Mechanical Complications of Shunts

Fig. 8.1

Scanning Electron Microscopy of a shunt valve of diaphragm type, removed from a patient due to shunt malfunction. There is precipitation of crystals on the diaphragm, which with time deteriorates and interferes with the physical properties of the membrane and can block the gap between diaphragm and case (magnification ×100)

Immune Response

Investigation of previously implanted shunts revealed eosinophils and giant cells as parts of the fibrous sleeve, indicating that a hypersensitivity reaction occurs [74]. Most of these studies applied plain radiographs, scanning electron microscopy (SEM), spectral analysis, and simple histology. The cell-mediated immunity was supported from the presence of T-cell granulomas. Others have found immunoglobulin G antibodies specifically directed against silicon, as a part of humoral immunity involvement [30]. On the other hand, some investigators only found dense hyalinized connective tissue with fibroblasts and calcific deposits [85]. Heggers et al. supported a chronic inflammatory reaction – predominantly a foreign body giant cell granuloma type – that was initiated from the release of silicon particles from the aging material [34, 38]. Some authors support that different kind of particles are responsible for the foreign body reaction such as cotton fibers, talc granules, and hair that inadvertently enter the host during surgery [66]. The most profound tissue reaction over the ventriculoperitoneal (VP) shunt tubing was noticed at the subcutaneous course of the catheter and especially at the neck area [9, 75] (Fig. 8.2). Calcification was present only in catheters placed in subcutaneous or vascular areas [46]. It is supported that the cerebral parenchyma and the peritoneal cavity are far less reactive for the silicon catheters [42]. This suits well with the “cellular reaction” theory since the subcutaneous tissue and the intravascular space are areas where migration of immunocompetent cells can occur but it does not explain the lack of calcification in the peritoneal cavity. Furthermore, some case reports of distal catheter insertion into abdominal hollow viscera suggest that a key point for that is the anchoring of the catheter to the organ serosa, which appears to be initiated by a local inflammatory reaction around the distal tip that facilitates erosion and perforation [41, 53, 64]. The clear mechanism is not yet fully understood. Regarding the most proximal compartment of the host involved, there is no cellular immunity in the cerebral parenchyma (similar to the one described for the rest of the body) and ventricular catheters do not induce a gliotic reaction inside the brain tissue as noticed by the absence of adherent cells on the silicone [9, 19].

Fig. 8.2

Distal catheter removed during shunt revision. There is obvious calcification on its outer surface at the area corresponding to the neck region, where the shunt was implanted

Degradation and Altered Physical Properties

The interaction of the silicone catheter with the surrounding tissue leads to the calcification of tube. The implant becomes rigid and fragile. Besides the calcified covering sheet, the degradation process evolves. It has been shown that the dynamic properties of the silicon rubber begin to deteriorate about 6 months after the implantation; the ultimate tensile strength and extensibility gradually decreases over the first 3 years and becomes remarkably altered after 5 years [23, 76]. Different studies have found reticulation damage as an important factor [9, 23]. The four ways that biopolymers deteriorate were described by Kronenthal [43]: (a) the structure is altered by hydration; (b) some covalent bonds of the chain are weakened; (c) these bonds are broken; and (c) soluble fragments are digested by the macrophages. In the human body, the silicones are not subjected to extreme temperatures or irradiation, which are known to contribute to degradation, but the subsequent chemical and mechanical insults (especially in prolonged time periods) can have the same result. This is more evident in prosthetic heart valves that are exposed to strong biological activity (blood cells) and intense mechanical stress (heart beats and blood flow) [65, 78]. In VP (and ventriculoatrial – VA) shunts, the more mobile segment is in the neck [23, 46]. Repeatable head movements – especially extension and contralateral rotation – increase the tension of the mobile neck segment over the fixed parts. The three more vulnerable zones were found to be: (a) the connection between the distal catheter and the reservoir or the valve, (b) the point of traversing the galea, and (c) the crossing of the clavicle [24]. Shunts that are inserted in young children and remain for long time periods are exposed to additional mechanical stress due to the increasing height of the child. The vulnerable part is this case is from the occiput to the peritoneal insertion [24] (Fig. 8.3). There are reports that even a minimal-to-moderate external tension over the tethered and weakened catheter led to fracture as it was the case in two boys where the shunt was fractured over the occipital area after a haircut with clippers [45]. In addition, children have a greater tendency for shunt calcification from a physiological point of view and this maybe related to the increased serum phosphorus levels, compared to adults [9, 18]. A reduction in catheter tubing tensile strength and extensibility was demonstrated when the apparatus had been implanted for more than 6 years [23]. These changes were related to mineralization of the catheter, which resulted in a 30–40 % reduction in tubing thickness. Similar findings were reported from Tomes et al. who showed that catheters become weaker the longer they are implanted and that tubing with a greater cross-sectional area requires greater force to fracture [79].

