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Uncommon (Group 4.0) Primary Headaches: Less Familiarity and More Missed Diagnosis.

Background
Other primary headaches make up group 4.0 of the International Classification of Headache Disorders third edition (ICHD-3). This group includes a clinically varied group of headache entities whose pathophysiology is not well understood and whose treatments are based on open-label reports.


Objective
To review and update the uncommon primary headaches included under group 4.0 in ICHD-3.


Methods
We reviewed the headache disorders listed under group 4.0 of ICHD-3 for the clinical features, diagnostic criteria, and management, and described recent updates of these relatively rare disorders.


Results
The entities included in this group have a characteristic presentation in practice. Some of them are activity-related and can be provoked by Valsalva maneuver (primary cough headache), some by prolonged exercise (primary exercise headache), and some by sexual excitation (primary headache associated with sexual activity); primary thunderclap headache has also been included here because all the above-listed entities can sometimes present in thunderclap fashion too. Two of the entities in this group 4.0 are linked to direct physical stimuli viz. cold stimulus headache and external pressure headache. Also included in this group are primary stabbing headaches and nummular headaches that are presumed to be due to the involvement of the terminal branches of the sensory nerves supplying the scalp and are, therefore, termed "epicranial headaches." Hypnic headache syndrome and new daily persistent headache are included here as "miscellaneous headaches" because we still do not know enough about their causation.


Conclusion
The headache disorders included under group 4.0 in ICHD-3 are uncommon, heterogeneous entities, which may pose diagnostic and management challenges to the physicians. The diagnosis may be missed owing to a lack of familiarity.