Headache caused by colloid cyst of the third ventricle, presenting very characteristically as recurrent attacks with thunderclap onset, often triggered by postural change or Valsalva-like manœuvre, and associated with reduced level or loss of consciousness.

Diagnostic criteria:
  1. Headache fulfilling criterion C
  2. A colloid cyst of the third ventricle has been demonstrated
  3. Evidence of causation demonstrated by both of the following:
    1. headache has developed in temporal relation to development of the colloid cyst, or led to its discovery
    2. either or both of the following:
      • a)headache is recurrent, with thunderclap onset and accompanied by reduced level or loss of consciousness
      • b)headache has significantly improved or resolved in temporal relation to successful treatment of the colloid cyst
  4. Not better accounted for by another ICHD-3 diagnosis.

The vast majority of colloid cysts of the third ventricle are discovered incidentally, having been asymptomatic. Nevertheless, their position immediately adjacent to the foramen of Monro can, on occasion, result in sudden obstructive hydrocephalus, causing headache with thunderclap onset and reduced level or loss of consciousness. This highly characteristic presentation should lead to rapid diagnosis. Headache attributed to colloid cyst of the third ventricle signals a life-threatening emergency.