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Encephalic Arachnoid Cyst

Described for the first time in 1831 by BRIGHT, the arachnoid cyst is an arachnoid formation whose walls are purely arachnoid and does not differentiate itself from the neighboring arachnoid tissue. It can develop wherever there is arachnoid but it tends to localize in the peri-cephalic arachnoid cisterns. This cyst contains cerebro-spinal fluid (LCS) of the same composition as the neighboring LCS and communicates with the joined arachnoid lakes allowing a regular exchange of the intracystic fluid. Under certain conditions, this cystic formation is enclosed and the intracystic LCS does not communicate or communicate with the neighboring pericerebral LCS. The formation can then increase in volume with an accumulation of LCS under pressure which it can itself secret (GO, 1986). The cyst then becomes symptomatic and compresses the neighboring formations. Microtraumatism responsible for small local haemorrhages or meningeal infections can symphise the communication of the cyst with the neighboring LCS, thus being able to cause a valve phenomenon with intermittent fluid circulation, or even a complete occlusion of the intracystic circulation. This will also cause compression of the neighboring parenchymal formations.

Encephalic Arachnoid Cyst

A. Origin of arachnoid cysts the congenital origin of arachnoid cysts is admitted by most authors (DI ROCCO, 1990, GO, 1986, LESOIN, 1983, RENGACHARY, 1985). It results from the development of arachnoid tissue.
The histological study of the arachnoid cysts finds identical cell layers to the normal arachnoid tissue and the ultramicroscopic study reveals cerebrospinal fluid secretion capacities (HUME ADAMS, 1984).
B. Arachnoid Seat The site of these arachnoid cysts is variable, but all the publications find, in order of frequency, arachnoid cysts, the distribution of which is as follows:
sylvan in nearly 50% of the cases seller region in 10 % of cerebellar pontine region 10% of and are distributed with an almost identical frequency in 5% of cases, cysts in the vermis, the large tank, the fissure inter Hemispherical, and hemispherical convexity.
The discovery of an intracranial arachnoid cyst has become commonplace since the use of modern imaging methods (scanner and MRI). Their frequency is difficult to quantify because we do not yet have precise neuroradiological statistics.
The autopsy series study found a frequency of more than 1% of arachnoid cysts in a non-selected population. HUME ADAMS in 1984 found on a series of 3OOO neonatal examinations, 5 cases of arachnoid cysts, bringing a frequency of 1.6% of arachnoid encephalic cysts.
It is only very exceptionally that encephalic arachnoid cysts can be a source of sometimes dramatic complications that are difficult to predict.
In a series of 1500 intracranial neoformations hospitalized during the last 10 years in the department of neurosurgery of the Beaujon hospital, we found only 14 cases of symptomatic arachnoid cysts, that is to say in 0.9% of the cases. This frequency is found by all the authors. In most published series so far, the frequency of symptomatic arachnoid cysts accounts for about 1% of intracranial neoformations (ROBINSON, 1971).
The clinical symptoms of poorly tolerated arachnoid cysts will depend on the site of the arachnoid cyst but will always show signs of intracranial hypertension with signs related to the anatomical situation of the cyst.
A. Cysts of the Sylvan Valley
If they can be symptomatic at any age, they are often revealed in two conditions:
1. In children In children with a symptomatology mainly due to intracranial hypertension and accompanied by a particular dysmorphy (blowing and protrusion of the corresponding temporal fossa) with on the X-rays of the face skull an asymmetry of Rocks and a thinner and bulging aspect of the temporal scale.This aspect is one of the arguments in favor of the congenital nature of these cysts which, during the evolution of the fetal and neonatal life, will shape the temporal scale which molds on the arachnoid cyst under pressure. Signs of intracranial hypertension are in the forefront in children and will require the practice of cerebral CT to visualize the arachnoid cyst under pressure compressing the neighboring parenchyma and causing a ventricular tilt.
2. Adult In adults, the arachnoid cyst in the Sylvan Valley can also be revealed by intracranial hypertension with headache, but is sometimes associated with critical phenomena.
Another form revealing the arachnoid cyst of the sylvan valley is the sudden hemorrhagic accident in the cyst cavity following a cranial trauma which may be minor but which is sufficient to break the arterial and venous vessels that cross the arachnoid and which are weakened by the pressure exerted by the arachnoid cyst on them. Diagnosis in these cases is more difficult because it can be resolved to that of extra cerebral haemorrhage under Dural, post-traumatic. It is the procedure that will correct the diagnosis of the hemorrhage enclosed in the cystic cavity.
The cyst may also rupture at the periphery and be responsible for symptomatic subdural detachment.
B. The cyst of the sedative region these cysts are observed mainly in children and young adults and are revealed by a semiology associating often severe intracranial hypertension, an endocrine symptomatology by hypothalamic-pituitary compression which is of a rather deficient type, and a visual symptomatology by compression of the chiasma and the optical channels. Revealed initially by a bitemporal hemianopia as in all supra-sarcomas tumors chiasma compressing, the visual symptomatology will degrade rapidly and can lead to blindness.
Neuroradiological investigations (CT and MRI) visualized a rounded lesion with a density and signal of the LCS that occupied the seller region and the third ventricle. It causes a dilation of the lateral ventricles by blocking the holes of MONRO.
It is sometimes difficult to distinguish between the arachnoid cyst and the third dilated ventricle, which can cause therapeutic problems because the treatment must lower the cyst to allow free circulation of the LCS. It would be dangerous to place a ventricular bypass in one of the ventricles, thinking that it may be a atrioventricular hydrocephalus because the lesion is left in place and the contralateral ventricle remains blocked, leading to a hemispherical rocking.
In addition, the clinical symptomatology related to the local volume of the cyst remains.
C. Other arachnoid cysts the other arachnoid cysts are revealed by a symptomatology of intracranial hypertension and compression of local structures:
1. Cysts of the cerebellar cistern the cysts of the cerebellopontic cistern will be revealed by a syndrome of the ponto-cerebellar angle with involvement of the VII, the VIII and the V, associated with a cerebellar syndrome.
2. The cysts of the quadrijumelle platform the cysts of the quadrijumelle platform will be revealed by a syndrome of PARINAUD with intracranial hypertension
3. Cysts of convexity the cysts of convexity are not a localized deficit syndrome depending on the situation of the cyst, sometimes associated with comital crises and especially intracranial hypertension.
4. Cysts of the large cistern the cysts of the large cistern are not a vermic syndrome associated with intracranial hypertension.
In all these cases, it is the neuroradiological assessment that will allow the diagnosis. This assessment focuses on the cerebral CT scan, and secondarily the MRI which will better situate the arachnoid cyst with respect to neighboring parenchymal structures.