Fig. 8.3

Distal catheter fracture in the region of the neck. Plain radiographs of a 10-year-old boy, who had a shunt inserted when he was 6 months old, and he presented on this occasion with symptoms of raised intracranial pressure. (a) Lateral skull radiograph. The distal shunt tube is abruptly discontinued in the neck region, a few centimeters below the valve. (b) Abdominal plain radiograph. The remaining distal tube has “fallen” in the peritoneal cavity and coiled up at the region of Douglas pouch

8.1.3 Material Selection and Importance of the Hardware

As previously mentioned, there are reports of allergy to the implanted silicone catheters [35, 36]. In these cases, the presentation was similar to shunt infection or obstruction, but the importance is that the authors used as an alternative catheters that were made from polyurethane. Until recently, such choice was not available. Polyurethane is a polymer composed of a chain of organic units joined by carbamate (urethane) links. It shares many properties with silicone such as heat resistance, which allows it to be thermally sterilized, high biocompatibility, low toxicity, and high tensile strength. On the other hand, it is more rigid, making such systems less easy to manufacture than silicone-based ones [35]. Polyurethane breast implants tended to produce thicker and longer-lasting surrounding capsules compared to the silicon-based ones [70]. On the field of CSF diversion shunt systems, the two materials have not yet been properly compared on a clinical basis.

In order to be radiographically visualized after implantation, shunt catheters must be either homogeneously impregnated with barium or only have a stripe of it. It has been demonstrated that barium precipitates locally as barium salts, enhancing the local immune response and increasing the focal tethering effect and subsequently the risk for catheter fracture [9]. Furthermore, catheters with barium that were tested for their tensile strength without being previously implanted, showed significantly lower values in terms of both ultimate strain and ultimate stress [79]. Similar reports were not found regarding antibiotic-impregnated shunt catheters. It is not mentioned if their physical properties are altered in vitro due to the presence of the antibiotics. Aryan et al. did not find significant local reaction in such externalized catheters [6]. It is unknown if their long-term fracture rates are different from the conventional type of catheters.

The valve type may influence the mechanical features and durability of the shunt system. The valve profile has a major impact [9]. A tubular valve in the catheter line may have a tendency to migrate especially in a growing child as it is pulled by a fixed calcified catheter. Round-bodied valves are more resistant to migration distally. If such valves are placed in the hypochondriac region (to prevent cranial MRI artifacts and skin erosion issues), there is a tendency to fracture at the side of the outlet connector as this is the point of maximal stress force due to the continuous abdominal movements over the fixed valve [2]. Shunt failure caused by valve collapse has been reported in the past [50] (Fig. 8.4) and regarded 11 patients with the Mini-Holter valve where intussusception of the valve ends into the valve tubing was noticed, possibly due to the siphoning effect. The same type of valve was mechanically disrupted in a case report in which the proximal metallic portion was “dislocated” out of its silicone sheath, possibly due to the growth-related traction applied to the fixed valve [86]. In a recent retrospective study, the fixed pressure–gravity-assisted valve paediGAV (Aesculap AG & Co. KG, Tuttligen, Germany) and the programmable differential pressure Codman Hakim (Codman &Shurtlef, Inc., Raynham, USA) were compared and there was no significant difference regarding shunt failure of mechanical etiology [7].

