Shunt Infections and Their Treatment

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CHAPTER 193 Shunt Infections and Their Treatment

Hydrocephalus is a common neurological condition in children and accounts for approximately 2% of all pediatric hospital admissions.¹ The real costs of these admissions are substantial, with total hospital charges estimated to be between $1.4 billion and $2 billion annually.¹ Although shunt insertion greatly mitigates the long-term disability caused by hydrocephalus, the complications associated with shunt insertion are a challenge in the long-term management of these patients. One shunt-related complication is the postoperative occurence of infection, which leads to increased hospital stays, additional negative impacts on the patient’s developmental progress, and increased mortality.² This chapter summarizes the clinical features of shunt infection, its treatment, the benefits of preventive measures, and the long-term outcomes of shunt infection in the pediatric population.

Infection Rates

General

Shunt infection rates differ among reported studies.²^,³ Borgbjerg and associates³ studied 884 individuals (440 of whom were younger than 14 years) who underwent placement of a new shunt from 1958 to 1989. The infection rate in this group was 6.2% for the first postoperative month, with an overall rate of 7.4%. Robust data are available from randomized clinical trials evaluating shunt designs in patients recruited from several medical centers. In one study, 344 patients were prospectively randomized to receive three different shunt designs.⁴ Nearly 40% of newly inserted shunts failed in the first year, with a shunt infection rate of 8.1% over the follow-up period of 1 to 3 years. These data reflect the shunt infection rate following a single shunt procedure. If patients are followed for longer periods, the cumulative likelihood of shunt infection related to multiple shunt procedures ranges from 19% to 38%.⁵^,⁶ Despite the presence of a self-reporting bias, the Hydrocephalus Association database demonstrated that nearly 40% of patients who had hydrocephalus for at least 10 years experienced at least one shunt infection.⁶ Although preventive measures (see later) may reduce shunt infection rates, a reasonable conclusion is that the overall infection rate for new shunts is between 3% and 8%. Compared to historical data, it seems that there has been a gradual decline in shunt infection rates.⁷ It is not entirelly clear what factors have contributed to this decline, although greater attention to sterile technique, preoperative antibiotics, and improved surgical technique may all play a role.⁸

Timing of Infection

Among pediatric patients, the majority of shunt infections occur relatively soon after operative placement of the shunt. In one of the larger series of pediatric patients with extended follow-up, Casey and colleagues ⁵ reported that among children with shunted hydrocephalus who underwent a first shunt revision for infection, 92% of infections occurred within 3 months of the initial shunt placement. Similarly, in infants, who appear to have higher shunt infection rates, the majority of infections occurred in the first 3 months after surgery.⁹ However, in a longer term analysis from a randomized trial, there appeared to be delayed shunt infections that occurred 2 to 3 years after shunt insertion.¹⁰

Risk Factors

Although many variables are proposed to affect shunt infection rates, the most consistent factor is patient age, with neonates and very young children at greatest risk.⁵^,¹¹ In one cohort study, children 6 months or younger had a 19% rate of infection, versus 7% among older children; this finding is similar to the reports of other groups.⁵

A variety of explanations accounts for the increased shunt infection rate in very young children, including the presence of age-related changes in the density and identity of bacterial populations on the skin of neonates, as well as increased susceptibility to pathogens due to the relative deficiency of the neonatal immune system. Although maternal breast-feeding has been associated with the maintenance of immunoglobulin G levels in neonates, no data exist on the potential role of breast-feeding in reducing the risk of shunt infection. Some data suggest that more highly adherent strains of coagulase-negative Staphylococcus, the most prevalent organism in shunt infections, occur in neonates.¹²