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This problem is extremely complex because unanimity is far from being made as regards the therapeutic attitude to have with these cysts. While it is now generally accepted that only symptomatic cysts should be used and that treatment of asymptomatic cysts of chance discovery should not be offered, even if they reach a considerable volume, the proposed therapy oscillates between two main options:
Resection with resection of the cyst walls
The fluid derivation of the cyst
This therapeutic will also detract from the seat of the arachnoid cyst.
A- The cysts of the sylvan valley the treatment of these symptomatic cysts of the sylvan valley is not simple. The logic would be to approach the cyst widely and surgically remove both outer and inner membranes, to prevent it from being reformed. This gesture is however dangerous and causes post-operative vacuo hemorrhages on a brain remaining at a distance.
The current therapeutic tendency is towards a less dangerous gesture and consists in the derivation of the fluid from the cyst by installing a cystoperitoneal bypass valve allowing the cyst to be permanently emptied and to avoid does not reform. The surgical complications are then those of any derivation that diminishes the important life-threatening risk of cystic exacerbations, which, moreover, expose to a postoperative comitiality because the internal membrane of the cyst must be carefully peeled from the underlying corex to which it adheres very often.

Encephalic Arachnoid Cyst
If the symptomatology of intracranial hypertension is rapidly relieved by the fluid derivation, the symptomatology of painful headache that patients complain of is generally not relieved by the intervention.
B- The arachnoid cyst of the sellar region. The treatment of this cyst is also quite complex. The direct approach of the arachnoid cyst under the frontal approach was carried out before the stereotaxic processes became generalized. However, this surgical procedure is very cumbersome and causes post-operative haemorrhagic or infectious complications and does not protect against cystic recurrence because in this situation it is very difficult to completely remove the two internal and external membranes of the cyst Adhere to vital vascular and nervous structures.
The stereotactic puncture of this cyst thus appears to be the ideal solution and allows the cystic cavity to be marsupialised in the ventricular system or the neighboring arachnoid. If this gesture is not enough, it is then possible to set up a peritoneal kysto bypass to restore the hydraulics of the LCS.
It is in these cases that the endoscopic techniques take all their interest in allowing the realization of a ventriculo-kysto-cisternostomy.
C- The treatment of the other arachnoid cysts. It will be guided by the neuroradiological assessment which will specify the exact situation of the cyst with respect to the neighboring parenchyma. This therapeutic gesture is increasingly tending to be minor: simple withdrawal of the cyst rather than an intervention by radical excision (VENES, 1993).
D- Treatment of intracystic haemorrhage Finally, treatment of the intracystic haemorrhage which is particularly frequent in the arachnoid cysts of the sylvian valley and the convexity, requires a two-stage treatment: initially the evacuation of the haemorrhage takes place in Emergency conditions either by flap or enlarged craniectomy in acute hematoma, or by trepanopuncture and drainage in chronic forms which are of course confused with a simple chronic dural hematoma. It is only secondarily that the peritoneal kysto drainage of the cyst will arise if it is reformed and if it remains symptomatic

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