Book - Contributions to Embryology Carnegie Institution No.44
The Experimental Production Of An Internal Hydrocephalus
by Lewis H. Weed.
The pathological condition of internal hydrocephalus is intimately connected with abnormalities of the production or absorption of cerebro-spinal fluid. The first explanation of this condition — an increased elaboration of the fluid by the choroid plexuses — remains a hjqDothetical possibility. Obstruction to the normal pathways of drainage of the fluid has been many times demonstrated anatomically, so that now this idea of the origin of the disease may be considered to rest on a firm basis. It is with the experimental production of that type of hydrocephalus, due to obstruction to the flow of the cerebro-spinal fluid through its normal channels, that this paper will deal.
To insure a more complete understanding of the anatomical problems underlying internal hydrocephalus, a short review of the essential morphological and physiological features of the pathwaj^s of the cerebro-spinal fluid will be given. The conception that the greater part of the cerebro-spinal fluid is produced by the choroid plexuses of the cerebral \entricles is no longer strongly controverted; when first advanced, it had only the glandular histology of the plexus to support it (Faivre, 1853; Luschka, 1855), but much later a combination of pharmacological, histological, and pathological observations indicated beyond question the essential role of these plexuses in the elaboration of this body-fluid. From a summary of this evidence and from personal study IMott (1910) suggested the term "choroid gland" for these vascular structures. More recently embryological studies related the first extraventricular flow of the fluid to the initial tufting of the plexuses (Weed, 1916 a and h; 1917). Pathologically, the production of an internal hydrocephalus by tumors occluding the ventricular passages has been recognized for years as affording evidence of an intraventricular source of the fluid.
From the choroid plexuses the fluid is poured into the cerebral ventricles which are lined by ependj^mal cells of ectodermal origin. The fluid from the lateral ventricles escai^es through the foramina of IMunro into the third ventricle, thence through the aqueduct of Sylvius into the fourth. Additions to the fluid are made by the choroid plexuses of the third and fourth ventricles. From the fourth ventricle this fluid passes into the mesodermal subarachnoid spaces through the foramina of IMagendie and of Luschka, if these be actual openings, or at least through permeable membranes if the patency of the foramina be questioned.
The cerebro-spinal fluid leaving the ventricular system, is distributed from the cisterna cerebello-meduUaris, which it first reaches on leaving the fourth ventricle. In the comparatively large channels about the base of the brain the flow is rapid and great; down the cord the dispersion is also fairly efficient, but over the convexities of the cerebral hemispheres a very slow and less eflEicient distribution occurs. Everj'where this extraventricular course is through the meshes of the subarachnoid spaces— a complicated, interrupted fluid-channel, completely clothed by low or flat mesothelial cells. In the cisternal regions, and to a lesser extent about the ujjper spinal cord, the mesh of this fluid channel is rather large, but over the hemispheres it becomes quite small. The pial reflection of the mesothelial cells lining the subarachnoid space is broken by the openings of the perivascular spaces (Robin) ; into these cuff's the mesodermal cells continue for a short distance. By way of these perivascular spaces a small but rather important fluid, representing probably the elimination of the brain tissue, is added to the subarachnoid cerebro-spinal fluid.
It seems quite well estabUshed that the cerebro-spinal fluid is returned directly into the blood stream. This escape of fluid is wholly from the subarachnoid space; all observers are agreed that the intraventricular absorption is minimal. The most important hj'potheses regarding the mode of absorption of the fluid are those which assume the Pacchionian vilh as the pathways into the dural sinuses (Key and Retzius, 1876) ; that the fluid reaches the cerebral capillaries by way of the perivascular sheaths (Mott, 1910); or that a direct absorption into the capillaries of the subarachnoid space occurs (Dandy and Blackfan, 1913, 1914). More recently evidence that the arachnoid villi (normal projections of arachnoidea into the dural sinuses) are the essential structures in the pathway of escape, has been presented (Weed, 1914, a, b, c). The direction of flow (toward subarachnoid space) in the perivascular spaces argues against Mott's idea of drainage, while the absence of capillaries in the arachnoid renders the hypothesis of Dandy and Blackfan untenable. The strongest evidence, both anatomical and physiological, favors the idea of a major absorption from the cranial portion of the subarachnoid space.
