This study is unique in that it evaluated a homogeneous large group of cluster headache patients, well defined by a self-help organization, who regard themselves as cluster headache patients and for whom a diagnosis of a cluster headache was made by their physician. The results confirm that the majority of patients with cluster headaches have clinical features that are mapped by the diagnostic criteria of ICHD-3 . However, due to the variability of the symptoms, there is a significant proportion of clinical phenotypes that are not captured by the ICHD-3 criteria. Severe or very severe pain intensity (criterion B of the ICHD-3) applies to 93.2% of patients. However, 6.8% showed mild or moderate pain; 85.7% of the patients showed unilateral pain, 14.3% showed a sequential change of side meaning that cluster attacks can alternate bilaterally. This phenomenon is not explicitly listed in the diagnostic criteria and can therefore lead to delays and incorrect diagnoses in patients with cluster headaches [1, 9, 46, 53]. The change of side is not specifically referred to in the diagnostic criteria and in the notes of the ICHD-3. The most common pain location was orbital, supraorbital, and/or temporal. However, around a third of patients also experienced pain in the upper jaw, neck, and back of the head. These pain locations have not yet been considered in the ICHD-3 criteria. The duration of the headache was 15–180 min in 89.8% of patients. There were, however, 10.2% of patients with a longer duration of pain. Patients with this attack duration are not included in the current headache classification. The autonomic symptoms lacrimation, nasal congestion, rhinorrhea, sweating, reddening of the eye, ptosis, eyelid edema, and miosis are found in high frequency as accompanying symptoms, particularly lacrimation, nasal congestion and rhinorrhea. Lacrimation was the most common accompanying symptom in 58.7% of our patients. In other current analyses, lacrimation was also found as the most common accompanying symptom in 58.8% . Psychomotor accompanying symptoms such as restlessness and agitation were found in 54.6% of our patients. The data support the special consideration of these psychomotor symptoms in addition to the autonomic symptoms as an independent diagnostic criterion that alone can justify the diagnosis of a cluster headache within the framework of criterion C in the ICHD-3 criteria .
Persisting pain between individual cluster attacks has not yet been explicitly mentioned in ICHD-3. These were found in 47% of patients between attacks. Persisting pain occurred ipsilaterally in 37%, bilaterally in 9%, and contralaterally in 1% of patients. They are more common in chronic cluster headache than in episodic cluster headache. They can contribute to a further increase in the level of suffering. Since persisting pain was reported by almost half of those affected, it could lead to other headache and facial pain being considered as a diagnosis delaying the correct diagnosis of cluster headache [9, 14,15,16, 19, 46,47,48, 51]. The intensity of the persisting pain was mild or moderate in 67.3%. However, 24% reported severe and 6.7% even very severe persisting pain. If the persisting pain in the interval between attacks is severe or very severe, it can delay and make a correct diagnosis more difficult [10, 11, 30, 31, 56]. Persisting pain may not respond to the prophylactic treatment effective for cluster headache attacks and requires further therapeutic considerations [9, 33, 45, 46, 48]. Little attention has been paid to this situation so far, and further studies are required.
Regarding the attack frequency, it was found that 6.6% of the patients had more than eight attacks per day. The most common attack frequency was between one and four attacks per day. If more than eight attacks occur per day, criterion D of the ICHD-3 criteria for cluster headache is not fulfilled  and the diagnosis cannot be made. When using the classification for making a diagnosis, it should therefore also be taken into account that some of the patients can experience more than eight attacks per day. With chronic cluster headache, significantly more and longer attacks occur than with episodic cluster headache. This fact, besides chronicity itself, is another cause of increased suffering in patients with chronic cluster headache [34, 45, 47].
The most frequent comorbidities shown by this study were psychiatric disorders in the form of depression, sleep disorders, and anxiety. Comorbidities were recorded as they were known to the patients based on their medical diagnosis and treatment initiated by their physician(s). These comorbidities can significantly increase the level of suffering. Psychiatric disorders should receive careful attention in the care of patients with cluster headaches [7, 22,23,24,25, 39, 40, 47].
The analysis shows that the participants in the study received treatment for acute therapy and prevention of cluster headaches largely in line with existing guidelines. This positive finding may be related to the involvement of the patient in specialized self-help groups for cluster headaches with extensive exchange about therapeutic options. In addition to acute and prophylactic pharmacological treatment of cluster headaches, the focus should also be on providing education, psychotherapeutic intervention and the involvement of relatives.
