Cluster headaches (CH) are part of a group of 'primary' headaches named Trigeminal Autonomic Cephalgias (TACs). There are three other types of TAC, namely Paroxysmal Hermicrania (PH) SUNCT Syndrome [Short lasting unilateral headache with conjunctival injection and tearing] and Hemicrania Continua [HC]. Primary headaches are defined as those where the headache is the condition itself, as opposed to 'secondary' headaches, which are caused by external factors, such as accidents or infection.
All three types of TAC can be identified by the fact that severe pain focuses on one side of the head (often including the face and eye), together with other symptoms occurring on the same side as the pain. These are called 'cranial autonomic' symptoms or features.
These are called 'autonomic' because they are involuntary or automatic responses not caused by any conscious effort or external forces. These include reddening and tearing of the eye, a runny or blocked nostril, droopy eyelid, constriction of the pupil, flushing and facial sweating. If none of these autonomic features occur at the same time as the headache, then they are more likely to be other types of 'short-lasting headaches' of which there are five main types (see separate links).
The first three listed TACS are defined as short lasting. However, defining 'short lasting' headaches is difficult but as a guideline, the period of pain for most TACs is less than four hours compared with migraine headaches, which tend to last for more than four hours. HC attacks can last for several hours with exacerbations in the pain levels.
The key feature of TACs is the severity of pain involved, and unlike other severe headaches, there is often a sense of restlessness or agitation during the period of pain, whereby the sufferer finds it almost impossible to keep still.
It is extremely important to differentiate between the three TACs because the treatment of each type is very different. All three are relatively rare, when compared to migraine for example, which is one of the reasons why GPs fail to diagnose these syndromes. The importance of recognising and differentiating these conditions is emphasised by their often excellent response to different types of treatment.