Fig. 8.4

Shunt valve of Holter type, removed during shunt revision, 20 years after implantation. The valve case has completely disintegrated due to material distruction. Extensive calcification is seen on the outer surface

A critical point in mechanical complications of shunts is the number of shunt components. The more the pieces connected to each other, the higher the failure rates [3]. A typical shunt includes one proximal catheter, connected to a reservoir or a burr-hole-type valve and a distal catheter connected to the valve. This means two components with one connection at best, but frequently three components with two connections. In a review of 275 shunt revisions, disconnection of the system accounted for 15 % of the malfunctions [3]. It was also found that the more distal the connection was from the ventricle, the higher the likelihood of disconnection. One-piece shunt systems are described to decrease early fractures and disconnections by avoiding distal connectors [31, 59].

8.1.4 Surgical Technique

Although significant amount of data have highlighted the contribution of the surgical technique in the prevention of shunt infection and obstruction, too little evidence exists regarding the correlation of mechanical complications and surgical choices or maneuvers [22]. When shunts other than one piece are utilized, an important determinant is the competence of the connection between two parts. Most times this requires reinforcement with surgical knots. The knots have to be strong enough to secure the connection but not too tight to prevent rupture of the catheter or the suture. The choice of the suture material may have an impact. Multifilament nonabsorbable sutures are mostly preferred (Silk). They are better manipulated with minimal memory effect. Monofilament sutures are regarded safer for the prevention of infection (bacteria are attached less easily compared to multifilament material) but may tear the silicon tube. For the same reason, the size of the suture is also important. Sizes 2-0 or 3-0 are preferred. The knot has to be efficient but not too big and preferably should face downward so that it is hidden in cases of thin-skinned patients [37, 72].

Several studies and reviews highlight the significance of handling the shunt components only with instruments, avoiding any contact with gloved hands, to reduce the infection rates [10, 11, 71, 81]. If these instruments are sharp and improperly used, they may tear the shunt catheters leading to shunt malfunction if the damage is not noticed. Similarly, when passing the catheter through the tunneler, an acute angle of insertion should be avoided in order to prevent abrasions to the tube caused by the sharp end of the tunneler. Lubrication of the catheter with sterile saline is a useful maneuver. When performing a shunt revision (i.e., for proximal obstruction) many surgeons use a scalpel blade to cut the knot and disconnect the shunt components. This is hazardous for the catheter’s integrity and should be performed with great caution if there is no option for insertion of new hardware. The surgeon must target the knot – not the thread – and the direction of the movement should be away from the catheter. After that, the tube is inspected for unnoticed tears. If such exist, a small piece of the catheter – containing the tear – is cut away and the rest is reconnected as usual.

The choice of the burr-hole site may influence the mechanical complications rate. As previously explained, a shunt with a frontal burr hole usually needs an additional connection and this adds to the failure likelihood. On the other hand, Aldrich et al. found that occipitally placed shunts have a significantly higher rate of dislocation than frontally placed shunts [3]. Langmoen et al. studied the differences between ventriculoperitoneal and ventriculoatrial shunts with respect to the problem of mechanical failures and they found that the fracture rates of the distal catheter were almost the same (2.9–3 %) [46].

8.2 Dislocation

Dislocation of a shunt component refers to the migration of a previously correctly placed catheter or valve. The component could be disconnected from the shunt system or not. There is a lack of uniformly applied definitions regarding mechanical complications of shunts. In addition, there is paucity in the recent literature about this specific type of shunt malfunction. Most authors focus on the rheological properties of these systems.

