Anatomically, choroid plexus tissue is floating in the cerebrospinal fluid of the lateral, third, and fourth ventricles. This tissue is well perfused by numerous villi, each having a central capillary with fenestrated endothelium. A single layer of cuboidal epithelium then covers each of these vessels. This unusual cellular anatomy forms the blood CSF barrier characterized by tight junctions at the apical end of the choroid epithelial cells rather than at the capillary endothelium within each villus[ 2 , 11 , 12 ].

Due to its glandular appearance and ventricular location, the choroid plexus has been suggested to be the major site of CSF secretion. This view was mainly based upon the historical canine experiments of Dandy. In these experiments the foramen of Monro was occluded and a choroid plexectomy of one lateral ventricle was performed. The authors reported collapse of the ventricle without choroid plexus and dilatation of the other ventricle[ 13 ].

They concluded: "From these experiments we have the absolute proof that cerebrospinal fluid is formed from the choroid plexus. Simultaneously it was proven that the ependyma lining the ventricles is not concerned in the production of cerebrospinal fluid"[ 14 ].

Interestingly, the experiments of Dandy were based upon observations from only a single dog[ 1 ]. Furthermore, the experiments could not be reproduced by others[ 15 — 17 ]. From this value and the estimated arterial blood flow through the choroid plexus, a CSF secretion rate was calculated that came very close to the estimated rate of total CSF absorption[ 18 ].

Second, these findings were substantiated by concordance with experiments in which the CSF production rate was assessed in the isolated and extracorporally-perfused choroid plexus[ 19 — 22 ]. These experiments, however, were criticized because of inherent large errors possible in the experimental technique since the various preparations all required considerable operative manipulations[ 1 , 2 , 11 ]. Furthermore, other experimental studies, including those with radioactive water provided evidence that at least some CSF must come from a source other than the choroid plexus, presumably the brain tissue itself[ 23 — 25 ].

An even higher rate of ependymal fluid secretion was derived from experiments investigating spinal cord ependyma[ 27 ]. Again, these experiments were criticized because of the "drastic experimental procedures" used.

It was concluded that "it may be wise to reserve final judgment on this question"[ 11 ]. The capillary-astrocyte complex of the blood—brain barrier BBB has been implicated as an active producer of brain interstitial fluid ISF. The ISF secreted at the blood—brain barrier is coupled with shifts of extracellular fluid between brain and CSF, eventually leading to the net formation of CSF[ 28 , 29 ].

The rate of ISF formation was estimated from the clearance rate of tracer substances, which were injected into the brain parenchyma. It was assumed that the rate of clearance provides an estimate of the rate of ISF secretion at the blood—brain barrier. Accordingly, even a recent review concluded that "the working hypothesis that the BBB is a fluid generator, although attractive, needs substantiation"[ 4 ].

Historically, the absorption of CSF into the circulating blood is most notable across the arachnoid villi[ 3 , 31 , 32 ]. It was stated: "From a purely anatomical point of view, these arachnoid villi are obvious regions for the drainage of CSF into the vascular system…" page in[ 33 ].

The notion of the arachnoid villi being the major site of CSF absorption is actually based on the early experiments of Key and Retzius who injected colored gelatin into the CSF space of human cadavers.

They reported the distribution of the dye throughout the entire CSF system and its passage across the arachnoid villi into the venous sinuses[ 34 ]. However, their results were questioned since the dye was injected at a pressure of up to 60 mmHg. It was suggested that the high pressure during the dye injection could cause rupture of the arachnoid villi and absorption into the sinuses[ 35 ].

Therefore, Weed performed dye injection experiments at pressures of only 9—13 mmHg that also attempted to determine whether or not the injected dye particles themselves could obstruct the normal drainage pathways. Isotonic solutions of non-toxic dyes ammonium citrate and potassium ferrocyanide were infused that precipitated granules of Prussian blue before the animals were intravitally fixed with acidified formalin.