The age at the onset of the cluster headache disorder differed between men and women (28.69 ± 12.3 versus 29.67 ± 11.73 years). Cluster headache occurs later in life than migraine. On the other hand, women suffer significantly longer cluster headache attacks than men, so that there are significant gender differences analogous to other studies [26, 27, 29, 41, 52]. The intensity of the attacks is also reported to be higher in women than in men. These data indicate an increased level of suffering in women with cluster headaches. Perhaps this is one reason why the proportion of women in our study was relatively high. The high proportion of female participants could also be related to a higher affinity of women to social networks . Overlap with other headache disorders such as migraine or medication-overuse headache is also possible. However, since these headache phenotypes differ markedly, a potential confound is unlikely. The ICHD-3 specifies a male: female gender ratio of 3:1. This is not reflected in the participating population. The study was not designed to analyze prevalence in an epidemiological sense. The aim was to record the headache phenotype in an existing cohort of patients who were diagnosed as cluster headaches in the healthcare setting and who are organized in cluster headache self-help groups.
In the chronic form, the duration and frequency of the attack were significantly higher than in the episodic form. It is therefore important to formulate the correct diagnosis quickly in order to initiate an effective therapy, especially in the chronic form. The frequency of attacks was positively correlated with the age of the patient and the intensity of the pain. This also emphasizes the importance of making a correct diagnosis as well as initiating therapy in a timely manner.
The difficulties in diagnosing cluster headaches are manifold. They include the relative rarity of the disorder compared to other forms of headache, the episodic course and the neglected training in the diagnosis and treatment of headaches [4, 6, 9, 51, 53, 55]. The variable clinical picture, as the results of this study show, is a further contributing factor. The very strict explicit criteria of ICHD-3 may exclude some of the patients from the diagnosis. For the pain criteria (criterion B), this applies to pain intensity, pain localization, and pain duration. In particular, the sequential change of side between active periods should be taken into account in the diagnosis and should not be an argument against the diagnosis of a cluster headache. The accompanying symptoms of criterion C of the ICHD-3 comprehensively describe the most important diagnostic criteria. This also applies to restlessness and agitation, which have been given a separate number under the accompanying criteria and are highly relevant for the differentiation from migraine forms. The occurrence of persisting pain between cluster attacks should also be enquired about with regard to their high prevalence and intensity as well as the effect on the level of suffering in cluster headache patients [5, 8, 12]. When devising the therapeutic plan, it should be checked whether such persisting pain also requires a specific treatment; the therapy should not focus solely on the cluster headache attacks . Criterion D covers the frequency of attacks per day. It turns out that 6.6% of the patients have more than eight attacks per day. If this situation is not taken into account, misdiagnoses may result. In the foreground of comorbidities in cluster headache are psychiatric disorders such as depression, sleep disorders, and anxiety. When treating patients with cluster headaches, these comorbidities should also be taken into account carefully. They should not result in cluster headache being regarded as a psychosomatic illness in the event of diagnostic uncertainty .
This study has several limitations. By voluntarily participating in the study, it is possible that participants with particularly pronounced headaches preferentially participated. The resulting clinical headache phenotype could thus represent particularly severe forms. We have tried to capture the headache phenotype in the most precise way possible by recruiting a large number of participants. The analysis could not capture the diagnostic criteria according to which the patients' headaches were classified and diagnosed by their physicians. The diagnosis could have been made by the physician using to the ICHD-3 criteria, but this could not be verified as part of the study. The patients were active in cluster headache self-help groups. It can therefore be assumed that their headache picture corresponded to the ICHD-3 criteria for cluster headaches, including probable cluster headache or a broader cluster headache-like disorder, which led to the diagnosis of cluster headache. The basis of the survey was the headache phenotype, which is treated as cluster headache in the real-world healthcare setting. It is possible that some of the cluster headaches explicitly met the ICHD-3 criteria over certain periods of time. Others might not meet the ICHD-3 criteria due to the inter- and intra-individual variability. In particular, the possible intra-individual variability should be examined in further studies. The analyses are based on the answers given in the questionnaire. False-positive medical diagnoses cannot therefore be ruled out. However, this seems unlikely, as the patients were actively participating in cluster headache support groups with extensive experience with the condition. A large proportion of the patients reported the phenotype from memory; at the time of the survey, 67.7% had no active cluster episode. A recall bias can therefore also be assumed. In view of the very long course of the disorder, however, the influence of such a bias can be assumed to be minor.