An Imperfect Science: Diagnosis of CSF Shunt Malfunction

Clinical scenario: Your patient is a 20 yo male with a history of VP shunt placement as a child for obstructive hydrocephalus. He was brought to the emergency department by his family because of decreased responsiveness over the past day. On arrival to the emergency department, he has aniscoria (L greater than R), no verbal response, and withdraws his extremities symmetrically. An emergent non-contrast head CT shows no change in ventricular size from prior CT scan one year prior and a VP shunt series demonstrates no evidence of fracture of the shunt line. Clearly, something is critically wrong with the patient, but is it his VP shunt to blame?

Clinical question: What is the spectrum of shunt complications? What is the sensitivity of clinical exam and various imaging modalities in detecting shunt malfunction?

Literature Review: There are multiple forms of CSF shunts, the most common of which is the Ventriculo-Peritoneal shunt (as opposed to ventriculo-atrial & ventriculo-pleural) which shunts CSF into the peritoneal cavity. A CSF shunt is composed of a proximal catheter, reservoir, valve and distal catheter [1]. The proximal catheter starts in the frontal horn of the lateral ventricle and exits through a burr hole to connect to the reservoir which is located in the subcutaneous tissue (this is what is accessed when neurosurgery taps a shunt). Flow from the reservoir to the distal catheter is regulated by a one way valve. Programmable shunts allow for the setting of a specific pressure above which fluid drains through a valve. This is sometimes adjusted in one direction or another for VP shunt patients who experience headaches, lightheadedness or other symptoms related to the pressure when their evaluation is negative for obstruction, infection etc. For VP shunts, the distal catheter is then tunneled into the peritoneum

Image Source: Cancer Research UK / Wikimedia Commons

As an emergency physician, one must be familiar with the presentation and diagnosis of shunt complications because they are relatively common; incidence of VP shunt failure is close to 40% at one year and 50% at two years from initial shunt placement, at least in the pediatric population where it has been most actively studied[2]. There are multiple types of shunt malfunctions leading to increased intracranial pressure, including but not limited to:

1. Mechanical Obstruction - Most proximally, the catheter can be obstructed by blood, debris or in-growth of the choroid plexus. The catheter position within the lateral ventricle can also migrate. Kinking or fracture along the catheter track at any point will also lead to shunt failure, as will distal obstruction which can occur when the catheter adheres to the omentum or erodes into intra-abdominal organs.

2. Infection - This often presents with shunt failure, and occurs most commonly within 6 months of placement due to intraoperative contamination with skin flora. The overall incidence of shunt infection is common (8-10%).

3. Ventricular Loculations - Loculations within the ventricle can create non-communicating pockets of CSF that are not drained by the VP shunt. If these grow, they can cause symptoms of hydrocephalus.

At least in very young children, depressed level of consciousness, nausea/vomiting, headache, irritability, and fluid tracking along the shunt site are highly predictive of shunt malfunction (see positive LR below). However, none of these clinical signs and symptoms are adequately sensitive to rule out shunt malfunction in their absence [2,3]. Some signs like abdominal pain/peritonitis are less commonly seen, but more highly predictive of shunt infection.

LR, Sensitivity, & Specificity for clinical signs and symptoms associated with shunt failure in two large pediatric studies

In addition to overall clinical exam and picture, radiographic imaging plays a central role in the emergency department evaluation of VP shunt malfunction.

CT scans are the most commonly used imaging modality to evaluate for shunt malfunction. While enlarged ventricles (when compared with prior imaging studies) are the canonical feature of shunt obstruction, other CT findings correlated with increased intracranial pressure include effacement of the cortical sulci, loss of the basal cisterns and periventricular edema due to transependymal CSF absorption [4]. Based on multiple retrospective pediatric studies using surgical shunt revision as a "gold standard", CT has a sensitivity for shunt malfunction of anywhere between 53% to 92% [4,5; see Table below]. In one small retrospective study of 174 adults evaluated for shunt malfunction with both shunt series and head CT, head CT had a sensitivity of only 52%, a specificity of 78% and negative predictive value of 88% for shunt malfunction [6]. This study only included patients who had had shunt series performed, so it may have underestimated the sensitivity of CT by excluding patients who were evaluated with CT alone. While this is a wide range of estimations for sensitivity, the important point is that a negative head CT does not completely rule out a shunt malfunction.

