Diagnosis of a CSF shunt infection requires either isolation of a pathogen from ventricular fluid, lumbar CSF, or blood (for VA shunts) or the presence of CSF pleocytosis (usually defined as >50 white blood cells/mm3 in the context of a CSF shunt) in combination with either shunt malfunction or one or more of the signs or symptoms listed in Table 70–3.
The CSF should be sent for cell count, glucose, protein, Gram stain, and aerobic and anaerobic bacterial culture (Table 70–6).22,23 A CSF fungal culture should also be performed in premature infants and in children with other immunocompromising conditions.13 A mild CSF pleocytosis, low CSF glucose level (hypoglycoracchia), and elevated CSF protein are usually present in cases of ventricular infection. CSF white blood cell counts typically range from 100 to 2500/mm3 in VP shunt infections although normal CSF parameters (including CSF white blood cell count) have been reported in 17–35% of children with VP shunt infections.4,10,22 CSF pleocytosis alone is not diagnostic of infection. Mild to moderate pleocytosis (20–500 white blood cells/mm3) also occurs as a consequence of postsurgical or foreign body (i.e., shunt)-associated inflammation. Furthermore, infections caused by indolent organisms such as P. acnes may fail to induce a vigorous inflammatory response.
Table 70–6. Diagnostic Tests for Patients with a CSF Shunt and Suspected Infection ||Download (.pdf)
Table 70–6. Diagnostic Tests for Patients with a CSF Shunt and Suspected Infection
Step 1: Detect shunt malfunction
Assess shunt patency (Box 70–1)
Shunt series (radiographs of skull, neck, chest, and abdomen)
CT or magnetic resonance imaging to diagnose ventriculitis, intracranial abscess, and empyema and identify changes that suggest elevated intracranial pressure
Step 2: Detect infection
Blood cultures (especially if ventriculoatrial shunt)
- Gram stain
- Aerobic and anaerobic culture
- Cell count and differential
- Glucose, protein
Urinalysis, serum C3 and C4 complement in patients with a ventriculoatrial shunt to diagnose shunt nephritis
The types of white blood cells present in the CSF also facilitate the diagnosis of infection. McClinton et al.24 found that the presence of >10% CSF neutrophils had a specificity of 99% for shunt infection (i.e., almost all patients without shunt infection had very few CSF neutrophils). Furthermore, the positive predictive value of >10% CSF neutrophils was 93% (i.e., almost all patients with >10% CSF neutrophils had a shunt infection).24 CSF eosinophilia (>5% of total CSF white blood cell count) has also been associated with both shunt infection and malfunction but may also occur in response to intrathecal antibiotics or as a reaction to the shunt catheter.24
Ideally, fluid from the reservoir should be obtained by percutaneous aspiration under sterile conditions. Shunt drainage should be performed by a neurosurgeon or a clinician with experience in performing this procedure. The potential complications of draining CSF directly from the shunt include bleeding at the puncture site, CSF leakage, mechanical damage to the valve, and introduction of infection. In addition, draining CSF too rapidly may cause intraventricular or subdural bleeding. Bacteria are identified by Gram stain of CSF obtained from the reservoir in up to 80% of cases although the likelihood of a positive Gram stain depends on the causative organism. S. aureus and aerobic gram-negative rods such as E. coli typically have positive Gram stain results while P. acnes, coagulase-negative staphylcoccci, and viridans group streptococci are positive in <40% of cases.10 Therefore, a negative Gram stain does not exclude the diagnosis of shunt infection. Although most bacteria causing shunt infections grow within 48–72 hours, cultures should be held for 5–7 days since fastidious organisms such as P. acnes may take longer to grow. Contamination and true infection cannot be readily differentiated when bacteria are identified by culture in the context of normal CSF parameters. In such cases, infection should be strongly considered and shunt aspiration should be repeated; a positive culture with the same bacteria usually indicates true infection.
Isolation of bacteria from CSF obtained by lumbar puncture suggests CSF shunt infection in the appropriate context. However, children requiring a CSF shunt often have impaired CSF flow. As a consequence, the ventricular fluid may have little or no communication with the lumbar spinal fluid and CSF obtained by lumbar puncture may not suggest infection despite the presence of ventriculitis.
Blood should be routinely obtained for culture from patients evaluated for suspected shunt infection. While a negative peripheral blood culture does not rule out a shunt infection, a positive blood culture often influences the choice of antimicrobial therapy. Among patients with confirmed VP shunt infection, blood cultures are positive in 20–30% of cases.4,10 Peripheral cultures are more likely to be positive in patients with VA shunt infection where blood cultures are positive in 90% of cases.1,7 Laboratory manifestations of shunt nephritis include anemia, azotemia, hypocomplementemia, as well as hematuria and proteinuria.
Neuroimaging studies including X-rays of the skull, neck, chest, and abdomen (the “shunt series”) and computed tomography (CT) should be performed as part of the routine evaluation of a child with a suspected CSF shunt infection. Neuroimaging studies may provide evidence of shunt malfunction that accompanies some cases of infection. Specific abnormalities that can be visualized on the shunt series include disconnection of the distal catheter, retraction of the distal catheter tip, and discontinuity near the proximal shunt bulb. Routine performance of shunt series has a low overall yield but on rare occasions detects abnormalities that are missed by CT.25 Both CT and MRI of the head will detect increased ventricular size; this finding may reflect either increased intracranial pressure or hydrocephalous ex vacuo, a condition where the increased ventricle size reflects shrinkage of brain parenchyma rather than an increase in the intracranial pressure. Ventriculitis and meningitis can be visualized on CT and MRI as enhancement of the ventricular ependymal lining or cerebral cortical sulci.26 In rare cases, subdural empyema or brain abscess may be the first indication of shunt infection. Radiologic imaging of other areas should be considered depending on the location of the distal catheter tip. CT or ultrasound of the abdomen may identify abdominal peritoneal pseudocysts at the distal portion of a VP shunt (Figure 70–2). Some free fluid in the peritoneal cavities is normal but larger amounts should raise concern for infection. Chest radiography detects pleural effusions associated with ventriculopleural shunt infection.