Along with age, numerous other factors have been examined for their role in shunt infection, including the timing of shunt placement, educational level of the surgeon, length and time of surgery, use of antibiotics before and after surgery, surgical method for placement of the distal catheter, type of shunt, reason for shunting (e.g., posthemorrhagic versus congenital hydrocephalus), previous shunt history, spinal dysraphism, number of early revisions, and concurrent infection. In some studies, the reason for shunt placement ⁵^,⁹ and the presence of spinal dysraphism were associated with increased rates of infection.¹¹ Two studies reported that patients with congenital hydrocephalus had lower rates of infection than did patients with either postinfectious or posthemorrhagic hydrocephalus. One study reported that half the children in the postinfectious and posthemorrhagic group had at least one episode of shunt infection by the end of 1 year.⁹ Although earlier studies suggested that the type of shunt affected infection rates, more recent randomized studies examining shunt types have not confirmed this finding. A caveat with regard to these results is that the findings are not consistent across individual studies, suggesting that other variables are leading to an observed bias.

With respect to surgical factors, there is a paucity of convincing data. Shunt infection requires the exposure of shunt hardware to an infectious agent, usually bacteria, that subsequently leads to an inflammatory response of varying severity. An infection can result from several sources: colonization at the time of surgery, skin breakdown and subsequent colonization, hematogenous spread, and retrograde infection involving either a perforated viscus or coincident peritoneal infection. The timing of shunt infection and the predominance of skin organisms suggest that colonization from the patient’s skin is the likely cause of most shunt infections. However, some data suggest that the patient’s own skin flora may not be the primary source of these bacteria.¹³^,¹⁴ A postoperative cerebrospinal fluid (CSF) leak leads to a very high risk of shunt infection (odds ratio of 19), presumably by allowing a direct path from the patient’s skin to the shunt hardware.¹⁵

Clinical Evaluation

Signs and Symptoms

Fever, headache, and pain in the setting of a recent shunt procedure are the usual symptoms reported to occur with infection. The actual presentation can vary and is dependent on the virulence of the organism and the location of the infection. It is useful to consider shunt infections as being present in three locations: within the ventricles and shunt, in the abdomen, or around the shunt hardware in the subcutaneous tissues. With low-virulence organisms, a shunt infection causes a pleocytosis and subsequent shunt obstruction, with corresponding symptoms such as headache and vomiting. Absence of a fever does not exclude shunt infection. More important, repeated shunt failure is sometimes caused by very low virulence organisms that may not be identified using routine culture techniques. Infections within the peritoneal cavity usually present with abdominal pain and shunt failure. Finally, wound infections at either the cranial or abdominal end may not affect shunt function but can result in wound dehiscence, cellulitis, and erosion of the skin over the hardware.

Symptoms and signs of shunt infection are also age dependent. Infants may present with increased irritability or, in more severe cases, apnea and bradycardia. Other clinical features include fever, gait disturbances, seizures (particularly in patients with a baseline seizure disorder, although new-onset seizures are possible), and visual disturbances, including upward gaze palsy and papilledema. The clinical presentation can also be affected by the type of infecting organism. For example, patients with shunt infection caused by gram-negative bacilli such as Escherichia coli frequently present in a relatively acute fashion with severe abdominal pain or septicemia, whereas shunt infections caused by skin organisms such as Staphylococcus epidermidis tend to be more indolent in nature. Staphylococcus aureus infections may be associated with erythema along the shunt tract. Individuals with ventricular-vascular shunts may present with subacute bacterial endocarditis as well as shunt nephritis, an immune complex disorder of the kidneys that is similar to glomerulonephritis.

Imaging Studies

Imaging studies play an indirect role in the diagnosis of shunt infection. Plain radiographs are used primarily to determine whether a shunt system is in continuity. Rarely, plain radiographs demonstrate a specific finding, such as air within the peritoneal cavity, that is strongly suggestive of a shunt infection.

