypnic Headache: a new case]. Rev Neurol. 1999;28(9):883-4.
22. Peatfield RC, Mendoza ND. Posterior fossa meningioma presenting as Hypnic Headache. Headache. 2003;43(9):1007-8.
23. Patsouros N, Laloux P, Ossemann M. Hypnic Headache: a case report with polysomnography. Acta Neurol Belg. 2004;104(1):37-40.
24. Newman LC, Lipton RB, Solomon S. The Hypnic Headache syndrome: a benign headache disorder of the elderly. Neurology. 1990;40(12):1904-5.
25. Mullally WJ, Hall KE. Hypnic Headache secondary to haemangioblastoma of the cerebellum. Cephalalgia. 2010;30(7):887-9.
26. Mulero P, Guerrero-Peral AL, Cortijo E, Jabary NS, Herrero-Velazquez S, Miranda S, et al. [Hypnic Headache: characteristics of a series of 13 new cases and proposal for modification of the diagnostic criteria]. Rev Neurol. 2012;54(3):129-36.
27. Morales-Asin F, Mauri JA, Iniguez C, Espada F, Mostacero E. The Hypnic Headache syndrome: report of three new cases. Cephalalgia. 1998;18(3):157-8.
28. Moon HS, Chung CS, Hong SB, Kim YB, Chung PW. A case of symptomatic Hypnic Headache syndrome. Cephalalgia. 2006;26(1):81-3.
29. Mitsikostas DD, Vikelis M, Viskos A. Refractory chronic headache associated with obstructive sleep apnoea syndrome. Cephalalgia. 2008;28(2):139-43.
30. Marziniak M, Voss J, Evers S. Hypnic Headache successfully treated with botulinum toxin type A. Cephalalgia. 2007;27(9):1082-4.
31. Martins IP, Gouveia RG. Hypnic Headache and travel across time zones: a case report. Cephalalgia. 2001;21(9):928-31.
32. Manni R, Sances G, Terzaghi M, Ghiotto N, Nappi G. Hypnic Headache: PSG evidence of both REM- and NREM-related attacks. Neurology. 2004;62(8):1411-3.
33. Lisotto C, Mainardi F, Maggioni F, Zanchin G. Episodic Hypnic Headache? Cephalalgia. 2004;24(8):681-5.
34. Liang JF, Fuh JL, Yu HY, Hsu CY, Wang SJ. Clinical features, polysomnography and outcome in patients with Hypnic Headache. Cephalalgia. 2008;28(3):209-15.
35. Kocasoy Orhan E, Kayrak Ertas N, Orhan KS, Ertas M. Hypnic Headache syndrome: excessive periodic limb movements in polysomnography. Agri : Agri (Algoloji) Dernegi’nin Yayin organidir = The journal of the Turkish Society of Algology. 2004;16(4):28-30.
36. Klimek A, Sklodowski P. [Night headache: report of 2 cases]. Neurol Neurochir Pol. 1999;33 Suppl 5:49-54.
37. Kerr E, Hewitt R, Gleadhill I. Benign headache in the elderly–a case report of Hypnic Headache. Ulster Med J. 2006;75(2):158-9.
38. Karlovasitou A, Avdelidi E, Andriopoulou G, Baloyannis S. Transient Hypnic Headache syndrome in a patient with bipolar disorder after the withdrawal of long-term lithium treatment: a case report. Cephalalgia. 2009;29(4):484-6.
39. Jimenez-Caballero PE, Gamez-Leyva G, Gomez M, Casado-Naranjo I. [Description of a series of cases of Hypnic Headache. Differentiation between sexes]. Rev Neurol. 2012;54(6):332-6.
40. Ivanez V, Soler R, Barreiro P. Hypnic Headache syndrome: a case with good response to indomethacin. Cephalalgia. 1998;18(4):225-6.
41. Holle D, Wessendorf TE, Zaremba S, Naegel S, Diener HC, Katsarava Z, et al. Serial polysomnography in Hypnic Headache. Cephalalgia. 2011;31(3):286-90.
42. Holle D, Naegel S, Krebs S, Katsarava Z, Diener HC, Gaul C, et al. Clinical characteristics and therapeutic options in Hypnic Headache. Cephalalgia. 2010;30(12):1435-42.
43. Holle D, Naegel S, Krebs S, Gaul C, Gizewski E, Diener HC, et al. Hypothalamic gray matter volume loss in Hypnic Headache. Ann Neurol. 2011;69(3):533-9.
44. Holle D, Gaul C, Krebs S, Naegel S, Diener HC, Kaube H, et al. Nociceptive blink reflex and pain-related evoked potentials in Hypnic Headache. Cephalalgia. 2011;31(11):1181-8.
45. Guido M, Specchio LM. Successful treatment of Hypnic Headache with topiramate: a case report. Headache. 2006;46(7):1205-6.
46. Grosberg BM, Lipton RB, Solomon S, Ballaban-Gil K. Hypnic Headache in childhood? A case report. Cephalalgia. 2005;25(1):68-70.
47. Gould JD, Silberstein SD. Unilateral Hypnic Headache: a case study. Neurology. 1997;49(6):1749-51.
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