The hydrostatic indifference point is the point along the neuraxis where CSF pressure does not change when the patient moves from erect to recumbent. This is usually located in the lower cervical-upper thoracic region 1,2.
When recumbent, CSF pressure is equal along the entire length of the neuraxis from the vertex to the thecal cul-de-sac, being slightly positive (e.g. 7-15 cm H2O). When erect, however, CSF pressure below the hydrostatic indifference point is increased and above the hydrostatic indifference point is decreased; the zero CSF pressure point is located at the level of the posterior fossa or upper cervical spine, and supratentorial intracranial pressure is slightly negative at around -5 mmH2O 1,3. This explains why base of skull CSF leaks (resulting in otorrhea or rhinorrhea) do not routinely cause orthostatic headaches nor are they associated with imaging features typically seen in spontaneous intracranial hypotension 3.
It should be noted that although the CSF compartment can be thought of as a simple column of fluid, numerous physiological responses (e.g. heart rate, blood pressure, vascular compliance, internal jugular vein collapse, changes in abdominal pressure, CSF production) influence how intracranial and intraspinal pressure respond to normal and pathological changes (see Monro-Kellie hypothesis) 1.
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