In many cases shunt infection causes some degree of shunt obstruction and results in findings consistent with that diagnosis, such as increased ventricular size. This is usually easily determined on an ultrasound study if the fontanelle is open or with computed tomography (CT) or magnetic resonance imaging (MRI). Complex shunt infections, associated with multicompartmental hydrocephalus, severe ventriculitis, or resistant or virulent organisms, result in dramatic findings (Fig. 193-1). Imaging studies are needed in these situations to determine whether cystic collections should be fenestrated or multiple catethers are required for effective CSF drainage.

FIGURE 193-1 Axial and coronal T1-weighted magnetic resonance images of the ventricular system and surrounding brain of a 5-year-old boy with bilateral schizencephaly and multicompartmental hydrocephalus caused by severe ventriculitis.

Peritoneal infections usually result in the formation of loculated fluid collections that can be detected by sonography or CT. Formation of an abdominal “pseudocyst” is usually caused by a localized reaction of the omentum and progressive accumulation of fluid within that space (Fig. 193-2). Drainage of the pseudocyst is rarely needed unless there is strong suspicion of a true abscess or the collection fails to resolve after treatment. Antibiotic treatment usually results in rapid improvement. It is our practice to confirm resolution of the fluid collection with sonography or CT before placement of a new shunt.

FIGURE 193-2 Sagittal ultrasonographic view of an abdominal fluid collection in a 7-year-old child with a ventriculoperitoneal shunt infected with methicillin-resistant Staphylococcus aureus.

Laboratory and Bacteriologic Studies

Shunt infection is a recognized complication of shunt insertion, and clinical suspicion should guide the investigations performed to either confirm or exclude the diagnosis of infection. If there is an obvious skin infection directly over the shunt hardware, or if the hardware becomes exposed owing to skin breakdown, infection should be assumed and antibiotic treatment should be started, along with removal of the device. Skin swabs are rarely helpful in isolating the exact organism because cross-contamination invariably occurs. In most other situations, confirmation of a shunt infection and identification of the causative organism require a CSF sample. This is most commonly obtained by percutaneous aspiration of the shunt through the cranial reservoir. Even if there is some degree of proximal shunt obstruction, CSF can usually be obtained. If the proximal portion of the shunt is completely obstructed and symptoms are suggestive of shunt infection, there are two options. If there is no evidence of raised intracranial pressure, a small-volume lumbar puncture (2 mL) can be done. It should be noted that a negative lumbar puncture result does not completely exclude a shunt infection. If the patient has symptoms of raised intracranial pressure, shunt exploration is required. An intraoperative Gram stain and cell count may clarify the issue. If there is sufficient doubt and the CSF analysis is inconclusive, the shunt should be externalized until final cultures are obtained.

CSF obtained from a shunt tap is tested for glucose, protein, and cell counts, and a separate specimen is sent for culture. Low glucose, high protein, and high nucleated cell counts are generally indicative of infection. Depending on the initial event causing hydrocephalus, CSF cell counts can remain elevated for weeks or months. A differential count can help clarify the origin of an elevated white cell count, with lymphoctyes representing the predominant population after intraventricular hemorrhage or craniotomy. CSF cultures may also be positive for infectious organisms, although negative cultures can occur and should not be considered proof of absence of a shunt infection. Repeated shunt failure is sometimes a warning that an indolent shunt infection is present.

CSF ventricular shunt catheter infections occur via three routes: contamination of the shunt material with skin organisms at the time of surgery, contamination from the bloodstream, and contamination along the shunt tubing from an abdominal source (generally associated with bowel perforation, although in some cases, no gross disruption of the bowel is observed). The source of the infection is reflected in the causative bacterial agent. The most common organisms found in shunt infections are typical skin flora such as coagulase-negative S. epidermidis,^5,^9,¹⁶ followed by S. aureus in a roughly 2 : 1 ratio. Interestingly, data suggest that in addition to its easy access by virtue of being a skin contaminant, S. epidermidis secretes a mucoid material that enhances its ability to adhere to foreign bodies such as shunt material.¹⁷ Shunt infections in which enteric organisms are isolated, including gram-negative bacteria such as E. coli, Proteus, and Klebsiella, are also common. Delayed infections with anaerobic diphtheroids such as Propionibacterium are particularly difficult to assess and treat because cultures may remain negative for more than a week.¹⁸ In the setting of repeated shunt failure, an indolent infection with Propionibacterium acnes should be considered, and CSF cultures should be followed for at least 1 week. Fungal infections are reported but are less common than bacterial infections. These often lead to repeated shunt failures and are notoriously difficult to treat, often requiring prolonged administration of antifungal agents.

