This theory proposed that slit ventricle syndrome should be considered as a consequence of early suture ossification, which was a secondary effect associated with drainage of CSF via a working shunt. Insertion of a VPS in newborns and infants has as a consequence the remodeling of the skull, which leads to a craniocerebral disproportion. In the majority of the aforementioned patients, a shunt has been inserted during the first year of life and they were associated with remarkable microcephaly. Based on the doctrine of Monro-Kellie, we could explain the resultant intracranial hypertension, along with the existence of tonsillar herniation below the foramen magnum, which is described as acquired or secondary Chiari I. Imaging finding consistent with tonsillar herniation were, initially, more frequently encountered in patients harboring lumboperitoneal, rather than ventriculoperitoneal shunts. However, Rekate[8] was unable to verify this proposed relationship between lumboperitoneal shunts and Chiari I, a finding that is in contradistinction to that theory. However, the management protocol that is considered to be the most suitable for the patients that are categorized under this entity (acquired craniocerebral disproportion) is the expansion of the supratentorial cranial compartment, while the existing shunt is maintained.

The development of gliotic scar tissue around the ventricles has been considered as been associated with the existence of a chronic CSF drainage system. When insertion of a new ventricular catheter is performed, this process could result in an increased resistance when canulation of the ventricular system is attempted. This is called “stiff ventricle” and it may be an obstacle to the development of ventricular enlargement when failure of the shunt system is verified. The pathological finding of periventricular gliosis could be the equivalent, in anatomical terms, to a system that is characterized by the presence of increased elastance. Contrary to that theory are studies centered on patients that are characterized by overdrainage, which did not reveal significant differences related to the existence of reactive astroglia in the periventricular white matter when normal subjects were compared with children who had died due to shunt failure and associated ventricular dilatation.[8] Additionally, If the inability of the ventricular system to increase its dimensions was merely associated with periventricular gliosis, this would not permit the transmission of pressure. Because of that, a pressure gradient would be established. This concept was not verified by experimental work which compared intraventricular pressure with intraparenchymal pressure using bolus infusion.[8]

Patients who are subject to episodes of hypotension which is alternated with hypertension of shunt overdrainage are amenable to the development of a state characterized by reduced compliance, or, in different words, increased elastance or stiffness. This pathological situation renders patients particularly susceptible to minor alterations of ICP measurements. Elastance constitutes an intrinsic property of the brain and it refers to the resistance to distortion (viscoelastic properties). It is largely determined by the volume of the venous compartment, along with the functionality of the mechanisms that determine the venous outflow of the cerebrum. Indeed, when overdrainage of CSF is evident, this is related to ICP values below zero this fact is associated, based on Monro-Kellie doctrine, with venous congestion. This pathophysiologic cascade leads to increased brain elastance. When shunt failure occurs, ICP increases and this may provoke the development of increased transmural pressure, with the resultant sudden collapse of bridging veins to the major venous sinuses. This sequence of events could be the generative cause for the development of a model of positive feedback between venous and intracranial hypertension. This mechanism could only support the concept of treatment of a group of patients suffering from shunt overdrainage via the aid of a lumboperitoneal shunt.

This theory has been suggested based on the changes on the morphometric characteristics of the ventricles that are encountered in cases of chronically inserted drainage systems. When overdrainage is evident, asymmetry in the dimensions of the ventricular system may develop, which consists of collapse of the ventricular system harboring the central catheter, resulting in its occlusion. Additionally, enlargement of the contralateral ventricle may ensue. This sequence of events may be transitory, a concept that is compatible with the intermittent nature of the clinical presentation which characterizes the slit ventricle syndrome. When the distal tip of the central catheter is occluded, the CSF compartment that refers to the ventricular system becomes entrapped, and this could be related to the contralateral ventricular enlargement, along with the resultant intracranial hypertension. Based on that concept, slit ventricle syndrome could be regarded as synonymous to ventricular isolation. This model provides support to the management plan which consists of treating a specific subgroup of patients suffering from shunt overdrainage by the establishment of communication of the isolated compartments with the rest of the ventricular system via the aid of neuroendoscopy.