Weed reported the distribution of the dye particles throughout the entire CSF space, filling the arachnoid villi along the sagittal sinus, eventually invading the dural wall of the sinus. Notably, only some granular material was found in the lumen of the sinus[ 35 , 36 ]. The authors also stated as another important result: "No evidence has been afforded in our observations of the escape of cerebrospinal fluid into cerebral veins or capillaries"[ 37 ].

Weed performed numerous pilot experiments in his effort to identify a dye solution that was best suited for his studies: "Many solutions were tried, but all proved unsatisfactory because of their toxicity or their diffuse tissue staining"[ 36 ].

One could argue, therefore, that Weed inadvertently excluded those solutions in which the absorption of CSF throughout the entire brain parenchyma would have been the result. Electron microscopy studies performed on arachnoid villi revealed a pressure-sensitive vacuolation cycle of pores, which act as one-way valves and allow for the transcellular bulk transport of fluid[ 38 , 39 ]. Extracorporal perfusion of excised dura demonstrated the passage of particles up to the size of erythrocytes[ 40 ].

Considerable portions of CSF may be absorbed into the cervical lymphatics[ 2 ]. The perineural subarachnoid space of cranial nerves, which is connected to the cranial CSF space, was suggested as a pathway for the drainage of CSF into the lymphatics of the extracranial soft tissue at the skull base[ 2 ].

Though it is obvious that CSF drains into the lymphatics, the physiological significance of this CSF absorption route is still a matter of debate. This finding led to the conclusion that only a small fraction of CSF drains via the lymphatic channels. Interestingly much RISA was drained via the cerebral perivascular spaces as well as by the passage from the subarachnoid space of olfactory lobes into the submucosal spaces of the nose and thus to the lymphatics [ 43 ].

Intravital microscopy of the exposed cervical lymph nodes during the cisternal infusion of ink revealed that particle movement was dependent on the respiratory cycle: during inspiration the speed of particle movement was 10—20 mm s -1 , while no movement was observed during the expiration phase[ 44 ]. It is important to note that the CSF and ISF spaces communicate with the cervical lymphatics via two anatomically different routes, i. Extracranial organs feature fluid exchange across the capillary bed that is driven by hydrodynamic and osmotic pressure gradients.

However, absorption of CSF into cerebral capillaries has been disputed because it was thought that the absorption of CSF is not dependent on osmotic forces. This notion was based on experiments in which dextran solutions of different osmolality were infused into the ventricles of cats at a constant pressure of 27 mmHg.

The measured infusion rate, which should equal the CSF absorption rate, decreased by the same extent. The decrease of the absorption rate was explained by the increased CSF viscosity[ 33 ].

Interestingly, a more recent animal study failed to reproduce these earlier experiments, since it was shown that 3 H 2 O from the bloodstream enters osmotically loaded cerebrospinal fluid significantly faster[ 46 ]. Since, historically, osmolality was assumed to not be relevant for CSF absorption, hydrodynamic pressure gradients would be the only driving forces for CSF drainage into the brain capillaries and post-capillary venules.

It was also assumed that any absorption would require a CSF pressure higher than the intravascular pressure and that this would cause the collapse of the vessels and prevent absorption of CSF[ 2 , 47 ].

These statements from the s and s were actually defining the understanding of CSF physiology for decades until BBB and aquaporin AQP studies clearly indicated the involvement of osmotic forces in brain water homeostasis for discussion see below. In , Masserman calculated the rate of CSF formation in patients by measuring the time needed for the CSF pressure to return to its initial level following drainage of a standard volume of CSF by lumbar puncture[ 48 ].

After drainage of 20 to 35 mL of CSF, pressure was restored at a rate of about 0. The validity of results obtained in this way was criticized because the Masserman technique assumes that neither formation nor absorption rates are changed by alterations in pressure. However, the absorption of CSF varies greatly with changes in intracranial pressure[ 49 , 50 ]. Modifications of the Masserman technique applied sophisticated infusion and drainage protocols, which recorded and controlled the CSF pressure during the measurement period see for example[ 51 ].

