Low-pressure hydrocephalus

Overview

Low-pressure hydrocephalus (LPH) is a serious disease characterized by ventricular dilatation and clinical signs of intracranial hypertension, although the pressure of the cerebrospinal fluid (CSF) in the cerebral ventricles is below the normal range. The pathophysiology of LPH remains complex and not fully understood, but it seems that a combination of mechanisms involving isolation of the ventricular system from the subarachnoid space, changes in brain tissue elasticity and compliance, and increased brain tissue permeability may play a key role. The diagnosis of LPH should be considered in patients with clinical signs of intracranial hypertension and ventricular dilatation on imaging with preserved patent CSF drainage and normal pressure settings. Treatment of LPH includes temporary therapy to ensure patient stabilization with external ventricular drainage and gradual weaning from drainage with the use of increasing intracranial pressure. Permanent treatment includes endoscopic ventriculostomy of the third ventricle and implantation of a shunt with a low differential pressure setting. The choice of the type of shunt may be individualized; only the lumbo-peritoneal type is not recommended. It should be emphasized that recognition and proper treatment of LPH are crucial, as improper treatment can lead to fatal consequences. Despite the ongoing challenges in the diagnosis and treatment of LPH, it is important that the disease is well known to the professional community.

Keywords:

Ventriculostomy – shunt – low-pressure hydrocephalus – external ventricular drainage

This is an unauthorised machine translation into English made using the DeepL Translate Pro translator. The editors do not guarantee that the content of the article corresponds fully to the original language version.

 

Introduction

Hydrocephalus is traditionally understood as an accumulation of cerebrospinal fluid in the intracerebrum, due to an imbalance between fluid production and absorption [1]. According to Dandy et al. it has historically been divided into communicating and obstructing types [2]. In 1965, Adams et al. described chronic hydrocephalus with normal liquor pressure [3], the existence of which has long been a subject of controversy and it still cannot be said that this disease is widely known outside the professional community. Even less is known about low-pressure hydrocephalus (LPH). It was first mentioned in the literature in the 1970s, but at that time it was rather confused with normotensive hydrocephalus [4,5]. In their 1994 paper, Pang and Altschuler clearly defined low-grade hydrocephalus and described the diagnostic criteria [6]. Like high-pressure hydrocephalus, its low-pressure variant presents with ventricular dilatation with an acute clinical state, especially impaired consciousness, but the pressure of the liquor in the cerebral ventricles is below normal. This apparent paradox, together with inadequate brain imaging capabilities and a lack of understanding of the complicated pathophysiology, initially led to inappropriate treatment and poor outcomes [7]. Distinguishing BPH from other types of hydrocephalus is crucial. Treatment of BPH according to the principles of high-pressure hydrocephalus treatment leads to fatal consequences. Even today, the mortality rate is 11%, which is alarming [8]. In this review paper, we aim to summarize the current knowledge on the pathophysiology, diagnosis and treatment of BPH.

 

Pathophysiology

Patients with BPH show clinical signs of intracranial hypertension, which corresponds to ventricular dilatation. However, lymphatic pressure is below normal values. This paradox makes it clear that the pathophysiology of BPH is complicated and there is not yet a general consensus. It is likely to be an interplay of several mechanisms. Certain risk factors for the development of BPH are known. In adult patients, the disease develops most often after intracranial hemorrhage or trauma. In pediatric patients, the most common risk factor is intracranial neoplasia. Furthermore, the risk of developing BPH in intracranial infection, the presence of a shunt to drain the liquor, liquorrhea (postoperative or posttraumatic) is described. A significant risk factor is also the lumbar puncture performed. Especially in children with BPH, a history of puncture can be traced in 31% [6,8].

The theory of isolation of the ventricular system from the convexity of the subarachnoid space, described by Rekate et al. [9], seems to be the simplest. Under normal conditions, these two compartments communicate with each other, but due to hemorrhage, tumor or inflammation, their separation may occur. Under physiological conditions, the pressure in both compartments is in equilibrium. In isolation, however, they may differ. If there is leakage of fluid from the cortical subarachnoid spaces due to lumbar puncture or due to liquorrhea, a pressure gradient is created even with normal intraventricular pressure, leading to dilatation of the ventricular system. This theory is supported by the finding of narrowing or obstruction of the aquaeductus mesencephali or the outflow tract of the IV ventricle in 10% of patients with BPH. It has also been mentioned that in a significant proportion of patients, lumbar puncture or postoperative liquorrhea can be traced [8]. However, in the majority of patients a similar history is not present, therefore a more complex mechanism can be assumed.

