Cerebrospinal fluid, occupying the subarachnoid space, is elaborated in an active process by the choroid plexus. It supports the brain and spinal cord and acts in lieu of a lymphatic system for central nervous tissue. Whether or not absorption of CSF into dural venous sinuses is an active or passive process is still controversial. A very thin layer of bone separates the posterior ethmoid air sinus from the subarachnoid space. There appears to be potential in man for flow of cerebrospinal fluid into the perilymphatic space of the inner ear, but it seldom occurs 1).
Cerebrospinal fluid (CSF) is continuously produced at a 0.4-ml per-minute rate with an average rate of 20-ml per-hour.
CSF flows in a pulsatile manner, dependent on the cardiac rhythm. CSF, being associated with increasing intracranial blood flow and pressure in the cardiac systole, made from the cerebrum, passes through the lateral ventricles by means of Foramen of Monro to the third ventricle, then to the fourth ventricle by the aqueduct cerebri, and to the pontine by passing through the cistern and flows within the spinal canal in the subarachnoid gap. In diastole, there is a return flow toward the lateral ventricles.
The CSF contains approximately 0.3% plasma proteins, or approximately 15 to 40 mg/dL, depending on sampling site, and it is produced at a rate of 500 ml/day. Since the subarachnoid space around the brain and spinal cord can contain only 135 to 150 ml, large amounts are drained primarily into the blood through arachnoid granulations in the superior sagittal sinus. Thus the CSF turns over about 3.7 times a day. This continuous flow into the venous system dilutes the concentration of larger, lipid-insoluble molecules penetrating the brain and CSF.
Several studies have cast doubt on the traditional cerebrospinal fluid (CSF) circulation theory. Some of the criticism has been due to the results of magnetic resonance imaging (MRI) studies, which have indicated that the CSF does not move in a laminar flow in the CSF space. The CSF can move in a turbulent, swirling, oscillating fashion in various parts of the CSF space. Today, many researchers believe that CSF does not act like water in a river, moving in a unidirectional fashion throughout several parts of the CSF space, and the terms CSF flow and cerebrospinal fluid circulation in the CSF space have been replaced with cerebrospinal fluid motion or cerebrospinal fluid movement.
These new concepts arose from several different types of MRI sequences. Unfortunately, most of these imaging techniques do not produce quantitative data, and interpretation is limited to subjective impression. This may not provide a full understanding of the true CSF motion, and cannot be used to identify diseases such as hydrocephalus that involve disturbance of the CSF motion. Additionally, the question has arisen as to how CSF motion changes with aging. Understanding the relation between CSF motion and aging is important to understand the CSF environment. Idiopathic normal pressure hydrocephalus (iNPH) is caused by the disturbance of CSF motion distribution, usually found in elderly individuals, and for this reason it is important to identify the CSF motion in elderly individuals.
The Cerebrospinal fluid (CSF) moves in a pulsatile manner throughout the CSF system with a nearly zero net flow, as shown on an MRI.
The cerebrospinal fluid flow speed and its direction can be measured non-invasively via Phase contrast magnetic resonance imaging (Cine-Contrast MR). When CSF flow is obstructed at any level, hydrocephalus occurs 3) 4).
Cerebrospinal fluid flows throughout the inner ventricular system in the brain and is absorbed back into the bloodstream, rinsing the metabolic waste from the central nervous system through the blood–brain barrier. This allows for homeostatic regulation of the distribution of neuroendocrine factors, to which slight changes can cause problems or damage to the nervous system. For example, high glycine concentration disrupts temperature and blood pressure control, and high CSF pH causes dizziness and syncope.
To use Davson's term, the CSF has a “sink action” by which the various substances formed in the nervous tissue during its metabolic activity diffuse rapidly into the CSF and are thus removed into the bloodstream as CSF is absorbed.
Both fast cine-PC and pencil beam imaging demonstrated expected changes in CSF flow with Valsalva maneuver in healthy participants. The real-time capability of pencil-beam imaging may be necessary to detect Valsalva-related transient CSF flow obstruction in patients with pathologic conditions such as Chiari I malformation 5).
Real-time MR imaging noninvasively showed a transient decrease in CSF flow across the foramen magnum after coughing in symptomatic patients with Chiari I malformation, a phenomenon not seen in healthy participants. The results provide preliminary evidence that the physiology-based imaging method used here has the potential to be an objective clinical test to differentiate symptomatic from asymptomatic patients with Chiari I malformation 6).
Disturbed cerebrospinal fluid (CSF) dynamics are part of the pathophysiology of normal pressure hydrocephalus (NPH) and can be modified and treated with shunt surgery.
Intracranial pressure (ICP) pulsations are generally considered a passive result of the pulsatility of blood flow. Active experimental modification of ICP pulsations would allow investigation of potential active effects on blood flow and cerebrospinal fluid flow and potentially create a new platform for the treatment of acute and chronic low blood flow states as well as a method of CSF substance clearance and delivery.