The prevalence of pediatric shunt malfunction was reported to be 39 % at the first year and 53 % at 2 years [21]. The rate of shunt discontinuity (fracture or disconnection) is up to 10 % and refers to either ventriculoperitoneal or subduroperitoneal shunts [48]. Migrations of proximal or distal catheters have been reported on case reports and accurate frequencies extracted from efficient series or reviews are missing.

8.2.1 Proximal Catheter

The dislocated proximal catheter could enter deeper into the cranial cavity or retract distally from the previously desired location. Most systems include an angled port where the distal part of the ventricular catheter is secured in a 90° position before the connection to the distal part. Others prefer to use burr-hole reservoirs or burr-hole-type valves which are connected to the ventricular catheter. The connection should be secured with a suture properly knotted. This connection is not always straightforward, especially in cases of slitlike ventricles where even minimal manipulation of the proximal catheter can lead to improper placement. A loose connection of the ventricular catheter may result in spontaneous proximal migration. This can also happen iatrogenically in future shunt revisions when one tries to lift the burr-hole reservoir or valve and misses the disconnected catheter. Besides the surgical aspect, recently the material of the catheter was found to be responsible for the disconnection and intracranial migration [15]. The Bioglide® catheter (Medtronic, Minneapolis, US-MN) designed to potentially decrease cell adhesion and thereby reduce shunt obstruction or infection by becoming hydrophilic and lubricious when hydrated, was prone to disconnection because of this slippery surface. It was eventually recalled by Medtronic in 2009.

8.2.2 Distal Catheter

The most case reports in the literature concerning catheter migration are about the distal ventriculoperitoneal components. The distal catheter in the majority of VP shunts starts from the distal end of the valve – usually located occipitally or frontally – and ends into the peritoneal cavity in a variety of lengths, related to the manufacturer’s guidelines and the surgeon’s preferences. The length varies from 15 to 120 cm. Some surgeons prefer to cut short the catheter especially in newborns. The long tube and the intestinal peristaltic movements were considered responsible in rare cases of spontaneous knot formation [14, 41]. On the other hand, short catheters may add to overdrainage as explained by the Poisseuille law. A intra-abdominal length of 30–40 cm is sufficient.

The distal catheter can migrate either distally from its previous position or proximally in a retrograde direction. As already mentioned, it can be disconnected from the shunt system or not. Usually it remains coiled into the peritoneal cavity (if disconnected) or a herniated loop protrudes through the peritoneal incision and part of it is fixed subcutaneously. There are case reports of distal catheter dislocation in almost every compartment of the human body. In the peritoneal cavity, the catheter can entry a hollow [40] or (much more rarely) a solid viscus [80]. The large intestine is the more probable entry site [28] and from there it protrudes through the anus. In cases of stomach perforation, transoral protrusions have been reported [8]. Additional reports include urinary bladder perforation and transurethreal protrusion [13], dislocation through the vagina [55], into the gallbladder [58], the scrotum [60], and extrusion through the abdominal wall [82], the umbilicus [1] and the lumbar region [39].

Several theories have been presented in order to explain the “transperitoneal” dislocations of the distal catheters. One is the iatrogenic perforation of the intestine during the insertion of the shunt. Some surgeons use the trocar technique, which may predispose to this complication in inexperienced hands [29]. Studies that compared mini-laparotomy and laparoscopic-assisted insertion techniques, failed to show an advantage of one against the other [44]. In obese patients and in multiple peritoneal reoperations, the laparoscopic-assisted technique should be strongly considered [29]. In general, anal protrusion of the catheter can have an early (a few weeks) or delayed presentation (from months to years) (Fig. 8.5). The “iatrogenic” theory applies mostly in the early presentation cases. For delayed presentations different explanations are given. Spring-coiled catheters had a higher tendency to relate with bowel perforation. The plain silicone catheters can initiate an allergic reaction and become adherent to the intestinal serosa with subsequent erosion into the lumen [12]. Local infective adhesions were also mentioned [53]. Some authors recommend the hydration of the silicon tube prior to insertion based on the tendency of the silicon rubber to become sticky when it remains in a dry state [41]. Another theory focuses on children with myelomeningoceles (MMC), which are reported to be more susceptible to bowel perforation due to their weaker intestinal musculature [25]. Glatstein et al. in 2011 reviewed the reported pediatric cases with catheter protrusions through the anus and showed 23 patients with ages ranging from 0.9 to 72 months (6 years) [29]. Sathyanarayana et al. in 2000 reviewed 45 adult patients and reported a bowel perforation rate of less than 0.1 % in VP shunt insertions [64].