Lesniak et al. explain the development of BPH by changes in the elasticity and compliance of the brain. They suggest that initially there is a dilatation of the ventricular system due to the high pressure of the liquor. The ventricular system causes compression of brain tissue, increasing its compliance and decreasing its elasticity. Thus, after the pressure of the liquor is reduced, e.g. by its drainage, there is no immediate reduction of ventriculomegaly -⁠ the tissue is too compliant and the reduced elasticity does not allow the restoration of the original brain volume and ventricular width. This phenomenon is referred to as hysteresis, i.e. the delayed effect behind its cause. The clinical symptoms of BPH are then not caused by white matter compression by tension ventriculomegaly, but by stretching of the periventricular fibers due to ventricular dilatation [10]. Increased brain tissue compression has also been demonstrated by MR elastography, although this is a case report [11].

Another theory posits changes in the viscoelastic properties and turgor of brain tissue. Already Hakim et al. suggested that mathematical modelling of hydrocephalus should treat brain tissue as a porous sponge that has certain viscoelastic properties [12]. We can also call it a poroelastic model. If we apply this model to the brain, we achieve ventricular dilation at a lower intraventricular pressure than when using the elastic model [13,14]. The porous material has two components. The first component is solid, in this case the brain tissue itself, and the second component is liquid, i.e. extracellular fluid. It appears that it is the volume of the extracellular fluid of the brain that may play a significant role in the pathophysiology of BPH. Pathologic insults may result in increased brain tissue permeability and increased extracellular fluid volume. This phenomenon has been demonstrated in patients with BPH especially in the periventricular regions. In general, the presence of periventricular fluid on imaging is considered a sign of high intraventricular pressure with transependymal transfer of fluid. However, in BPH, this picture can develop even at low pressure due to just increased permeability [15]. Akins et al. noticed that in a patient with established external ventricular drainage, its drainage decreases during the development phase of BPH. They explain this precisely by the transfer of fluid into the brain tissue and the expansion of the ventricular system. When the drainage overflow is reduced to negative values, the drainage then increases, from which they conclude that due to "negative pressure drainage" water is returned from the extracellular spaces back into the ventricular system [16]. Brain tissue with higher extracellular fluid volume has been shown to exhibit higher compliance and lower elasticity. Simply put, the tissue is the "muddy" equivalent of an imaginary sucked sponge [6]. But why does ventricular enlargement occur? One possibility is that BPH develops secondary to an existing ventriculomegaly, and because of the change in elasticity and compliance, the ventricular system does not narrow. The disproportionate distribution of increased permeability also plays an important role. The latter is highest in the periventricular regions and in the white matter [13,14]. Areas with higher extracellular fluid volume may then expand into areas where the volume is lower. Again, an analogy is offered with respect to the porous sponge. A wrung-out sponge does not have the same resistance as a sponge soaked in water. Therefore, the periventricular region expands towards the convexity and the cerebral ventricles then dilate secondarily [16].

 

Diagnostics

Diagnosis of BPH is not complicated. The main problem is to think about the possibility of BPH. The diagnosis of BPH is likely in a patient who shows clinical signs of severe intracranial hypertension. In adult and pediatric patients, the most common symptoms are impaired consciousness, headache, and cranial nerve lesions [8]. On imaging, ventricular dilatation is present, and the drainage of the liquor is patent and set at standard pressure. It is not uncommon for high liquor pressure to be present initially and for BPH to occur secondarily. It is therefore important to monitor patients clinically and graphically on a regular basis [17]. The diagnostic criteria were already summarized by Pang and Altschuler in 1994 [6] (Table 1).

 

Treatment

Once we think of the diagnosis of BPH, we need to stabilize the patient. If the stabilisation steps are effective, we will also confirm the diagnosis definitively. The guiding principle is to achieve a significant reduction in ventricular width. In the first phase, we indicate a supportive or temporary treatment, and after the patient is stabilised, a permanent treatment.

 

Temporary treatment of low pressure hydrocephalus

External chamber drainage

The definition of the disease implies that the clinical and graphic manifestations of acute hydrocephalus are present despite functional cerebrospinal fluid drainage with standard pressure settings. This drainage may be of the type of implanted external ventricular drainage (EVD) or a previously established shunt. It is the latter option that predominates in paediatric patients (92%). In the case of an established shunt, a ZKD version is often required. The latter not only allows stabilization of the patient's condition, but it is possible to attempt gradual weaning from drainage and reverse the risk of requiring permanent drainage [8].