With this conception of the pathway of the cerebro-sjjinal fluid, it becomes evident that the obstruction to outflow of fluid may be either intraventricular or extraventricular. Pathologically, the differentiation of hydrocephalus into two classes with reference to the point of obstruction has repeatedly been made. In the one, the block to outflow occurs usually at the points of constriction of the ventricular system, as at the foramina of Munro, witliin the aqueduct of Sylvius, or (in rarer cases) within the fourth ventricle. In the second variety of cases the obstruction to flow of the fluid occurs within the subarachnoid space. The mechanism of this intrameningeal block is not well understood, except in the cases of obvious fllling of the cisterna cerebello-meduUaris with fibrinous exudate, occasioning a macroscopic closure of the exit.
The occurrence of a typical internal hydrocephalus due to block witliin the narrow portions of the ventricular systems seems of easy explanation. With continued elaboration of cerebro-spinal fluid by the choroid plexuses, and practically no intraventricular absorption, it seems inevitable that dilatation of the ventricles should take place. Such enlargement reaches its fullest expression in the lateral ventricles with compression and thinning of the cerebral cortex. With the increased pressure of the fluid there is ultimately brought al)out a ])ahincc between the production of fluid by the choroid i)lexuses and the minimal intraventricular absorption, plus other potential agencies of escape.
But when consideration is given to the enlargement of the lateral ventricles from obstruction to flow within the meninges, a plausible explanation is more difficult. The mere mechanical explanation does not fairlj^ meet the question as to why the meningeal block does not cause compression of the cerebral cortex, dilatation of the subarachnoid spaces, and a typical external hydrocephalus. In one variety of this type (obstruction at the foramina of the fourth ventricle) the mechanical explanation holds. But actually, a diffuse block to the outflow of cerebro- spinal fluid within the meninges results almost inevitably in the later production of an internal hydrocephalus.
CHnically, a differentiation between the two types of internal hydrocephalus may be made. In children Gushing (1908) found that the cerebral ventricles of one kind of internal hydrocephalus (due to block in the meninges) could be drained by lumbar puncture, while in the other (due to block in the ventricular system) the cerebral ventricles could be emptied only from the ventricular needle. Similarly, Dandy and Blackfan (1913, 1914) recovered by lumbar needle phenolsulphonephthalein injected into the cerebral ventricle in the "communicating" type, while the subarachnoid absorption was markedly retarded. In the obstructive type these writers showed a negligible absorption from the cerebral ventricles, but an unim- paired absorption from the subarachnoid space. In these cases the dye injected into the dilated ventricles did not appear as normally in the lumbar fluid.
Experimentally, the obstructive type of hydrocephalus alone seems to have been produced. Dandy and Blackfan (1913, 1914) were able to cause a typical internal hj^drocephalus in dogs by two methods, the first of which gave the pathological picture of an intraventricular block. Pledgets of cotton were introduced from the occipital region through the fourth ventricle into the aqueduct of Sylvius. Such a foreign obstruction caused signs of cerebral pressure (lethargy, vomiting) and a rather acute dilatation of the ventricles was produced. In a somewhat different way the same enlargement of the ventricles was accompUshed b}' Ugation of the vein of Galen, but this occurred in only one of the ten animals used. In the other nine animals the higher Ugation apparently permitted sufficient collateral circulation. Dandy and Blackfan used young dogs 2 to 6 months of age; at this time in the dog the cranial sutures are strongly united, so that no enlargement of the head occurred. The time allowed for the dilatation of the ventricles varied from three to eight weeks after the operation. At the end of this period a fair degree of hydrocephalus was present.
At about the same time Thomas (1914) published the results of his experiments on the production of an internal hydrocephalus by the intraventricular injection of aleuronat in starch. This protein caused a marked inflammatory reaction, blocking finally the ventricular pathway at one or other of the narrow parts. Dogs were used throughout for the observations. Thomas found Httle enlargement of the ventricles present in the first week, but in the chronic stage of the inflammatory process incited bj^ the protein, an internal hydrocephalus developed, with symptoms of increased intracranial pressure. The dilatation of the lateral ventricles occurred slowly and reached its maximum in about two months. Subsequent intraventricular injection of India ink demonstrated that the obstruction to flow of the cerebro-spinal fluid occurred, in the different animals, either at the foramen of Munro, in the aqueduct of Sylvius, or most frequently, at the foramen of Magendie. It must be emphasized that in many cases the ventricular dilatation was markedly asymmetrical (the greater enlargement being on the side of the injection) and that the ventricular wall was often irregularly eroded. The intraventricular accumu- lation of fluid was associated with a sterile inflammatory process.
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Cite this page: Hill, M.A. (2020, January 26) Embryology Book - Contributions to Embryology Carnegie Institution No.44. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Contributions_to_Embryology_Carnegie_Institution_No.44
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