Despite numerous research efforts, more sophisticated experimental protocols did not yield CSF formation rates that differed from earlier work. The ventriculo-cisternal perfusion "Pappenheimer" technique represents a more quantitative approach for the assessment of CSF formation rate.

Inulin or other macromolecules, which pass through the ventricular space without being absorbed, are infused at a constant rate into the cerebral ventricles. CSF formation is calculated from the measurement of the extraventricular cisternal or spinal CSF concentration of inulin. It is assumed that any dilution of inulin between the inflow cannula and outflow cannula results from the admixture of freshly formed CSF.

In addition, the test procedure allows for the calculation of the CSF absorption rate from the clearance of inulin at the extraventricular site in animals the cisterna magna, in man the lumbar space [ 49 ]. An important disadvantage was that the procedure was difficult to apply in clinical settings because of its invasiveness: The hour long infusion required both a ventricular and extraventricular CSF catheter.

Also, both infusion rate and infused volume exceeded the physiological range of CSF flow by far. Despite these obstacles, clinical measurements were performed in brain tumor patients who received ventricular catheters for chemotherapy purposes: In patients 9—61 years old the average flow rate was 0.

These results were confirmed in children with brain tumors[ 53 ]. Furthermore, similar data are available from hydrocephalus patients[ 54 ]. Though more precise, the ventriculocisternal or ventriculolumbar perfusion techniques yielded results remarkably close to those assessed by the Masserman technique[ 2 ]. Findings from both the Masserman and the Pappenheimer techniques were supported by neuroradiological investigations applying serial CT scans to assess the ventricular washout of metrizamide, a water soluble contrast media.

The rate of right lateral ventricular CSF formation ranged from 0. Hence, the assessment of the CSF formation and absorption rates remains a matter of debate even today. It has been suggested that a method that is less invasive than the Pappenheimer method ventriculo-cisternal perfusion and more reliable than the Masserman method is sorely needed[ 50 ]. The concept of the "third circulation" suggesting that CSF flows through the ventricles, cisterns and subarachnoid space SAS and is reabsorbed into the blood at the arachnoid villi, was introduced by Cushing in [ 57 , 58 ].

This notion was a radical departure from the contemporary view that the CSF moved by ebb and flow[ 1 ]. Since Cushing, the circulatory, bulk flow character of the CSF system has remained unquestioned by the majority of researchers.

Even recent reviews assume a directed CSF circulation through the ventricles and the subarachnoid space toward the arachnoid villi[ 1 , 5 , 32 ]. Nevertheless, as will be discussed below, this understanding of CSF circulation appears to be a rough simplification of a much more complicated situation.

Anatomically the VRS refers to a histologically-defined space, which surrounds blood vessels arterioles and venules when penetrating from the subarachnoid space into the brain tissue. Originally, it was thought that the VRS is connected to the subarachnoid space, allowing for a free fluid communication. It was suggested that interstitial fluid may be outwardly drained along these pathways into the SAS and eventually towards the arachnoid villi[ 35 ]. Later this concept was questioned on the basis of light microscopic examinations, which depicted perivascular spaces as cul-de-sacs, open to the subarachnoid space but closed towards the parenchyma and therefore not a channel for flow[ 59 ].

The first systematic electron microscopic study of blood vessels entering the cerebral cortex confirmed this view. In addition it was reported that small arterioles entering the cortex carry with them to the point at which they become capillaries an extension of the subarachnoid space[ 60 ]. Actually, these findings, showing the obliteration of the VRS at the capillary bed, led to the rejection of the earlier theories on the existence of a perivascular CSF circulation.

As discussed by others[ 61 ], these morphological findings eventually supported the general belief that the interstitial fluid ISF is stagnant in the central nervous system. Morphology of Virchow Robin and perivascular spaces. Delineated by basal membranes of glia, pia and endothelium, the Virchow Robin space VRS depicts the space surrounding vessels penetrating into the parenchyma. The VRS is obliterated at the capillaries where the basement membranes of glia and endothelium join.

