Shunt overdrainage represents an incompletely understood condition of variable clinical presentations and imaging findings that still represents a challenge for neurosurgeons, in part due to the lack of agreement or uniformity concerning the entity. Important problems exist relating to its definition.
The real incidence of the entity its unknown.
There are also problems with the classification, and the pathophysiological theories behind the various treatment strategies proposed. Recent reports have suggested that the evidence about overdrainage and its consequences is not so robust as presumed. Consequently, the topic requires more detailed examination 1).
There are three types of overdrainage: related to body posture, related to excessive vasogenic pressure waves and related to excessive pumping of shunt prechamber (less common, but worth considering, particularly in paediatric cases). Generally, posture related overdrainage is derived from the fact that in most of shunts flow is controlled by the differential pressure between inlet and outlet. Distance between ventricles and peritoneal space may produce in vertical body position additional pressure gradients. In ventriculo-peritoneal shunts this gradient generates distal negative pressure caused by the fluid column in peritoneal drain. In lumbo-peritoneal shunts this will be proximal positive pressure exerted by CSF column ranging from ventricles to lumbar catheter 2).
see also Ventriculoperitoneal shunt overdrainage.
There is mounting evidence that changes in cerebrospinal venous system dynamics are a key factor to the pathophysiology of chronic overdrainage syndrome.
Despite the prevalence of these conditions, their mechanisms are still not fully understood.
Although several improvements have been observed in the past few years in shunt technology, currently available systems still present several associated problems. Among these, overdrainage along with its complications remains one of the great challenges for new shunt designs 3).
A medical literature search and review were performed via Google Scholar. Of 565 publications, 3 primary papers were identified and a timeline was developed demonstrating the convergence of the aforementioned concepts. From the primary papers, 25 relevant publications were selected and further analyzed searching for hypothesis, evidence, and conclusions.
Overdrainage, siphoning, and slit-ventricle syndrome are associated concepts that have converged into a pathophysiological theory where siphoning of CSF leads to overdrainage, which is then hypothesized to cause slit-ventricle syndrome in a small subset of patients.
The data suggests that while there have been numerous reports regarding overdrainage and its consequences, the evidence is not as robust as currently presumed and this subject requires prospective exploration 4).
Devices to avoid overdrainage when the patient is upright:
a) antisiphon devices (ASD): prevents siphoning effect when patient is erect. Some valves have ASDs integrated into the valve. ASDs always increase the resistance of the shunt
Shunt overdrainage, whether intermittent proximal obstruction or low pressure states, are best managed with valve upgrades and the addition of devices that retard siphoning. Increased ICP without ventriculomegaly at the time of shunt failure is best managed by shunting devices that access the cortical subarachnoid space such as lumboperitoneal shunts or shunts involving the cisterna magna. Cranial expansion operations and subtemporal decompression should be limited to patients with craniofacial syndromes 5).