Pressure gradients change flow velocities and affect flow near the dura and pia differently than flow in the middle of the channel. When a pressure gradient is applied to a laminar flow, the resulting velocities depend in part on the initial velocity of the fluid. Acceleration and deceleration in CSF require the application of pressure. Inertial forces resulting from the mass and velocity of CSF produce other effects in CSF flow. Laminar CSF flow means that fluid moves with greater velocity in the center of a channel and with lesser velocity near a boundary resulting from frictional effects ( Fig 2).
Viscosity creates a laminar pattern of flow, which can be thought of as layers of flowing fluid with no disruption between the layers. Viscous and inertial properties, together with the complex anatomy of the subarachnoid space, determine CSF flow patterns. 3 With contrast media or radionuclides in the spinal subarachnoid space, a slow convection of fluid is observed resulting from the continuous oscillation of CSF. 2 Elastic properties of the tissues surrounding the subarachnoid space theoretically induce pressure waves, which to date are not fully characterized. CSF oscillations are coupled to CSF pressure oscillations that, in the healthy adult, are approximately 90° out of phase with the velocity fluctuations. CSF flow and the venous displacement diminish progressively from the cephalic end of the cervical canal to the caudal end of the thoracic canal. The fluid entering the spinal canal displaces blood from the epidural venous plexus in the spine.
Caudal CSF flow has greater velocities and shorter duration than cephalad flow ( Fig 1).
CSF in the spinal canal moves caudally when the systolic pulse wave reaches the brain and cephalad during diastole. In the spinal canal, oscillatory CSF flow results primarily from the displacement of approximately 1.5 mL of fluid 1 from the cranial cavity as intracranial blood vessels expand in arterial systole (Monro-Kellie doctrine).
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