Some researchers consider that in patients who manifest chronic overdrainage (in the setting of a ventriculoperitoneal shunting), the pressure within the ventricles, as well as within the subarachnoid space of the cerebrum ranges in low, or even in negative, settings.[6] When acute shunt malfunction becomes evident, there is no effective compensatory mechanism for the acute management of the increased intraventricular and extracellular fluid pressures. This condition is presumed to lead to cerebral edema due to the accumulation of extracellular fluid.

A newly proposed theory about the development of the slit ventricle syndrome is centered on an alteration in the transmission of the arterial cerebral pulsatility to the CSF. It mainly refers to patients with shunt systems draining CSF from the extrathecal compartment. According to this hypothesis, the treatment of shunt overdrainage could be based on the reduction of the pulse wave transmission to outside the extracranial compartment. To achieve that, they proposed the use of devices capable to regulate the antigravitational drainage, to prevent the uncontrolled drainage of the CSF during activities of daily living.

The theory of the siphon effect justifies the management of over-drainage by systems to offset the siphon effect, the so-called antisiphon devices, or other systems that increase the resistance to drainage across the valve.[30,31,32] These antisiphon systems could be used preventively in the first shunt implantation or at the time of revision due to malfunction, attempting to reduce the rates of ventricular catheter obstruction and the probability of developing symptomatic shunt over-drainage.[33] Conclusively, it appears that multifactorial pathophysiology should be responsible for the development of SVS.[1,3] These newer theories about the genesis of SVS share in common with previous studies the demographic characteristics and risk factors of the patients included in this group. More precisely, they mention that symptoms more often appear before the age of 10 years, generally between the ages of 2 and 5 years,[34] though others have reported a peak at 6.5 years.[35] A clinical observation related to slit ventricle syndrome is that it is less common in children with hydrocephalus associated with cerebral palsy and relevant atrophy.[29] Buxton reported a mean age at diagnosis of 7–9 years,[21] although the transition from normal ventricles to ventricular collapse can occur later and is therefore difficult to be predicted with accuracy.[36] One of the most widely accepted risk factors is shunt implantation during the first months of life.[29,34,37] Other risk factors are related to hemorrhage[38] (posthemorrhagic hydrocephalus of the newborn or premature infant), infection (neonatal meningitis), valve malfunction (more common in valves that have been functioning well for several years),[20] and the hydrodynamic properties of the valve.[20] The latter issue is more evident with the use of low differential pressure opening valves that is considered by some researchers some the most undisputed predictor for the development of slit ventricle syndrome.[39] The fact that the time of first intervention for treatment of hydrocephalus is an important part of the history is supported by a recent review article.[40]

Another important issue in the management of the entity of SVS is the surgical treatment of this entity and the orientation of our treatment goals. Based on our current knowledge about the pathophysiology of SVS, the standard treatment for shunt over-drainage is the reduction of CSF flow across the valve system. This can be accomplished by placement of a device that counteracts the siphon effect, ideally additionally increasing the valve opening pressure,[20] or by using self-adjusting valves or flow control systems.[39] A large cohort studies centered on the relevant efficacy of flow control versus anti-siphon valves[37] concluded that early development of SVS could be addressed by raising the opening pressure of the inserted valve early in the clinical course of the patient, an option that is easily to be accomplished when a programmable valve is inserted.[37,41]

Despite these modern treatment options, the rate of recurrence continues to be relatively high and other treatment options should be incorporated. One of them involves repositioning catheters or shunt transferring, placement of a complete second shunt system, “Y” connectors between the two ventricular catheters, or a single multi-perforated catheter in both lateral ventricles (trans-septal catheter). These procedures can be made easier with neuronavigation or neuroendoscopy.[42,43,44]

Another alternative option, in the case previously proposed therapeutic measures have failed, is considered to be the insertion of lumboperitoneal shunts and the performance of cranial expansion surgeries, which can increase the buffering capacity of the brain.[28,45,46,47] An additional role for these systems relates to their capacity to circumvent VPS mechanics and to directly drain the SAS.[48] Drainage of CSF through the subarachnoid space permits a certain degree of ventricular dilatation by creating a favorable pressure gradient through the cortical mantle.[3] Lumboperitoneal shunts can be used alone or in combination with the third ventriculostomy, or other treatment modalities, depending on the underlying pathology.[14] An interesting case report, referring that only bilateral shunting of the ventricular system was able to eliminate the symptoms permanently,[49] is recently reported.