Treatment

Most shunt infections caused by staphylococcal species do not cause significant tissue damage or a severe inflammatory response. Intravenous antibiotic treatment usually results in rapid bacteriogic clearance, with resolution of the CSF pleocytosis. Persistence of viable bacteria in or around the shunt hardware usually precludes the use of antibiotics as the sole treatment for shunt infections. The majority of shunt infections require surgical removal of the shunt, placement of an external CSF drain, and several days of intravenous antibiotics. A new shunt is usually not placed until CSF cultures are negative for bacterial growth. The recommended interval between shunt removal and reinsertion averages approximately 10 to 14 days, with at least 48 hours between the final negative CSF culture and reinsertion. A survey of centers to determine variations in practice patterns noted that the treatment time for shunt infection varied widely from 4 to 47 days.¹⁹

In the interval between shunt removal and reinsertion, a variety of methods can be used to support shunt-dependent patients, including shunt externalization or placement of external ventricular or lumbar drainage catheters. In patients for whom temporary external drainage systems are not appropriate, such as premature infants with very low birth weight, intermittent lumbar punctures or ventricular taps can be performed. Patients who might require these methods include those with high CSF protein levels and hence an increased likelihood of valve malfunction and those with repeated shunt infections, for whom the temporary removal of all foreign material may be necessary to clear material from the CSF. Replacement shunts are frequently placed at a new location relative to the old shunt.

CSF culture results are used to guide the selection of antibiotics during the interval between removal of the infected shunt and replacement of a new shunt. Given the preponderance of skin organisms such as S. epidermidis and S. aureus that are identified as infectious agents, intravenous vancomycin is frequently used initially, with rifampin occasionally added for increased coverage. Once the cultures have returned, antibiotics can be selected based on the identified organism and drug sensitivities. For example, in the absence of methicillin-resistant S. aureus organisms or penicillin-allergic patients, both S. epidermidis and S. aureus can be treated with oxacillin or nafcillin, with good results.

An alternative to surgical replacement of the infected shunt is the use of antibiotics alone. Although there have been cases of shunt infection eradicated by this method,²⁰^,²¹ purely medical treatment, administered either intravenously or by a combination of intravenous and intrathecal routes, appears to be less effective than surgical treatment.^2,^17,²² In the sole randomized trial to compare medical versus surgical treatment of shunt infection, 30 patients with ventriculoperitoneal shunts were randomized to one of three treatment arms: (1) removal of the shunt, accompanied by the use of systemic antibiotics and an interim period of external ventricular drainage or ventricular taps; (2) immediate surgical replacement of the infected shunt, along with antibiotic treatment; and (3) antibiotic treatment only. All study subjects were treated with intravenous and intraventricular antibiotics. The cure rates, as measured by negative CSF cultures 48 hours and 1 to 4 months after treatment, were 100%, 90%, and 30% for the three groups, respectively. In addition to lower rates of cure, medically treated patients had a mean hospital stay of 47 days, versus 33 and 25 days for the two surgical groups, respectively.¹⁷

