Book - Contributions to Embryology Carnegie Institution No.44
The Experimental Production Of An Internal Hydrocephalus
by Lewis H. Weed.
Introduction
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.
Content to be added----
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Cite this page: Hill, M.A. (2024, April 19) 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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