In case that the treatment armamentarium that is available for SVS (e.g., shunt revision, cranial expansion, lumboperitoneal shunt) does not work,[47,48,49] there are studies which have described endoscopic procedures for the treatment of SVS. These include shunt replacement, shunt removal, and endoscopic third ventriculostomy (ETV), which are safe and effective treatment options.[20,50,51] There are previous reports who support the concept of performing ETV as an effective strategy for the management of SVS that occurs following shunt placement.[46] Another recent report[52] supports the effectiveness of aqueductoplasty in the treatment of isolated fourth ventricle when is considered to be combined with slit-ventricle morphology on imaging studies. An algorithm, which indicates most currently accepted treatment options and alternatives associated with the different clinical manifestations of this syndrome, as well as its associated pathophysiologic substrate, follows [Figures 1-3].

Regarding the definition of the syndrome, the triad of headache, small ventricles, and slow valve refilling conform to the typical diagnostic criteria. Nonetheless, we have encountered different or atypical forms of presentation, as well as apparently silent forms but with intracranial hypertension.[53,54]

As far as the issue of the definition of the term SVS is considered, it is a common concept that there are different forms of presentation, as well as types of shunt over-drainage. Besides that, we could support the concept that the classifications proposed by Rekate, Khorasani, and Olson, albeit only coincide in some of the types, even though they are obviously using different terminology.

The pathophysiology of this condition is considered to be the key element in our effort to improve the efficacy of our treatment strategies. Currently, there is no theory that can unify the variously proposed models. A possible explanation should be that in any particular case, there should be more than one pathophysiological mechanism, perhaps with one predominating, and it is evident that this one should suggest the main treatment strategy. Despite progress in valve technology, CSF over-drainage continues to be one of the most usual complications in shunted hydrocephalus.[55] A multi-centered comparison of different treatment strategies for groups of patients who fulfill the same diagnostic criteria would be beneficial, which, in turn, would enhance our ultimate goal, which is improvement in clinical practice and patient care.[56]

In cases of refractory shunt over drainage, a final treatment option that is adopted is the execution of cranial expansion procedures. A variety of techniques have been employed the oldest and most basic technique utilized in patients suffering from secondary craniosynostosis related to a CSF is suturectomy.[57] Later on, a more complex surgical concept was introduced, which involved subtemporal decompression of 4 cm × 6 cm, which was eventually combined with a dural opening.[58,59] Nowadays, other surgical alternatives are introduced, including cranial vault expansion,[45,60] which are considered as more suitable for patients who are shunted early in their life. These patients commonly develop secondary craniosynostosis and microcephaly. Furthermore, this treatment option is suggested in cases of shunt over drainage without ventricular dilatation,[1] as well as in syndromic craniosynostosis. The current trend is not to consider these techniques as first-line management of slit ventricle syndrome. As such, they are usually selected for cases in which we are unable to adequately control CSF drainage via the selection of any type of valvular mechanism.[6]

Based on our research, we mention that propose ETV is proposed as a suitable alternative for this entity. It is remarkable that the success of this technique is irregular depending on the pathology that caused the hydrocephalus in the first place. Also, ETV is difficult to perform in these patients because of the collapse of the ventricular system. Researchers state that the cranial expansion technique in SVS cases allows ventricular expansion and favors the ETV procedure. Some cases have been treated this way with good results, but this approach is highly controversial and cannot be considered an alternative for all the cases.

In order to assume a thorough perspective of the entity of the slit ventricle syndrome, any figures or percentages would be helpful to depict the distribution of patients in diagnostic and therapeutic perspectives. It would be a helpful adjunct for our research to know how many patients with chronic headache and existing shunts got symptom-free after simple pressure adjusting of shunt valves and how many patients are refractory to conservative measures and go to cranial expansion surgery. To the best of our knowledge, we haven’t heard the answer yet according to our opinion, this is due to the lack of concordant data.

According to our opinion, several definitions are rendered mandatory to clarify the relevant entities which are discussed in this manuscript. As already discussed, the term “Classical” slit ventricle syndrome refers to the original definition of the condition and required three things (as previously depicted) and included the failure of refilling of the pumping mechanism. Many and maybe most contemporary valves do not have a pumping chamber and that definition is no longer applicable. According to our opinion, to define the condition of slit ventricle syndrome we should use the following definition: “SVS is a syndrome of intractable headaches in shunted patients with small ventricle.”

