Cerebrospinal fluid shunt malfunction diagnosis
The diagnosis of cerebrospinal fluid shunt malfunction based on a careful clinical history, examination, and investigations such as computed tomography (CT) scanning and plain X-ray shunt series is not always straightforward 1).
Admission for observation is expensive and excessive CT scanning carries a radiation burden. Many patients may be admitted and subjected to CT scanning on multiple occasions. There is a need to develop more reliable methods of assessing shunt function and monitoring intracranial pressure (ICP) 3) 4) 5) 6) 7) 8).
Optic nerve sheath diameter may be assessed using ultrasound or magnetic resonance imaging (MRI). Implantable ICP sensors within a shunt system have been blighted by poor long-term stability. Long-term studies of the recently introduced Raumedic NEUROVENT-P-tel and the Miethke SENSOR RESERVOIR are awaited with a keen interest 9).
Several attempts have been made to measure the cerebrospinal fluid flow velocity utilizing different Phase contrast magnetic resonance imaging techniques. In a study, König et al. evaluated 3T (Tesla) MRI scanners for their effectiveness in determining of flow in the parenchymal portion of ventricular shunt systems with adjustable valves containing magnets.
At first, an MRI phantom was used to measure the phase-contrasts at different constant low flow rates. The next step was to measure the CSF flow in patients treated with ventricular shunts without suspected malfunction of the shunt under observation.
The measurements of the phantom showed a linear correlation between the CSF flow and corresponding phase values. Despite many artifacts resulting from the magnetic valves, the ventricular catheter within the parenchymal portion of shunt was not superimposed by artifacts at each PC MRI plane and clearly distinguishable in 9 of 12 patients. Three patients suffering from obstructive hydrocephalus showed a clear flow signal.
Cerebrospinal fluid flow detected within the parenchymal portion of the shunt by phase contrast magnetic resonance imaging may reliably provide information about the functional status of a ventricular shunt. Even in patients whose hydrocephalus was treated with magnetic adjustable valves, the CSF flow was detectable using PC MRI sequences at 3 T field strength 10).
Non-invasive techniques to assess ‘semi-quantitatively’ whether intracranial pressure is raised or not include optic nerve sheath diameter (ultrasound or MRI), tympanic membrane displacement and transcranial Doppler but none have yet been shown to be sufficiently accurate for routine clinical use in patients with potential shunt malfunction. Provision of a separate subcutaneous CSF reservoir is of proven benefit in allowing access to the cerebral ventricles to measure ICP and allow removal of CSF in an emergency
Various invasive diagnostic test procedures for the verification of shunt function have been described:
Invasive CSF pressure and flow measurements
CSF tap test and drip interval test
By comparison, publications addressing the noninvasive pumping test are rare.
Noninvasive pumping test of all formerly published results are values derived from tests with a variety of reservoirs and valves (at least 2 types).
In a few reports, the shunt/reservoir type used is not even specified, although the technical parameters of such reservoirs and valves are obviously essential.
To judge occlusions distally from the reservoir other authors have had to close the pVC transcutaneously by manual compression.
This is never possible with a sufficient certainty and, if ever undertaken, it usually does provide a source of error.
Rapid cranial MRI was not inferior to CT for diagnosing ventricular shunt malfunction and offers the advantage of sparing a child ionizing radiation exposure 11).