Fig. 8.5

Extrusion of distal shunt catheter through the anus, on a 28-month-old girl. The shunt was inserted in the peritoneal cavity 12 months before, through open mini-laparotomy (not with trocar)

A retrograde migration of the peritoneal catheter is also well described. In these cases, the tube can end at the thoracoabdominal subcutaneous tissue (Fig. 8.6), the breast [69], the heart [57, 61], the lateral ventricle [4, 52], the subdural space [77], or extruded through the neck incision [83]. The mechanism here is more complex and multifactorial. The migration may be the result of abdominal wall contractions that can expel the shunt catheter into the fibrous tract surrounding the catheter [69]. Increased intraabdominal pressure due to obesity, constipation, or pseudocyst formation may contribute. Anchoring of the shunt tube to a more proximal calcified point may act like a “windlass” resulting in traction of the tube. The retained memory effect of the catheters that are coiled into the manufacturer’s package could be another possible explanation [20]. Forceful rotation or flexion–extension movements of the head and neck can pull out the distal catheter from the peritoneal cavity [69]. In girls the distal tube that is tunneled closely to the breast may become adherent to it and eventually, when the breast grows, can – theoretically – facilitate the upward migration of the catheter. Similar result can have the deterioration of severe scoliosis in the calcified shunt.

Fig. 8.6

Lateral abdominal radiograph of a 5-year-old girl who presented a few months after shunt insertion with symptoms of raised intracranial pressure. The distal catheter has coiled up in the region of the abdominal wound, outside the peritoneal cavity

The migration of the distal tube into the heart or more distally in the pulmonary vasculature is also described. Until 2011, there were 13 reported cases [57]. One mechanism proposed transvenous placement of the catheter, followed by proximal migration into the pulmonary artery. The transvenous placement must have occurred during the initial subcutaneous tunneling and further migration due to negative intrathoracic pressure and venous flow [26]. A different mechanism involved chronic erosion of a neck vein by the adherent shunt tube with subsequent proximal migration into the pulmonary artery [54].

The migration of a lumboperitoneal shunt catheter intracranially has also been described. In this case report [5], it occurred 10 months after the insertion and the tip of it was found in the deep parenchymal structures. Possibly it was caused by spinocerebral CSF flow.

8.2.3 Presentation

Fractures, disconnections, and dislocations of shunt components usually present as shunt malfunctions. But they do not always become clinically evident. In a retrospective study it was found that from the 25 cases of disconnections in 22 patients, only 9 (40.9 %) had nonfunctioning shunts and these shunts were removed in 8 patients [48]. In the above mentioned study, only 3 out of 9 patients with nonfunctioning shunts had no symptoms [48]. It should be kept in mind that shunt failure is a neurosurgical emergency until proven otherwise and that a high index of suspicion must be maintained when receiving a shunted patient with symptoms [56]. A patient shunted in his early childhood for posthemorrhagic hydrocephalus or after MMC closure and presents after a few years with evidence of shunt disconnection is extremely likely to have signs and symptoms of increased intracranial pressure. All these patients should have proper clinical evaluation, including fundoscopy. On the other hand, many disconnections are found incidentally, usually in scheduled follow-up. The typical finding is the discontinuation of the radiolucent catheter strip, between the retroauricular area and the clavicle, in a formal shunt series (plain X-ray films). Less frequently, the ventricular catheter is found disconnected from its burr-hole attachment.

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