The present study demonstrated that 18.9% of the participants in the initial questionnaire survey experienced HAMP more than 10 times during the last 1 year. HAMP exhibited mostly stereotyped clinical features such as mild intensity, localisation ipsilateral to the side of MP use, dull or pressing quality, association with a burning sensation, and provocation by prolonged MP use.
These results are similar to those of a study on MP users in Sweden and Norway, in which 8.4-13% of MP users reported HAMP [21
]. In Saudi Arabia, HAMP was observed in 22.4% of MP users [22
]. Santini et al. reported that 10-20% of MP users complained of HAMP in a questionnaire study conducted in France [23
]. Discrepancies in the proportion of MP users who experience HAMP may be due to differences in MP types, demographic features of users, social level of concerns about MP use or media reporting about them [24
]. In addition to headache, a burning sensation and dizziness have been commonly observed among MP users. Burning sensation and dizziness accompanying headache were also frequently observed in our study.
Headache during or after MP use could be induced by altering conditions during MP use including radiofrequency fields (RFs), psychological factor, temperature change, noise and various combinations thereof.
Exposure to RFs during MP use has been suggested to trigger a variety of symptoms such as headache, fatigue, concentration difficulties, and nausea [11
]. A series of double-blind provocation studies failed to demonstrate a causal relationship between RFs exposure and these symptoms [9
]. A meta-analysis of 46 blind or double-blind provocation studies reported no robust evidence in support of a connection between RFs exposure and symptoms as a biological entity [17
To date, no specific effects of active RFs exposure have been identified, nor has any difference in triggering symptoms been noted between active and sham conditions. Instead, various responses have been observed in repeated exposures of the same participants. Thus, these provocation studies suggest the role of psychological factor, a nocebo effect, in symptom provocation [25
]. Nocebo effect refers to harmful, unpleasant, or undesirable effects a subject manifests after receiving a sham treatment [26
]. In the present study, we used an initial questionnaire survey and subsequent telephone interviews, and did not evaluate psychological factors or nocebo effects associated with HAMP provocation.
Additional altering conditions on subjects during MP use are local temperature change and noise [29
], both of which can provoke headache [30
]. Local temperature elevation in the area of MP use has been noted in several studies [34
]. The sound of a MP is not the same as an actual human voice and usually includes noise [36
]. One double-blind cross-over provocation study has taken into account thermal effects of MP use in a provocation test [11
]. These investigators used a heated (39 ± 0.2°C) ceramic plate to mimic the sensation caused by using a warm MP during active and sham exposures of RFs. However, they did not investigate whether temperature change during MP use induced symptoms solely or in combination with RFs exposure. The effects of local temperature change and noise in connection with MP use in symptoms triggering by blinded tests were not reported yet. In our study, a burning sensation was reported by 71.1% of participants with HAMP and phonophobia was more prevalent among participants with HAMP compared to those without HAMP. These findings suggest the possibility that local temperature change and/or noise resulting from MP use are associated with HAMP provocation.
Proposals for new standard general diagnostic criteria for secondary headaches include suggestions in evidence of causation of headache: (1) headache has occurred in temporal relationship to the onset of the presumed causative disorder, (2) headache has occurred or has significantly worsening of the presumed causative factors, (3) headache has improved in temporal relation to improvement of the presumed causative factors, (4) headache has characteristics of typical of the causative disorder, (5) other evidence of causation. Evidence of causation of headache is inferred when at least 2 of the aforementioned 5 conditions are met [18
]. Regarding the results of our study, HAMP satisfied: (1) temporal relationship to onset of headache (experiencing HAMP more than 10 times during or after MP use), (2) occurring or worsening by worsening of the presumed factor (more individuals experienced HAMP with longer duration of MP use), and (3) characteristics typical of the causative disorder (stereotyped headache features and associated symptoms).
Headache provocation by regular telephone use was reported by only 1 participant with HAMP and HAMP did not occur when using hands-free equipment in the present study. Rare headache provocation by regular telephone use suggests that HAMP is related to certain conditions that are specific to MP use and not to regular telephone use. The lack of HAMP occurrence when using hands-free equipments also suggests that certain factor(s) nullified by hands-free equipments are related to HAMP provocation.
One possible mechanism for headache provocation by regular telephone use was her chronic headache disorder. Her HNAMP occurred 4 days per week with migrainous features (nausea, aggravation by routine physical activity and phonophobia). Clinical features of her HNAMP suggested that she had chronic migraine [37
]. Her headache provocation by regular telephone use probably resulted from enhanced sensitivity to environmental stimuli in chronic migraine. The other possible mechanism for her headache provocation by regular telephone use is idiopathic environmental hypersensitivity to electromagnetic fields (IEI-EMF) [17
]. However, we did not assess the symptoms during or after using electrical devices other than MP and regular telephone and could not know whether she had IEI-EMF.
Our study has some limitations. First, this study was not a blinded attempt to provoke headache under specific conditions. Instead, we have described the self-reported prevalence and clinical features of HAMP among MP users under actual conditions. Thus, we were not able to evaluate the causal relationship between headache provocation and specific factors related to MP use. Second, the participants in this study were all medical students in Korea. Most of them were 20-30 years of age and used MPs; therefore, our study participants did not reflect the general population. Third, the initial questionnaire survey assessed only the occurrence of headache and HAMP, and we conducted follow-up interviews with only those participants who reported that they were currently experiencing or had previously experienced HAMP. We assessed other MP-associated symptoms such as burning sensation, dizziness, and orbital or periorbital pain during the telephone interviews, but because they were not included in the initial questionnaire survey, we were unable to independently evaluate MP-associated symptoms other than headache among MP users. Fourth, the study was conducted in 2005 when only code division multiple access (CDMA) type MPs were available in Korea. CDMA-type MPs and global system for mobile communication (GSM) type MPs differ in their operating frequency range and electromagnetic filed output power. Thus, the effects of CDMA-type MPs on humans may differ from those of GSM-type MPs. Nevertheless, the prevalence of HAMP among CDMA-type MP users in our study was similar to those of previous reports of HAMP among GSM-type MP users. Fifth, we screened for HAMP by using an initial questionnaire survey in which the participants were asked to recall HAMP during the last 1 year, and there is the possibility of erroneous memory of their headaches. In the present study, most participants with HAMP reported their HAMP occurred frequently during or after MP use (Figure ). These findings suggest that most participants with HAMP had experienced HAMP recently and might have answered positively in the initial questionnaire survey.