Ventriculoperitoneal (VP) shunt implantation is the standard treatment for patients suffering hydrocephalus, a condition of excess accumulation of cerebrospinal fluid (CSF) in the liquor space. The main reason for hydrocephalus is an obstruction of normal CSF flow, which makes it a life-threatening neurological disorder [1]. First symptoms are heterogeneous and include dementia, incontinence and reduced vigilance. Subsequently, it can cause severe damage to the brain within a short period of time by secondary hemorrhage, infection, trauma or stroke.
The VP shunt placement is a simple and effective neurosurgical procedure, which came up in 1960 as a novel treatment approach [2]. The shunt system allows drainage of excess CSF to regulate intracranial pressure by absorption of the fluid in the abdomen. Shunt implantation was first performed using thin elastic tubes, which subcutaneously extend to the abdomen. The shunt systems are equipped with a pump reservoir and a valve in order to allow flow control and prevent reflux. Once a patient has a shunt system implanted, he/she remains dependent on the system for lifetime.
In case patients with a VP shunt system present with acute symptoms of hydrocephalus in an acute setting, occlusion of the VP shunt system has to be excluded in order to plan further clinical treatment, potentially leading to surgical revision of the VP shunt system. The evaluation of the patient requires high clinical expertise, as acute symptoms are unspecific, including headache and vomiting. The diagnosis of shunt obstruction and the localization of the obstruction site are still lacking adequate mechanical instrumentation. Upon hospital admission, patients showing clinical signs of shunt obstruction undergo investigations using ionizing radiation, invasive tests and open surgical examination, bearing several risks including the danger of possible brain damage. In the first year following implantation, up to 40% of patients need repeated surgery, and 10% additionally will undergo surgery annually after the first year [3]. Possible reasons for shunt obstruction include improper shunt placement, disconnection of shunt segments, breakage of the shunt catheter as well as infection, component malfunction and accumulation of cellular materials mainly in the ventricular catheter portion.
The clinical evaluation in patients with symptoms of shunt obstruction is frustrating for clinicians in the acute setting, as there are no feasible tests available. Neurosurgeons routinely apply pressure on the silicon membrane of the pump reservoir, which is an integral part of the shunt system distally to the ventricular part and proximal to the peritoneal part of the shunt system. Simultaneously, the distal end of the catheter is obstructed by manual pressure in a subcutaneous localization. Upon release of the pressure, the reservoir of the system refills in an antegrade manner, which should be visible as the silicon membrane of the pump reservoir causes a small pouch on the skin level. However, this clinical evaluation does not allow any conclusion on the localization of the obstruction.
Ultrasound (US) is an attractive imaging approach in this setting, as it is readily available, not time consuming and can be performed in different locations according to the course of the CSF shunt system. The tubing of CSF shunt systems typically has a lumen diameter of 1 mm and is made of silicone rubber, which is a transparent layer for US, while the inner surface of the shunt systems is not completely smooth, causing disruptions of a potentially laminar flow system [4, 5]. Due to the low flow rates that result from the small diameter of CSF shunt systems and large flow fluctuations, conventional Doppler US systems have a limited sensitivity to detect flow within the catheter, especially as CSF is a clear colorless fluid, which consists of water to 99%. The shunt systems are reported to allow very low flow rates in combination with a high variability of these flow rates in correlation to patient positioning. Drake et al. reported flow rates in CSF shunt systems between 3 and 40 ml/h [6], corresponding to 0.05 ml/min and 0,7 ml/min. In accordance, Kadowaki et al. published data with a physiologic flow rate between 0.01 and 0.1 ml/min [7]. From a technical point of view, modern shunt systems offer programmable, hydrostatic and flow-controlled walves as well as anti-siphon devices to prevent unphysiological flow rates, when vertical body position is taken. In vertical body position, the flow rates observed reach from 93 to 232 ml/h, corresponding to 1.5 to 3.9 ml/min respectively [8].
Superb microvascular imaging (SMI) is a novel US imaging technique allowing the visualization of low velocity blood flow in small vessels. The essential technical aspect of this new US procedure is based on the fact that noise generated from motion artifacts can be suppressed. The images contain a monochrome colour map of blood flow, which is registered onto the B-mode image. We therefore wanted to evaluate the technical feasibility of SMI US visualizing patent flow in VP shunt systems simulating physiologic flow rates in a phantom model.