Shunt series radiographs are used to identify mechanical shunt defects such as shunt discontinuity or kinking. Studies in both children [4,7] and adults [6] support the conclusion that although the yield and sensitivity of radiographic shunt series is very low (see Table below), it is not zero. Shunt series rarely (~ 1-2%) detect abnormalities not identified on initial CT that prompt surgical revision. Therefore, shunt series are still indicated in the evaluation of potential shunt malfunction.

Table 2 from Boyle and Nigrovic, 2015. Reference 4.

In some cases, more commonly in pediatric institutions, MRI protocols have been instituted to reduce cranial radiation in children [4,8,9]. This has been made possible in part due to advances in MRI technology that have allowed for development of "ultra-fast" or Rapid MRI protocols that can acquire images in a span of ~ 1-4 minutes. Rapid Cranial MRI has been studied in comparison to CT for detection of ventricular shunt malfunction in the pediatric population, and appears to be comparable at least with respect to specificity and accuracy [8]. When considering using MRI in place of CT, the provider should be aware that some VP shunts have a programmable shunt valves that can be affected by the magnetic force of the MRI machine and may need to be readjusted after the exam. For this reason, it is common practice to obtain coned-down radiographs of a small indicator usually located near the proximal portion of the distal catheter to identify the setting prior to MR and then again after MR. If the programmed setting has changed, the neurosurgeon can use a magnet to reprogram the setting. The radiologist uses an indicator that looks like a clockface to determine the settings. 

Image source: http://www.ajnr.org

Take home Points: Malfunction and infection are common complications of CSF shunts. No single clinical exam finding or image study is sufficient to rule out shunt malfunction, and clinical management should take into account patient history, overall clinical picture, diagnostic data and neurological assessment.
 
Submitted by Maia Dorsett @maiadorsett
Faculty Reviewed by Peter Panagos and Richard Griffey
 
References: 
1. Wallace, A. N., McConathy, J., Menias, C. O., Bhalla, S., & Wippold, F. J. (2014). Imaging Evaluation of CSF Shunts. American Journal of Roentgenology, 202(1), 38-53.2.Garton, H. J., Kestle, J. R., & Drake, J. M. (2001). Predicting shunt failure on the basis of clinical symptoms and signs in children. Journal of neurosurgery, 94(2), 202-210.3. Piatt Jr, J. H., & Garton, H. J. (2008). Clinical diagnosis of ventriculoperitoneal shunt failure among children with hydrocephalus. Pediatric emergency care, 24(4), 201-210.4. Boyle, T. P., & Nigrovic, L. E. (2015). Radiographic Evaluation of Pediatric Cerebrospinal Fluid Shunt Malfunction in the Emergency Setting. Pediatric emergency care, 31(6), 435-440.
5.Lehnert, B. E., Rahbar, H., Relyea-Chew, A., Lewis, D. H., Richardson, M. L., & Fink, J. R. (2011). Detection of ventricular shunt malfunction in the ED: relative utility of radiography, CT, and nuclear imaging. Emergency radiology, 18(4), 299-305.
6. Griffey, R. T., Ledbetter, S., & Khorasani, R. (2007). Yield and utility of radiographic “shunt series” in the evaluation of ventriculo-peritoneal shunt malfunction in adult emergency patients. Emergency radiology, 13(6), 307-311.
7. Desai, K. R., Babb, J. S., & Amodio, J. B. (2007). The utility of the plain radiograph “shunt series” in the evaluation of suspected ventriculoperitoneal shunt failure in pediatric patients. Pediatric radiology, 37(5), 452-456.
8.Boyle, T. P., Paldino, M. J., Kimia, A. A., Fitz, B. M., Madsen, J. R., Monuteaux, M. C., & Nigrovic, L. E. (2014). Comparison of rapid cranial MRI to CT for ventricular shunt malfunction. Pediatrics, 134(1), e47-e54.
9. Koral, K., Blackburn, T., Bailey, A. A., Koral, K. M., & Anderson, J. (2012). Strengthening the argument for rapid brain MR imaging: estimation of reduction in lifetime attributable risk of developing fatal cancer in children with shunted hydrocephalus by instituting a rapid brain MR imaging protocol in lieu of head CT. American Journal of Neuroradiology, 33(10), 1851-1854.10.