According to Olson,[1] slit ventricle syndrome should probably be referred to as the non-compliant ventricle syndrome, to avoid confusion of radiological “slit ventricles” with the syndrome. He also supports the concept that as the true slit ventricle syndrome is represented by the on-off symptom complex, this should be known as the noncompliant ventricle syndrome to avoid confusion.[21] It is estimated that 6%–22% of children with imaging characteristics compatible with slit ventricles and associated headaches may suffer from the non-compliant ventricle syndrome.[45,58]

Another pathologic entity whose definition remains an unresolved problem is the shunt over-drainage syndrome. Nowadays, the most accepted concept of shunt over-drainage is related to what has become known as “shunt-related headaches.” Rekate[8] considered it to be an entity characterized by the appearance of severe headache, i.e. that which interferes with activities of daily living, in patients harboring a CSF shunt valve and normal or smaller than normal ventricles.

The charts that are included in our manuscript do recognize that there are several sub diagnoses for this syndrome. Figure 1 essentially does this by calling the SVS “Headache (Shunt Associated). On Figure 1 there is a block for headaches that respond to conservative treatments such as antimigraine medication. These patients do not, therefore, meet the criteria for SVS being “Disabling-refractory” headaches. Basically, then SVS patients may require management of the shunt or the hydrocephalus. At this point, we recognize three subtypes of SVS.

Figure 2 is an algorithm for the management of “ambiguous cases.” As in the Dandy classification, we recognize two subtypes, “obstructive” and “communicating.” Unfortunately, this too may become confusing. To Dandy and most papers now “communicating” relates to flow from the ventricles to the spinal subarachnoid space. It does not mean that the CSF can go from the ventricles to the cortical subarachnoid space. It would be better to ventricles expand or ventricles do not expand to clarify.[49]

In Figure 3, we limit the management of “classical” SVS to the manipulation of the valve. This is the usual treatment and often works. There are times however when it is necessary to access the cortical subarachnoid space as well as the ventricle in this treatment. This is dealt with in the discussion related to the lumbar shunts and cisterna magna shunts.[3,40]

Shunt infusion study is still used in some institutes, although it constitutes an old diagnostic method. Actually, some researcher says it is the only method to determine the true functional state of the shunt in situ. Clinical practice has shown that accurate diagnosis of shunt malfunction is a clinical problem presenting many challenges.[61,62] More precisely, shunt malfunction can come to clinical attention with nontypical symptomatology and/or ventricular dimensions that do not differ from normal. Moreover, there is a subgroup of children which may not be clinically affected, but, on the other hand, they do not demonstrate the anticipated decrease of ventricular dimensions, along with the widening of external CSF spaces following a seemingly successful shunt placement. When the shunt-opening pressure is low or is lowered (in cases of programmable valves) and the previously recorded situation is our case, this constitutes a more difficult to resolve the clinical challenge.[63,64,65]

When we are confronted with equivocal cases, an option is to perform open shunt revision with intra-operative shunt testing however, this is very invasive method. Serial follow-up examination could be another option. However, this has the drawback of adding an element of delay in treatment, or long-term neglect, especially in patients with complex postural over-drainage or intermittent obstruction.[66,67,68,69] Shunt testing in vivo using shunt infusion studies has been reported as a diagnostic adjunct for over 25 years and is utilized, in selected cases, in clinical practice.[70,71,72,73] It is an accurate and relatively low-invasive method that helps us to assess shunt function and detect over-drainage, underdrainage as well as proximal or distal blockage.

It seems that the shunt infusion study is an accurate and radiation-free diagnostic tool which provides a quantitative shunt assessment in the pediatric population, aiming to rule out or prove not clinically evident shunt malfunction. We consider that silent shunt dysfunction simply represents another form of compensated, even though not sufficiently treated, chronic hydrocephalus. This situation is accompanied by all devastating consequences to the development of the child, so shunt infusion studies constitute an important tool for the pediatric neurosurgeon. The recent bibliographic review considers that it provides a significant degree of certainty that children with shunted hydrocephalus either are harboring working shunts, or revision is necessary. A clinical benefit accompanied by radiologic improvement is clearly demonstrated.[61]

Nil.

There are no conflicts of interest.