In the acute phase, a single aspiration of 30-50 ml of lysate from the ZKD is recommended. This should be followed by lowering the drainage overflow height below the level of the external ear canal, i.e. to negative pressure values. The drainage volume should be approximately 10-15 ml of fluid per hour. This often requires lowering the drainage overflow level 5-20 cm below the level of the ear canal [8].

If the diagnosis of BPH is confirmed and a reduction in the width of the ventricular system is evident with negative pressure drainage, we can try to gradually increase the level of drainage, i.e. intracranial pressure. The rate of increase should be approximately 1-2 cm every 3-5 days, i.e. very slow, while the volume of drained fluid should still be 10-15 ml per day. Such a long interval between each step of increase is necessary because clinical deterioration may occur after 5 days. The assumption is that by gradually increasing the pressure, the compliance and elasticity of the brain will be returned to physiological values and drainage can be extracted. However, this is achieved in only 20% of paediatric patients and 1% of adults [8]. The rest of the patients must be referred for definitive surgical treatment. The above rate of drainage increase suggests that withdrawal requires long-term implantation of a ZKD. We know that the risk of infection increases with the length of implantation [18]. Thus, the question is whether to attempt gradual weaning from drainage in an adult patient when we know that the chance of success is while small. In pediatric patients, however, this approach is entirely rational. The risk of infection can be reduced by the use of catheters impregnated with silver or antibiotics [18]. If these catheters are not implanted as standard in a given institution, they may be recommended when BPH is suspected.

In some papers, gentle bandaging of the neck or abdominal area is described as a complementary method to ZKD. By compressing these areas, we achieve a worsening of the return from the intracranial venous system, i.e. an increase in venous pressure and a limitation of resorption of fluid. This mechanism may then help to increase brain turgor and restore physiological conditions [19].

 

Permanent treatment of low pressure hydrocephalus

Endoscopic stoma of the third ventricle

The aforementioned theory of the origin of BPH speaks of the isolation of the intraventricular and subarachnoid compartments. Therefore, it is not surprising that endoscopic third ventriculostomy (ETV) may be effective in some patients [20]. In this procedure, an ostomy of the base of the third ventricle is performed at the tuber cinereum into the prepontine cistern. This creates communication between the ventricular system and the subarachnoid space. From there, physiological resorption of the lysate can then take place. A higher ETV success rate is naturally achieved in patients who have evidence of obstruction on MRI of the brain (e.g. tumours, cysts, ureteral narrowing) and who already have a shunt in place. Conversely, a lower success rate is described in patients who develop BPH after inflammation and subarachnoid hemorrhage because the subarachnoid space is subject to fibrosis [8]. In general, if ETV is anatomically possible, it should be considered as a first step. After an ETV is performed, a rapid weaning from ventricular drainage should be attempted. A dramatic reduction in ventricular width cannot be expected after ETV and the patient should be evaluated mainly clinically. If the patient is still dependent on drainage, even if the stoma is patent, the signs of BPH often disappear and a subsequent shunt can be implanted with a standard valve setting. Hydrocephalus then no longer shows the typical features of high-pressure hydrocephalus [20]. In cases where ETV did not lead to definitive treatment, at least the time required for subatmospheric LKD was reduced [20].

 

Implantation of shuntu

If the ETV is unsuccessful or was not anatomically possible and the patient is still dependent on a ZKD, we must consider permanent drainage of the lysate with a shunt. Due to the low-pressure nature of this type of hydrocephalus, the use of a valve with a low differential pressure setting is advisable, preferably a programmable valve. The choice of the type of shunt is an important issue. As already mentioned, lumbar puncture has been identified as a risk factor for hydrocephalus in a proportion of BPH patients. Therefore, the introduction of a lumbo-peritoneal shunt is not recommended. No difference was found between the effectiveness of ventriculo-peritoneal and ventriculo-atrial shunts, and both types of drainage can be used [8].

 

Conclusion

Low-pressure hydrocephalus is a rare disease characterized by clinical signs of intracranial hypertension but subnormal cerebrospinal fluid pressure. In the treatment of BPH, ZKD is most commonly used in the acute phase, with endoscopic stoma of the third ventricle or shunt placement being the definitive treatment. Incorrect diagnostic and therapeutic procedure leads to fatal consequences. It is therefore very important that the issue of BPH is well known in the professional community.

 

Table 1. Diagnostic criteria of BPH according to [6].

Diagnostic criteria for low pressure hydrocephalus
1	worsening of the patient's neurological condition during normal drainage of the liquor
2	ventriculomegaly
3	persistent ventriculomegaly with normal or subnormal blood pressure
4	Clinical and radiological response to "negative pressure" drainage of the lymph node
5	exclusion of shunt malfunction or liquor drainage