Glossary term:

Pressure-dissociation headache

A specific type of headache that develops because the normal free communication of cerebrospinal fluid, across the craniovertebral junction, is partially obstructed. Normally, during coughing, sneezing, laughing, straining, bending over or similar Valsalva-like activities, cerebrospinal fluid is squeezed from the spinal canal into the cranial cavity, only to then return once the action ceases. In the presence of herniated cerebellar tonsils (otherwise referred to as a hindbrain hernia, or Chiari malformation), a valve mechanism operates, such that the cerebrospinal fluid can pass readily into the head but returns only very slowly. The individual has, in effect, injected a volume of her or his cerebrospinal fluid – a normal body constituent – into the head, thereby transiently increasing the intracranial pressure and generating a short-lived but intense headache. This may be generalised in distribution but is often worse in the occipital region.