In general, studies reporting high rates of cure from the use of antibiotics alone are limited by small sample sizes or the failure to randomize patients in treatment groups. Although it is likely that some shunt infections can be treated by antibiotic therapy alone, there is often doubt about the organism’s identity at the time of diagnosis and a desire to avoid progression of a CSF infection. Prudence usually prompts the surgeon to proceed with shunt removal and broad-spectrum antibiotic coverage. Clearly, for resistant or recurrent infections, the intrathecal administration of antibiotics leads to increased antibiotic levels within the ventricular system; however, the role of routine intrathecal antibiotics in addition to intravenous antibiotics is unclear.²³

Outcome

A primary concern is the success of treatment. Kestle and colleagues ¹⁹ noted that the reinfection rate in their study was 26%. Two thirds of the reinfections were caused by the same organism, suggesting a failure to completely eradicate the original infection. Surprisingly, the reinfection rate was not affected by the duration of antibiotic therapy. This suggests that other factors such as resistance or lack of antibiotic penetration may account for reinfection.

An important question is whether patients with shunt infections have a poorer health-related quality of life as well as higher overall medical morbidity and mortality. Although some studies report no difference in the rate of death in children with infected versus noninfected shunts,⁵ the data from other studies suggest an increased mortality risk in the former. In a series of 108 infants presenting with hydrocephalus at birth and operated on from 1971 to 1981, the 10-year mortality rate was 71% in children who had a shunt infection, compared with 51% in children who did not.²⁴ Similarly, Walters and colleagues² reported a mortality rate of 34% among infected patients, versus 18% in a group of shunted patients without infection.

Few studies include data on the long-term outcomes for patients with shunts, and those that do provide conflicting results. Casey and colleagues ⁵ followed 155 shunted hydrocephalic children for 10 years and found no evidence that the number of infection-related episodes affected the final intellectual outcome, with a good outcome defined as the child’s ability to attend school. Renier and associates,²⁴ however, found that shunt infection was associated with poor functional results; the mean “late” postoperative IQ was 40 among children who had at least one shunt infection, versus an IQ of 60 among children without infection. There has been no examination of the possible correlation between intellectual outcome and clinical or demographic patient characteristics, such as the severity of infection or the age at which a child suffers an infection. Future studies are needed to examine these relationships in greater detail.

Prevention

The overwhelming preponderance of skin flora as the causative organisms in shunt infections suggests that minimizing contact between shunt equipment and the skin and strict adherence to sterile technique may reduce colonization by organisms. Specific intraoperative factors contributing to shunt infection were analyzed, with only postoperative CSF leak, prematurity, and a possible breach in the surgeons’ gloves reaching statistical significance.¹⁵

Data regarding the use of prophylactic antibiotics are strongly suggestive, but confirmation is lacking. Meta-analyses from pooled data suggest a 50% reduction in infection rates when prophylactic antiobiotics are used.²⁵^,²⁶ In one study, the benefit of prophylactic antibiotics was lost if the baseline infection rate was below 5%. A recent systematic review noted that although there was a trend toward a reduction in infection rates with the use of antibiotics, there was no support for continuting antibiotics beyond 24 hours after surgery.²⁷ All these analyses are limited by variations in technique and treatment of shunt infections.¹⁹

Recently, antibiotic-impregnated shunt (AIS) components have been introduced for use as temporary external ventricular catheters and implanted shunts.²⁸ The antibiotics found in AISs include rifampin, clindamycin, and minocycline. A number of studies have shown a reduction in infection rates, but these studies are usually observational or cohort studies without the benefit of randomization to control for other changes in surgical technique. Nevertheless, some of these results are particularly compelling. For example, using AIS components in a population of very young infants, Sciubba and associates²⁹ reported a shunt infection rate of 4.6% (5 of 74 patients) with at least 9 months’ follow-up. This represents a substantial reduction when compared with historical controls. A similar finding was noted by other investigators who compared groups of patients receiving AIS components with their own historical cohorts.³⁰^,³¹ A particular benefit was noted in the neonatal subgroup receiving first-time shunts. In contrast, an observational study of mainly adult patients did not find any reduction of shunt infection rates using AIS components.³²