Cluster headache and its variants.

When I first joined Lord (then Dr) Walton's unit in September of 1962, I was immediately impressed not only by his unique expertise in muscle diseases, but also by his encyclopaedic knowledge of medicine, and his versatility. Not surprisingly this caused digressions which strayed from the myofibril; one such diversion took him and my colleague, John Balla, into the field of cluster headache or migrainous neuralgia as it was then called in England.' Years later, Lord Walton had remained faithful to myology, Balla branched into the methodology ofmedical learning and Bayesian theory, and I became entangled in the more banal and mundane problems of headaches. For many years cluster headache was known as periodic migrainous neuralgia of Wilfred Harris, erythroprosopalgia of Bing, histaminic cephalgia, sphenopalatine neuralgia, Vidian or Sluder's neuralgia and, in the USA, Horton's headache. The condition was thought to be a variant of migraine. However, since there is no specific diagnostic test for either condition and their aetiologies remain uncertain, the nosology remains confused. Opinion has shifted from regarding cluster headache as a migraine variant, and I shall consider and compare their differences which are important. Essentially we are dealing with clinical phenomena,2 because although a variety of laboratory abnormalities have been shown to accompany both types of attacks, no one of them is of causal importance; many are epiphenomena. The diagnostic criteria of the International Headache Society are given in Table I.