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 colleagues⁵ 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

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