The complex pial architecture may be understood as an invagination of both cortical and vessel pia into the VRS. The pial funnel is not a regular finding. The pial sheath around arteries extends into the VRS, but becomes more fenestrated and eventually disappears at the precapillary section of the vessel.

Unlike arteries as shown in this figure , veins do not possess a pial sheath inside the VRS. ISF may drain by way of an intramural pathway along the basement membranes of capillaries and arterioles into the lymphatics at the base of the skull green arrows.

It should be noted that the figure does not depict the recently suggested periarterial flow from the SAS into the parenchyma and an outward flow into the cervical lymphatics along the veins for discussion see text "Current research". Also, it is still a matter of debate whether the Virchow Robin space, extending between the outer basement membrane of the vessel and the glia, represents a fluid-filled open space see text.

The current understanding of the microscopic anatomy of the VRS is more complex Figure 1. Actually, its fine structure is built upon endothelial, pial, and glial cell layers, each of them delineated by distinct basement membranes[ 62 — 64 ].

The glial membrane glia limitans covering the brain parenchyma forms the outer wall of the VRS[ 65 ]. At the capillary bed, the basement membrane of the glia fuses with the outer vascular membrane thereby occluding the Virchow-Robin space[ 66 , 67 ]. Arterial and venous vessels running within the cortical subarachnoid space are covered with a pial cell layer, which ensheaths the vessels.

The pial sheath creates a space next to the vessel wall, which is referred to as perivascular space PVS [ 68 ]. At the site of the entrance of the cortical vessels into the VRS, their pial sheath joins with the pial cell layer covering the brain surface forming a funnel like structure, which accompanies the vessels into the VRS though for a short distance only[ 69 , 70 ].

However, the pial sheath of the arterial, but not venous, vessels extends into the VRS. Near the capillary bed, the pial sheath becomes more and more fenestrated and leaky[ 68 ]. It is important to note that the nomenclature is not used consistently.

Some authors use the terms "Virchow Robin space" and "perivascular space" as synonyms[ 71 ], while others use the terms to name different spaces as discussed above[ 72 ]. Ultrastructural electron microscopic studies agree that pial membranes separate the VRS from the cortical subarachnoid space[ 65 , 68 , 70 ].

Since electron microscopy of human brain specimens shows that the VRS and the PVS are collapsed[ 68 ], it is a matter of debate whether these histologically-characterized compartments are actually open or just potential spaces.

However, studies in rodents have demonstrated the VRS filled with fluid, electron microscopic dense material[ 70 ], macrophages and other blood born inflammatory cells[ 64 , 67 ]. Possibly, different fixation procedures may explain this discrepancy: rodent brains undergo intra-vital perfusion fixation, while the studies in man have to rely on specimens, which are fixed extra-corporally.

Although pial cell layers obviously separate the VRS from the cortical subarachnoid space, physiologically there is strong evidence indicating that fluid circulates along the VRS Figure 2.

Following the injection of horseradish peroxidase HRP into the lateral ventricles or subarachnoid space of anesthetized cats and dogs, light microscopic examination of serial brain sections has been performed utilizing a sensitive histochemical technique tetramethylbenzidine incubation [ 73 ]. The authors reported the distribution of tracer reaction product within the VRS and along the basal laminae around capillaries.

The influx into these spaces was very rapid since the intraparenchymal microvasculature was clearly outlined 6 min after the infusion of HRP. Electron microscopy of sections incubated after 10 or 20 min of HRP circulation confirmed the paravascular location of the reaction product, which was also dispersed throughout the extracellular spaces ECS of the adjacent parenchyma.

The rapid paravascular influx of HRP could be prevented by halting or diminishing the pulsations of the cerebral arteries by aortic occlusion or by partial ligation of the brachiocephalic artery.

However, it should be noted that others were not able to reproduce these findings; Krisch et al. Also, another study reported that following microinjection into the VRS or the subarachnoid space of rats, tracers e. India ink, albumin labeled with colloidal gold, Evans blue, rhodamine remained largely in the VRS, the cortical subpial space and the core of subarachnoid trabeculae.

Nevertheless, bulk flow of fluid within the VRS, around both arteries and veins, was suggested from video-densitometric measurements of fluorescently labeled albumin. However, the observed flow was slow and its direction varied in an unpredictable way[ 71 ]. Furthermore, it was shown that, following intracerebral injection, India ink particles concentrated in the VRS, but were then rapidly ingested by perivascular cells.

Notably, very little movement of carbon-labeled perivascular cells and perivascular macrophages was seen after 2 years[ 74 ]. Diagram representing fluid movements at the Virchow Robin space.

Glial blue lines and pial yellow lines cell membranes enclose the VRS and control fluid exchange. Note, that it is a matter of debate whether the VRS represents an open fluid fill space see text for discussion. Both experimental and clinical evidence indicate the existence of a pathway along the basement membranes of capillaries, arterioles, and arteries for the drainage of ISF and solutes into the lymphatic system red lines and green arrows.

It is unclear, whether the subpial perivascular spaces around arteries and veins light blue serve as additional drainage pathways. Also, the proposed glymphatic pathway connecting the arterial and venous VRS with the venous perivascular space black arrows is still a matter of debate. Step 1: prepare the patient You may be given a sedative to make you drowsy and relaxed. A doctor and at least one assistant will be in the room. You will lie on your side with your knees drawn to your chest so that your spine is curved; in some cases you may sit on the table and lean forward onto some pillows instead.

After cleaning your back with a cooling antiseptic, the doctor will numb the area of your lower back where the needle will be inserted. This may cause some brief stinging. Step 2: insert the needle Next, a hollow needle is inserted between the third and fourth lumbar vertebrae into your spinal canal Fig.

The needle doesn't touch the nerves of your spinal cord. Your doctor will collect between 5 to 20 ml of cerebrospinal fluid in 2 to 4 tubes. You will probably feel pressure when the needle is inserted, and some people feel a sharp stinging sensation when the needle goes through the protective dural layer that surrounds the spinal cord.

Although you may feel some discomfort, it is important that you lie still. Let your doctor know if you are feeling pain. Step 3: measure CSF pressure optional You will be asked to straighten your legs to decrease abdominal pressure and increase cerebrospinal fluid pressure. The needle is attached to a meter and the pressure in your spinal canal is measured. Step 4: insert a lumbar drain optional In cases of hydrocephalus, a catheter may be inserted to continuously remove CSF and relieve pressure on the brain.

The CSF is contained within a system of fluid-filled cavities called ventricles. The ventricles are shown in blue on the following midsagittal section of the brain. The Ventricles CSF is produced mainly by a structure called the choroid plexus in the lateral, third and fourth ventricles.

CSF flows from the lateral ventricle to the third ventricle through the interventricular foramen also called the foramen of Monro. Physicians have long known that rising intracranial pressure may help create leaks. Now, a multidisciplinary team is developing protocols to better monitor rising pressures, especially in postsurgical patients. Health Home Conditions and Diseases. What is cerebrospinal fluid? What is a CSF leak? Analysis of the nasal fluid: This test is used to detect beta-2 transferrin, a protein found almost exclusively in CSF.

CT scan: This noninvasive diagnostic imaging procedure uses X-rays and computer technology to produce detailed imaging of bones and planes of the brain. MRI scan: This uses a large magnet, radiofrequencies and a computer to produce detailed images of organs and structures in the body.

Cisternogram — CT or nuclear medicine : These two tests, each performed in a similar way, require a spinal tap also known as a lumbar puncture to administer a fluid to the CSF that allows your doctor to identify if a CSF leak is present and the source of the leak. During this test, a contrast medium is injected into the spinal fluid through a spinal tap, and then a CT scan is performed.